University of Potsdam
Institute of Chemistry
Group of Prof. J. Koetz
Versatile uses of halogen-free Ionic Liquids for the
formulation of non-aqueous microemulsion and synthesis
of gold nanoparticles
Dissertation
zur Erlangung des akademischen Grades
"doctor rerum naturalium"
(Dr. rer. nat.)
in der Wissenschaftsdisziplin "Kolloid- und Polymerchemie"
eingereicht an der
Mathematisch-Naturwissenschaftlichen Fakultät
der Universität Potsdam
von
Oscar Mario Rojas Carrillo
Potsdam, 2012
“ A person who never made a mistake
never tried anything new“.
Albert Einstein
To my family for their support and inspiration
ACKNOWLEDGEMENTS
I would like to sincerely thank Prof. Dr. Joachim Koetz, from the University of
Potsdam, for giving me the opportunity to do my PhD thesis in his group. I am
particularly grateful for his constructive discussions and support on practical and
theoretical aspects. I would like to express my appreciation to him for giving me the
opportunity to work independently and for his supervision in this interesting subject.
I would like to thank Prof. Dr. Ulla Wollenberger, from University of Potsdam, for the
collaboration, the helpful discussions and introduction to the fascinating topic of
bioelectrochemistry and biosensor systems. Furthermore, I would like thank her for
providing access to her lab to perform the cyclic voltammetric and UV-vis
measurements. I also thank Dr. Stefano Frasca and Ting Zeng for their contribution
to my work and constant support.
I express my gratitude to all the members of the Colloid Chemistry Group from the
University of Potsdam. I would like to especially thank to Dr. S. Kosmella for her
stimulating discussions and to Dr. B. Tiersch, S. Rüstig and Dr. C. Prietzel for the
TEM and Cryo-SEM pictures. I would like to give special thanks to my colleagues, for
their support, friendship and great atmosphere.
I thank C. Rabe and R. Stehle from the Helmholtz-Zentrum Berlin for their assistance
with the SAXS measurements and valuable discussion. I thank also the fruitful
collaboration with Dr. Erik Dujardin and Aniket Thete from the NanoSciences Group,
CEMES CNRS, from Toulouse, France.
Finally, I would like to acknowledge the National University of Costa Rica, the
National Science Bureaus of Costa Rica (MICIT and CONICIT) and DAAD for the
financial support.
Table of Content
List of Abbreviations .................................................................................................I
Abstract ................................................................................................................... III
Zusammenfassung.................................................................................................. V
1
Introduction .......................................................................................................... 1
1.1
Surfactants in aqueous solution ..............................................................................................1
1.1.1
Surfactants......................................................................................................................1
1.1.2
Micellization ....................................................................................................................2
1.1.3
Emulsions and microemulsions ......................................................................................5
1.2
Surfactants in Ionic Liquids .....................................................................................................6
1.2.1
Ionic Liquids (IL) .............................................................................................................6
1.2.2
Physicochemical properties of ionic liquids .....................................................................8
1.2.3
IL-based microemulsions with non ionic surfactants .....................................................12
1.2.4
IL-based microemulsions with ionic surfactants ............................................................15
1.2.5
Microemulsions with ionic liquid-like surfactants (IL-S) .................................................16
1.2.6
Fields of application of IL-based microemulsions .........................................................18
1.2.7
Characterization methods of microemulsions ...............................................................20
2
Aim of the work .................................................................................................. 22
3
Materials and Methods....................................................................................... 23
3.1
Materials ...............................................................................................................................23
3.1.1
Chemicals .....................................................................................................................23
3.1.2
Synthesis ......................................................................................................................23
3.1.3
Phase diagrams ............................................................................................................24
3.2
Methods ................................................................................................................................24
3.2.1
Surface tension .............................................................................................................24
3.2.2
Rheology.......................................................................................................................25
3.2.3
Conductometric measurements ....................................................................................25
3.2.4
Dynamic Light Scattering (DLS) ....................................................................................29
3.2.5
Small Angle X-Ray Scattering (SAXS) ..........................................................................32
3.2.6
Cyclic voltammetry (CV) ...............................................................................................35
3.2.7
Cryo-Scanning Electron Microscopy .............................................................................36
3.3
Gold nanoparticles (AuNPs) in Ionic Liquids .........................................................................36
3.3.1
Synthesis of AuNPs in IL-based microemulsions ..........................................................36
3.3.2
Synthesis of gold nanoparticles in ionic liquids .............................................................37
3.3.3
Characterization of gold nanoparticles ..........................................................................37
3.3.4
Electrode modification...................................................................................................38
3.3.5
Electrochemical experiments on Human sulfide oxidase (hSO) modified gold
electrodes. ....................................................................................................................................38
4
Results and Discussion...................................................................................... 39
4.1
Self-aggregation of ionic liquids studied by surface tension measurements .........................39
4.2
Microemulsions with [Bmim][OctSO4] ...................................................................................42
4.2.1
Phase Diagram .............................................................................................................42
4.2.2
Shear Viscosity .............................................................................................................44
4.2.3
Electrical Conductivity ...................................................................................................46
4.2.4
Cyclic Voltammetry (CV) ...............................................................................................50
4.2.5
Dynamic light scattering (DLS) .....................................................................................51
4.2.6
Cryo-scanning electron microscopy (Cryo-SEM) ..........................................................52
4.2.7
Small angle X-ray Scattering (SAXS)............................................................................53
4.3
Microemulsions with [Bmim][DodSO4] ..................................................................................56
4.3.1
Phase Diagram .............................................................................................................56
4.3.2
Shear Viscosity .............................................................................................................57
4.3.3
Electrical Conductivity ...................................................................................................58
4.3.4
Dynamic Light Scattering (DLS) ....................................................................................63
4.3.5
Small Angle X-ray Scattering (SAXS) ...........................................................................64
4.3.6
Cryo-Scanning Electron Microscopy (Cryo-SEM) .........................................................67
4.3.7
Conclusions ..................................................................................................................68
4.4
Synthesis of gold nanoparticles in ionic liquids .....................................................................70
4.4.1
General aspects............................................................................................................70
4.4.2
Synthesis of gold nanoparticles in IL-based microemulsion..........................................72
4.4.3
Synthesis of gold nanoparticles in pure ionic liquids .....................................................75
4.4.4
Gold nanoparticles prepared in ionic liquids used as a seed for the preparation of gold
rods…….. .....................................................................................................................................79
4.4.5
Application of gold nanoparticles in an electrochemical biosensing system .................81
4.4.6
Conclusions ..................................................................................................................88
5
Summary and Outlook ....................................................................................... 90
6
References ........................................................................................................... i
7
Appendix............................................................................................................ xx
7.1
Tables ................................................................................................................................... xx
7.2
List of publications ............................................................................................................. xxiv
7.3
List of presentations........................................................................................................... xxiv
List of Abbreviations
1
H NMR
C6mimPF6
AIBN
AOT
ATRP
AR
BaSO4
[Bmim][PF6]
BHDC
[Bmim][BF4]
[Bmim][OctSO4]
[C16mim][Cl]
Brij-35
CdS
C-480
C6F14
CD
CiEj
cmc
Cryo-SEM
CTAB
hSO
CV
cyt c
DLS
DMSO
DODMAC
DTSP
Dapp
DET
DNA
EDC
ET
EAN
EG
k
EGFP
[Emim][EtSO4]
[Emim][HexSO4]
Fc
FFEM
FRET
FTIR
fa
ΔG°mic
Au
AuNPs
AuNRs
1
H nuclear magnetic resonance
1-hexyl-3-methylimidazolium hexafluorophosphate
2,2-azobisisobutyronitrile
1,4-bis(2-ethylhexoxy)-1,4-dioxobutane-2-sulfonate
Atom-transfer radical polymerization
Aspect ratio
Barium sulfate
1-butyl-3-methylimidazolium hexafluorophosphate
Benzyl-n-hexadecyldimethylammonium chloride
1-butyl-3-methylimidazolium tetrafluoroborate
1-butyl-3-methylimidazolium octyl sulfate
1-hexadecyl-3-methylimidazolium chloride
Polyoxyethylene 23 lauryl ether
Cadmium sulfide
Coumarin 480
Perfluorohexane
Circular dichroism
Alkyl oligoethyleneoxide
Critical micelle concentration
Cryo-scanning electron microscopy
Cetyltrimethylammonium bromide
Human sulfite oxidase
Cyclic Voltammetry
Cytochrome c
Dynamic light scattering
Dimethyl sulfoxide
Dioctadecyldimethylammonium chloride
Dithiobis-N-succinimidyl propionate
Apparent diffusion coefficient
Direct electron transfer
Deoxyribonucleic acid
N-ethyl-N’-(3-dimethylaminopropyl)carbodiimide hydrochloride
Electron transfer
Ethylammonium nitrate
Ethylene glycol
Electrical conductance
Enhanced green fluorescent protein
1-ethyl-3-methylimidazolium ethyl sulfate
1-ethyl-3-methylimidazolium hexyl sulfate
Ferrocene
Freeze-fracture electron microscopy
Fluorescence resonance energy transfer
Fourier transformation infrared spectroscopy
Amphiphilic factor
Gibbs free energy of micellization
Gold
Gold nanoparticles
Gold nanorods
I
ILs
IL-S
ITC
K4Fe(CN)6
KPF6
L1
L2
MMA
W0
Moco
MD
MPT
MU
MUA
µE
N3111Tf2N
[Omim][Cl]
PAF
PAN
PEG-400
Φp
[Pmim][BF4]
(PS-PMMA)
(PEO-PPO-PEO)
PEI-5K
PEI-25K
PEI-IL
p-PNB
q
R6G
Rg
RTIL
SANS
SAXS
SAM
Triton X-100
Tween 20
TEM
XRD
YADH
Ionic liquids
Ionic liquid-like surfactants
Isothermal titration microcalorimetry
Potassium ferrocyanide
potassium hexaflourophosphate
Oil-in-water microemulsion
Water-in-oil microemulsion
Methyl methacrylate
Molar ratio
Molybdenum cofactor
Sulfite oxidase Moco domain
Molybdopterin
11-mercapto-1-undecanol
11-mercapto-1-undecanoic acid
Microemulsion
N,N,N-trimethyl-N-propyl ammonium
Bis(trifluoromethanesulfonyl) imide
1-octyl-3-methylimidazolium chloride
Propylammonium formate
Propylammonium nitrate
Polyethylene glycol, 400 (molar mass)
Percolation threshold
1-pentyl-3-methyl-imidazolium tetraflouroborate
Polystyrene-block-poly(methylmethacrylate)
Poly(ethylene oxide)-poly(propylene oxide)-poly(ethylene oxide
Poly(ethylenimine) 5 000 g mol-1
Poly(ethylenimine) 25 000 g mol-1
poly(ethyleneimine)-ionic liquid
p-nitrophenyl butyrate
Scattering vector
Rhodamine 6G
Apparent gyration radius
Room temperature ionic liquid
Small-angle neutron scattering
Small-angle X-ray scattering
Self assembled monolayer
Polyethylene glycol p-(1,1,3,3-tetramethylbutyl)-phenyl ether
Polyoxyethylenesorbitan monooleate
Transmission electron microscope
X-ray diffraction
Yeast alcohol dehydrogenase
II
Abstract
This thesis is divided into three main topics. The first subject of this work regards to
the formulation of non-aqueous microemulsions containing ionic liquids (ILs). The
ionic liquids i.e. 1-butyl-3-methylimidazolium octyl sulfate, [Bmim][OctSO4] and 1butyl-3-methylimidazoilium
dodecyl
sulfate,
[Bmim][DodSO4]
were
used
as
surfactants. The room temperature IL i.e. 1-ethyl-3-methylimidazoium ethyl sulfate
([Emim][EtSO4]) was used as the polar phase and toluene as the non polar
compound. The ternary systems were prepared at room temperature and
characterized by different techniques. Three different zones were identified in the
isotropic phase region, by means of conductometric experiments, which suggest the
formation of oil-in-IL, bicontinuous and IL-in-oil microemulsions.
Dynamic light scattering (DLS) experiments performed on the IL-in-oil region
indicates the formation of droplets of about 10nm in diameter. Small angle X-ray
scattering (SAXS) shows a single broad peak in both systems, similar to that
observed in aqueous based microemulsions. Additional information with regard to the
microstructure of the studied systems was obtained by modeling the scattering
intensity by using the Teubner-Strey model. The microemulsions were characterized
by cryo-scanning electron microscopy (Cryo-SEM) which indicated the formation of
droplets and a sponge phase in the range of the IL-in-oil and bicontinuous
microemulsions, respectively. Therefore, one can conclude that the investigated
systems may represent an interesting alternative as a water-free reaction medium to
perform chemical or enzymatic processes.
The following part of this work comprises the uses of IL as a solvent for the synthesis
of gold nanoparticles (AuNPs) by the chemical reduction of Au+3 to Au0, employing
poly(ethyleneimine) (PEI) as a reducing agent. Two strategies to synthesize AuNPs
were employed. One approach shows that AuNPs of about 20nm can be obtained in
IL-based microemulsions. The second approach shows that spherical and quite
narrow distributed AuNPs are synthesized in pure ILs. The formation of an IL-polymer
complex on the surface of the gold nanoparticles can be the reason for the longer
stability and solubility characteristics of the AuNPs.
The final part of this work describes the application of the prepared AuNPs in pure
ILs. In the first approach, AuNPs coated with PEI and IL were used as “seeds” for the
preparation of anisotropic particles, such as gold nanorods. In the second approach,
III
AuNPs were successfully incorporated in the design of a biosensor system. The
AuNPs protected with poly(ethyleneimine) were attached to an Au-electrode surface,
previously modified by a self assembled monolayer (SAM). The positively charged
surface provides an adequate platform for the adsorption of the enzyme i.e. Human
sulfite oxidase (hSO). Cyclic voltammetric (CV) experiments demonstrated that the
absorption of hSO was achieved. One can conclude that the immobilized AuNPs
enhance the electrocatalytic activity of the protein. Moreover, the proposed biosensor
exhibited a quick steady-state current response, and the ability to work at high ionic
strength. Additionally, the use of ionic liquids as alternative electrolyte in order to
perform enzymatic reactions was briefly explored.
In conclusion, the versatile uses which ionic liquids can posses, such as amphiphilic
molecules and co-surfactant for the formulation of non-aqueous microemulsions or as
solvent for the preparation of nanomaterials or as an electrolyte to perform enzymatic
reactions were demonstrated.
IV
Zusammenfassung
Die vorliegende Arbeit wurde in drei Hauptteile gegliedert. Teil 1 befaßt sich mit der
Herstellung und Charakterisierung von nicht-wässrigen Mikroemulsionen unter
Verwendung von zwei ionischen Flüssigkeiten (IF) und einer Öl-Komponente. Die
beiden
amphiphilen
IF-Komponenten
1-butyl-3-methyl-imidazolium
octylsulfat
([Bmim][OctSO4) und 1-butyl-3-methyl-imidazolium dodecylsulfat [Bmim][DodSO4])
weisen dabei tensidartige Eigenschaften auf. Die Raumtemperatur-Ionischen
Flüssigkeiten (RTIF) 1-ethyl-3-methylimidazolium ethylsulfat ([Emim][EtSO4])
und
1-ethyl-3-methylimidazolium hexylsulfat ([Emim][HexSO4]) wurden als polare Phasen
und Toluol als nicht-polare Komponente verwendet. Die ternären Systeme wurden
bei Raumtemperatur hergestellt und mit verschiedenen Techniken charakterisiert.
Konduktometrische-Messungen zeigen drei Typen von Mikroemulsionen, das heißt
Öl-in-IL, bikontinuierliche und IL-in-Öl-Mikroemulsionen. Strukturelle Umwandlungen
innerhalb des BmimOctSO4-Systems wurden durch die zyklischen Voltammetrie-und
Scherviskositäts-Messungen belegt. Dynamische Lichtstreuungsmessungen (DLS)
gaben zunächst Hinweise für die Bildung von Tröpfchen in beiden
IL-basierten
Mikroemulsionen. Die Resultate wurden durch SAXS Messungen bestätigt und durch
Cryo-Rasterelektronenmikroskopische Aufnahmen (Cryo-REM) visuell belegt. Das
untersuchte System repräsentiert ein wasserfreies System, das als Nano-Reaktor für
chemische oder enzymatische Prozesse verwendet werden kann.
Der zweite Teil dieser Arbeit befasst sich mit der Synthese von Gold-Nanopartikeln
(AuNPs) in ionischen Flüssigkeiten. Im ersten Ansatz wurden sphärische Au-NP
(~20nm) im Mikroemulsion-System hergestellt. Im zweiten Ansatz wurden ionische
Flüssigkeiten als Lösungsmittel verwendet, und AuNPs mit einem Durchmesser
kleiner als 10nm synthetisiert.
Der dritte Teil beinhaltet die Anwendung der in ionischen Flüssigkeiten hergestellten
AuNPs. Dabei werden diese an einer Gold-Elektrode immobilisiert und in
Kombination mit einem Enzym für biosensorische Zwecke verwendet. Aternativ
wurden AuNPs in der ionischen Flüssigkeiten synthetisiert und als “Keime“ für die
Nanorod (NR) –Synthese benutzt.
V
1
Introduction
1.1 Surfactants in aqueous solution
1.1.1 Surfactants
Amphiphilic molecules are of special interest for many industrial applications. The
innumerable uses of amphiphilies is wide varied and can be found in cleaning
products, detergency or as emulsifiers for creams in pharmaceutical applications.
The word surfactant is the acronym for “surface acting agent”. They are
characterized by their tendency to adsorb on surfaces or interfaces, resulting in a
reduction of the interfacial tension between two phases. The structure of a surfactant
consists of two parts. Firstly, a hydrophobic or nonpolar, segment generally referred
to as the “tail” and constituted by a hydrocarbon chain containing between 8 and 20
carbon atoms. The second part is a hydrophilic segment, refers as the “head” group.
One can distinguish between nonionic, cationic, anionic or zwitterionic (dipolar)
surfactants.
Other important classes of surfactants are dimeric or oligomeric surfactants. Dimeric
surfactants or so called Gemini surfactants consist of two amphiphilic moieties
connected to the head group by a spacer group [1]. Block copolymers can also be
considered as amphiphilic molecules, as one part can be designed from a monomer
with polar characteristics, whereas a second part can be built up by a monomer with
no polar features. Depending on the different blocks present, one can find
e.g.
diblock copolymers such as polystyrene-block-poly(methylmethacrylate) (PS-PMMA)
or
poly(ethylene
glycol-co-poly(styrene)
and
triblock
copolymers
such
as
poly(ethylene oxide)-poly(propylene oxide)-poly(ethylene oxide)(PEO-PPO-PEO),
respectively [2-6].
Perhaps one of the most important properties of surfactants is their spontaneous
aggregation in water and the possibility to form different structures such as micelles,
cylinders, bilayers etc. Due to their hydrocarbon chain, surfactants tend to adsorb at
the water-air interface, with their nonpolar chains oriented toward the air phase. By
increasing the surfactant concentration, the surface tension is drastically decreased,
until it reaches a constant value. The concentration at which the surface tension
1
remains constant is called the critical micellization concentration (cmc) or critical
aggregation concentration (cac). This phenomenon can be detected by different
methods such as turbidity, conductivity, surface tension and osmotic pressure. Table
5 (in Appendix) shows some common surfactants and their respective cmc.
Different conditions such as pH, ionic strength and temperature can affect the cmc.
In the case of ionic surfactant, the cmc is slightly affected by temperature but much
more stronger influenced by the addition of salt. This leads to the Kraft temperature,
at which the surfactant solubility equals the critical micelle concentration. At this point
the surfactants solubility is low and less effective in most applications. On the other
hand, nonionic surfactants are strongly affected by the temperature. By increasing
the temperature, precipitation of large aggregates can be observed. The temperature
at which a nonionic surfactant shows this behavior is referred to as the cloud point.
1.1.2 Micellization
Different approaches have been proposed in order to derivate the free energy of
micellization ΔG°mic from the critical micelle concentration (CMC) of a surfactant in a
diluted solution.
One proposed model is the phase separation model, which considers that the micelle
formation has several features in common with the formation of a separate phase [7].
This means that at the beginning of a surfactant addition, the dissolved surfactant
has a certain chemical potential:
(1)
When the surfactant concentration match the cmc, the chemical potential of the
surfactant in the solvent
surfactant in the micelle
equals the chemical potential of the
and the equilibrium condition can be written
as:
(2)
However, this model is limited to uncharged surfactant systems. To include ionic
surfactants in the discussion, the mass action model has been proposed. R and T are
the gas constant and temperature respectively. This approach represents the micelle
formation by the following reaction A:
2
Where S- represents the surfactant ions, n represent the degree of aggregation, M+
the surfactant counter ions and Smic the formed aggregate. Considering this process
as a chemical reaction and expressing the equilibrium constant by taking the
activities of every species, we can write:
(3)
Taking the free energy equation
, and applying to equation (3) the
given ΔG° for the reaction A can be express as follows:
(4)
Dividing the equation (4) by n, the free energy change can be expressed in terms of
mole of surfactant. At the CMC,
, and the equation can be written as:
(5)
For large n (for example n ≥ 50), the second term of the equation (5) can be
neglected, and the expression becomes:
(6)
By replacing the activity “
for the surfactant concentration at the CMC, the free
energy of micellization can be written as:
(7)
As already discussed, surfactants aggregate to form micelles in aqueous solutions. In
particular, the solubility of the nonpolar moiety, such as the tails of the amphiphilic
molecules in water, can be understood in terms of hydrophobic-water interactions.
The hydrophobic tails are transfered out from the aqueous medium and confined into
the oil-like interior of the micelle, whilst simultaneously orienting the polar head group
at the water interface. This process leads to entropic changes associated with the
3
changes in the water structure, whereas enthalpic changes vary slightly. This
phenomena has been considered the driving force of micellization and is known as
the “hydrophobic effect”. As a consequence of the aggregation of surfactant
molecules into a micelle, the repulsion interaction between the head groups and
steric forces must be considered as a competitive factor. As the head groups come
closer, thermal fluctuations become smaller, resulting in a decrease of mobility, which
reduces the entropy.
Spherical micelles are considered the simplest aggregates, when taking into account
the different microstructures that amphiphilics can form. The critical packing
parameter (℘c) is one useful indicator to relate the amphiphilic structure with the self
assembly that they could form. The critical packing parameter ℘ ) can be defined
according to the equation (8):
(8)
℘
where vt is the volume of the tail, lt is the critical length of the tail and ah is the optimal
head group area. Therefore, using equation (8), one can predict the different
aggregates of surfactants:
spherical micelles
bilayers
,
cylindrical micelles
,
vesicles for
inverted structures
Figure 1. Packing parameters for different surfactant aggregates.
4
1.1.3 Emulsions and microemulsions
An emulsion is a dispersion of two immiscible liquids. According to the stability of the
system, one can distinguish between two kinds of emulsions, i.e. macroemulsion and
microemulsion. Macroemulsions are kinetically stable dispersions and appear milky
due to the larger droplet size; typically above 1 µm. Microemulsions are
thermodynamically stable, optically clear isotropic solutions consisting of nano
droplets (2-20 nm) of one liquid in another, surrounded by a membrane of surfactant
and/or co-surfactant molecules. In this system, one can also distinguish between
different types of microemulsions: oil-in-water microemulsion (L1), reverse water-in-oil
microemulsion (L2), and the bicontinuous microemulsion which can be represented
by a sponge phase structure in the region between the L1 and L2 phase [8-9].
Oil phase
Inverted
micelles (L2)
Bicontinuous structure
Lamellar
2Φ
Cylindrical micelles
Cubic micelles
2Φ
Aqueous phase
Spherical and elliptical micelles
Surfactant
(L1)
Figure 2. Schematic representation of some self-assembling structures in a ternary water-surfactantoil system.
The most important parameter of a microemulsion is their spontaneous curvature
(H0) of the surfactant film, in which surfactant molecules adopt its lowest free energy
state. This parameter can be tuned within the range of +0.5nm-1 to – 0.5nm-1,
5
according to the following expression H = 1/Rm, (Rm is the radius of the micelle) by
changing the oil chain length [10-11], ionic strength [12] or by varying the polymer
and temperature conditions [13].
Water-based microemulsions are the most studied and widely applied systems in
cleaning
processes,
lubricants,
cosmetics,
pharmaceutical,
agrochemical
formulations and as a reaction media [14]. However, non aqueous systems in which
water is replaced by organic solvents represents an interesting alternative in
particular to those applications where anhydrous conditions are required. These
systems have been intensively studied by several authors [15-19], but especially by
Friberg et al. [20-23]. More recently, these types of microemulsions have attracted
more attention since ionic liquids are being incorporated together with different
surfactants (e.g., zwitterionic, nonionic and anionic).
1.2 Surfactants in Ionic Liquids
1.2.1 Ionic Liquids (IL)
Ionic liquids are defined as organic salts with a melting temperature below 100°C,
consisting of one organic cation and one organic or inorganic anion [24]. In recent
years ionic liquids (ILs) have received special attention as potential alternatives to
traditional organic solvents, which has motivated different research groups to carry
out reactions under anhydrous conditions or at high temperature ranges. The first
report concerning the preparation of ionic liquids is dated back to 1914 with the
synthesis of ethylammonium nitrate (EAN). Since then, an increasing number of
reports are being published every year. The main field in the application of ionic
liquids has been centered in electrochemistry [25-26]. However, the increasing
interest in these compounds has resulted in numerous developments in their
synthesis. Thus, it has opened potential interests in the preparation of nanomaterials
[27-29], in polymer synthesis [30-34], material processing [35], as well as in many
other industrial applications such as cleaning agents, as extraction media or
compatibilizers of pigments [36].
The most common cations and anions used to prepare ionic liquids are illustrated in
Figure 3.
6
Cations
R
R'
R''
R
R
N
N + N
N
R'''
N
N+
O
+
R'
N
R''
+
R''
N
+
R'
R
R
R'
Imidazolium
R'
N
Ammonium
Benzotriazolium
Morpholinium
R
R
+
N R
R'
+
S
Piperidinium
N
+
R
+
R'
N
N+
R''
R
Pirrolidinium
Pyridinium
Pyrazolium
Pyrrolinium
Sulfonium
Anions
_
_
F
F
F
F
Br
Cl
I
B
F
Chloride
Bromide Iodide
_
O
P
F
F
F
F
R
N
O
_
O
N
H3C
O
Alkyl sulfate
_
_
S
O
F
Tetraflouroborate Hexaflourophosphate
O
Acetate
_
O
N
N
O
O
CF3
O
Dicyanamide
O
S
N
O CF3
S
O
Nitrate
(Trifluoromethylsulfonyl)imide
Figure 3. Most common cations and anions used for the preparation of ionic liquids.
Two different strategies can be used for the synthesis of ionic liquids. On the one
hand, the quaternization of the imidazole by means of haloalkanes has been used for
the preparation of the majority of cations for ionic liquid synthesis. The reaction
conditions require a careful control of temperature, inert atmosphere and a dry
environment. The synthesis of ionic liquids using 1-methylimidazole as a starting
material can be done in the presence or absence of solvents with the corresponding
alkylating agent. On the other hand, there is a second procedure to prepare ionic
7
liquids by using an anion exchange reaction which can be divided into two types:
direct reaction of halide salts with Lewis acids and a synthetic strategy which involves
an anion metathesis [24]. However, it should be mentioned that no matter which
strategy is used, the purification of the product is one of the most difficult steps during
synthesis and becomes an issue of relevant importance because the physical
properties of ionic liquids are strictly correlated with the type and amount of impurities
in the final product.
1.2.2 Physicochemical properties of ionic liquids
The influence of the shape, size, and type of the cations and anions on the
physicochemical properties of ionic liquids such as, melting temperature, density,
viscosity and other physical properties have been a topic of intense investigation for a
longer time now [37-39].
The thermal behavior of ionic liquids, in particular their melting temperature, is of
great importance. The effect of the variation of the cation or the anion size on melting
temperature has been explained by considering two effects: the disruption of the
crystal packing and the crystal lattice energy. Considering that Coulombic attraction
forces are the dominating forces between ions in ionic liquids, we can use the
following equation to describe the lattice energy of the ionic compound,
(9)
Where (Z+, Z-) are the ion charges, r is the ion separation and M the Madelung
constant, which is related with the packing efficiency of the ions. Therefore, the
overall lattice energy is proportional to the ion charge and inversely proportional to
the inter ion separation (r). This implies that small ion charges are preferred, whereas
larger distances, r, between the ion species will contribute to reduce the overall lattice
energy, and as a consequence, a decrease in the melting temperature of the ionic
liquid can be expected. On the other hand, large ions enable charge delocalization,
which also contributes to reducing the lattice energy. In addition, it is preferred for low
melting temperatures that the product of the net charges and the packing efficient
(derived from the Madelung constant, M) is also small. Table 6 (in Appendix) shows
the melting temperature of some common ionic liquids resulting from the combination
of different anions and cations. As already mentioned, Coulombic interactions in ionic
8
liquids are relatively small compared to common inorganic salts, which can reflect in
the melting temperature as seen in Table 6. The presence of flexible substituents,
such as uncharged hydrocarbon groups, in ionic liquids may also contribute to charge
separation, disruption of the lattice isotropy and introduce many rotational and
vibrational degrees of freedom between the ions, which finally lead to a reduction of
the melting temperature. Further, the reduction of symmetry of the cation part can
contribute to the distortion in the ion-packing, causing similar effects. In the case of
imidazolium cations, the changes in the ring substitution can not only affect
symmetry, but can also cause changes in the cation structures.This can induce
aromatic stacking or methyl-π interactions, which leads to an increase of the melting
temperature [40].
Similar outcomes are observed by the introduction of functional groups [41-42]. The
increase of the alkyl chain can initially have a favored effect on the depression of
melting temperatures. However, after a certain number of carbons (between 8-10
carbons) the melting temperatures start to increase again. This effect has been
explained in terms of van der Waals interactions between the long alkyl chains. The
attractive forces between the long hydrocarbon chains result in a high structural order
phase, which can be seen in the formation of structured liquid crystalline material
[43].
The viscosity of ionic liquids is another important property to be considered in order
to use ILs for technological applications. Room temperature ionic liquids (RTIL) are
viscous liquids if compared to conventional solvents like water (0.890 cP), ethylene
glycol (16.1 cP) or glycerol (934 cP) at the same temperature [44]. The viscosity
values can vary between 10 cP to 500 cP and are influenced by several factors, in
particular, by temperature.
Table 7 (In Appendix) lists viscosity values for some
representative RTILs. As seen, some correlations between viscosities of the RTILs
with regard to the nature of the ion size have been observed for a given cation. On
the one hand, larger anions like bis(trifluoromethanesulfonyl)imide, [Tf2N] show
lowest viscosities; whereas on the other hand, nonplanar symmetric anions such as
hexafluorophosphate, [PF6], result in higher values; however, it cannot always be
treated in such a straightforward manner. The relative basicity of the ions, their ability
to form hydrogen bonds or to allow van der Waals attractions have also to be taken
into account. In addition, the variation of the cation and anion structure can lead to
large entropic effects and rotational and vibrational freedoms in the alkyl groups,
9
which can break up the lattice structure, as mentioned above. On the other hand,
cations influence viscosity by increasing the length of alkyl substituents or by
branching them [38], due to the increase in van der Waals attractions between the
hydrocarbon chains.
One could expect that strong hydrogen bonding can introduce order into a system
which results in an increase in viscosity or in the melting temperature. For example,
for reduction of these interactions, it can be suggested the introduction of a methyl
group at the 2-position in the imidazolium cation.
However, experimental results performed on 1-butyl-2,3-dimethyl imidazolium
chloride ([Bmmim][Cl]) have shown that the viscosity and the melting temperature
tends to increase, when compared to its analogous 1-butyl-3-methyl imidazolium
chloride [Bmim][Cl]. The reason for this behavior has been explained in terms of
entropic and steric effects. A significant reduction in the number of stable ion-pair
conformers causes a depression of entropy in the case of the [Bmmim] based ionic
liquid, which results in a more ordered structure [45]. In addition, the free rotation of
the butyl chain is restricted due to the steric repulsion from the methyl group causing
further molecular organization.
Viscosity can be affected by the presence of impurities such as halides [46] and
water [47]. For example, the presence of chloride increases the viscosity of the ionic
liquids, whereas the presence of water, or other cosolvents, reduces the viscosity.
Further, the reduction of viscosities can also be reached by mixing different ionic
liquids or by adding a co-surfactant. Conversely, the density of ionic liquids seems to
be least affected by varying temperature or by the presence of some impurities [46].
Another interesting property that ionic liquids offer is their conductivity, which may
allow them to be used as an alternative electrolyte in electrochemical applications.
The conductivity of an electrolyte is a measure of the available charge carriers and
their mobility. Although ionic liquids are entirely composed by ions, they show
moderate conductivities in the order of 10 mS/cm in the case of 1-ethy-3methylimidazolium based cation [48]. The temperature dependence of the ionic
conductivity can be expressed by the Vogel-Fulcher-Tamman (VFT) equation (10),
(10)
10
Herein, A and B are constants and T0 is the ideal glass transition temperature [49].
The viscosity of room temperature ionic liquids is strongly correlated with their
conductivity. Since, the conductivity  is proportional to the number of charge
carriers, the dependence on viscosity is, in the majority of cases, inversely
proportional [50]. By increasing the length of the alkyl chains the viscosity tends to
increase and a reduction in conductivity is observed. The increase in the anion size
lowered the viscosity but did not necessarily affect the conductivity. Therefore, the
viscosity alone cannot be taken to explain the conductive behavior. The relatively low
can be attributed to the reduction of available charge carriers due to ion pairing
and/or ion aggregation and the reduced ion mobility related to larger ion sizes. It was
demonstrated in a comparative study between ionic liquids containing imidazolium
and
pyridinium
cations
with
tetrafluoroborate
([BF4])
and
bis(trifuoromethanesulfonyl)imide ([Tf2N]) anions, that the observed viscosity of the
ionic liquids with the [Tf2N] anion is lower than that of the BF4 anion, a comparable or
lower ion conductivity was determined. A possible reason for this was explained in
terms of ion-pairs and neutral ion aggregates in the ionic liquids with the [Tf2N] anion,
which dilutes the ionic concentration and reduces Coulombic interaction between the
ionic species causing the depression in viscosity [51]. Approaches to explain the
effect of impurities like water [46,52] and halogens [46] on the ion conductivity have
also been reported.
Moreover, tailored polarity, low vapour pressure and high thermal stability are other
interesting properties of liquid salts. In particular, the possibility to tune the polarity of
ILs, is of special interest. By varying the molecular structure of the cation and anion,
it might be possible to achieve ionic liquids with different polarities. The polarity of
ionic liquids could be within the range of molecular solvents, such as alcohols. Ionic
liquids
which
are
hexafluorophosphate
insoluble
in
water
([Bmim][PF6])
such
or
as
1-butyl-3-methylmidazoium
1-butyl-3-methylimidazoium
bis(trifuoromethanesulfonyl)imide ([Bmim][Tf2N]) and ionic liquids which are miscible
in water, i.e. 1-butyl-3-methylmidazoium tetrafluoroborate ([Bmim][BF4]) or 1-ethyl-3methylimidazoium acetate ([Emim][Acetate]) can be found.
Although derived from their behavior and properties, several research groups are
being motivated to use ILs as environmentally friendly solvents in a wide range of
applications [36,53]. Approaches where ILs have been used as solvents for the
micellization of common surfactants (non ionic, ionic surfactants), polymers and ionic
11
liquids like surfactants (IL-S) have been reported in the last years [54-55]. The
formulation of microemulsions containing ionic liquids and their application in
biotechnology, polymer chemistry and as templates for the metal nanoparticles
preparation were also reviewed [56-57] and some examples are shortly described in
the next chapter.
1.2.3 IL-based microemulsions with non ionic surfactants
The special solvating properties and the different types of available ionic liquids with
variable functionalities have motivated research into their ability to formulate
microemulsions. Evans and co-workers show for the first time the ability of
surfactants to form micelles in ethylammonium nitrate (EAN) as a solvent [58]. In the
following period of time, this concept has been extended to other ionic liquids, in
particular those based on alkylimidazolium cations.
In this context, the addition of nonpolar solvents to the mixture of ionic liquids and
nonionic, ionic, amphoteric and blockcopolymer surfactants has taken relevant
interest recently, in particular for the formation of microemulsions [54-55,57]. For
instance, microemulsions containing imidazolium based ionic liquids, i.e. 1-butyl-3methylimidazolium tetrafluoroborate BmimBF4/Triton X-100/cyclohexane, have been
formulated [59] and extensively studied. Conductometric titrations were performed in
the isotropic phase area shown in this system. The authors could distinguish three
different regions which were adjudicated to the formation of IL-in-oil, oil-in-IL and
bicontinuous microemulsions. Additional characterization, including freeze-fracture
electron microscopy (FFEM) and dynamic light scattering (DLS) determined the size
and size distribution of the microemulsion droplets in the IL-in-oil region. Results
indicate that the droplet size increases from 10nm to100nm as the IL content rises.
However, the experimentally obtained droplet size was significantly larger than
expected for normal inverse microemulsion systems. Eastoe et al. [60] demonstrated
by means of contrast variation Small-Angle Neutron Scattering (SANS) experiments,
in the same microemulsion system (BmimBF4/TX-100/cyclohexane-d12) that the
inverse microemulsion droplet becomes progressively more elongated when the IL is
added, in the order of 2nm in radius. The effect of a co-surfactant on the phase
behavior of bmimBF4/TX-100/ cyclohexane has been reported by Cheng et al. [61]. It
was shown that the addition of 1-butanol favours the stabilization and magnifies the
isotropic phase region. Moreover, the addition of 1-butanol leads to an increase in
12
the hydrodynamic radius of the inverse microemulsion droplets from a mean radius of
12nm to 19.8nm. In contrast, the addition of a small amount of water decreases the
microemulsion droplet size and increases the number of microemulsion droplets. It
was suggested that there is a transition from a large elliptical structure to small
spherical droplets (IL-in-cyclohexane) stabilized by a surfactant layer. This transition
in the structures was explained based on enthalpy changes derived from hydrogen
bonding, which compensates the entropy loss [62]. The effect of the temperature in
the same system was investigated by the same authors, showing an increase in the
size of single microemulsion droplets [63]. However, the relatively high temperatureindependence, compared to traditional aqueous systems, is related to the effect of
electrostatic attractions that favored the IL microemulsion formation. Similar to the
aqueous based microemulsions, the effect of adding polymer, i.e. a low molecular
weight polyethylene glycol (PEG-400), was also reported.
Li and co-workers
concluded that the solubilized polymer could affect the stability of the inverse IL-incyclohexane droplets. Based on electrostatic interactions, it has been suggested that
the interaction between the imidazolium cation of the ionic liquid and the ethoxy
group of the surfactant might destabilize the microemulsion [64]. The formation of
microemulsions, using IL as a polar domain and nonionic surfactants, have been
demonstrated in other organic solvents, such as toluene [65-66], xylene [67],
benzene
[68],
triethylamine
[69]
and supercritical
carbon
dioxide
[70-71].
Microemulsions using the hydrophobic ionic liquid 1-butyl-3-methylimidazolium
hexafluorophosphate ([Bmim][PF6]), water and the surfactants Tween 20 and TX-100
has been also documented [72]. A nearly independent behavior of the measured
hydrodynamic diameter was observed at fixed BmimPF6-surfactant molar ratio, which
indicates that the structure of the inverse water micelles remains constant. The
interest in microemulsions with high thermal stability, has motivated research groups
to substitute water with ethylene glycol (EG). Han and co-workers have reported a
microemulsion consisting entirely of nonvolatile components, i.e. BmimPF6/EG/TX100 at 30°C [73]. Surface tension measurements demonstrated that TX-100 can form
dry micelles in an EG solution and shows lower activity than in water. The reason for
that behavior has been explained due to weaker hydrophobic interactions in the EG
system than in the aqueous system. A linear dependence between the hydrodynamic
radius and the IL/surfactant molar ratio was shown by DLS. The behavior is
understood in terms of the droplet swelling, due to addition of EG to the
13
BmimPF6/TX-100 mixture. It should be noted, that the most common ionic liquids
used for microemulsion formulation are based on the 1-butyl-3-imidazolium cation
with tetrafluoroborate or hexafluorophosphate anions. Another class of ionic liquids
based on pyridinium and alkylammonium cations have been also studied.
Benzylpyridinium bis(trifluoromethanesulfonyl) imide in TX-100/toluene mixtures,
were recently investigated by different techniques [74]. The inverse IL-in-oil
microemulsion droplets of about 2-3nm in radius were determined by SANS and
present considerable advantage as a nanoreactor to perform organic reactions. In a
recent investigation, Pramanik et al. [75] have studied the behavior of the ionic liquid
N,N,N-trimethyl-N-propyl
ammonium
bis(trifluoromethanesulfonyl)
imide
([N3111][Tf2N]) in the TX-100/cyclohexane mixture, using steady-state and timeresolved fluorescence spectroscopy as a technique and coumarin 480 (C-480) as a
fluorescence probe. An average droplet diameter between 10nm and 23nm
wasdetermined by DLS from the [N3111][Tf2N]) /TX-100 molar ratio (R) in the range of
0.08 to 0.3.
Atkin et al. [76] have prepared a microemulsion system containing nonionic alkyl
oligoethyleneoxide surfactants, alkanes, and the ionic liquid ethylammonium nitrate
(EAN). Phase behaviour studies show similar features in comparison to the
analogous aqueous systems. However, higher surfactant concentrations and longer
surfactant tail groups are required to stabilize the surfactant molecules in the ionic
liquid. Systems with propylammonium nitrate (PAN) were recently investigated by the
same group [77]. From the binary phase behavior one can conclude that longer alkyl
chains are required to form lyotropic phases in PAN in comparison to EAN or water,
whereas, higher surfactant concentrations are needed to favor the spontaneous
microemulsion formation in EAN or in aqueous microemulsions. The strong solvation
properties and the greater molecular volume of the alkylammonium cation, results in
higher molecular areas in the self-assembled aggregates, according to SANS
experiments. This favours a smaller surfactant packing parameter or more highly
curved aggregates in PAN than EAN or water. Phase behaviour and conductivity
experiments reveal that the PAN/CiEj/oil microemulsions are all weakly structured
systems. By increasing the alkyl chain length of the oil compound and the surfactant
amphiphilicity, a slight effect on the ternary phase behavior and the microemulsion
efficiency is observed. The longest alkyl chains investigated displayed a ternary
14
lamellar phase adjacent to the microemulsion. These results are all attributed to high
surfactant solubility in the ionic liquid. Finally, these studies indicate that the
propylammonium ion may act as a co-surfactant in these self-assembled structures.
1.2.4 IL-based microemulsions with ionic surfactants
In contrast to the well characterized IL-based microemulsions with nonionic
surfactants, discussed above, investigations of microemulsions with ionic surfactants
are rather scarce. Nevertheless, these kinds of microemulsions are of special interest
from the academic, as well as, from the application point of view, due to the
synergistic effects induced by electrostatic interactions between the functional
groups. Therefore, different research groups focused their interest on IL
microemulsions with ionic surfactants, their phase behaviour, and structural features
by using several characterization techniques. Falcone et al. [78] have investigated
the
microemulsions
formed
by
mixing
two
ionic
liquids,
i.e.
1-butyl-3-
methylimidazolium tetrafluoroborate ([Bmim][BF4]) and 1-butyl-3-methylimidazolium
bis(trifluoromethylsulfonyl)imide
([Bmim][Tf2N])
in benzene,
with
the cationic
surfactant benzyl-n-hexadecyldimethylammonium chloride (BHDC), in comparison to
the nonionic surfactant TX-100. Larger IL reverse micelles in the system containing
[Bmim][BF4] as the polar domain were determined in comparison to the BmimTf2Nbased system, independent on the type of surfactant used. The authors suggest that
a decrease in the surfactant packing parameter due to the stronger interactions of the
[Bmim][BF4] with the cationic surfactants, might be the reason for an increase in the
size of the inverse droplets. More recently, the same group has postulated the
formation of reverse micelles of [Bmim][Tf2N] dispersed in chlorobenzene by adding
two different types of ionic surfactants, i.e. BHDC and sodium 1,4-bis(2-ethylhexoxy)1,4-dioxobutane-2-sulfonate (AOT). However, based on nuclear magnetic resonance
experiments (NMR), the combination of the [Bmim][Tf2N] ionic liquid with the cationic
surfactant results in a strong interaction between the positively charge BHDC and the
negatively charged anion. This electrostatic interaction causes a surfactant
counterion exchange of [Cl]- with [Tf2N]-, which produces significant changes in the
structural organization of the IL-in-oil micelle. Conversely, the addition of the anionic
AOT surfactant seems to be less effective, this means, that small changes in the
structure of the encapsulated ionic liquids can be expected due to poorly electrostatic
15
interactions of the bulky ions [79]. The effect of the ionic liquids on the
physicochemical properties of a microemulsion has been reported by Eastoe at al.
[80]. Surface tension, SANS, and SAXS experiments show that the ionic liquid acts
as a polar phase, and co-surfactants influencing the interfacial stiffness of the
surfactant film. Furthermore, it was suggested that the ionic liquid [Bmim][BF4] is
much more effective to adjust the ionic microemulsion curvature, compared to NaCl.
The quaternary system based on AOT/water/[Bmim][BF4]/decane has been also
investigated [81]. The results show that the addition of [Bmim][BF4] significantly
reduces the critical temperature, contrary to the effect of adding KCl. Similar effects
were
observed
in
our
methylimidazolium
group
by
hexyl
adding the
sulfate
ionic
liquid,
i.e.
1-ethyl-3-
([Emim][HexSO4]),
to
1
water/toluene/cetyltrimethylammonium bromide (CTAB) microemulsions [82]. HNMR
diffusion coefficient measurements and cryo-SEM micrographs were combined to
determine structural transitions in the system. The relative long alkyl chain of sulfate
anion was suggested to act as a co-surfactant, oriented in a palisade layer together
with the cationic surfactant. The work in this field was extended by the contribution of
Rabe & Koetz [83]. In these studies it was demonstrated that inverse micelles of the
polar ionic liquids, i.e. 1-ethyl-3-methylimidazolium ethyl sulfate ([Emim][EtSO4]) and
1-ethyl-3-methylimidazolium hexyl sulfate ([Emim][HexSO4]) can be formed using the
cationic surfactant (CTAB) and a toluene-pentanol mixture as the oil phase in the
absence of water. A detailed study with regard to the effect of the co-surfactant
(pentanol) and temperature on the system was presented.
Attempts to formulate microemulsions with two ILs have been also reported by
dispersing the hydrophobic IL [Bmim][PF6] in the hydrophilic IL propylammonium
formate (PAF) with AOT as a surfactant. However, relatively larger droplets of about
30nm to 100nm were determined by freeze-fracture electron microscopy (FFEM)
[84].
1.2.5 Microemulsions with ionic liquid-like surfactants (IL-S)
Ionic liquids containing long alkyl chains are attractive compounds because of their
amphiphilic
behavior.
For
example,
ILs
formulated
based
on
fluorinate
alkylmidazolium hexafluorophosphate ([C6mim][PF6]) (Figure 4) have shown their
capacity to act as agents to promote and stabilize dispersions of fluorinated
16
molecules [85].
F
+
N
R N
PF6-
CF3
n
F
1 R = Me
2 R = n-Bu
3 R = Me
4 R = n-Bu
n=5
n=5
n=7
n=7
Figure 4. General structure of fluorous ionic liquids.
The field of application of these kinds of ionic liquids has been extended to formulate
microemulsions. Surfactant-based imidazolium ionic liquids, with and without
polymerizable groups (Figure 5), were introduced for the first time by Texter, showing
the great potential with regard to their application in polymerization processes [86].
10
N
+
N
O
10
Br
N
+
N Br
O
(a-Br)
(b-Br)
O
10
O
N
+
(c-BF4)
N
BF4-
Figure 5. Chemical structure of 1-dodecyl-3-methylimidazolium bromide and the polymerizable 1-(2methyl acryloyloxyundecyl)-3-methylimidazolium bromide (b-Br) and 1-(2-methyl acryloyloxyundecyl)3-methylimidazolium tetrafluoroborate.
Room
temperature
ionic
liquids
like
1-butyl-3-methylimidazolium
hexafluorophosphate ([Bmim][PF6]) [87], ethylammonium nitrate (EAN), 1-butyl-3methylimidazolium tetrafluoroborate ([Bmim][BF4]) [88] have been dispersed in water
or non polar organic solvents using IL-like surfactants. The studies open new
possibilities to use microemulsions at high temperature as a reaction medium, for
lubricant formulations, and as a template for the synthesis of nanoparticles [89][90].
17
1.2.6 Fields of application of IL-based microemulsions
Thermodynamically stable microemulsions consisting of structurally well-defined
nanosized domains of water (or respectively ionic liquids) and oil, separated by a
surfactant film have multiple applications. The use of ionic liquids as a medium for
different polymerization processes is a topic of prospective investigations. In
particular, the preparation of new kinds of polymer electrolytes has received special
attention [91]. However, the successful preparation of these materials has been
limited due to the lack of compatibility between the IL and polymers.
The synthesis of new kinds of IL surfactants has permitted the development of
polymerizable microemulsion systems. Polymerization processes involving such
polymerizable ILs monomers led to the formation of a transparent and stable
monodisperse polymer latex dispersion. In addition, the copolymerization between
monomer/IL-surfactant is possible. The resulting material can be tuned between a
hydrogel and a porous structure by changing the anions in the IL moiety of the
copolymer. In other words, the porous polymer can change their properties by
immersion in organic solvents or in water [86,92]. Different applications of these
materials have been suggested, such as in tissue scaffolding, bicontinuous materials
templating, antimicrobial filtration, fire-resistant foams and drug delivery systems.
Additionally,
a
polymerization
[Bmim][BF4]/polystyrene
in
membrane
was
[Bmim][BF4]/styrene/1-(2-methyl
produced
from the
acryloyloxyundecyl)-3-
methylimidazolium bromide (b-Br) microemulsions [93], whereas the polymerization
of methyl methacrylate (MMA) was conducted in a [Bmim][BF 4]-based microemulsion
stabilized by 1-dodecyl-3-methylimidazolium bromide (a-Br) [94].
The application of water inverse droplets as a nanoreactor have provided significant
advantages with respect to the separation, nucleation and growth processes of metal
nanoparticles. Additionally, self-assembled systems might act as a template to
control the growth and morphology of these materials [95-96]. However, the
preparation of nanostructured materials in IL microemulsions is rather scarce. The
synthesis of porous silica microrods with nano-sized pores was reported by Li et al.
[97] in water/TX-100/BmimPF6 microemulsions. Ellipsoidal nanoparticles and hollow
silica spherical nanoparticles were prepared under acidic and alkaline conditions by
Zhao et al. [98]. A sustainable photo reduction strategy was documented by Harada
[99] to prepare silver nanoparticles in water-in-IL microemulsions using Tween 20 as
18
a surfactant. Nevertheless, it was demonstrated that under high-pressure CO2 the Ag
particles were more effectively regulated than under ambient conditions, due to the
fact that the size of water droplets are more stable under these conditions. Moreover,
the presence of ionic liquids seems to prevent the aggregation and precipitation of
the nanomaterials
[100].
In a recent
approach,
gold
nanoparticles were
electrodeposited directly using a water-in-IL microemulsion. The nanoparticles of
about 25nm in size were directly electrodeposited from microemulsion. The modified
gold bar electrode shown high electrochemical activities toward glycerol oxidation.
The reason for this has been explained in terms of an increase of the specific surface
area of the gold electrode due to the nanoparticle immobilization. [101].
The application of IL-based microemulsions in biotechnology has become an area of
increasing interest in recent years. Several reports have been published concerning
the effects of the ionic liquids properties on enzyme performance [102-103]. Based
on that, enzyme preparations in ILs as non-aqueous solvents can enhance, in some
cases, conversion rates, enantioselectivity, recoverability and recyclability. Strategies
that comprise microemulsion based on imidazolium cations as a non polar phase
have been also investigated. For example, the encapsulation of proteins, such as the
enhanced green fluorescent protein, in reverse micelles have shown some
advantages concerning their activity [104]. Furthermore, the catalytic activity of lipase
in water-in-ionic liquid microemulsions exhibited higher values than the analogous
system using isooctane as a continuous phase. The addition of co-surfactants to that
system, apparently changes the aqueous microenvironment and the partition of the
substrate between the aqueous phase and the IL, which may enhance the activity of
the protein [105]. Similar microemulsion systems containing ionic liquids have been
successfully used in other enzymatic processes, such as the enzymatic oxidation of
pyrogallol catalyzed by horseradish peroxidase [106] or in lipase-catalyzed
esterification reactions of natural fatty acid with aliphatic alcohols [107]. Recently, the
catalytic activity of trypsin in water/IL-in-oil microemulsion has been demonstrated by
Debnath and coworkers [108]. The authors suggested that the IL provided a better
balance between size and water pool solubility. In other words, it can offer a suitable
environment, which results in an activating effect to the trypsin by improving the
nucleophilicity of water in the vicinity of enzyme, through hydrogen bonding. The
preservation of the secondary structure in the large-sized reverse micelles was also
postulated.
19
1.2.7 Characterization methods of microemulsions
Microemulsions have been the topic of comprehensive research in the last decades.
Several techniques have been used in order to study the microstructure of
microemulsions [109], for example by determining macroscopic properties such as
rheology [110], surface tension [80] and conductivity [67]. Substantial improvements
of experimental characterization techniques have allowed studying microemulsions
by direct visualization of their microstructure, by means of transmission and scanning
electron microscopy [111-112]. Techniques which use the scattering of radiation like
dynamic light scattering (DLS), small-angle X-ray scattering (SAXS) and, in
particular, small-angle neutron scattering (SANS) [80,113-114], have been applied
successfully in a large collection of reports. In the next paragraphs, techniques which
have been used to characterize water based microemulsions system, as well as,
more complex systems, such as microemulsions containing ionic liquids, are
presented.
Other methods have been successfully applied to complement the characterization of
ionic liquid based microemulsions. For example, the interaction between ionic liquid
droplets in a microemulsion has been determined by means of isothermal titration
microcalorimetry (ITC). The result obtained by this method might also contribute to
explain the relatively large droplet size of the IL-in-oil microemulsion systems
compared to traditional water-in-oil microemulsions [115]. Nuclear magnetic
resonance (NMR) is a powerful tool to study microemulsion systems. On the basis of
electrostatic attraction forces between the components in the mixture, (e.g. the
interaction of the positively charged imidazolium cation of [Bmim] + and the
electronegative oxygen atoms of Triton X-100), the formation process of IL-in oil
microemulsion has been investigated [112]. However, the major strength of NMR is
by using the Fourier transform pulsed gradient spin–echo (FT-PGSE) method, which
studies the diffusive behavior of the molecular components of a microemulsion. The
method of NMR self-diffusion measurements has been used to characterize mainly
water based systems [116-119], however some effort has recently been made in our
group to study ionic liquid-modified microemulsions [82]. Alternatively, cyclic
voltammetry (CV) using potassium ferrocyanide K4Fe(CN)6 as an electroactive probe
was reported to determine the different microemulsion regions in aqueous based
microemulsions [120-121] and has been successfully applied in the characterization
20
of ionic liquid systems [72]. Significant changes of the apparent diffusion coefficient of
ferrocyanide in the system have helped to solve structural transitions in the isotropic
phase area. UV-vis spectroscopy using dyes (e.g. methyl orange and methyl blue) as
a probe has been used to provide information about the microenvironment of inverse
droplets [66,122]. Other techniques such as Fourier transform infrared spectroscopy
[123] and fluorescence spectroscopy have also been used in the characterization of
ionic liquid based microemulsions [124-125].
21
2
Aim of the work
The aim of this work is explore the versatile uses of ionic liquids as alternative
organic solvents in the self assembling of amphiphilic molecules or as solvents to
prepare metal nanoparticles.
The formulation and characterization of new types of non-aqueous microemulsions
consisting of ionic liquids and one oil compound has been investigated. This work
proposes the use of the halogen-free ILs, i.e. 1-ethyl-3-methylimidazolium ethyl
sulfate and 1-ethyl-3-methylimidazolium hexyl sulfate as polar phase and the 1-butyl3-methylimidazolium octyl sulfate and 1-butyl-3-methylimidazolium dodecyl sulfate as
an ionic liquid-like surfactant. In addition, toluene was employed as a non polar
compound.
Furthermore,
different
techniques,
such
as
conductometry,
rheology
and
voltammetry, can be used to determine structural variation in the isotropic phase of
the ternary system. Complementarily, scattering techniques and electron microscopy
may reinforce these results. The ternary mixture could be of interest as a template
phase to prepare nanomaterials or as a reaction media to perform organic reactions.
Chemical reactions in pure ionic liquids can provide a structured solvent platform that
may result in the formation of small metal and crystalline nanoparticles. Therefore,
taking advantage of the polar features and thermal stability of [Emim][EtSO 4] and
[Emim][HexSO4], the synthesis of colloidal gold nanoparticles has been proposed
using poly(ethyleneimine) as a reducing agent. In addition, the presence of ILs may
influence the solvation properties of the nanoparticles and thus, induce their
dispersion in aqueous and/or in organic solvents. Colloidal gold nanoparticles are of
current interest due to their electronic, optical and magnetic properties that can be
efficiently used in a wide range of technological applications.
Therefore, the prepared polymer-caped AuNPs might contribute to the development
of a biosensor system or participate in the preparation of controlled anisotropic
materials such as gold nanorods.
22
3
Materials and Methods
3.1 Materials
3.1.1 Chemicals
1-butyl-3-methylimidazolium
octyl
sulfate
([Bmim][OctSO4])
(99%),
1-ethyl-3-
methylimidazolium ethyl sulfate ([Emim][EtSO4]) (99%), 1-ethyl-3-methylimidazolium
hexyl sulfate ([Emim][HexSO4]) and ferrocene were purchased from Merck
(Darmstadt,
Germany).
N-ethyl-N’-(3-dimethylaminopropyl)
carbodiimide
hydrochloride (EDC), dithiobis-N-succinimidyl propionate (DTSP) was purchased
from Sigma (Steinheim, Germany). 11-mercapto-1-undecanoic acid (MUA), 11mercapto-1-undecanol (MU) and hydrogen tetrachloroaureate HAuCl4, were provided
by Aldrich (Taufkirchen, Germany). Sodium dodecyl sulfate (SDS) (>99%), toluene
(99%) and pentanol (99%) were purchased from Roth (Karlsruhe, Germany).
Branched poly(ethyleneimines) with a molecular weight 5000 g/mol and 25000 g/mol
(Lupasol G100 and Lupasol WF) were obtained from BASF (Ludwigshafen,
Germany). Other reagents like 1-butyl-3-methylimidazolium chloride ([Bmim][Cl]) and
1-ethyl-3-methylimidazolium ethyl sulfate ([Emim][EtSO4]) were obtained from Iolitec
(Denzlingen, Germany). All products were used as received. Human Sulfite oxidase
(hSO) were expressed and purified in the molecular enzymology laboratory of Prof.
Dr. Leimkühler at Potsdam University as described in references [126-127].
Goodfellow (Bad Nauheim, Germany) supplied gold and platinum wires with a
diameter of 0.5 mm. All the solutions were prepared in Millipore water.
3.1.2 Synthesis
1-Butyl-3-methylimidazolium
dodecyl
sulfate
([Bmim][DodSO4])
was
prepared
according the protocol reported by Wasserscheid [128]. Firstly, 15.3 g of 1-butyl-3methylimidazolium chloride and 26.6 g of sodium dodecyl sulfate were dissolved in
50 ml of hot water. After 2 h reaction at 65°C under stirring, the water was removed
using a rotational evaporator. The resulting solid was dissolved in 50 ml of
dichloromethane and the solution was filtrated. The filtrate was washed with water
until the water phase was colorless (at least 5 times). The organic phase was
23
concentrated and dried under high vacuum at 80°C for 48 h. The product (65.5% of
the theoretical yield, based on [Bmim][Cl]) consists of a beige waxy solid.
Melting point (DSC) 45°C,
Decomposition temperature (TGA-onset): 210°C
1
H-NMR (300 MHz, d6-DMSO, [ppm])
δ = 9.107 (s,1H, N-CH-N), δ = 7.761 (s,1H,N-CH-CH-N) δ = 7.693 (s,1H, N-CH-CHN), δ = 4.149 (t, 2H, N-CH2CH3), δ = 3.836 (s, 3H, N-CH3), δ = 3.658 (t, 2H, S-OCH2), δ = 1.749 (qnt, 2H, CH3-[CH2]9-CH2-CH2-O-S) δ = 1.456 (qnt, 3H, N-CH2-CH3),
δ =1.225 (m, 23H, (CH3-[CH2]10)-CH2-O-S), δ = 0.950-0.775 (m, 3H, CH3-CH2, 3H,
CH3-[CH2]3-CH2-).
13
C-NMR (300 MHz, d6-DMSO, [ppm])
137.033, 124.077, 122.739, 65.925, 61.179, 48.950, 36.185, 33.002, 31.825, 31.754,
29.559, 29.479, 29.424, 29.249, 29.163, 25.966, 22.543, 19.230, 14.378, 13.712.
Water content (Karl Fischer) < 0.3 %.
3.1.3 Phase diagrams
The
ternary
systems
consisting
of
BmimOctSO4/toluene/EmimEtSO4
and
BmimDodSO4/toluene/EmimEtSO4 were investigated at 25°C. The isotropic phase
was determined by visual inspection titrating the toluene/IL-S mixture with
[Emim][EtSO4] at 25°C. After adding each drop, the mixture was equilibrated in a
thermostatic bath to guarantee steady state conditions of the optically clear solution.
3.2 Methods
3.2.1 Surface tension
Surface tension experiments are regularly used to estimate the critical micelle
concentration of amphiphilic molecules in aqueous or non aqueous solution. The
surface tension of a liquid is defined as the work required to produce a new surface
per unit area. The addition of surface active species such as amphiphilic molecules at
the interface generally results in a reduction of the surface tension γ as the
concentration of amphiphilies increases.
The surface tension measurements were carried out on a digital tensiometer K10St
(Krüss) equipped with a platinum ring. The temperature of the samples was
controlled by a thermostat with an uncertainty of ±0.1.
24
3.2.2 Rheology
The time dependence of the deformation of the matter is the field of studies of
rheology. In particular, viscometry is the subfield of rheology concerned with fluid
matter (e.g. dilute surfactant or polymer solutions and microemulsion systems). The
energy dissipated during this process per unit volume can be used to define viscosity
[110]. In general, every fluid shows certain resistance to flow due to internal friction.
In a viscometry experiment using cone/plate geometry, a fluid is placed in between
the cone and the plate and a force is applied, shear stress, σ. The rotation of the
upper cone generates a velocity gradient or shear rate,
. The viscosity is
calculated by the ratio of these two quantities according to Newton’s law.
(11)
In respect to the viscosity of fluids, two types of fluids can be identified: Newtonian
and non-Newtonian. Fluids are called Newtonian if the shear viscosity behaves
independent on the shear rate or time.
In case of microemulsion systems, a Newtonian flow behavior and low viscosity are
part of their characteristics. In principle, viscosity measurements, as part of rheology
mapping, gives the possibility to obtain fundamental information about changes in
colloidal structures and their interactions. In addition, the measurements are of
special interest as a sensitive indicator for association phenomena in polymersurfactant based systems.
In the present investigation, rheological experiments were performed to study the
behavior of the resulting isotropic region from the ternary mixture. The shear viscosity
of different IL/toluene compositions were studied with the Gemini 150 Rheometer
(Malvern, United Kingdom) with cone-plate (1/40) geometry. The experiment was
performed at 25°C and it was controlled by a thermo-controller supplied by the
rheometer. The viscosity was calculated by linear regression of the Newtonian flow
curves over the shear rate between 1 s−1 and 1000 s−1.
3.2.3 Conductometric measurements
Electrical conductivity experiments can provide relevant information about the internal
dynamics occurring between droplets in microemulsion systems. The experiment
25
consists of registering the variation in electrical conductivity
in dependence on the
water content.
To describe the composition of the system, the water-surfactant molar ratio (W0) and
the volume fraction of water plus surfactant (Φ) have to be defined:
(12)
(13)
Figure 6 shows an idealized view of the variation of the electrical conductivity in
 / a.u.
dependence on the water content expressed as volume fraction.
Φcritical
Φp
Φ
Figure 6. Variation of the conductance in dependence on the water volume fraction.
The curve of conductivity as a function of the volume fraction of water can depict
different behaviors. With high oil content (low water content, beginning of the curve),
diluted droplets of water dispersed in an oil continuous phase are expected. Previous
experiments on water droplets in oil continuous phase with ionic surfactants have
shown that at low water volume fraction (Φ < 0.05) low conductivity values in the
order of (10-5 – 10-6 S/m) are registered, but the values are relatively higher
compared to the oil component (10-12 – 10-16 S/m). This suggests that the
conductance observed might come from the motion of inverse droplets that carry
charges [129-131]. By raising the volume fraction of the polar component (water or in
case of non aqueous microemulsions, i.e. ionic liquids) a percolation transition is
26
reached. Beyond this point, a linear increase in
is observed. Several models have
been proposed; in particular to explain the sharp increase in conductivity in water-inoil microemulsions. One of the most accepted models was proposed by Eicke et. al.
[130].
According
to
this
model,
the
electrical
conductivity
of
water-in-oil
microemulsions can be explained by the movement of spherical charged
microdroplets formed by spontaneous fluctuations of ions on the droplets. The model
leads finally to an equation that predicts the specific conductivity in dependence on
and the volume fraction of the droplets:
(14)
Here the volume fraction is defined as Φ4r3/3 , kB is the Boltzmann constant, T is
the temperature, r the radius,  the relative permittivity, 0 the electric permittivity of
free space and  the dynamic viscosity. As can be seen, the droplet radii can be
determined by this equation, however the calculated radii rd have been observed to
be larger [130]. Modified versions of Eickes’s model have been reported by other
authors [132-134]. For example, Kallay et al. [132] distinguished between the
hydrodynamic radius rd and the effective radius rc of the charge separation within the
micelle. The relation between the radius takes into account the thickness of the
surfactant layer according to
. The resulting adjustment gives:
(15)
The equation (15) gives a better approximation of the radii, however, the thickness of
the surfactant layer might be determined or estimated from the bond distances
between the different atoms that constitute the molecule.
The increases in electrical conductivity when the water content is raised have been
related by an increase in the concentration of water droplets, according to Lagües et
al. [135]. The volume fraction at the percolation threshold (Φp), defined as the point at
which an “infinite” cluster spans the length between the two electrodes [136] can be
determined by plotting
versus Φ. From the linear fit of part of the curve, the
resulting straight line is extended to intersect the Φ axis providing the Φp [131].
Alternatively, Φp can be calculated from the inflection point in the plot log
versus Φ
and fitting the data to a fourth-order polynomial in Φ. The second derivative is set
27
equal to zero and the value for Φp is obtained [137]. The observed phenomenon
(above the percolation threshold Φp) has been explained in terms of the existence of
a progressive droplet interlinking and clustering process [131]. In addition, the
location of the percolation threshold can also be influenced by varying the
temperature, pressure or water to surfactant molar ratio [138]. In addition to the
fluctuation model describing the increase in electrical conductivity of microemulsions,
the movement of spherical droplets can be described by two models, i.e. static and
dynamic percolation models [139-140]. The static model describes a system of
conducting regions randomly dispersed in an insulating matrix. The dynamic
percolation model suggests that below the percolation threshold the increase in
conductivity is due to the Brownian motion of the charged droplets, whereas beyond
this point the charge carriers themselves diffuse in the reverse micellar cluster.
Once Φp has been determined, the scaling exponents μ and s can be calculated from
plots of the In versus In (Φ - Φp) and In (Φp - Φ), respectively. The width of the
transition interval (Δ) = δ + δ‘ can be approximated by Δ ≈ (2/1)1/(µ+s). Therefore, for
water-in-oil microemulsions in which (2/1) « 1, the width of the transition interval is
considered small.
The conductivity behavior above and below the percolation threshold Φp can be
described by asymptotic power laws [141].
(16)
(17)
(above percolation threshold)
(below percolation threshold)
Here 1 and 2 correspond to the conductivities of the percolating droplets and of the
homogeneous phase, respectively, Δ is the width of the transition region, and µ and s
are the characteristic exponents of the two power laws. By means of computer
simulations on percolating systems, the scaling exponents are expected to be μ ≈ 2
and s ≈ 1.2, [142][143]. For static percolation, different scaling exponents are
expected, namely μ ≈ 2 and s ≈ 0.6-0.7 [142].
After the critical water content (Φcritical), the increase in is slower, reaches a
maximum and then decreases to the value of bulk water. Sometimes a plateau or
even an increase in  can also be observed [135]. The anomalous conductivity
behavior observed at high Φ (between Φcritical and the maximum) in the conductivity
curve (Figure 6) suggests a new conduction mechanism that implies the formation of
28
an intermediate structure reported in the literature as a bicontinuous structure [131].
Furthermore, the descension in conductivity at most higher water content has been
adjudicated to the progressive dilution due to the addition of water. Therefore, this
part of the curve has been related to the formation of oil-in-water microemulsions.
These observations have been extended to non aqueous microemulsions, in
particular, to those systems using ionic liquids as polar phase [88].
In the present investigation, conductivity measurements were carried out on the
isotropic range of the ternary mixture in order to determine structural transitions in the
microemulsion systems. A microprocessor conductometer LF 2000 (WTW) was used
to perform the measurements at 25°C.
3.2.4 Dynamic Light Scattering (DLS)
Dynamic Light Scattering is a regularly used method to obtain information with
regards to the diffusion coefficient, size and size distribution of the dispersed colloidal
particles. When the scattering medium (dispersion of colloidal particles) is irradiated
by a laser, the particles will scatter the light in all directions. Due to Brownian motion
the scattering particles may vary their positions causing a fluctuation in the scattering
pattern from one random configuration to another (Figure 7) [144-145].
Incident light
Sample
Speckle pattern
Figure 7. Light scattered by colloidal particles giving a random speckle pattern.
Taking this into account, valuable information can be subtracted from the fluctuating
intensity (Figure 8a), by constructing its time correlation function:
(18)
The function compares the signal I (q, t) with a delayed expression of scattered
intensity I (q, t + τ). When delay time proximate to zero the expression is reduced to:
29
(19)
whereas for delay times greater than the fluctuation time Tc of the intensity,
fluctuation of I(q, 0) and I(q, t + τ) are uncorrelated and can be defined as:
(20)
This means that the autocorrelation function has its maximum value at t = 0, and its
decay from the mean square intensity
2
to the square of the mean
at
long time, as shown in Figure 8b.
(a)
(b)
t
Figure 8. (a) Illustration of the fluctuating intensity of the scattering light with time, (b) variation of the
autocorrelation function in dependence on the delay time
[144].
The ratio of the autocorrelation function to its asymptotic value
can be written
as:
(21)
Applying the so called Sieglet relation results [144]:
(22)
Here β has been introduced and represents an instrumental constant approximate to
unity. By inverting the measured intensity correlation function g(2) (q, τ), the
intermediated scattering function results and can be written as:
30
(23)
Assuming the case of a suspension of spheres, this function can be given as:
(24)
Where ΔR (τ) ≡ R (τ) - R (0) represents the displacement of a particle due to the
Brownian motion in time τ. Considering that in a real case the movement of the
particle is in three dimensions, the random displacement can follow a Gaussian
probability distribution, where the particle mean-square displacement in time τ is
given by:
(25)
Therefore, the result of the evaluation of the field correlation function and the
Gaussian probability distribution gives:
(26)
In other words, the equation (26) tells that the field correlation function, g(1) (q, τ)
decays with time in a single exponential mode with a rate D0q2. The scattering vector
q can be determined by applying equation (27):
(27)
From equation (27), n is the refractive index of the medium, λ is the laser wavelength
and θ corresponds to the scattering angle with respect to the incident light. Knowing
the value for D0, the hydrodynamic radius can be calculated using the StokesEinstein equation:
(28)
However, in real cases the analyzed suspensions are polydisperse, are in constant
interaction and may have different shapes. Therefore, the solution form becomes
more complex and models are necessary. Assuming that the Brownian motion of
31
each particle is different, the intermediated scattering function should be written as a
sum of all exponential decays which correspond to every particular particle, weighted
by the intensity scattered by the particle. This can be expressed as:
(29)
Here the P(D) is the normalized intensity-weighted distribution of diffusion constant. A
number of methods are available, but one of the simplest and commonly used is the
known cumulant expansion of the exponential function g(1)(q,τ) expressed as
(30)
Here, the first order cumulant is related to the average diffusion coefficient
of the
particles and the second term to the standard deviation σ of the diffusion coefficients.
Dynamic Light Scattering (DLS) measurements were used to determine the size and
size distribution of the microemulsion droplets at 25 °C. The measurements were
carried out with a Nano Zeta sizer 3600 (Malver Instruments) at a fixed angle of 173°
(backscattering detection) equipped with a He-Ne laser (λ= 633nm; 4 mW) and a
digital autocorrelator. For all samples, the results were calculated using the multimodal distribution mode, and 10 runs of 20 seconds were performed. For the
measurements, the samples were placed in a glass cuvette.
3.2.5 Small Angle X-Ray Scattering (SAXS)
Small angle X-ray and neutron scattering (SAXS and SANS) have wavelengths (1100 nm) comparable to atomic size which allows to proximate with good accuracy the
microstructure of microemulsions. Therefore, such techniques are some of the most
powerful tools used in the characterization of microemulsions. Basically, a typical
scattering experiment can be described as follows: an incident beam (X-rays or
neutrons) interacts with a sample which occupied a scattering volume. Part of the
incident radiation is scattered by the scattering medium, such as a suspension of
colloidal particles, polymer or surfactant solution, but some is absorbed or
transmitted. The scattered intensity is then measured by a detector set at a certain
angle and distance from the sample [146]. A schematic representation of the
scattering experiment is shown in Figure 9.
32
q
Incident radiation
Sample
Figure 9. Schematic representation describing scattering experiment.
In solution, SAXS measures the difference of electron density of molecules, whereas
SANS measures the difference of a coherent neutron scattering lengths of the
particle and the solvent. Three general strategies have been reported to determine
the microstructure of a microemulsion from the scattering data. One can extract
structural parameters such as the specific area and the size domain by direct
determination of the scattering curve. Another methodology implies the use of a fitting
procedure assuming the presence of simple structures like spheres. Finally, another
strategy uses a microstructural model test, for example, using dilution lines for
several samples that are confronted to one specific model [147-148].
One of the most used models to describe the scattering of microemulsions was
proposed by Teubner and Strey [149-150]. This model uses three fitting parameters
to describe the broad scattering peak and the q-4 decay at large q values. The
scattering function has the form of the following equation (31):
(31)
As can be seen, the Teubner-Strey expression provide the possibility to extract two
length scales by fitting three parameter a2, c1 and c2. From the peak position the
periodicity on the oil and water domains, d, can be determined, whereas from the
width of the peak, the correlation length ξ can be obtained:
(32)
(33)
33
Additionally, one can obtain information regarding to the amphiphilic strength by the
calculation of the amphiphilic factor using equation (34):
(34)
For strong amphiphilic factor fa = -1 a lamellar liquid crystal may be expected.
Structured microemulsions are found in the range -1>fa <0, however, between -0.9
and -0.7 microemulsions are suggested to be much more structured. The decrease in
amphiphilicity leads to the Lifshitz Line (fa = 0), consequently showing a vanishing in
the structure peak. Values for fa = 1 are correlated to a loose in the quasiperiodic
ordering of the microemulsion (disorder line) [151-152].
Another method to evaluate small angle data is the free-form model approach
proposed by Glatter, which is based on the model-free transformation of the
measured scattering pattern (reciprocal space) to real space by the indirect Fourier
transformation (IFT). The method allows to determine the form factor P(q) and the
structure factor S(q), simultaneously [153-154].
The characterization of microemulsions at different composition were complemented
by means of Small angle X-ray scattering (SAXS). The SAXS measurements were
conducted at the beam line 7T-MPW-SAXS at the BESSY II storage ring in Berlin,
Germany. Microemulsion samples were enclosed in 2mm quartz mark tubes from
Hilgenberg and placed in a tempered sample holder. All measurements were carried
out at 25°C. The photon energy was selected by a silicon double crystal
monochromator (Si 110) and was set to 12 keV corresponding to a wavelength of λ =
1Å. By using sample detector distances of 1.2 m and 3 m a q-range between 0.1nm-1
and 3nm-1 was covered. Sample and empty cell transmissions were determined using
a photo sensitive diode inserted behind the sample. The individual sample path
length was determined by transmission measurements of the samples moved
perpendicular to the incident beam. The intensity normalization was performed using
glassy carbon, with a thickness of 90µm as reference. Silver behenate was used for
the radial calibration. The data reduction including corrections of the background,
detector noise, sample thickness, radial and absolute calibration was done with the
software package SASredTool (version V1.1.) developed by Haas.
34
3.2.6 Cyclic voltammetry (CV)
Cyclic voltammetry (CV) is a suitable method to study structural changes in
microemulsions [121]. CV consists in the cyclic variation of the applied potential in a
pre determinated potential range and the measurement of the resulting current
[43,155]. The peak current
of a reversible system is described by the Randles–
Sevčik equation.
(35)
Where (A) is the electrode area given in cm2, the diffusion coefficient (D) in cm2 s−1,
the concentration of the electroactive probe (c) in mol cm-3 and the scan rate ( ) in V
s−1 and n is referred to the number of electrons involved in the redox process.
According this equation,
increases linearly with
, for a given electrode area and a
constant probe concentration. The diffusion coefficient, D, of each solution in
examination can be obtained from the slope of the linear regression in the plot of the
anodic peak current
versus
. In the case of microemulsions, the electro active
species can be solubilized in the core of the droplet or in the continuous phase.
Therefore, the diffusion coefficient will be affected by the structural state of the
medium. Due to the different constituents in the microemulsion, i.e., oil, surfactant, and
polar phase, the solubility of the probe might vary, i.e., the concentration. Hence, it is
reasonable to report the diffusion value as an apparent diffusion coefficient, Dapp.
Cyclic
voltammetric
experiments
on
the
ternary
system
EmimEtSO4/toluene/BmimOctSO4 were performed in an electrochemical assemble
with a platinum wire as the counter electrode, a glassy carbon as working electrodes
and a Ag/AgCl/KCl glass reference electrode. Before each experiment, the working
electrode was polished with slurry containing 0.3 µm and then 0.05 µm sized
aluminum oxide particles for 5 min. After each treatment the electrode was washed
and ultrasonificated in distilled water for 5 min to dislodge retained aluminum oxide
particles on the surface. The working electrode surface area was determined in 1 M
KCl aqueous solution, using cyclic voltammetric experiments on a reversible system
(4mM K3Fe(CN)6). Employing the Randles Sevčik equation (35) and a diffusion
coefficient of 0.76×10−5 cm2 s−1 for the Fe(CN)6−3/−4 [156], the electrode area of
6.78x10-2 cm2 was estimated. All scans were started in the negative potential range
35
with a scan rate range between 20 mV s−1 and 100 mV s−1. The experiments were
performed at 25 °C.
Figure 10. Electrochemical cell 1 elements and the final set up with Au wire (right) and a glassy
carbon working electrode (left). (Courtesy of Dr. Stefano Frasca from the Laboratory of Bioanalytic
Chemistry from University of Potsdam) [157].
3.2.7 Cryo-Scanning Electron Microscopy
The structure of the optically clear microemulsion was examined by Cryo-high
resolution Scanning Electron Microscopy (Cryo-SEM). The samples were cooled by
liquid nitrogen, freeze fractured at −180 °C, etched for 60 s at −98 °C, sputtered with
platinum in the GATAN Alto 2500 Cryo preparation chamber, and then transferred
into the Cryo-SEM S-4800 (Hitachi).
Moreover, high resolution scanning electron microscopy was also used to
characterize the surface of a gold wire electrode in order to visualize gold
nanoparticles immobilized on the surface. The immobilization strategy will be
described in section 3.3.4.
3.3 Gold nanoparticles (AuNPs) in Ionic Liquids
3.3.1 Synthesis of AuNPs in IL-based microemulsions
Two microemulsions containing HAuCl4 and PEI (0.5 wt.%) were mixed. After shaking
over a few minutes, the resulting microemulsion was heated up to 45°C and 100°C.
At 100°C, the solution becomes red after 20 minutes. At 45°C the solution was left
over night. Further, the mixture was cooled down and the toluene phase was
36
evaporated in a vacuum chamber at 40 °C for 48 h. Due to the lower vapor pressure
of the ionic liquids, a solution of gold nanoparticles in the ionic liquids was finally
obtained. The gold nanoparticles dispersed in the ionic liquid were finally
characterized by TEM.
3.3.2 Synthesis of gold nanoparticles in ionic liquids
A solution of poly(ethyleneimine) in ionic liquid was prepared by using two strategies.
In the first strategy an adequate amount of PEI was weighted in a glass tube and
mixed with a defined amount of ionic liquids, to obtain a final polymer concentration
of 0.5 wt.% and 1 wt. %. To dissolve the polymer in IL, the mixture was heated at
50°C, applying ultrasound for 2 h. The second strategy involved dissolving the
polymer in methanol and then adding the ionic liquid to the polymer/alcohol solution.
Finally, the alcohol was removed from the mixture by applying high vacuum for 3 h at
75 °C. Poly(ethyleneimine) (0.5 wt. %) and the tetrachloroaurate solution of 2 mM in
ionic liquids were mixed with 1:1 ratio (wt/wt) at room temperature. Then the mixture
was heated up to 45°C, 100°C and 150 °C. The ionic liquids used for the synthesis of
AuNPs were: [Emim][EtSO4], [Emim][HexSO4] .
3.3.3 Characterization of gold nanoparticles
3.3.3.1 UV-Vis Spectroscopy
Gold nanoparticles prepared in ionic liquids were characterized by means of UV-vis
spectroscopy. The experiments were performed with a Cary 5000 UV-vis NIR
spectrophotometer (Varian). Quartz cuvettes with a path length of 1 cm were used.
3.3.3.2 Transmission Electron Microscopy
Transmission electron microscopy (TEM) micrographs of the nanoparticles were
recorded on an EM 902 microscope from Zeiss. The nanoparticle solution was diluted
in chloroform (volume ratio 1:10 of AuNPs:CHCl3). Methanol can alternatively be
used. The sample was prepared by dropping a small amount of gold solution on
copper grids, dried and examined in the transmission electron microscope at an
acceleration voltage of 80 kV.
37
3.3.4 Electrode modification
A gold wire (Goodfellow, Bad Nauheim, Germany) with a diameter of 0.5 mm was
cleaned by boiling for 4 h in 2 M KOH and kept 10 minutes in concentrated HNO 3. A
careful rinsing with Millipore water followed every successive step. The electrodes
were stored in concentrated H2SO4 when not in use. The cleaned electrodes were
incubated in an ethanol mixture of 5 mM 11-mercapto-1-undecanoic acid (MUA) and
5 mM 11-mercapto-1-undecanol (MU) with a volume ratio of 1:3, for at least 24 h at 4
°C. After rinsing with water, the MU/MUA modified electrodes were incubated in a
freshly prepared [Emim][EtSO4] solution of AuNPs for 2 h at room temperature.
Further, 5 µL of 0.2 M EDC in H2O was added to the solution in order to couple
amino functions of the AuNPs to the carboxylic acid functionalized SAM gold
electrode. The coupling time used in the experiment was 20 min. The resulting
assembled film {Au/MUA-MU/AuNPs-PEI} was characterized by SEM.
For the protein functionalization, the AuNPs modified gold wires were washed
carefully with 0.5 mM Tris buffer at pH 7. Further, the electrodes were dipped in a 2
μM hSO solution prepared in the Tris buffer (0.5 mM, pH 7). After 10 min at 4°C, the
electrodes were careful washed with 5 mM Tris solution at pH 8.4 to remove not
tightly bound enzyme. Modified electrodes were stored dry at 4 °C until use.
3.3.5 Electrochemical experiments on Human sulfide oxidase (hSO)
modified gold electrodes
The electrochemical measurements for hSO at AuNP were performed in a home-made
three-electrode electrochemical cell, with a total volume of 1 mL employing a platinum
wire as the counter electrode, an Ag/AgCl (1M KCl) reference electrode. The working
electrode was a gold wire electrode. Cyclic voltammetric and amperometric
experiments were performed with PalmSens potentiostat and analyzed with PSLite 1.8
software or with Gamry Reference 600TM potentiostat (Gamry, USA) and analyzed with
Gamry Echem Analyst 5.50 software.
38
4
Results and Discussion
4.1 Self-aggregation of ionic liquids studied by surface
tension measurements
The self assembly of amphiphilic molecules in aqueous solutions is of special interest
and an issue of relevant importance in many applications, such as in nanomaterials
synthesis [158-159], in extraction processes [160], in drug delivery systems [161] and
other applications [162]. The aggregation behavior of surfactants in non aqueous
media has been explored [163], particularly, in ionic liquids, as shown in some
recently published reviews [54-55,164-165].
Figure 11 shows the surface tension (γ) as a function of the logarithm of the ionic
liquid
like-surfactant
(IL-S)
i.e.
1-butyl-3-methylimidazolium
octyl
sulfate
([Bmim][OctSO4]) and 1-butyl-3-methylimidazolium dodecyl sulfate ([Bmim][DodSO4])
concentration at 25°C. A significantly low surface tension was observed in the case
of [Emim][EtSO4] ionic liquid (48.6 mN m-1) compared to water (71.8 mN m-1), at
25°C. Note that a pronounced decrease of the surface tension was observed when
IL-S was added. In the case of the [Bmim][OctSO4]-water and the [Bmim][DodSO4][Emim][EtSO4] systems, a near plateau region was observed (Figure 11a and Figure
11d, respectively), whereas a slight change in the slope of the isotherm curve was
registered in the case of [Bmim][OctSO4]-[Emim][EtSO4] and [Bmim][DodSO4]-water
(Figure 11b and Figure 11c, respectively). The critical micellization concentration
(CMC) in water and in [Emim][EtSO4] was estimated at the intersection lines shown
in the surface tension isotherms (Figure 11). The slight variation in the slope for the
[Bmim][OctSO4]-[Emim][EtSO4] system, near to 1.7 mol L-1, provides a hint about an
aggregation process of the IL surfactant, however, the relatively high cmc value
indicates that the micelle formation process is possibly less spontaneous than in
aqueous media. On the one hand, the high solubility of the [Bmim][OctSO 4] in the IL
solvent can be a possible explanation. On the other hand, the presence of highly
surface active impurities in the IL-S can alter the surface tension data near the
inflection points. As it was pointed out, the aggregation of amphiphilic ionic liquids is
a new field which has received considerable attention during the last years. In
contrast to the sodium octyl sulfate surfactant (cmc = 120 mM [166]), the analogous
39
[Bmim][OctSO4] surfactant shows a significantly low cmc (cmc = 32 mM)
corresponding to a ten times lower value [167-168]. By increasing the alkyl chain
from 8 to 12 carbons in the hydrocarbon tail anion, the cmc in water is 100 times
lower than the cmc of sodium dodecyl sulfate, cmc = 8.25 mM [169]). These results
indicate that the imidazolium surfactants have a stronger aggregation tendency
because the repulsions between the head groups might be minimized, due to the
presence of the bulky [Bmim] cation. However, the critical micelle concentration of
IL-S in [Emim][EtSO4] is significantly higher than in water, indicating that the
aggregation process might be less favorable. The behavior can be comparable to
other common ionic surfactants (Table 5). Therefore, the differences can be
explained in terms of the lower polarity and the relatively less structured nature of the
ionic liquids solvent, when compared to water.
70
(a)
46
60
-1
-2
cmc = 3.2 10 mol/L
40
 / mN m
 / mN m
-1
44
50
1E-3
0.01
-1
c / mol L
variation in the slope
40
38
34
0.1
0.1
c / mol L-1
1
46 (d)
44
(c)
50
variation in the slope
40
 / mN m
-1
-1
60
/ mN m
42
36
30
70
(b)
42
40
38
36
cmc =0.116 mol/L
34
32
30
1E-6
1E-5
1E-4
-1
c / mol L
1E-3
30
0.01
0.1
c / mol L-1
Figure 11. Surface tension (γ) isotherms of [Bmim][OctSO4] in water (a), in [Emim][EtSO4] (b) surface
tension (γ) isotherms of [Bmim][DodSO4] in water (c) and in [Emim][EtSO4] (d) at 25°C.
40
Additional parameters such as, the surface excess concentration (Γmax) , the
minimum areas occupied by the surfactant at the liquid/air interface (Amin) and the
Gibbs free energy of micellization (
) (Table 1) were calculated from the surface
tension isotherms using equations (36) and (37), respectively.
(36)
(37)
Where R is the gas constant, T is the temperature and NA is the Avogadro constant.
Table 1 summarizes the estimated critical micelle concentrations for the 1-butyl-3methylimidazolium alkyl sulfate ionic liquid surfactants in water and in the
[Emim][EtSO4], as well as other calculated parameters.
Table 1. Critical micelle concentration (cmc), surface tension (γ) at cmc, surface excess concentration
(Γmax) , area at the liquid/gas interface (Amin ) and the Gibbs free energy of micellization (
IL-S
[Bmim][OctSO4]
).
[Bmim][DodSO4]
Solvent
Water
[Emim][EtSO4]
Water
[Emim][EtSO4]
cmc (mol L-1)
3.2 x10-2
1.76
8.59 x 10-5
0.116
γcmc (mN m-1)
31
37
33.8
31.7
Γmax x 10-6 (mol m-2)
3.63
1.39
4.35
2.42
Amin (nm2)
0.46
1.19
0.38
0.69
-8.52
+1.40
-23.20
-5.33
(kJ mol-1)
The surface excess concentration is an expression related to the effectiveness of the
adsorption of amphiphilic molecules at the liquid/air interface. From the results
observed, one can conclude that the increase of the alkyl chain length in the IL-S
leads to an increase in the number of amphiphilic molecules that can attach at the
interface. The decrease of the surface area at the interface from 0.46 nm2 to 0.38
nm2 in water and from 1.19 nm2 to 0.69 nm2 in [Emim][EtSO4] indicates that the
surfactant molecules are more closely packed when the alkyl chain increases.
41
However, a higher Amin value was determined in ionic liquids when compared to
water, which can be explained again due to the different solvent nature.
As was already stressed in section 1.1.2, the self-assembly of surfactants in water is
governed by the hydrophobic effect and the formation of aggregates in aqueous
media is mainly entropy driven. However, the aggregation in water or in other
solvents involves additional contributions derived from van der Waals, hydrogen
bonding and electrostatic interactions. Taking this into account, one can express the
equation for free energy of micellization as:
. In the case of ionic
liquids, because of the high ionic strength, the electrostatic contributions can be
considered negligible due to the surface charge screening, and only the hydrophobic
term might be considered resulting in
. However, it is still unclear which
model (the phase separation or the equilibrium model) is better in order to determine
the free energy of micellization. Nevertheless, Evans et al. [170] have suggested that
the phase separation model is more relevant in determining the Δ
, since the
micellization process is supposed to occur at a very high ionic strength, which implies
the absence of dissociation. Thus, the resulting aggregates can be thought as a
micelle covered by a counterion shell, similar to that presented by Sarkar and coworkers [168]. In the systems investigated here, the Gibbs free energy was
calculated applying the phase separation model (summarized in Table 1). The
aggregation of the IL-S seems to be more spontaneous in water than in the ionic
liquid ([Emim][EtSO4]). Moreover, the more negative Δ
for [Bmim][DodSO4] IL-
surfactant in [Emim][EtSO4] indicates a more favorable aggregation process in
comparison
to
[Bmim][OctSO4].
In
the
next
chapters
the
formulation
of
microemulsions using [Bmim][OctSO4] and [Bmim][DodSO4] as IL-surfactants are
presented.
4.2 Microemulsions with [Bmim][OctSO4]
4.2.1 Phase Diagram
Room temperature ionic liquids (RTIL) such as [Emim][EtSO4] and [Emim][HexSO4]
with the IL-S, [Bmim][OctSO4], have been mixed together with toluene and the
resulting ternary system has been studied. The addition of pentanol as a cosurfactant was also investigated.
42
The phase diagrams were determined by preparing different compositions of
[Bmim][OctSO4]/toluene and [Bmim][OctSO4]/RTIL into glass tubes. The binary
mixtures, initially transparent, were titrated with the RTIL or toluene, respectively,
until a turbid appearance was obtained. In the case of systems containing a cosurfactant, pentanol was added to the oil component at the appropriate ratio, and the
determination of the phase behavior was performed as before. Subsequently, the
tubes were weighted and the amount of RTIL or toluene was calculated.
The phase diagrams resulting from an optical inspection of the ternary and pseudoternary system are illustrated in Figure 12. Two regions can be distinguished. The
grey area corresponds to the turbid or two phase region, whereas the unfilled area to
the isotropic phase.
Toluene : pentanol (5:1)
Toluene
0.0
0.0
1.0
(a)
0.2
1.0
(b)
0.2
0.8
0.4
0.4
0.6
Turbid phase
0.8
0.6
a
0.6
0.8
g
h
0.2
e
f
0.4
d
b
c
0.6
0.0
0.8
1.0
BmimOctSO4
EmimEtSO4
Turbid
phase
0.2
1.0
0.0
0.0
0.2
0.4
0.0
0.2
0.8
Turbid phase
0.4
0.6
1.0
0.0
0.6
0.6
Turbid
phase
0.8
0.4
0.8
0.2
0.4
0.8
0.6
0.4
0.8
EmimHexSO4
1.0
(d)
0.6
0.2
1.0
BmimOctSO4
1.0
0.2
0.0
0.8
Toluene:pentanol (20:1)
(c)
0.4
0.6
EmimEtSO4
Toluene
0.0
0.4
0.8
0.2
1.0
0.0
0.6
0.4
1.0
0.0
1.0
BmimOctSO4
0.2
0.0
0.2
0.4
0.6
EmimHexSO4
0.8
1.0
BmimOctSO4
Figure 12. Phase diagrams of the IL-base system using (a) EmimEtSO4/toluene/BmimOctSO4, (b)
EmimEtSO4/toluene/pentanol/BmimOctSO4,
(c)
EmimHexSO4/toluene/BmimOctSO4,
and
(d)
EmimHexSO4/toluene/pentanol/BmimOctSO4.
43
Note that the isotropic region becomes larger when the [Bmim][OctSO4]
concentration increases. However, a significant increase of the isotropic phase range
is observed by adding pentanol in both systems. This phenomenon can be
understood in terms of the major solubilization capacity of the swollen micelles in
combination with the change of the spontaneous curvature of the surfactant film.
Comparatively, the addition of pentanol or butanol as a co-surfactant in water-based
microemulsions, results in a major amount of water molecules that can be
accommodated into the swollen micelles. The addition of a co-surfactant induces an
arrangement of the surfactant film that causes the preservation of a negative
interfacial curvature [11,155,171]. The effect of adding a co-surfactant in
microemulsion containing ionic liquids has been reported by Cheng et al. [61]. In the
current study [61] the addition of butanol favors the stabilization of the microemulsion
and increases the isotropic phase region, similar to our results. Nevertheless, the
effect seems to be more significant using the ionic liquid [Emim][HexSO4] in
combination with pentanol as a co-surfactant. This “boosting” effect observed in the
[Emim][HexSO4] based system can be explained by a major packing of the ionic
liquid surfactant molecules at the interface. One can assume that a palisade layer of
the hexylsulfate anion, pentanol and IL-surfactant molecules might be formed, which
results in the enlargement of the isotropic area. This means that this arrangement
can not only have a remarkable effect over the spontaneous curvature of the
interfacial film, but also can influence the droplet size of the micelles [82]. However,
the next section is focused on the system EmimEtSO4/toluene/BmimOctSO4.
[Emim][EtSO4], bearing short alkyl chain substituents can work more effectively as a
polar phase for the self assembly of amphiphilies. In addition this RTIL can have
more technical advantages from the point of view of application due to the relatively
low viscosity, the broad window of fluidity, low toxicity and commercial availability on
ton-scale [172].
4.2.2 Shear Viscosity
Viscosity is a relevant quantity for many practical applications. It depends largely on
the shape of the aggregates, on the interactions of the suspended particles and on
the concentration of the system. Therefore, one can deduce valuable information
regarding to the microstructural changes in a microemulsion by determining the
viscosity of a system [110]. Microemulsion systems could contain different types of
44
aggregates which depend on the volume ratio of their components (oil, surfactant, cosurfactant and water), i.e. oil-in-water micelles, water-in-oil micelles or bicontinuous
microemulsions [173].
A
series
of
microemulsion
EmimEtSO4/toluene/BmimOctSO4
samples
compositions
containing
were
prepared
different
along
the
experimental path shown in Figure 12a (line a). The result is illustrated in Figure 13.
Shear viscosity,  /(Pas)
0.06
0.05
0.04
0.03
0.02
0
10
20
30 40 50 60
EmimEtSO4 / (wt.%)
70
80
90
Figure 13. Mean shear viscosity as a function of [Emim][EtSO4] content.
The different microemulsion compositions show a Newtonian flow behavior along the
shear rate studied. The results demonstrate that a relatively low viscosity is found for
all analyzed samples, which is a characteristic feature of a microemulsion system.
The progressive addition of [Emim][EtSO4] results in a linear increase in shear
viscosity up to 48 wt.%. A further addition of IL leads to a steeper increase in η up to
the viscosity region of the pure [Emim][EtSO4] (0.085 Pa s). The continuous increase
in shear viscosity suggests that the system might pass through a dynamic structural
transition, possibly to the bicontinuous microemulsion. Furthermore, beyond 50 wt.%
[Emim][EtSO4] the steeper jump of the curve could be explained by the transition to
the oil-in-IL microemulsion. The relatively high viscosity can be attributed again to the
attractive droplet–droplet interactions. The results are comparable to other ionic liquid
based microemulsions [174].
45
4.2.3 Electrical Conductivity
Conductometric measurements were carried out to investigate possible transitions in
the microstructure of the isotropic phase region. As mentioned, conductometric
measurements can be used to determine different types of microemulsions (water-inoil, oil-in-water and bicontinuous microemulsions) in the optically clear phase region
by varying the water content. Particularly, in the water-in-oil microemulsion, a
percolation boundary and a linear increase in conductivity has been identified and
explained in terms of progressive droplet–droplet interactions and clustering
processes [131]. Non-aqueous microemulsions containing ionic liquids can be
studied in a similar manner. Figure 14 shows that the progressive addition of
[Emim][EtSO4] to the binary [Bmim][OctSO4]/toluene mixture results in a consistent
increase of the conductivity, along line a (Figure 12a). The problem in determining a
percolation boundary, even at a low RTIL content, may be related to the relatively
high ionic conductance of the ionic liquids ([Emim][EtSO4] and [Bmim][OctSO4]),
compared to the analogous water-based microemulsions. Another possible
explanation could be that the system is already percolated, this means that the IL-inoil droplets are in continuous interaction. By adding more [Emim][EtSO4], at about 45
wt.% a change in the slope of the curve is observed. The slight variation in the slope
of the curve could be a hint for a change in the microstructure of the mixture,
assuming again that droplet-droplet interaction may occur.
0.35
Conductivity,  (S/m)
0.30
0.25
0.20
0.15
0.10
0.05
0.00
0
10
20 30 40 50 60 70 80
EmimEtSO4 content / (wt.%)
90 100
Figure 14. Conductivity of the EmimEtSO4/toluene/BmimOctSO4 system at 25 °C along the line a
46
Several groups [61,67,74-75,122] have proposed an alternative strategy to identify
the different microstructures that could exist in IL-based microemulsions, e.g. by
adding the oil component to the polar phase/surfactant mixture and registering the
conductance. The different types of microemulsions determined by this strategy have
been observed by means of freeze-fracture transmission electron microscopy (FFTEM) [175]. Therefore, in order to study our system in more detail, we decided to add
toluene to the pure [Bmim][OctSO4] and also to a given mixture of ionic liquids
expressed in molar ration (W 0) of [Emim][EtSO4] in [Bmim][OctSO4].
0.10
Conductivity,  (S/m)
ratio = EmimEtSO4/BmimOctSO4
100 % BmimOctSO4
0.08
10/90 (W 0 = 0.16)
20/80 (W 0 = 0.36)
0.06
0.04
0.02
oil/IL
0.00
0
10
20
IL/oil
bicontinuous
30
40
50
60
70
80
90
Toluene content / (wt.%)
Figure 15. Electrical conductivity, , of the binary mixture (BmimOctSO4/toluene) and the
EmimEtSO4/toluene/BmimOctSO4 systems at 25 °C at W 0 = 0.16 and W 0 = 0.36 respectively.
Figure 15 shows the variation of the conductivity κ (S/cm) as a function of the toluene
concentration following the experimental path shown in Figure 12a (lines b and c). As
seen, an increase in κ is registered after the first volume of toluene is added. The
solubilization of a small amount of oil in the IL system suggest the formation of micro
droplets of toluene dispersed in the IL mixture, or in the [Bmim][OctSO4] itself.
Nevertheless, the more detailed droplet structure is an open question [176].
Considering primarily the conductivity results of the binary mixture of [Bmim][OctSO4]
and toluene in more detail, it has been reported that the addition of oil can decrease
the ion pairing between the anionic and cationic constituents of the molten salts
[177], consequently increasing the ionization of the IL. Therefore, it has been
postulated that at high concentrations of [Bmim][OctSO4], toluene droplets might be
47
formed and can be stabilized by a surfactant film consisting of the octyl sulfate anion,
whereas the [Bmim]+ counterion can be expected to be partitioned between the IL-S
palisade at the interface and the continuous phase. Further addition of toluene to the
mixture causes an increase in  until a maximum value [161]. After this point a nonlinear decrease is observed followed by linear decay in the conductivity. In the range
of non-linearity, a bicontinuous microstructure can be assumed.
The linear decline of the conductivity at high toluene concentration might be
associated to the formation of IL droplets dispersed in the continuous toluene phase
(IL-in-oil microemulsion). A similar explanation can be deduced for the ternary system
composed of EmimEtSO4/toluene/BmimOctSO4. First of all, it has been mentioned
that the [Emim][EtSO4] (0.3700 S/m) has a higher conductance than [Bmim][OctSO4]
(0.0190 S/m). Hence, the conductivity of the initial mixture with a larger amount of
[Emim][EtSO4] is about 0.0100 S/m higher (Figure 15). However, the strong increase
in conductivity by adding toluene can be explained only by a significant change of the
molar ratio of the two ILs in the continuous phase. Therefore, one can conclude that
by adding toluene to the IL mixtures, the molar ratio of EmimEtSO4/BmimOctSO4 in
the IL continuous phase is increased. As long as the [Bmim][OctSO4] can contribute
to the stabilization of toluene droplets, the increase in κ could be the effect of the
migration of [Bmim][OctSO4] from the continuous phase to the interface of the
droplets, resulting in an increase of a relative amount of [Emim][EtSO4] in the IL
mixture (as a continuous phase) surrounding the toluene droplets [161]. As explained
above, further addition of toluene leads to the bicontinuous phase and reverse
micelle formation. The results are comparable to other microemulsion systems, such
as
cetylpyridinium
choride/butanol/water
bis(trifluoromethanesulfonyl)imide/TX-100/toluene
[178],
[74]
benzylpyridinium
and
1-butyl-3-
methylimidazolium tetrafluoroborate/TX-100/toluene [65]. For a further detection of
the transition points between the IL-in-oil, the bicontinuous and the oil-in-IL
microemulsion,
additional
conductometric
measurements
starting
from
EmimEtSO4:BmimOctSO4 molar ratios W 0=0.60 (or 30:70 weight ratio), W 0=0.94
(40:60 weight ratio), W 0=1.41 (50:50 weight ratio), W 0=2.10 (60:40 weight ratio),
W 0=2.33 (70:30 weight ratio), were performed (Figure 12a, lines d-h).
48
0.3
Conductivity,  / (S/m)
Molar ratio EmimEtSO4:BmimOctSO4
W 0 = 2.33
W 0 = 2.10
0.2
W 0 = 1.41
W 0 = 0.94
W 0 = 0.60
W 0 = 0.36
0.1
0.0
W 0 = 0.16
0
10
20
30
40
50
60
70
80
90 100
Toluene content/ (wt.%)
Figure 16. Additional conductometric measurements at different EmimEtSO4:BmimOctSO4 molar
ratios.
Based on the conductometric breaking points from Figure 16, one can assume the
existence of a narrow bicontinuous phase range separating the small reverse IL-in-oil
microemulsion phase at higher toluene concentration from the oil-in-IL microemulsion
phase. Figure 17 illustrates the resulting phase diagram showing the different
microemulsion regions in the studied system.
Toluene
0.0
1.0
0.2
0.8
0.4
0.6
Turbid phase
IL/oil
(Line a)
0.6
(Point A)
0.4
bicontinuous
0.8
0.2
Isotropic phase region
oil/IL
1.0
0.0
EmimEtSO4
0.0
0.2
0.4
0.6
0.8
1.0
BmimOctSO4
Figure 17. Phase diagram of the EmimEtSO4/toluene/BmimOctSO4 system at 25 °C.
49
4.2.4 Cyclic Voltammetry (CV)
Cyclic voltammetry (CV) was used to investigate structural changes in the
EmimEtSO4/toluene/BmimOctSO4 system. To carry out the experiments, ferrocene
(Fc) was used as an electroactive probe. Fc shows a high solubility in toluene and a
relatively low solubility in the two ionic liquids. The water soluble ferricyanide was
employed to determine the area of the electrode in aqueous solution, as described in
the chapter 3.2.6, but it was discarded as the electroactive probe because of its low
solubility in [Emim][EtSO4]. The polar [Emim][EtSO4] was added to the mixture of
[Bmim][OctSO4] and toluene (55/45 wt.% ratio), corresponding to line a in Figure 12.
The mixture remained optically clear after adding the [Emim][EtSO4]. Nevertheless,
structural transitions might be expected from the oil rich area to the [Emim][EtSO4]
corner. The apparent diffusion coefficient of Fc as a function of [Emim][EtSO4] is
shown in Figure 18. At a low IL content (<10 wt %), large deviations of Dapp were
registered in the highly diluted system. At about 10 wt. % of [Emim][EtSO4] the Dapp
of Fc (14×10−8 cm2 s−1) can be determined and tends to decay as the [Emim][EtSO4]
concentration is raised. The observed linear decrease may correspond to the
bicontinuous microemulsion, similar to the results reported by Mo et al. [121].
Dapp (10
-8
2 -1
cm s )
14
12
10
8
6
4
2
0
0
10
20
30
40
50
60
70
80
EmimEtSO4 content (wt.%)
Figure 18. Apparent diffusion coefficients, calculated by cyclic voltammetry of ferrocene in
BmimOctSO4/toluene (55/45 wt. % ratio) in dependence on the [Emim][EtSO4] concentration at 25 °C
along line a (marked in Figure 12a).
50
At a higher [Emim][EtSO4] concentration (~47 wt.%), the variation in the decay of
Dapp is observed to be less pronounced. At such a high [Emim][EtSO4] concentration,
the oil droplets are expected to be dispersed in the polar IL phase. The relatively low
diffusion coefficient of ferrocene can be explained by the fact that at this
concentration range, the ferrocene molecules are placed inside of the oil-in-IL
droplets.
Nevertheless, the transition between the IL-in-oil to the bicontinuous microemulsion is
not detected, whereas that from the bicontinuous phase to the oil-in-IL microemulsion
is only slightly detectable by this method.
In summary, the almost linear decay of the diffusion coefficient and the increase in
shear viscosity of the system can reinforce the conductivity results regarding the
existence of a dynamic structural transition process within the system. However, the
most significant change in the electrochemical and rheological results occurs at about
45–50 wt. %, in good agreement with the conductivity measurements. Based on
these results, a structural transition from bicontinuous to an oil-in-IL microemulsion
might be possible.
In order to elucidate the existence of the inverse microemulsion droplets formed by
[Emim][EtSO4] dispersed at high toluene concentration, we investigated the isotropic
area where IL-in-oil microemulsion may exist using Dynamic Light Scattering, CryoScanning Electron Microscopy and Small Angle X-ray Scattering.
4.2.5 Dynamic light scattering (DLS)
Dynamic light scattering measurements have been used to determine the size and
size distribution of microemulsion droplets in the IL-in-oil phase. Note, in Figure 19
the addition of [Emim][EtSO4], following the experimental pathway shown in Figure
17 (line a), results in the appearance of scattering peak, indicating the formation of
droplets in the range between 2nm and 10nm. Conversely, the binary mixture of
BmimOctSO4/toluene (48/52 wt. %) does not show a scattering peak intensity. Taking
into account that in the ternary mixture, the [Emim][EtSO4] should be located in the
inner part of the droplets, in contrast to the [Bmim][OctSO4], which is incorporated
more in the interfacial film, the swelling effect can be well understood. The results are
in agreement to the system composed of EmimEtSO4/toluene–pentanol/CTAB
studied by Rabe & Koetz [83] and similar systems reported by other authors
[67][87][88]. The increase in the toluene up to 60 wt. % (40 wt. % [Bmim][OctSO4],
51
results in the appearance of the a scattering peak, indicating the formation of IL
aggregates in toluene.
25
10
20
10
5
1
S iz e
/ nm 10
0
E tS O
0
4
15
5
w t. %
15
E m im
Intensit
y/%
20
Figure 19. Size distribution of the EmimEtSO4/toluene/BmimOctSO4 system in dependence on
[Emim][EtSO4] concentration following line a from Figure 17. Additional measurement of the
BmimOctSO4/toluene mixture at 40/60 wt. % respectively (red curve).
4.2.6 Cryo-scanning electron microscopy (Cryo-SEM)
Cryo-SEM has been successfully used to characterize microemulsion systems [179].
For example, the bicontinuous microemulsion sponge phase and the inverse droplet
phase were identified for aqueous and IL-based microemulsion systems using this
technique [82]. Based on that, we analyzed point A (marked in Figure 17) in more
detail. At this point, the micrograph (Figure 20) shows spherical structures of about
10nm, which suggests the presence of droplets. The droplet sizes are slightly
deviated from the diameter detected by dynamic light scattering, but still in the same
size order.
52
Point A_IL-in-oil droplets
Figure 20. Cryo-SEM micrographs of the EmimEtSO4/toluene/BmimOctSO4 microemulsion at point A
4.2.7 Small angle X-ray Scattering (SAXS)
In order to reinforce the above reported results and to detect any variation in the
microstructure of the system, small angle X-ray scattering experiments were
performed, keeping the [Bmim][OctSO4] concentration fixed at 40 wt. %. When the
oil content was sufficiently high (60 wt. %), in the absence of [Emim][EtSO4], the
SAXS spectra shows a broad Bragg peak, which can be correlated to the presence of
[Bmim][OctSO4] micelles in toluene (Figure 21a). The result confirms the previous
observation determined by DLS. Further, the addition of RTIL apparently causes a
shift of the peak maximum to smaller scattering vector q (up to 10 wt. %). This can be
correlated to a possible increase in the droplet size, similar to that observed for
water-based systems [149]. However, this tendency is broken off when the content of
[Emim][EtSO4] reaches 15 wt. %. Assuming the presence of spherical aggregates,
the shift of the maximum peak toward larger q values can be correlated to a reduction
of the droplet size. The results seem to be surprising, considering that the addition of
RTIL may be located at the inner part of the polar domain, may consequently
increase their size.
53
0.5
(a)
0 wt. % EmimEtSO4
5 wt. % EmimEtSO4
I(q) / cm
-1
0.4
10 wt. % EmimEtSO4
15 wt. % EmimEtSO4
0.3
20 wt. % EmimEtSO4
25 wt. % EmimEtSO4
0.2
0.1
0.0
1
2
-1
q / nm
3
4
5
-1
I(q) / [cm ]
(b)
0.1
1
q/
[ n m -1
]
10
5
0
Em
im
15
Et
20
SO
4
25
t. %
/w
Figure 21. SAXS spectra plotted in linear scale, of EmimEtSO4/toluene/BmimOctSO4 microemulsion
system in dependence on [Emim][EtSO4] weight fraction (b) Log-log plots of the same SAXS data
fitted applying the Teubner-Strey model. Full line represents the fit by equation (31).
The scattering intensities were fitted using the Teubner-Strey equation (31).
As
defined, the periodicity d, can be extracted from the peak position and represents the
size domain, whereas ξ is calculated from the peak width and corresponds to the
correlation length. The description of the broad peak and the q-4 decay (for spherical
geometry) can be made by fitting three coefficients a2, c1 and c2, obtained from the
order parameter expansion of the free energy density [149]. Thus, valuable
information regarding to the microstructure of microemulsions can be delivered.
Additionally, the amphiphilic factor fa and the d/ξ ratio are dimensionless parameters
54
that provide information regarding to the order within the system. All calculated
parameters are summarized in Table 2.
Table 2.Values of the Domain Size d, Correlation Length ξ, Amphiphilic Factor fa from the TeubnerStrey Model for different [Emim][EtSO4] weight fractions at 25°C.
[Emim][EtSO4]
qmax
I (qmax)
d
ξ
d/ξ
fa
0.84
3.64
-0.50
3.12
0.86
3.62
-0.50
0.42
3.19
0.89
3.59
-0.51
1.72
0.45
3.06
0.89
3.45
-0.54
20
1.86
0.40
2.92
0.98
2.96
-0.64
25
1.98
0.38
2.83
1.08
2.61
-0.71
-1
-1
(wt.%)
(nm )
(cm )
(nm)
(nm)
0
1.68
0.35
3.07
5
1.51
0.40
10
1.56
15
As can be seen in Table 2, the size domain d increases with the [Emim][EtSO4]
content (up to 10 wt. %), however, it tends to decrease upon 15 wt.% of RTIL. This
behaviour might suggest a reorganization of the microstructure. Contrary to the
behaviour observed by d, ξ seems to increase linearly with the addition of
[Emim][EtSO4].
The formation of structured microemulsions can be proximate by analysing the
amphiphilic factor (fa) and the d/ξ, ratio. It has been suggested that the formation of
“good” microemulsions are usually found in the range of negative amphiphilic factor
values close to -1 [151]. In particular, well-structured bicontinuous microemulsions
may vary from -0.9 and -0.7 according to reference [152].
In the present work, the values for the amphiphilic factor (fa), slightly varies between 0.50 to -0.51 and finally decreases from -0.54 to -0.71 in the concentration range
between 15 wt % to 25 wt% of [Emim][EtSO4]. The decrease in fa suggests that the
system becomes more ordered denoting the formation of a well-structured
microemulsion at about 25 wt. % of RTIL. Parallel to this, the marked decrease of the
d/ξ ratio observed between 15 wt% to 25 wt% of [Emim][EtSO4]), indicates a
tendency to form a more organized structure. The results seems to match very well
with the electrical conductivity results, which also suggest a transition in the
55
microstructure of the microemulsion at about 25 wt. % [Emim][EtSO4] from IL-in-oil to
bicontinuous microemulsion.
4.3 Microemulsions with [Bmim][DodSO4]
As mentioned before, the great diversity of ionic liquid structures with particular
physicochemical properties makes them attractive for use in several chemical
processes, as a potential alternative to conventional solvents. Halogen-free have
relatively low melting temperature and a high stability to hydrolysis [172]. Moreover,
the decomposition of these compounds at relatively high temperatures can produce
less corrosive gases, compared to those containing halogenated anions, such as
[Bmim][PF6] [180]. Additionally, their preparation can be considered relatively easy
and environmentally sustainable [165,181].
Ionic liquids with a long hydrophobic chain are potential alternative candidates in
comparison to traditional surfactants which can be used to form microemulsions. The
preparation of surfactant based ILs, based on N,N’-dialkylimidazolium cations and
long alkyl chain sulfate anions, can have considerable advantages in terms of strong
electrostatic interactions between the anion and the cation. This may facilitate the
formation of stable interfaces compared to the analogous alkylimidazolium chloride
ionic
liquid.
The
synthesis
of
1-butyl-3-methylimidazolium
dodecyl
sulfate
[Bmim][DodSO4], was performed by exchanging the chloride anion from the 1-butyl3-methylimidazolium chloride ionic liquid ([Bmim][Cl]) with sodium dodecyl sulfate.
The product consists of a beige waxy solid with a melting temperature of 45°C. The
ionic liquid surfactant has shown considerable advantages regarding the formation of
aggregates, possibly micelles, in water in comparison to the analogous sodium
dodecyl sulfate surfactant. Moreover, [Bmim][DodSO4] can be dispersed in
[Emim][EtSO4], as well as in organic solvents, such as toluene. Based on these
properties, the phase behavior of the ternary system was investigated and the
resulting isotropic phase was characterized by different techniques.
4.3.1 Phase Diagram
The ternary system consisting of EmimEtSO4/toluene/BmimDodSO4 was determined
in a similar manner as described in the section 4.2.1. The resulting phase diagram is
illustrated in Figure 22. As can be seen, two phases can be identified in analogy to
the system studied before. Note that a larger isotropic area is obtained by varying the
56
ionic liquid surfactant. An increase in the alkyl chain of the anion part results in more
[Emim][EtSO4] ionic liquid being solubilized in the oil continuous phase at higher
toluene concentrations. This is of critical interest as a large inverse microemulsion
area can be used as a nano reactor to perform chemical reactions.
Toluene
0.0
1.0
0.2
0.8
a
0.4
0.6
Turbid phase
0.6
0.4
0.8
0.2
f
1.0
0.0
EmimEtSO4
0.2
0.4
e
d
0.6
b
c
0.8
0.0
1.0
BmimDodSO4
Figure 22. Phase diagram of the IL-based system using EmimEtSO4/toluene/BmimDodSO4.
Therefore, the isotropic area is of most importance regarding the different types of
self assembling structures that can be formed. Viscometry and conductivity
experiments were used to obtain preliminary information about the microstructure
and structural transitions in the systems. Additionally, complementary techniques
such as DLS, SAXS, and Cryo-SEM are used to provide a detailed mapping of the
investigated system.
4.3.2 Shear Viscosity
Shear viscosity experiments were performed over a series of compositions on the
EmimEtSO4/toluene/BmimDodSO4 system along the experimental pathway shown in
Figure 22 (line a) at 25°C.A Newtonian flow behavior and low viscosity were
observed up to 14wt. % RTIL. After this point, a moderated increase of the shear
viscosity was observed as a function of the RTIL content (Figure 23). However, a
significant change in the slope is observed at about 50 wt. % RTIL. On the one
hand, the sharply increase in the slope can be interpreted in terms of an increase in
the interactions between the droplets, leading possibly the formation of a temporary
droplet cluster. On the other hand, the increase in the viscosity could also be
57
explained by the presence of anisotropic droplets. However, in order to reinforce
these interpretations additional experiments can be suggested, i.e. scattering or
electron microcopy experiments
Shear Viscosity,  / (Pa s)
0.100
0.075
0.050
Point A
0.025
0.000
0
10
20
30
40
50
60
EmimEtSO4 content / wt. %
70
Figure 23. Mean shear viscosity as a function of [Emim][EtSO4] content of the [Bmim][DodSO4] based
system.
4.3.3 Electrical Conductivity
Additional information regarding to structural transitions in the isotropic phase region
can be obtain by mean of electrical conductivity experiments. The experimental
pathway followed corresponds to line a, from Figure 22. In the experiment, a starting
BmimDodSO4/toluene solution of 70/30 wt. %, respectively, was titrated by an ionic
liquid mixture, (EmimEtSO4/BmimDodSO4) at the 70/30 wt. % in composition.
Knowing the amount of IL mixture added, the resulting RTIL [Emim][EtSO4] content
was calculated and plotted as a function of  at 25°C (Figure 24).
58
Conductivity,  S/m )
0.25
Point B
0.20
0.15
0.10
Point A
0.05
0.00
0
10
20
30
40
50
60
EmimEtSO4 content/ wt. %
70
Figure 24. Electrical conductivity of the EmimEtSO4/toluene/BmimDodSO4 system at 25 °C at a fixed
surfactant concentration of 30 wt. %.
As to be seen in Figure 24, low conductivity values up to 14 wt.% of RTIL are
obtained. These results can adduce the formation of diluted IL-in-oil droplets
stabilized by a surfactant layer dispersed in the continuos toluene phase. However,
beyond this point “A”, a continuous increase in , similar to the system using
[Bmim][OctSO4] as a surfactant, is observed. Nevertheless, the relatively short range
(<14 wt.%) and smooth increase in  makes it difficult to determine a defined
percolation threshold in the system.
Further, the increase in [Emim][EtSO4] content in the system up to 50 wt%, shows a
slight variation in the conductance. The resulting behavior is in good agreement to
that observed in shear viscosity experiments. Therefore, one can suggests that at the
concentration range between 45 wt % to 50 wt. % of [Emim][EtSO4], a change in the
conduction mechanism seem to occurs, which can associated to a transition in the
structure of the microemulsion.
Despite of the preliminary conductivity results suggesting a possible variation in the
microstructure, the limits of the different sub-regions are still difficult to identify.
Therefore, based on our previous experiments, the addition of toluene may provide a
suitable strategy concerning the determination of an IL-in-oil, bicountinuous, as well
as IL-in-oil microemulsions. The dependence of the conductivity,  (S/cm), on the
toluene
weight
fraction
is
shown
in
Figure
25.
The
investigated
59
EmimEtSO4:BmimDodSO4 molar ratios are represented in Figure 22 (line b to f, for
W 0 = 0.43, 0.74, 1.14, 1.71, 2.58, respectively).
Conductivity,  / S/m
0.20
Molar ratio EmimEtSO4:BmimDodSO4
W 0= 0.43
W 0= 0.74
0.15
W 0= 1.14
W 0= 1.71
0.10
W 0= 2.58
0.05
0.00 Oil-in-IL
µE
0
Bicontinuous
µE
10
20
30
IL-in-oil
µE
40
50
60
70
80
90 100
Toluene wt.%
Figure 25. Conductivity of the EmimEtSO4/toluene/BmimDodSO4 systems at 25 °C and different
EmimEtSO4:BmimDodSO4 molar rations as a function of toluene content.
For low EmimEtSO4:BmimDodSO4 molar ratios (W 0 = 0.43, 0.74), four regions can be
observed. In the range of low toluene content (up to 20 wt. %), an increase in  can
be registered until a maximum value is reached. This behavior was explained in
terms of the formation of microdroplets of toluene dispersed in the ionic liquids,
similar to the previous BmimOctSO4-based microemulsion and to other studied
systems [67,74]. After this point, a second region can be correlated by the non-linear
decrease of . This zone has been identified as bicontinuous microstructure. The
subsequent addition of toluene leads to a linear decline of the conductivity, which
might be associated to the formation of IL droplets dispersed in the continuous
toluene phase (up to 65 wt. %). Finally, the curve at very high toluene content, >65
wt. %, shows again a region of nonlinearity, which may be related to the existence of
isolated IL-in-oil droplets. On the basis of the conductometric breaking points shown
in Figure 25, three different microemulsions regions were identified, i.e, IL-in-oil,
bicontinuous and oil-in-IL microemulsions illustrated in Figure 26.
60
Toluene
0.0
1.0
0.2
0.8
0.4
0.6
Turbid phase
IL/oil
0.6
0.4
bicontinuous
0.8
oil/IL
1.0
0.0
0.2
0.2
Isotropic phase
0.4
0.6
0.0
0.8
1.0
BmimDodSO4
EmimEtSO4
Figure 26. Resulting phase diagram of the EmimEtSO4/toluene/BmimDodSO4 system obtained at
25°C showing the different types of microemulsions.
In order to explore the zone near to the percolation transition in more detail, the
behavior of the curve has been described using asymptotic power laws, according to
equations (16) and (17). The composition of the systems was expressed in terms of
volume fraction of [Emim][EtSO4] plus surfactant (Φ) using equation (13) in a similar
manner as Lee. et al. [137], by substituting water for the [Emim][EtSO4]. The density
of [Bmim][DodSO4] was assumed to be ≈ 1. The resulting curves are shown in Figure
27a. The percolation threshold (Φp) was determined by plotting the conductivity κ5/8
as a function of the droplet volume fraction (Figure 27b) in similarity to Lagourette et
al. [131].
0.15
(b)
-1 5/8
(a)
Molar ratio EmimEtSO4:BmimDodSO4= 0.43
)
0.04 Molar ratio EmimEtSO4:BmimDodSO4
W 0= 0.43
0.03
Conductivity (S m
Conductivity (S/m
0.05
W 0=0.57
W 0=0.74
0.02
0.01
0.00
0.0
0.1
0.2
0.3

0.4
0.5
0.6
0.10
0.05
0.00
0.0
p= 0.157
0.2

0.4
0.6
Figure 27. (a) Conductivity κ in dependence on the [Emim][EtSO 4] volume fraction (Φ), (b)
Determination of the percolation transition by plotting κ 5/8 in dependence on [Emim][EtSO4] volume
fraction (Φ).
61
Once Φp has been determined (Φp = 0.157), the scaling exponents u and s can be
obtained by plotting In κ versus In (Φ- Φp) and (Φp- Φ), according to equations (16)
and (17), respectively. Two straight lines were fitted to the data and from the resulting
slopes the values for u = 1.4 ± 0.03 and s = 1.9 ± 0.3 were determined (Figure 28).
On the one hand, according to the literature, values for the scaling exponents u and
s, of u between 1.4 and 2 and s ≈ 1.2 suggest a dynamic percolation process. On the
other hand, for static percolation, values for u ≈ 2 and s ≈ 0.6-0.7 are expected [142].
However, these values may depend on the investigated system, temperature and
volume fraction [182-184]. Note, that the main difference seems to be determined by
the index s. The s values (s = 1.9) is higher than either of the aforementioned s
values, therefore it is not clear whether the investigated system undergoes a dynamic
rather than of a static percolation phenomenon.
-3
-1
In ( /S m )
-4
-5
 In -3.62
-6
 = 1.4  = 0.03
-7
-8
s=1.9 = 0.3
-9
-10
-5
-4
-3
In (
-2
-1
- )
p
Figure 28. Determination of the percolation parameters u and s at 25°C for W 0 = 0.43.
Based on the results, the significant increase in (above the Φp) might be explained
due the motion of relatively small ions i.e. [Emim]+, [EtSO4]- and [Bmim]+ ions from
droplet to droplet. Therefore, the formation of temporary droplet clusters or/and the
transfer of ions from one droplet to another through formed channels, may require the
opening of the surfactant film, similar to water based systems [13,129,131,185]. The
anterior implies certain flexibility of the surfactant film which tends to be less rigid as
the droplet volume fraction increases, reaching finally an interfacial curvature near
62
zero, possibly above the critical content (Φcritical ≈ 0.6), where the bicontinuous
microemulsion is expected.
4.3.4 Dynamic Light Scattering (DLS)
As already discussed, the relatively low shear viscosity and conductivity determined
up to 10 wt% of [Emim][EtSO4] (see Figure 23 and Figure 24), supposes the
presence of isolated droplets composed of IL-in-oil aggregates surrounded by a
surfactant film. However, to obtain valuable evidence of the presence of these
colloidal structures, their sizes and size distribution, at this low concentration regime,
dynamic light scattering, SAXS and Cryo-SEM were performed as a function of the
amount of [Emim][EtSO4]. Figure 29 shows dynamic light scattering results of
different microemulsion compositions at a constant [Bmim][DodSO4] surfactant
concentration (30 wt. %). The presence of a peak, in absence of RTIL, suggests that
[Bmim][DodSO4] may be able to form micelles in toluene. A slight shift to a larger
droplet size is observed when [Emim][EtSO4] (up to 10 wt.%) is added, indicating a
swelling behavior, which is a characteristic feature of aqueous-based systems, but
also in non aqueous microemulsions containing ionic liquids [87][89].
20
0
tS O /
4 w t .%
10
20
15
10
1
5
Size 10
/ nm
0
E m im E
Intensity / %
30
Figure 29. Size distribution of the ternary EmimEtSO4/toluene/BmimDodSO4 system in dependence
on the [Emim][EtSO4] concentration at constant IL surfactant concentration (30 wt. %).
However, the droplet size tends to remain constant (circa 6nm in diameter) upon
increasing the [Emim][EtSO4] content (up to 15 wt.%).
63
The increase in the hydrodynamic diameter at low [Emim][EtSO4] (<10 wt. %)
suggests that the interfacial area occupied per surfactant molecule, in our case
occupied by the dodecyl sulfate anion, tends to be less affected by the addition of
RTIL. At 15 wt. % of [Emim][EtSO4], the interaction between the head groups at the
interface of the droplets may become less effective, due to the increase in the ionic
strength. Consequently, the interfacial area per sulfate head group of the surfactant
molecules is reduced, inducing the formation of a more packed interface.
Additionally, the reduction of repulsion interactions at the droplet interface may cause
high curvature of the surfactant film, inducing the formation of smaller IL-in-oil
droplets. The results are in good agreement to the system water/AOT/dodecane
studied by Shah et al. [186], showing that the increase of NaCl concentration reduces
the size of the water-in-oil microemulsion droplets. Behera and Pandey have been
pointed out that the addition of ionic liquid to water and anionic and zwitterionic
surfactants, i.e. sodium dodecyl sulfate (SDS) and N-dodecyl-N,N-dimetyl-3ammonio-1-propanesulfonate sulfobetaine (SB-12) respectively, has a significant
effect on the critical micelle concentration [155,187-188]. The authors have also been
explained this effect in terms of the reduction on the electrostatic interactions
between the cation of the ionic liquid and the anion site of the surfactant, favoring a
more efficient micellization process. On the other hand, the reduction of repulsive
forces seems to induce the formation of smaller and more monodispersed micelles.
Studies on microemulsion systems containing ionic liquids have shown some
similarities to our results with regard to the size of the micellar aggregates. Zech and
co-workers have investigated a microemulsion system composed of ethylammonium
nitrate (EAN) as a polar phase, 1-hexadecyl-3-methylimidazolium chloride as a
surfactant and dodecane and decanol as oil and co-surfactant, respectively [88][89].
From DLS results, it is shown that a slight increase in the size of the droplets is
detected, with a further tendency to remain constant upon 20 wt. % of RTIL. It may
also be considered that an increase in the number of droplets may also increase the
interaction and polydispersity in the system, creating certain problems with the DLS
data interpretation and causes deviations in the droplet size determination.
4.3.5 Small Angle X-ray Scattering (SAXS)
The ternary system was investigated by means of small angle x-ray scattering
(SAXS). SAXS experiments were performed following the experimental pathway
64
used in DLS experiments. In order to illustrate the subsequent shift on the Bragg
peak, the scattering intensities were plotted in a linear scale (Figure 30a).
1
(a)
0 wt.% EmimEtSO4
5 wt. % EmimEtSO4
-1
10 wt. % EmimEtSO4
I (q) / cm
15 wt. % EmimEtSO4
25 wt. % EmimEtSO4
0
1
2
3
-1
q / nm
4
5
-1
]
I(q) / [cm
1
(b)
0.1
1
q/
[n
Figure 30. (a)
SAXS
spectra
5
m -1
plotted
]
in
0
linear
scale
10
Em
of
15
im
25
20
S
Et
/w
t.
%
O4
EmimEtSO4/toluene/BmimDodSO4
microemulsion system in dependence on [Emim][EtSO4] weight fraction. (b) Log-log plots of the same
SAXS data fitted applying the Teubner-Strey model. Full lines represent the fit by equation (31).
SAXS results show a single broad peak, with characteristic q-4 decay at a large q
value, similar to SAXS spectra of aqueous microemulsions. The presence of a peak
maximum in absence of [Emim][EtSO4], suggests that [Bmim][DodSO4] may be able
to form micelles in toluene, as already concluded from DLS results. Note that the
position of the peak maximum and the corresponding intensity varies as a function of
65
the amount of [Emim][EtSO4]. As expected, an increase in the polar size domain is
correlated to the shift of the peak maximum to smaller q values, by increasing the
RTIL content until nearly 10 wt.%. The further movement to larger q values of the
peak maximum up to 15 wt. % [Emim][EtSO4], indicates a reduction in the droplet
size domain. Additionally, a slight variation in the scattering intensity at the maximum
q (I(qmax) is registered. The results show close similarities to that observed by
dynamic light scattering measurements. As already discussed, the swelling behavior
observed at low RTIL is explained by the incorporation of [Emim][EtSO4] into the
droplet core. The reduction of the droplet size observed at higher RTIL content may
arise from the decrease of the surface area per polar head group of the surfactant
molecules. This can be induced by the minimization of the repulsive interactions at
the interface of the droplet, due to the increase in the ionic strength.
As already mentioned in the experimental section, one of the most applied models to
describe the scattering data of microemulsions is the Teubner-Strey model.
According to the TS-model, the resulting scattering intensity (Figure 30b) can be
fitted applying equation (31) using three fitting parameters (a2, c1 and c2), which
describe the broad peak and the q-4 decay. From the fit, two length scales, the
periodicity d, which represent the domain size and the correlation length ξ can be
extracted from equations (32) and (33), respectively. Additionally, the amphiphilic
factor fa and the ratio d/ξ were determined and summarized in Table 3.
Table 3. Values of peak positions qmax, scattering intensities I(qmax) from experimental SAXS data.
Characteristic length scales d and ξ and dimensionless quantities calculated from the resulting fit
parameter applying the T-S model for the EmimEtSO4/toluene/BmimDodSO4 microemulsion system.
[Emim][EtSO4]
qmax
I (qmax)
d
ξ
(wt. %)
(nm-1)
(cm-1)
(nm)
(nm)
0
1.78
0.60
3.34
5
1.76
0.73
10
1.64
15
25
d/ξ
fa
1.92
1.74
-0.86
3.44
2.08
1.66
-0.87
0.81
3.67
2.19
1.68
-0.87
1.84
0.87
3.32
2.23
1.49
-0.89
1.86
0.84
3.32
2.19
1.52
-0.89
66
One can conclude that d and ξ increase only slightly with the [Emim][EtSO 4] content
(up to 10 wt. %). The addition of [Emim][EtSO4] (up to 15 wt.%) causes a decrease in
d, whereas ξ seems to be less affected. As mentioned, the amphiphilic factor (fa) and
the d/ξ, ratio can describe the formation of structured microemulsions. The ratio d/ξ,
is also a dimensionless quantity which can measure the polydispersity of the polar
and non-polar domains, the smaller the ratio, the larger the polydispersity [189].
Taking this into account, the relatively low values in the amphiphilic factor (fa), varying
between -0.86 and -0.89 and the relatively high d/ξ ratio (between 1.52 and 1.74),
over the whole range of investigation (0 wt% to 25 wt% of [Emim][EtSO4]), suggest
the formation of
well structured bicontinuous microemulsions. The d/ξ ratio is
comparable to those found by Atkin et al. in the system C18E4/propylammonium
nitrate (PAN)/octane. However, the anterior seems to contrast the results obtained
by DLS and conductivity measurements with regard to the formation of IL-in-oil
droplets at low RTIL content.
4.3.6 Cryo-Scanning Electron Microscopy (Cryo-SEM)
In order to obtain visual evidence about the microstructure of the system, two
different
compositions of
the EmimEtSO4/toluene/BmimDoSO4 system
were
evaluated by means of Cryo-scanning electron microscopy. Point 1 (Figure 31a)
represents the composition of the microemulsion with 10 wt % of [Emim][EtSO 4],
where IL-in-oil microemulsion is expected according to the experiments described
before. Point 2 (Figure 31b) corresponds to the composition (45 wt. % of
[Emim][EtSO4]) where a bicontinuous microemulsion may exists.
From the micrograph shown in Figure 31a, globular structures can be identified with
relatively high contrast in the complex matrix. The observed spherical aggregates,
are in the range of 10nm, similar to the previously investigated [Bmim][OctSO4]
system.
The micrograph at Point 2 (Figure 31b) seems to be more proximate to the structure
of a bicontinuous microemulsion. At this composition of IL and oil, certain differences
in the contrast can be noticed, indicating the coexistence of IL and oil micro
channels.
67
(a)
(b)
Figure 31. Cryo-SEM micrographs of different compositions of the EmimEtSO4/toluene/BmimDodSO4
system, (a) 10 wt.% [Emim][EtSO4] (Point 1) and (b) 45 wt. % [Emim][EtSO4] (Point 2).
The results reinforce previous observations determined by electrical conductivity
concerning the existence of a bicontinuous microemulsion within the investigated
concentration range. Moreover, it also supports the tendency determined by SAXS
measurements about the formation of a well-structured bicontinuous microstructure
when the [Emim][EtSO4] content is increased. The pattern observed in the
micrograph (Point 2) can be comparable to that observed by Gao et al.[175] using
FF-TEM on IL/surfactant mixtures with different organic solvents.
On the basis of the results, the investigated systems may represent an alternative
water-free reaction media. In particular, the investigated isotropic area can be of
interest as a template phase for the preparation of metal nanoparticles, as well as, to
perform other types of chemical or enzymatic processes. In the next chapter, the
synthesis of gold nanoparticles in IL-based microemulsions will be presented.
4.3.7 Conclusions
Ionic liquids bearing long alkyl chain substituents i.e. 1-butyl-3-methylimidazolium
octyl sulfate ([Bmim][OctSO4]) and 1-butyl-3-methylimidazolium dodecyl sulfate
([Bmim][DodSO4]) show a tendency to form aggregates in water and in the room
temperature ionic liquid (RTIL) [Emim][EtSO4]. The aggregation behavior of these
compounds seem to be more spontaneous in water than [Emim][EtSO 4]. However,
the increase in the amphiphilicity of the anion induces a more favorable behavior of
aggregation in [Emim][EtSO4].
68
Taking into account the amphiphilic/solvent features of the investigated ionic liquids,
non-aqueous microemulsion, consisting of [Emim][EtSO4] as the polar domain,
toluene as the oil component and the ionic liquids with surfactant i.e. [Bmim][OctSO4]
and [Bmim][DodSO4] were formulated and investigated.
The isotropic phase region was studied in more detail using different techniques. In
particular, conductometric experiments were successfully applied to determine three
different regions, correlating to the presence of oil- in-IL, bicontinuous and IL-in-oil
microemulsions. The results were reinforced using other techniques, such as, cyclic
voltammetry, rheology, dynamic light scattering (DLS), small angle X-ray scattering
(SAXS), and cryo-scanning electron microscopy (Cryo-SEM).
DLS measurements indicate the presence of droplets in the order of 10nm. A single
Bragg peak, determined by SAXS experiments, supports the formation of ionic liquid
micelles in toluene in both systems.
The modeling of the resulting peak using the Teubner-Strey equation was used to
proximate the microstructure of
the microemulsions.
In particular, in the
EmimEtSO4/toluene/BmimOctSO4 system, the decrease in fa from -0.50 to -0.71
suggests that the system tends to form a well-structured microemulsion. On the other
hand, similar results were obtained for the EmimEtSO4/toluene/BmimDodSO4
system. The slight variation and further decrease of the aggregates size determined
by DLS and SAXS can be a consequence of an increase of the ionic strength by
adding the RTIL. However, the calculation of the amphiphilic factor from the SAXS
data indicates that the current system is already in the transition range to
bicontinuous microemulsion.
Cryo-SEM provides valuable evidence about the formation of spherical aggregates in
the [Bmim][OctSO4] based system, however, the low contrast between the ionic liquid
and oil compound represent a difficulty.
A bicontinuous microemulsion was
successfully identified in the [Bmim][DodSO4] based system, reinforcing the
conductometric results.
Finally, the investigated systems may represent an interesting reaction medium to
perform chemical reactions, in particular as a template phase for the preparation of
metal nanoparticles.
69
4.4 Synthesis of gold nanoparticles in ionic liquids
4.4.1 General aspects
The preparation of colloidal metal nanoparticles by the controlled reduction,
nucleation, and growth of inorganic precursor salts has been intensively investigated
over the last decades [190].
Metal nanoparticles show a relative stability in solution towards aggregation because
of the acquisition of charges either from surface charged groups or by specific ion
adsorption from the bulk solution. Such charges lead to a repulsive double-layer force
between particles. Additionally, nanoparticles adsorbing a polymeric layer can be
sterically stabilized due to a steric barrier which protects the particles against
collision. A much more efficient stabilization is provided when the adsorbed polymer
is a polyelectrolyte. In this case, both types of stabilization can be combined to lead
to electrosterically stabilized systems [191-193]. The uses of polymers not only
provide stabilization to the nanoparticles but also functionalize their surface, which
made them attractive for different applications [194]. Particularly, colloidal gold
nanoparticles have important relevance, due to their distinct optical and electronic
properties when compared to the bulk metal [195-197]. The properties of small gold
particles, their size-dependent electrochemistry and their high chemical stability is of
current interest for different biotechnological applications [195,198-199].
The synthesis of AuNPs based on the reduction of precursors containing gold
complexes, like tetrachloroauric acid (HAuCl4), has been made in aqueous solution
and non aqueous medium [200], such as in ionic liquids. Several strategies have
been proposed for the reduction process, including chemical, photo-induced, thermal
decompositions or controlled solvent evaporation [195,201].
Taking advantage of the different combinations of ionic moieties, which can be made
for the IL preparation, the synthesis of nanoparticles in ILs, can provide the possibility
to obtain materials that can be dissolved and dispersed either in aqueous or in a non
aqueous medium. In addition, the relatively low surface tension and ordered
structured of ILs may induce the formation of small and stable nanoparticles, with the
possibility to tune their morphology. One of the first approaches with regard to the
synthesis of gold nanoparticles in ILs was reported by Itho et al. [202]. The
synthesized nanoparticles show hydrophilic and hydrophobic features which can be
tuned by an anion exchange of the ionic liquid. Another approach was reported by
70
Khare et al. where gold nanoparticles were prepared using glycerol as a reducing
agent in imidazolium based ionic liquids. The authors suggest a remarked effect on
the particle morphology resulting from the IL-gold interaction [203]. A similar
approach was also presented by Ren and co-workers [204]. A sustainable alternative
method was reported by Tetsuya et al. where gold (III) was reduced by irradiation
using accelerated electron beams and X-rays. The effect of the cation type in the
ionic liquid seems to play an important role during the synthesis process and for the
stabilization of the resulting nanoparticles without the addition of any stabilizing agent
[205]. The dissolution of biomolecules, e.g. cellulose in ionic liquids, is also a relevant
issue for simplifying its processing and chemical transformation. For instance,
cellulose has been dissolved in ionic liquids and used as a reducing agent for the
synthesis of gold nanoparticles [206]. The immobilization of ionic liquids in a polymer
matrix by covalent bonding is also an interesting approach to prepare AuNPs on the
nanometer scale [207].
On the one hand, the control of shape-dependent physical properties of gold
nanoparticles is of special interest in electronics and medicine. On the other hand,
the convergence of biotechnology and nanotechnology has led to the development of
hybrid nanomaterials that incorporate the highly selective catalytic and recognition
properties of biomolecules with the unique electronic, photonic, and catalytic features
of nanoparticles. A very interesting property of gold nanoparticles is that it provides a
suitable microenvironment for biomolecule immobilization, allowing the retention of
their biological activity. The incorporation of gold nanoparticles (AuNPs) over an
electrode surface has been successfully conjugated with a biocompatible matrix
(chitosan) [208-211], synthetic polyelectrolytes such as polyamidoamine and
polypropyleneimine [212-213], dinucleotides [214-215] and proteins [216]. Their
ability to facilitate electron transfer (ET) between immobilized proteins and electrode
surfaces have motivated their application in electrochemical biosensors [214,217]
[198,218]. It has been postulated that the high surface-to-volume ratio, high surface
energy and the ability to act as electron-conducting pathways between prosthetic
groups and the electrode surface, are the outstanding characteristics of gold
nanoparticles, which may facilitate an electron transfer between redox proteins and
electrode surfaces [219]. In the following sections, the preparation of AuNPs in ILs
will be presented. Additionally the application of the prepared nanoparticles in a
biosensor system or for the preparation of gold nanorods will be also described.
71
4.4.2 Synthesis of gold nanoparticles in IL-based microemulsion
The room temperature ionic liquid [Emim][EtSO4] was used as a polar phase and has
been dispersed in toluene by using ionic liquid like surfactants i.e. [Bmim][OctSO4]
and [Bmim][DodSO4] to formulate non-aqueous microemulsions.
As already
discussed, the isotropic phase depicts three different types of microemulsions, i.e. ILin-oil, bicontinuous and oil-in-IL. In particular, these phases are of special interest
because they can be used as a template for the preparation of nanomaterials or to
perform other chemical processes.
In our first approach, poly(ethyleneimine) 25 000 g mol-1 (PEI-25K) was used as
received and incorporated into the microemulsions. Branched PEI has demonstrated
to be an effective reducing agent for the synthesis of gold nanoparticles, contributing
to their growing process and providing additional protection against aggregation, due
to polyelectrolyte-nanoparticle interactions [192]. The chemical reduction of Au+3 to
Au0 at two different compositions in IL-based microemulsions was carried out by
mixing two adequate µEs containing PEI and HAuCl4, respectively. The interaction
between the microemulsion droplets allows the components to come into contact and
the reaction to take place. After combining the microemulsions at room temperature,
the mixture was left overnight at 45°C. Under these conditions two compositions
were evaluated and labelled as µE-Point 1 and µE-Point 2. The resulting gold AuNPs
were characterized by UV-vis spectroscopy and transmission electron microscope
(TEM).
(a)
AuNPs/µE-Poin1-EmimEtSO4/toluene/BmimOctSO4
AuNPs/µE-Point2-EmimEtSO4/toluene/BmimOctSO4
(b)
Absorbance a.u.
AuNPs/µE-Point2-EmimEtSO4/toluene/BmimDodSO4
µE-Point1
400
500
600
700
µE-Point2
800
Wavelenght / nm
Figure 32. UV-vis spectra of the IL-based microemulsions using [Bmim][OctSO4] and [Bmim][DodSO4]
as IL –surfactant, after the chemical reduction of Au+3 (b) TEM micrograph corresponding to sample
µE-Point 1 (10%-EmimEtSO4/50%-toluene/40%-BmimOctSO4).
72
Note that the color of the dispersions corresponding to µE-Point 1 and µE-Point 2
based on [Bmim][OctSO4] are pink and red, respectively (inserted photography in
Figure 32). The UV-vis spectra (Figure 32a) of the µEs show a maximum absorption
at about 550nm, indicating the formation of large gold nanoparticles. As to be seen in
the TEM micrograph (Figure 32b), spherical nanoparticles of about 20nm were
formed. In contrast, when the [Bmim][DodSO4] was used as surfactant, the
dispersion remained colourless and no evidence of absorption peak in the UV-vis
spectrum is noted. This means that gold nanoparticles were not produced. The result
can be explained in terms of the poor solubility of poly(ethyleneimine) in this µE
system. It was observed that the polymer precipitates when the microemulsion is
heated up to 45°, 65° and 100°C. Similar observations were made when binary
[Emim][EtSO4]-[Bmim][DodSO4] mixtures were used as a reaction medium. The low
solubility of the polymer in the microemulsion and in the binary mixture can be
understood in terms a less favoured IL-polymer interaction, in comparison to the IL-IL
interactions. However, the system represents an interesting alternative as a reaction
medium to perform other chemical reactions [74].
The effect of the molar mass of the polyelectrolyte was also investigated.
Poly(ethyleneimine) with a molar mass of 5000 g mol-1 (PEI-5K) was neutralized,
dried and incorporated into the ternary EmimEtSO4/toluene/BmimOctSO4 system.
The reaction, performed at 100°C, shows a change in color from colourless to red
during the reaction time, indicating the formation of colloidal gold particles. UV-vis
measurements performed on the resulting AuNPs showed a characteristic absorption
maximum at about 530 nm, suggesting the formation of gold nanoparticles. The
micrographs shown in Figure 33a confirmed the formation of spherical gold
nanoparticles between 10nm to 20nm.
However, the large polydispersity of the nanoparticles accomplished in µE-Point 1a
can be explained in terms of the variation of the droplets size, due to the high
temperature used. In order to reduce the negative temperature effect, the reaction
was performed at 45°C for a longer time (overnight). However, following the standard
strategy (mixing two microemulsions containing the reactants separately), the
samples (µE-Point 1 and µE-Point 2) remained colourless after the whole reaction
period. It can be assumed that the mixing of the droplets is less effective in the
presence of the neutralized polymer. Therefore, an alternative procedure was used
by combining the gold precursor and the polyelectrolyte (prior dissolved in
73
EmimEtSO4 IL) at room temperature, before being incorporated with the
[Bmim][OctSO4] and toluene to form the microemulsion. The composition labelled as
µE-Point 1b and µE-Point 2 were left in the oven at 45°C overnight. As a result, pink
and red solutions, respectively, were obtained, indicating the formation of AuNPs.
(a)
(c)
(b)
Figure 33. Gold nanoparticles synthesized in the EmimEtSO4/toluene/BmimOctSO4 microemulsion (a)
µE-Point1a
at 100°C, (10%-EmimEtSO4/50%-toluene/40%-BmimOctSO4), (b) µE-Point1b at 45°C,
and (c) µE-Point2 at 45°C, (30%-EmimEtSO4/30%-toluene40%-BmimOctSO4).
Figure 33b and c show the micrographs of AuNPs prepared in microemulsion under
the modified conditions. As can be observed, spherical particles between 10nm and
20nm were produced, however, a much more concentrated and polydispersed
particle size was obtained in µE-Point 2. Note that the composition at this point
represents a composition within the bicontinuous microemulsion area. Therefore, one
can conclude, that the size and shape of the nanoparticles seems to be less
controlled by the corresponding template phase. In addition, the tendency to form
mostly spherical particles, indicates that poly(ethyleneimine) favours the formation of
more isotropic gold nanoparticles, similar to other studies employing PEI [190].
74
4.4.3 Synthesis of gold nanoparticles in pure ionic liquids
The synthesis of AuNPs using ionic liquid as a solvent and poly(ethyleneimine) (PEI)
as a reducing and stabilizing agent is presented in this chapter. We have used
[Emim][EtSO4] and [Emim][HexSO4] ionic liquids as solvents. Two different molar
mass of poly(ethyleneimine) were used, 5000 g/mol (PEI-5K) and 25 000 g/mol (PEI25K). The polymers were neutralized and dried before use. The reaction was
performed at the mass ratio 1:1 between the metal precursor (2 mM of HAuCl4) and
PEI (0.5 wt %) in all experiments, at different temperatures. Figure 34 shows a TEM
micrograph of nanoparticles prepared in [Emim][EtSO4] at 100 °C.
(a)
270
Number of particles
(b)
180
90
0
0 1 2 3 4 5 6 7 8 9 10 11 12
Particle size / nm
Figure 34. (a) Electron transmission micrograph of gold nanoparticles synthesized in [Emim][EtSO4]
solvent, (b) Histogram showing the number of particles in dependence on the particle size (size
interval Δd of 1 nm).
The TEM micrograph shows well dispersed spherical nanoparticles with an average
core diameter of about 6 ± 2nm. The particles can be dispersed in water or in organic
solvents. In water, the gold nanoparticles have a hydrodynamic diameter, determined
by dynamic light scattering (DLS) of about 9 ± 1nm. However, the particles tend to
aggregate at longer times, perhaps due to ionization of the ionic liquid layer on the
surface of the particles and the relatively low surface potential +20 mV. Note, at low
concentration AuNPs, the particles remain more stable when dispersed in chloroform
or methanol. On can assume that an IL-polymer complex is formed protecting the
75
particles against aggregation. Moreover, it provides certain hydrophobicity, even
though, [Emim][EtSO4] has a short alkyl chain moiety.
Furthermore, gold nanoparticles were obtained using [Emim][HexSO4] ionic liquid
with longer alkyl chain anion as a solvent. Under the same conditions as before, the
reduction process needs more time. Figure 35 illustrate the TEM micrograph of gold
particles prepared in [Emim][HexSO4] at 100 °C.
(b)
Number of particles
(a)
150
100
50
0
0
1
2
3 4 5 6 7
Particle size / nm
8
9 10
Figure 35. (a) Electron transmission micrograph of gold nanoparticles synthesized in [Emim][HexSO4]
solvent, (b) Histogram showing the number of particles in dependence on the particle size (size
interval Δd of 1 nm).
One can see that more uniform, well dispersed and stable spherical AuNPs of about
5 ± 1nm were obtained. An interesting observation of the resulting gold solution is
their high stability against aggregation, which remains suspended for more than two
years.
Moreover, the AuNPs prepared in [Emim][HexSO4] can be dispersed in
organic solvents such as chloroform and dichloromethane. Contrary to the AuNPs
prepared in [Emim][EtSO4], these nanoparticles form a turbid dispersion in water.
Taking this into account, one can assume that in the presence of [Emim][HexSO4], a
stronger IL-polymer complex is formed on the surface of the gold nanoparticles which
is responsible for the longer stability and solubility characteristics over time. One can
suggest that the interaction between the amino group of the PEI and the hexyl sulfate
anion leads to the hydrophobization of the particle surface.
76
AuNPs_EmimEtSO4_PEI_5K_45°C_overnight
AuNPs_EmimEtSO4_PEI_5K_150°C_1min
150
Number of particles
Number of particles
200
150
100
100
50
0
9-13 14-18 19-23 24-28 29-33 34-38 39-43 44-48
50
0
0
Particle size / nm
2
3
4
6
7
8
9
10
11
12
13
AuNPs_EmimHexSO4_PEI_5K_150°C_2min
40
Number of particles
30
20
10
30
20
10
0
0
0
2
4
6
8
10
12
14
16
18
20
0
22
2
4
6
8
AuNPs-EmimEtSO4_PEI_25K_100°C_5min
10
12
14
16
18
20
22
Particle size / nm
Particle size / nm
AuNPs-EmimEtSO4_PEI_25K_100°C_20min
100
60
Number of particles
Number of particles
5
Particle size / nm
AuNPs_EmimHexSO4_PEI_5K_45°C_overnight
Number of particles
1
80
60
40
20
0
50
40
30
20
10
0
0
1
2
3
4
5
6
7
8
9 10 11 12 13 14 15 16
Particle size / nm
0
1
2
3
4
5
6
7
8
9
10
11
12
13
Particle size / nm
Figure 36. Gold nanoparticles prepared in ionic liquids as solvent at different temperatures and molar
masses of poly(ethyleneimine).
77
Using the same ionic liquids as solvents, several conditions were varied in order to
control the morphology of the resulting particles. Figure 36 illustrates AuNPs
synthesized by varying the temperature and the molar mass of poly(ethyleneimine).
From the micrographs shown in Figure 36, one can see that spherical particles are
obtained in all cases. The main results, with regards to the experimental conditions,
the UV-vis absorption of the final gold solution and the dimensions, are summarized
in Table 4.
Table 4. Main parameters used during the synthesis of gold nanoparticles, Uv-vis maximum
absorption and average particle diameters obtain from the TEM micrographs.
Molar mass
Temperature
UV-vis
TEM
(PEI 0.5wt %)
(°C)
(A/nm)
(d/nm)
2 mM
5 000
45
~20h
542
27 ± 8
AuNPs/EmimEtSO4
2 mM
5 000
100
5min
529
6±2
AuNPs/EmimEtSO4
2 mM
5 000
150
1 min
527
7±2
AuNPs/EmimHexSO4
2 mM
5 000
45
~20h
531
12.0 ± 4
AuNPs/EmimHexSO4
2 mM
5 000
100
20min
531
5±1
AuNPs/EmimHexSO4
2 mM
5 000
150
2min
529
11 ± 3
AuNPs/EmimEtSO4
2 mM
25 000
100
10min
529
6.8 ± 1.8
AuNPs/EmimHexSO4
2 mM
25 000
100
20min
531
6.3 ± 1.8
Sample
HAuCl4
AuNPs/EmimEtSO4
Time
Relatively small nanoparticles, ranging between 5nm and 12nm in size, were
obtained in most cases. Surprisingly, larger particle diameters were produced at a
relatively low temperature (45°C), using [Emim][EtSO4] as a solvent. The result can
be reinforced by means of UV-vis spectroscopy, showing an absorption maximum at
a wavelength of ~530nm for particles of about 6nm to 7nm in diameter, whereas
542nm for particles of about 27nm in size (Table 4). On the contrary, particles
prepared in [Emim][HexSO4], depict smaller diameters in TEM, without any clear
correlation to the UV-vis absorption data. On the one hand, the gold nanoparticles
dimensions seem to be less affected by varying the temperature or the molar mass of
the poly(ethyleneimine), which induce the formation of spherical particles in all cases.
78
On the other hand, the stabilization of the nanoparticles seems depend on the type of
the anion of the ionic liquid used. That means, larger alkyl chains of the anion provide
a better protection against aggregation and supply a partial hydrophobic surface.
4.4.4 Gold nanoparticles prepared in ionic liquids used as a seed
for the preparation of gold rods
As already presented, properties of ionic liquids, such as their low vapor pressure
and tailored solvent properties are of special interest, given their ability to act as a
reaction media to perform chemical reaction at elevated temperature and provide an
anhydrous
conditions
medium.
The
ionic
liquids
(e.g.
[Emim][EtSO4]
and
[Emim][HexSO4] proved to be an effective reaction medium to produce well dispersed
gold nanoparticles. A greater stability, due to the formation of an IL-PEI complex that
may also provide certain hydrophobicity to the nanoparticles, has been suggested.
The anterior might offer the possibility to use them in new fields of applications, e.g.
in nanoelectronics and molecular sensing. In particular, anisotropic particles like gold
nanorods are of current interest due to their potential as advanced materials for
medical purposes, such as bright contrast agents for diagnostic imaging [220] and
photothermal therapy of cancer cells [221]. Several approaches have been reported
in the literature for the preparation of gold nanorods, which include seed mediated
[222-225], electrochemical [226], photochemical [227] and microemulsions [228]
procedures. Moreover, the influence of gold seeds, purity and types additives given
to the growing solution has also been investigated [229-231].
In this context, gold nanoparticles synthesized in ionic liquids may represent an
interesting approach for the gold nanorod preparation. The AuNPs coated by PEI and
ILs may be an alternative seed, which can have certain advantages considering the
functional groups adsorbed on the nanoparticle surface. The present approach, uses
gold nanoparticles coated with PEI-IL as a new type of seed for the preparation of
gold nanorods based on the classical seed mediated method described by Murphy et
al. [222]. When the AuNPs were added to a colorless growing solution containing
HAuCl4, CTAB and ascorbic acid, a red color appears in the solution, indicating the
reduction of Au+ to Au0. Apparently, the added seeds may be incorporated into the
aqueous solution by assistance of the surfactant CTAB together with ethyl/ hexyl
sulfate anion. When the nanoparticles, which were prepared in [Emim][HexSO4] are
added to the growing solution, a bilayer (CTAB/HexSO4) can be expected. This
79
allows the dispersion and diffusion of the gold seeds in water. The reaction solution
was kept undisturbed at 25°C overnight to yield a light pink solution that was dropcasted onto substrates for electron microscopy characterization. The resulting
particles are presented in Figure 37.
Seeds-EmimEtSO4/5K-100°C
Seeds-EmimEtSO4/25K-100°C
Seeds-EmimHexSO4/5K-100°C
Seeds-EmimHexSO4/25K-100°C
Figure 37. Transmission electron micrograph of Au nanorods using gold nanoparticles prepared in
ionic liquids.
As to be seen in Figure 37, the added AuNPs seeds seem to have a marked
influence on the final particle morphology. Triangular, hexagonal, cubic, spherical
and cylindrical shapes can be distinguished from the micrographs. Nevertheless, a
systematic study about the evolution of these morphologies, in particular, gold
nanorods or gold nanowires, may be of interest to understand the influence of the ILs
and the polymer.
80
One can conclude that our IL-PEI AuNPs can be successfully used as a new type of
seed for the formation of nanorods. However, in the presence of higher molar mass
of PEI and [Emim][HexSO4], the resulting growth particles tend to be more isotropic
and the rods formed become shorter. Even though the procedure used herein formed
AuNRs in solution, the immobilization of gold seeds on a surface result in more
interesting challenge-growing anisotropic particles (e.g. nanorods) in an aligned
manner and with controlled spacing. The aligned assembly of the nanorods on a
surface may enhance the electric field for chemical sensing, in particular surface
enhanced Raman scattering applications [232]. Some efforts in this direction have
been reported in the literature [233-234].
The immobilization of spherical gold nanoparticles on substrate surfaces can be
performed following strategies that involve the use of organic molecules and
polymers. The chemisorptions of small molecules such as alkylthiols, silanes, alkyl
carboxylic, sulfonic, and phosphonic acids have been shown to form self-assembled
monolayer (SAM) on metal or metal oxide surfaces [235].
In addition, the immobilization of AuNPs prepared in ionic liquids may represent a
interesting approach to load small amount of gold solution (nano droplets) on a
substrate surface without any problem of solvent evaporation. The different
alternatives of available ionic liquids may also open new possibilities in order to tune
the aspect ratio (AR) of gold nanorods, for example by varying the nature of the anion
or the cation.
4.4.5 Application of gold nanoparticles in an electrochemical
biosensing system
As described in section 3.3.4, the immobilization of gold nanoparticles (AuNPs) on a
metal surface (gold wire) can be exploited for the development of a biosensing
system.
In this context, the incorporation of AuNPs has been reported to provide a favorable
surface for biomolecules, such as enzymes, to attach themselves without losing their
bioactivity. On the one hand, it has been claimed that gold particles of small
dimensions may provide a microenvironment, which supplies a favorable orientation
to the enzyme. On the other hand, nanoparticles may also act as nanoscale
electrodes that electrically communicate the redox proteins to the bulk electrode
81
material. Consequently, it makes the distance for electron transfer between the
protein and the metal electrode shorter, therefore, facilitating the interfacial electron
transfer process and the electrocatalytic activity [198,214,219]. In the present section,
the incorporation of AuNPs in combination with the redox enzyme, Human sulfite
oxidase (hSO) is investigated.
In the first step, a gold surface was modified by the mercaptoundecanoic acid (MUA)
and mercaptoundecanol (MU) (self assembling monolayer (SAM)). Subsequently,
AuNPs coated by a poly(ethyleneimine) were immobilized by means of the coupling
between the carboxylic group of the MUA/MU SAMs and the amino function derived
from the PEI layer. AuNPs prepared at 100°C in [Emim][HexSO4] and in
[Emim][EtSO4] using polyethyleneimine 5000 g mol -1 (PEI-5K) as a reducing agent
were selected in this investigation, however, the latter were studied in more detail.
The employed immobilization strategy leads to well dispersed single gold particles in
the size order between 5 and 10 nm, as to be seen in the SEM images (Figure 38a).
(a)
(b)
Figure 38. Scanning electron micrograph of the modified gold wire (a) modification using MUAMU/AuNPs-PEI-5K and (b) modification using DTSP/AuNPs-PEI-5K.
Note that the particles deposited on the electrode surface show no evidence of
aggregation after solvent evaporation. The AuNPs are clearly separated from each
other, forming an accessible surface for protein immobilization. Alternatively, the
surface modification of the gold wire can be performed by using dithiobis-Nsuccinimidyl propionate (DTSP). This compound is a commonly used thiol SAMs,
which provide distinctive surface properties, such as hydrophilicity, wettability, and
certain chemical reactivity toward polyamines, like poly(ethyleneimine). The
immobilization of gold nanopartilces coated by PEI is shown in Figure 38b. Similar to
82
the modification of MUA-MU, the immobilization of AuNPs on the electrode surface
can be observed in the SEM micrograph
Taking into account, that at the working pH 8.4 the polyelectrolyte coating may be
positively charged, the AuNP-modified electrode provide an adequate surface for the
formation of a protein film (hSO) on the surface. The immobilization of the protein
was performed by dipping the electrode in a hSO solution at 4°C. Based on the
electrostatic attraction between the negatively charged hSO and the positively
charged PEI, the attachment of the enzyme to the electrode surface, is to be
expected. After the enzyme immobilization, the modified electrode was dipped in a
protein-free buffer solution to perform cyclic voltammetry experiments. The
measurements were carried out using the buffer Tris (5mM and pH 8.4), to study the
direct (unmediated) electron exchange between the protein and the electrode.
Figure 39 shows the cyclic voltamogram of the electrode modified with hSO and gold
nanoparticles prepared in ionic liquids. As can be seen, in the case of the system
modified with AuNPs prepared in [Emim][EtSO4], an oxidation peak at about -100 mV
and a reduction peak at about -220 mV indicate the absorption and the direct
electron transfer of the enzyme on the electrode surface.
100
-1
I / nA
50
25 mV s
-1
50 mV s
-1
100 mV s
-1
200 mV s
0
-50
-100
-0.4
-0.3
-0.2
-0.1
0.0
0.1
0.2
E/V
Figure 39. Cyclic voltammogram of hSO adsorbed on AuNP-modified gold electrode surface prepared
in [Emim][EtSO4] for various scan rates.
Additional measurements at 750 mM of the buffer were performed (not shown) [236].
Note that the voltammetric peaks tend to separate and the peak current increases by
83
increasing the scan rate. From the peak area of the anodic wave an electroactive
protein surface concentration of 30 fmol cm-2 was calculated. No peak was registered
in absence of hSO.
The catalytic activity of hSO assembled together with gold nanoparticles was
investigated in presence of sulfite solutions. The experiments were performed at high
ionic strength condition (750mM) at pH 8.4. An increase in the oxidation current can
be observed for potentials higher than -150 mV, according to Figure 40.
100
80
I / nA
60
40
20
0
-20
-0.3
-0.2
-0.1
0.0
0.1
E/V
Figure 40. Cyclic voltammogram of a hSO/AuNP-functionalized (solid black line), hSO/PEIfunctionalized (dashed red line), AuNP-functionalized (dashed blue line) and bare (solid grey line)
SAM-modified gold electrode in a 750mM Tris buffer pH 8.4, 200µM SO 32- solution at 2 mV s-1 [236].
The electrocatalytic oxidation of sulfite in this potential region can be attributed to the
presence of hSO (solid black curve in Figure 40). By comparison, the gold electrode
modified with PEI, but without AuNPs, has also been loaded with hSO, but the
electrocatalytic sulfite oxidation current was significantly smaller, without AuNPs
(dashed red curve in Figure 40). Moreover, no evidence of catalytic activity was
observed at the bare and AuNP-modified gold electrode in the absence of enzyme.
According to these results, one can conclude that gold nanoparticles enhance the
electrocatalytic activity of the protein. Chronoamperometric experiments performed at
0 V potential to the system, shows a direct dependency between sulfite concentration
and current. The system additionally shows a quick response after sulfite addition,
as seen in Figure 41. The short response time of the AuNPs/hSO system enabled
84
nearly immediate detection.
These characteristics are of special interest for the
incorporation of this system in a biosensing device, where the specificity of the
enzyme can be used to quickly convert to an electric signal input by varying the
substrate concentration in the analyzed solution.
Inserted figure
I / nA
ss
I
25
/ nA
30
20
15
266 268 270 272
t/s
10
5
100
200
300
400
t/s
Figure 41. Current response I/nA after SO3-2 addition at 750 mM Tris buffer solution at pH 8.4 and
applied potential 0 V [236].
The investigated system provides advantages, such as, a quick response to detect
small concentration of sulfite and the ability to work at elevated ionic strength. These
properties may suggest that the system can be used as a sulfite biosensor or as
component for more complex bioelectronic device. However, efforts toward the
improvement of the long term stability and to understand the influence of gold
nanoparticles on the electron transfer process have to be explored more deeply.
Similar to the previous case, gold-DTSP modified electrodes incorporating AuNPs,
polyethyleneimine and hSO were characterized by means of CV. As seen in Figure
42a, the enzyme is successfully immobilized on the positively charged electrode
surface provided by the AuNPs coated with the polycation. A comparative study
employing AuNPs prepared in IL and in aqueous solution was conducted in the
presence of sulfite solution. As to be seen in Figure 42b, slightly larger catalytic
current was observed when AuNPs prepared in IL are used. This can give a hint
about the possible advantage of using a smaller and relatively homogeneous particle
size obtained in IL (<10nm in diameter), when compared to those prepared in
aqueous solution (~20nm). One can also suggest that the possible adsorption of the
85
remaining IL molecules on the particle surface may also contribute to enhance the
catalytic activity of hSO. This means that adsorbed ILs could provide an adequate
orientation of the enzyme. In addition, the increase in the polymer concentration to 1
wt. %, seems to have an insignificant contribution on the catalytic current.
1500
1000
(a)
AuNPs/IL-PEI-5K_0.5 wt. %
AuNPs/IL-PEI-5K_1 wt. %.
AuNPS/aqueous-PEI-5K_1 wt. %.
90
60
I / nA
I / nA
500
0
-500
30
0
2-
without SO3
-30
-1000
-1500
-0.4
(b)
AuNPs/IL-PEI-5K_0.5 wt. %.
AuNPS/IL-PEI-5K_1 wt. %.
AuNPS/aqueous-PEI-5K_1 wt. %
-60
-0.3
-0.2
-0.1
0.0
0.1
0.2
-0.3
E/V
-0.2
-0.1
0.0
0.1
0.2
E/V
Figure 42. (a) Cyclic voltammogram of hSO adsorbed on AuNP-modified gold electrode surface (a)
AuNPs prepared in [Emim][HexSO4], 50 mV s-1 (b) AuNPs prepared in [Emim][EtSO4] for various scan
rates (25, 60, 80, 120, 200 mV s-1).
The application of ionic liquids as an alternative electrolyte solution to perform
electrochemical reactions is of special interest considering their high ionic
conductivity, solvating properties, broad electrochemical window and electrochemical
stability [25,237]. It has been documented that the presence of ILs in the construction
of biosensors may induce electrostatic interaction and hydrogen bonding. These
interactions induce an effective kinetic barrier for the unfolding of the enzyme
structure, consequently may provide certain resistance to its denaturalization [238].
Therefore, base on these finding and on the results previously presented, enzymatic
reactions were performed in the presence of ionic liquids. Two ILs were selected
bearing different anion moieties, i.e. 1-ethyl-3-methylimidazolium ethyl sulfate
[Emim][EtSO4] and 1-ethyl-3-methylimidazolium acetate [Emim][Acetate], in order to
observe any influence on the catalytic activity of hSO.
86
60
(a)
0
40
I / nA
I / nA
20
0
-20
0 wt. % Emim EtSO4
-60
10 wt. % Emim EtSO4
50 wt. % Emim EtSO4
-0.3
40
20
-0.2
-0.1
0.0
E/V
0.1
-0.3
0.2
60
(c)
40
0 wt. % EmimEtSO4
10 wt. % EmimEtSO4
25 wt. % EmimEtSO4
I / nA
I / nA
I / nA
60
0 wt. % Emim Acetate
10 wt. % Emim Acetate
25 wt. % Emim Acetate
50 wt. % Emim Acetate
-75
25 wt. % Emim EtSO4
-80
80
-25
-50
-40
100
(b)
50 wt. % EmimEtSO4
bar SAM modified
electrode
20
-0.2
-0.1
E/V
0.0
0.1
(d)
0 wt. % Emim Acetate
10 wt. % Emim Acetate
25 wt. % Emim Acetate
50 wt. % Emim Acetate
0
-20
0
-20
-0.2
-0.1
0.0
0.1
E/V
0.2
-40
-0.2
-0.1
0.0
0.1
E/V
Figure 43. Cyclic voltammogram of hSO adsorbed on AuNP-modified gold electrode surface in
presence of (a) [Emim][EtSO4], (b) [Emim][Acetate] and their effect on the catalytic current (c) and (d),
respectively.
As shown in Figure 43a-b, the voltammetric peak indicates that the enzyme is
immobilized on the electrode surface. The addition of 10 wt. % of ILs induces a slight
decrease in the peak current and remains visible up to 25 wt. %, in both cases.
Further addition of ILs causes the complete vanishing of the peak, which can be
correlated to the desorption of the enzyme from the electrode surface. The addition of
ionic liquids conduce to an increase in the ionic strength of the solution, minimizing
the effective poly(ethyleneimine)-hSO electrostatic interaction and consequently loss
of the protein into the bulk, which causes a reduction in the catalytic efficient of the
modified electrode (Figure 43c-d).
87
4.4.6 Conclusions
The investigated ionic liquids i.e. [Emim][EtSO4] and [Emim][HexSO4] were
successfully used as a reaction media to perform the chemical reduction of Au+3 to
Au0, using poly(ethyleneimine) as a reducing agent.
Two strategies were applied in order to synthesize gold nanoparticles. The first
approach uses IL-based microemulsions as a template phase. The characterized
[EmimEtSO4]/toluene/[Bmim][OctSO4] and the [EmimEtSO4]/toluene/[Bmim][DodSO4]
microemulsions systems were used as a reaction media. In the case of the
[Bmim][OctSO4], the combination of two µEs containing PEI and HAuCl4,
respectively, result in the formation of gold nanoparticles between 10nm and 20nm,
observed by transmission electron microscope. Conversely, the [Bmim][DodSO4]
based system was less effective for the preparation of AuNPs, due to the poor
solubility of poly(ethyleneimine) in the microemulsion.
Much more effective is the simple approach, by using pure ILs as solvent to prepare
gold nanoparticles. Spherical nanoparticles with an average core diameter of about
6nm to 10nm were prepared in [Emim][EtSO4] and [Emim][HexSO4]. The formation
of an IL-polymer complex on the surface of the gold nanoparticles is responsible for
the longer stability and solubility characteristics of the AuNPs. In addition, the
stabilization of the nanoparticles seems to depend on the type of the anion of the
ionic liquid. That means that when the alkyl chain substituent in the anion becomes
larger, a better protection against aggregation can be provided.
The preparation of anisotropic particles and the assembly of a biosensor system
using the prepare AuNPs were explore. In the first approach, anisotropic particles,
such as gold nanorods were produced using AuNPs coated by PEI-IL as a new type
of seed. The second approach consists in the immobilization of gold nanoparticles on
a metal surface (gold wire) for the construction of a biosensor system. The adequate
surface of the AuNPs -poly(ethyleneimie) matrix provides a positively charged
platform for the adsorption of Human sulfite oxidase (hSO) on the surface.
The catalytic activity of hSO toward sulfite oxidation is enhanced by the incorporation
of gold nanoparticles. The biosensor system provides a quick answer to detect small
amounts of sulfite and is able to work at elevated ionic strength. The role of gold
nanoparticles in assembled biosensor system is not completely understood, however,
88
it was suggested that they provide an adequate microenvironment that favor a
suitable orientation of the enzyme.
Finally, the uses of ionic liquids as an alternative electrolyte to perform enzymatic
reactions was investigated, however, the addition of high concentration of RTIL (>10
wt. %), causes the desorption of the enzyme from the electrode surface.
89
5
Summary and Outlook
The present thesis has been divided into three main subjects:
1. The formulation and characterization of non-aqueous microemulsions
using two ionic liquids and one oil component.
The
amphiphilic
character
of
1-butyl-3-methylimidazolium
octyl
sulfate
[Bmim][OctSO4] and 1-butyl-3-methylimidazoilium dodecyl sulfate [Bmim][DodSO4]
was investigated by means of surface tension measurements at 25°C. The
aggregation process of the IL like surfactant (IL-S) showed significant differences
with regard to the Gibbs free energy of micellization in water, compared to the room
temperature ionic liquids (RTIL) solvent. Additional parameters such as, surface
excess concentration (Γmax) and the minimum areas of the surfactant at the liquid/air
interface (Amin) were calculated from the surface tension isotherms.
The anterior results indicate that the studied ionic liquids bearing long alkyl chain
substituents are surface active molecules able to form self assembled structures,
such as micelles. Therefore the formulation of non aqueous microemulsions based
on ionic liquids has been proposed. The ionic liquid 1-ethyl-3-methylimidazoium ethyl
sulfate ([Emim][EtSO4]) was used as a polar phase and toluene as a non polar
compound. The ternary systems were prepared at room temperature and
characterized by different techniques, such as, conductometric, cyclic voltammetry,
rheology, scattering techniques and cryo- scanning electron microscopy (Cryo-SEM).
Conductometric measurements performed on the isotropic area indicate the
presence of three types of microemulsions. At high concentrations of ILs, an oil-in-IL
microemulsion is expected. The formed toluene droplets are stabilized by a
surfactant film consisting of alkyl sulfate anions, whereas the [Bmim]+ counter ions
could act as co-surfactant and can also be dispersed in the IL continuous phase. At
medium ILs content, a network of micro-channels, where the ILs and the oil phase
coexist, is suggested. The formed structure can be identified as bicontinuous
microemulsion. At low IL content, inverse droplets composed of IL as polar domain
dispersed in toluene continuous phase are formed. The structural transitions within
90
the EmimEtSO4/toluene/BmimOctSO4 system were supported by cyclic voltammetry
and shear viscosity measurements.
Dynamic light scattering measurements provide preliminary evidence about the
formation of droplets in both IL-based microemulsion systems. In the case of
EmimEtSO4/toluene/BmimDodSO4 system, the droplet size increases until 10 wt. %
of RTIL, with a further tendency to decrease when RTIL content is raised. The results
were reinforced by SAXS measurements performed on the same experimental
pathway at 25°C. In addition, information regarding the microstructure of the µE
systems was obtained by applying the Teubner-Strey model to the SAXS data. The
microemulsions were investigated by Cryo-SEM showing visual evidence about the
formation of globular structures (micelles) and sponge like phase (bicontinuous
microemulsion).
The investigated system represent a water-free system which can be applied to
perform organic reactions or as a nano reactor for enzymatic processes and
preparation of nanomaterials.
2. Synthesis of gold nanoparticles in ionic liquid based microemulsions
and in bulk ionic liquids
The second part of this work is related to the synthesis of gold nanoparticles in ionic
liquids. In the first approach, spherical gold nanoparticles < 20nm were synthesized
in the EmimEtSO4/toluene/BmimOctSO4 microemulsion system at 45°C and 100°C.
Poly(ethylenimine) was successfully incorporated into the IL inverse droplets acting
as a reducing agent for Au+3. Unfortunately, the polymer precipitates when the
EmimEtSO4/toluene/BmimDodSO4 microemulsion system is used and consequently
AuNPs were not formed. Nevertheless, the investigated µE is of special interest to
perform other kinds of chemical processes.
In the second approach, ionic liquids were used as a solvent to prepare gold
nanoparticles. Spherical AuNPs <10nm in diameter were obtained in [Emim][EtSO4]
at 100°C and 150°C. Simultaneously, slightly smaller and relatively more stable
AuNPs were obtained in [Emim][HexSO4]. A more rigid polymer-IL complex may
provide a barrier against aggregation and certain hydrophobicity to the particle
surface.
91
The experimental conditions applied in this work can be extended to prepare other
types of colloidal nanoparticles such as, silver, iron oxide, platinum nanoparticles,
amongst others. In addition, multiple alternatives to combine anions and cations in
the ionic liquids may be of interest to understand their influence in the growing
process and the stability of the resulting nanoparticles.
3. Gold nanoparticles as seeds for the formation of anisotropic particles
and for biosensor applications
The first approach concerning the application of the prepared AuNPs in ILs,
([Emim][EtSO4] and [Emim][HexSO4]) was their used as seeds for nanorod (NR)
synthesis. In all cases, anisotropic particles were produced, however, a detailed
study to determine the effect of poly(ethyleneimine) and the ionic liquid in the growing
solution should be of interest, in order to tune the nanorod aspect ratio. The
immobilization of spherical gold seeds on substrate surfaces and their growth in a
controlled manner represent a major opportunity for specific applications, such as
chemical sensing, in particular Surface Enhanced Raman Scattering (SERS)and
deserves further research.
The second approach concerning the application of the prepared AuNPs in ILs
consists of their immobilization on a gold electrode and their influence on the catalytic
activity of Human sulfite oxidase (hSO), in order to design biosensing systems. The
spherical AuNPs obtained in [Emim][EtSO4] at 100°C were covalently attached to a
SAMs modified Au-electrode, providing a positively charged surface to attach
electrostatically the enzyme hSO.
The characterization of the system was carried out by means of voltammetry and
SEM. The designed biosensor exhibited a quick current response, a low detection
range and high sensibility toward the presence of sulfite. Additional advantages such
as, the possibility to work at low applied potential and at very high ionic strength were
also registered. Therefore, the assembled system is of special interest for
biotechnological applications, such as, sulfite biosensor or as a biofuel cell
component.
Nevertheless, the mechanism of how the AuNPs influence the catalytic activity of the
protein toward the sulfite oxidation is an open question. However, it was claimed that
the small nanoparticles provide a suitable microenvironment that favor the protein
92
orientation and may act as nanoscale electrodes, facilitating the communication
between redox proteins and bulk electrode. Investigations in this direction can be
extended by using other kinds of metal nanoparticles, such as cadmium sulfide
(CdS), silver (Ag), platinum (Pt) and barium sulfate (BaSO4) nanoparticles.
Alternatively, the nanoparticles loaded onto the electrode wire can be achieved using
alternative strategies. Similar to the MUA/MU self assembling monolayer, one can
use dithiobis-N-succinimidyl propionate (DTSP) to covalently bind the polyamine to
the SAM. Another strategy to bind directly the gold nanoparticles to the electrode
surface can be by using 4,4’-biphenyldithiol (BPDT).
Finally, the role of two different ILs [Emim][EtSO4] and [Emim][Acetate] was explored
as a potential electrolyte material to perform enzymatic reactions.
93
6
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xix
7
Appendix
7.1 Tables
Table 5. Some common surfactants and their critical micelle concentration (CMC) in water and in
ionic liquids
Ionic surfactants
Sodium dodecylsulfate (SDS)
Solvent
CMC (mM)
Reference
Water
8
[239]
[Bmim][Cl]
48
[239]
15.34(298K)
[240]
23(323K)
[170]
Dodecyltrimethylammonium bromide
Water
C12TAB
EAN
Tetradecyltrimethylammonium bromide
Water
3.943(298K)
[240]
C14TAB
EAN
6.97(323K)
[170]
Hexadecyltrimethylammonium bromide
Water
0.9642(298K)
[240]
C16TAB
EAN
2.23(323K)
[170]
Tetradecylpyridinium bromide
Water
2.7(298K)
[241]
EAN
80(298K)
[58]
0.64(298K)
[241]
EAN
20(303K)
[58]
1-hexadecyl-3-methyl-imidazolium bromide
[C16mim][Br]
Water
0.888
[242]
EAN
16.2
[242]
1-hexadecyl-3-methyl-imidazolium
tetrafluoroborate
[C16mim][BF4]
Water
1.37(333 K)
[242]
EAN
13.6
[242]
Pentyl ammonium nitrate
EAN
350(298K)
[243]
Octyl ammonium nitrate
EAN
79(298K)
[243]
Decyl ammonium nitrate
EAN
25(298K)
[243]
Dodecyl ammonium nitrate
EAN
7.9(298K)
[243]
1,4-bis(2-ethylhexoxy)-1,4-dioxobutane-
Water
0.91
[239]
2-sulfonate (AOT)
[Bmim][Cl]
4.7
[239]
Hexadecylpyridinium bromide
Water
xx
Nonionic surfactants
Solvent
CMC (mM)
Reference
0.06-0.09
[239]
[Bmim][Cl]
1.6
[239]
[Bmim][PF6]
115
[239]
[Emim][Tf2N]
50-60
[244,245]
[Bmim][PF6]
20-21
[239,244]
[Emim][Tf2N]
10-17
[244-245]
0.08
[244]
50-70
[244-245]
0.2
[244]
780
[246]
330
[246]
6.11(323K)
[170]
0.0005
[247]
9
[247]
0.009
[247]
[Bmim][BF4]
27
[247]
Water
0.1
[247]
[Bmim][BF4]
93
[247]
0.067
[247]
57
[247]
0.065
[247]
46
[247]
CMC (mM)
Reference
Water
29
[239]
[Bmim][Cl]
290
[239]
377-466
[239,244]
Water
Polyglycol alkyl ether (Brij-35)
Polyglycol alkyl ether (Brij-700)
Poly(ethylene oxide) sorbitan monolaurate
Water
(Tween-20)
[Emim][Tf2N]
Water
Poly(ethylene oxide) iso-octylphenyl ether [Bmim][PF6]
(Triton X-100)
[Bmim][PF6]
EAN
Alkylethylene glycol types
Water
(C16E8)
[Bmim][BF4]
(C14E8)
(C12E8)
(C12E6)
Water
Water
[Bmim][BF4]
(C12E4)
Water
[Bmim][BF4]
Zwitterionic surfactants
Caprylyl sulfobetaine (SB3-10)
Solvent
[Bmim][PF6]
xxi
Table 6. Melting point (°C) of some alkylimidazolium based ionic liquids.
Compound
Melting temperature
(°C)
Reference
NaCl
800.7
[44]
[emim][Cl]
82-89
[248-249]
[bmim][Cl]
41
[250]
NaBr
747
[44]
[emim][Br]
79
[249]
[bmim][Br]
77
[251]
[bmim][BF4]
37
[252]
[bmim]PF6
3.28
[253]
[emim][Tf2N]
-17
[252]
[bmim][Tf2N]
-2 (-6)
[252]
[pmim][Tf2N]
-9
[254]
[emim][CH3COO]
-10
[44]
[bmim][CH3COO]
-7
[47]
[emim][CH3CH2SO4]
-4
[255,172]
[emim][CH3(CH2)5SO4]
<0
[172]
[bmim][CH3(CH2)5SO4]
<0
[172]
[emim][CH3(CH2)7SO4]
<0
[172]
[bmim][CH3(CH2)7SO4]
<0
[172]
[bmim][CH3(CH2)11SO4]
44-48
[165]
xxii
Table 7. Viscosities, densities and ion conductivities of some common ionic liquids
Temperature
Viscosity
Density
Conductivity
(K)
 (cP)
 (g/cm3)
 (S m-1)
[Emim][BF4]
298
37-66.5
1.28
1.58-1.38
[38]
[Bmim][BF4]
298
219
1.26
0.35
[256]
[Bmmim][BF4]
298
243
0.023
[257-258]
[Bmim][PF6]
298
450
1.36
0.146-0.1
[256]
[Emim][Tf2N]
298
35
1.54
0.91
[256]
[Bmim][Tf2N]
298
69
1.43
0.40
[256]
[Bmmim][Tf2N]
298
88
[Emim][CH3COO]
298
162
[Bmim][CH3COO]
298
485
[Emim][CH3CH2SO4]
293
126.89
1.241
[Emim][CH3CH2SO4]
298
97.78
1.237
[259]
[Emim][CH3(CH2)5SO4]
293
437.53
1.133
[159,272]
[Emim][CH3(CH2)5SO4]
298
316.78
1.130
[259]
[Emim][CH3(CH2)7SO4]
293
717.8
1.097
[260]
[Emim][CH3(CH2)7SO4]
298
649
1.094
[160,272]
[Bmim][CH3(CH2)7SO4]
298
874-940
1.05
[172]
Ionic liquid
Reference
[257]
0.28(293K)
[47-48]
[47]
80
[259,272],
[255]
xxiii
7.2 List of publications
I.
O. Rojas, J. Koetz, S. Kosmella, B. Tiersch, P. Wacker, M. Kramer, Structural studies of
ionic liquid-modified microemulsions, Journal of Colloid and Interface Science. 333
(2009) 782–790.
II.
O. Rojas, B. Tiersch, S. Frasca, U. Wollenberger, J. Koetz A new type of microemulsion
consisting of two halogen-free ionic liquids and one oil component, Colloids and
Surfaces A: Physicochemical and Engineering Aspects 369 (2010) 82-87.
III.
O. Rojas, J. Koetz, Microemulsions with ionic liquids, Journal of Surface Science and
Technology, 26, (2010) 173-196.
IV.
S. Frasca, O. Rojas, J. Salewski, B. Neumann,K. Stiba, I. M. Weidinger, B. Tiersch, S.
Leimkühler, J. Koetz, U. Wollenberger. Human sulfite oxidase electrochemistry on
gold nanoparticles modified electrode, Bioelectrochemistry, 87 (2012) 33-41.
V.
Aniket Thete, Oscar Rojas, David Neumeyera, Joachim Koetz, Erik Dujardin,
Morphosynthesis of gold nanorods using polyethyleneimine-capped seeds in ionic
liquid, RSC Advances, (2012) submitted.
VI.
Oscar Rojas, Brigitte Tiersch, Christian Rabe, Ralf Stehle, Armin Hoell, Joachim Koetz
Non-aqueous microemulsions based on N,N’-alkylimidazolium alkylsulfate ionic
liquids, will be submitted.
7.3 List of presentations
I. Poster presentations:

25th European Colloid and Interface Society (ECIS) Conference. 4th -9th September
2011, Berlin (Berlin):
“Gold anoparticles formation in ionic liquids for Biosensor pplications”.

Second International Conference on Multifunctional Hybrid and Nanomaterials. 6th 10th March 2011, Strasbourg, France:
“Gold anoparticles with Defined Size as Support for the Direct Electron
Transfer and Catalysis of the human Sulfite Oxidase” and “Ultrafine gold
nanoparticles synthesized in halogen free ionic liquids”.

Polymers in Biomedicine and Electronics. Biannual meeting of the GDCh- Division of
Macromolecular Chemistry and polydays. 3- 5th October 2010, Berlin, Germany.
“Synthesis of gold nanoparticles in halogen-free ionic liquids for biosensor
applications”.
xxiv
II. Oral presentations:

8th Zsigmondy Colloquium. 05-07th March 2012, Darmstadt, Germany. “Ionic liquids
for microemulsion formulations and nanoparticles synthesis – application of gold
nanoparticles in a biosensor system”

Workshop on Biomimetic and Bioanalytical Systems. 22-23th September 2010,
Luckenwalde, Germany.
“Synthesis of gold nanoparticles in Ionic Liquids (ILs)”

6th Zsigmondy Colloquium. 22-24th March 2010 Chemnitz, Germany. “Structural
studies of a new type of microemulsion consisting of two ionic liquids and one oil
component”

12th European Student Conference. 15-18th July, 2009, Almería, Spain.
“Ionic liquid-modified microemulsions: A new template for the synthesis of gold
nanoparticles”.
xxv
Declaration
I, the undersigned, hereby declare that the work contained in this thesis is my own original
work and that I have not previously in its entirety or in part submitted it at any university for a
degree.
___________________
Oscar Mario Rojas Carrillo
Potsdam, September, 2012
xxvi