SYNTHESIS, EVALUATION AND MOLECULAR DYNAMIC SIMULATIONS
OF NOVEL ANIONIC POLMERIC SURFACTANTS BASED ON
POLYBENZOXAZINES
by
RIYAD AGELI SALEH MAHFUD
Submitted in partial fulfillment of the requirements
For the degree of Doctor of Philosophy
Department of Chemical Engineering
Case Western Reserve University
May 2014
SCHOOL OF GRADUATE STUDIES
We hereby approve the thesis/dissertation of
RIYAD AGELI SALEH MAHFUD
candidate for the
(signed)
Doctor of Philosophy
degree*
Dr. SYED QUTUBUDDIN
(chair of the committee)
Dr. DANIEL LACKS
Dr. MOHANN SANKARAN
Dr. HATSUO ISHIDA
(date)
December 19th, 2013
*We also certify that written approval has been obtained for any proprietary material
contained therein.
Copyright © by RIYAD A. S. MAHFUD
All Rights Reserved
DEDICATION
I would like to dedicate this work to my entire family. A special thank you goes out to
my parents (Mr. Ageli Mahfud and Ms. Fatma AlQade) who were always so proud and
supportive of their son. My wife (Wafa Zaroug) and my kids (Ahmed, Saviya and Omar)
who have always been next to me on this long journey.
TABLE OF CONTENTS
TITLE............................................................................................................
i
TABLE OF CONTENTS..............................................................................
v
LIST OF SCHEMES.....................................................................................
ix
LIST OF TABLES.........................................................................................
x
LIST OF FIGURES.......................................................................................
xi
ACKNOWLEDGMENTS.............................................................................
xvii
ABSTRACT..................................................................................................
xviii
CHAPTER 1. General introduction...........................................................
1
1.1
Background............................................................................
1
1.2
Classifications and synthesis of polymeric surfactants........
3
1.2.1
Block copolymers......................................................
4
1.2.2
Graft copolymers.......................................................
5
1.3
Polybenzoxazines..................................................................
8
1.4
Scope of present work..........................................................
12
1.5
References.............................................................................
15
CHAPTER 2. Synthesis and Evaluation of Novel Anionic Polymeric
Surfactants Based on Polybenzoxazines............................
2.1
Introduction..........................................................................
19
20
2.2
Experimental..........................................................................
22
2.3
Measurements........................................................................
25
2.4
Results and Discussions........................................................
27
2.4.1
The molecular structure of synthesized monomers
and
oligomers.............................................................
2.4.2
NMR analysis.............................................................
27
29
2.4.3
FTIR analysis.............................................................
30
2.4.4
DSC and TGA studies................................................
32
2.4.5
Determination of MW by SEC................................
38
v
2.4.6
Surface tension measurements...................................
40
2.5
Conclusions...........................................................................
49
2.6
References...........................................................................
50
CHAPTER
3.
Anionic
Surfactants
Based
on
Comb-like
Polybenzoxazine Oligomers: Effects of Salinity and
Temperature on Critical Micelle Concentration..............
54
3.1
Introduction...........................................................................
55
3.2
Experimental..........................................................................
56
3.2.1
Chemicals..................................................................
56
3.2.2
Preparation of solutions.............................................
57
Measurements........................................................................
57
3.3.1
Surface tension measurements...................................
57
3.3.2
Viscosity measurements............................................
58
3.3.3
Dynamic light scattering............................................
58
3.3.4
Foaming power measurements.................................
59
3.3.5
Conductance measurements.....................................
59
3.3
3.4
Results and Discussion..........................................................
60
3.5
Conclusions...........................................................................
85
3.6
References.............................................................................
86
CHAPTER
4. Gemini (dimeric) benzoxazine surfactants: Synthesis,
characterizations and molecular dynamics simulation of
self-assembly.........................................................................
90
4.1
Introduction...........................................................................
91
4.2
Experimental method.............................................................
93
4.2.1
Materials....................................................................
93
4.2.2
Preparation of di(4CaP-oca).....................................
93
4.2.3
Ionization of di(4CaP-oca) into di(4CaP-oca- Na+)
93
4.3
Measurements........................................................................
94
4.4
Computational Methods.......................................................
94
4.5
Simulations Details................................................................
96
vi
4.6
Results and Discussions.........................................................
98
4.6.1
Synthesis and Characterizations................................
98
4.6.2
Surface tension measurements...................................
102
4.6.3
Simulation Analysis...................................................
105
4.6.3.1 Bulk behavior of dimeric benzoxazine
surfactant.......................................................
105
4.6.3.1.1 Aggregation into spherical
micelles...........................................
105
4.6.3.1.2 Radial distribution function
analysis...........................................
106
4.6.3.1.3 Conformational analysis.................
110
4.6.3.2 Behavior of di(4CaP-oca-Na+) at air/water
interface.........................................................
114
4.6.3.2.1 Density profile...............................
114
4.6.3.2.2 The geometric shape.....................
115
4.7
Conclusions...........................................................................
119
4.8
References.............................................................................
120
CHAPTER 5. Molecular dynamic simulations of self-
assembly of amphiphilic comb-like anionic
polybenzoxazines..............................................................
123
5.1
Introduction...........................................................................
124
5.2
Computational Methods........................................................
125
5.3
Results and Discussions.........................................................
129
5.3.1
Simulations analysis..................................................
129
5.3.2
Surfactant concentration and micellar structure........
137
5.4
Conclusions...........................................................................
143
5.5
References.............................................................................
144
CHAPTER 6. CONCLUSIONS AND FUTURE WORK......................
147
6.1
Surface active anionic polybenzoxazines..............................
148
6.2
MD simulations....................................................................
150
vii
6.2.1
MD simulations of anionic dimeric benzoxazine......
150
6.2.2
MD simulations of anionic polybenzoxazines...........
151
Future work............................................................................
152
APPENDIX...................................................................................................
154
6.3
i.
The molecular dynamics algorithm.................................................
154
ii.
Integrating the equations of motion...................................................
155
iii.
Brendeson temperature and pressure coupling..................................
156
iv.
Trajectory analysis.............................................................................
157
v.
References.........................................................................................
157
CHAPTER 7. BIBLIOGRAPHY...............................................................
158
viii
LIST OF SCHEMES
Scheme No.
Legend
Page No.
2.1
The synthesis of benzoxazine monomers 4HBA-oca,
4HBA-dea and 4HBA-doa.
28
2.2
The proposed oligomer structures of poly(4HBA-oca),
poly(4HBA-dea) and poly(4HBA-doa) and the ionization
of these polymers into
poly(4HBA-oca- Na+),
+
poly(4HBA-dea Na ) and poly(4HBA-doa- Na+),
respectively; n in Scheme 1, and the symbol i is the
degree of polymerization.
29
4.1
Synthesis of dimeric benzoxazine 4CaP-oca.
98
4.2
The proposed dimeric benzoxazine structure of di(4CaPoca) and the ionization of this dimer into the ionized form,
di(4CaP-oca- Na+ ).
103
ix
LIST OF TABLES
Table No.
Legend
Page No.
2.1 Summary of analytical results for molecular weight
distributions and related properties determined from
SEC with triple detector.
39
2.2 Influence of NaCl concentration on the cmc, γcmc, Γmax
and ɑs of poly(4HBA-oca-Na+), poly(4HBA-dea-Na+)
and poly(4HBA-doa-Na+).
48
3.1 Summary of the surface activity properties of
poly(4HBA-oca- Na+), poly(4HBA-dea- Na+) and
poly(4HBA-doa- Na+).
62
3.2 The cmc, 𝛤𝑚𝑎𝑥 , minimum surface area per
molecule, ∆𝑚𝑖𝑐 𝐺 𝑜 for poly(4HBA-oca- Na+) at
different temperatures.
76
4.1 Summary of the surface activity properties of di(4CaPoca- Na+) compared with values for anionic monomeric
surfactant, SDS, from the literature.
104
4.2 The fractions of the guache and trans spacer conformers
in aqueuos media.
113
5.1 The chemical structures and the snapshots of the used
amphiphilic polybenzoxazines: The first column shows
the abbreviated names, the second column shows the
chemical structures using ChemDraw, and the third
column shows the snapshots of the molecules at the
minimum energy using VMD molecular viewer.
128
5.2 Structural Properties of the iBnXz clusters formed in
the 49.8 mM simulation systems.
136
x
LIST OF FIGURES
Figure No.
Legend
Page No.
1.1 Schematic represents the different types of block copolymers 5
containing alternating hydrophilic and hydrophobic blocks.
1.2.a Schematic represents the different types of graft copolymers 6
containing a hydrophilic backbone and hydrophobic side
chains (left), and hydrophobic backbone and hydrophilic side
chains (right).
1.2.b Synthetic methods for preparing Graft copolymers.
6
1.3 Formation of polymeric micelles from different types of 8
amphiphilic co-polymers (Extracted from Torchillin, 2001).
1.4 Scheme showing the phenolic-type polybenzoxazine.
9
2.1 Representative structures of the three anionic polymeric 28
surfactants, the symbol i is the degree of polymerization.
2.2
1
H NMR spectra of benzoxazine monomers 4HBA-oca, 30
4HBA-dea, and 4HBA-doa.
2.3 FTIR spectra of benzoxazine monomers 4HBA-oca, 4HBA- 31
dea, and 4HBA-doa.
2.4 FTIR spectra of 4HBA-doa after polymerization at 80 oC, 120 32
o
C and 160 oC for 30 min.
2.5 DSC thermograms of benzoxazine monomers 4HBA-oca, 34
4HBA-dea, and 4HBA-doa.
2.6 Dynamic DSC plots for 4HBA-oca polymerization at various 35
temperatures for 30 minutes.
2.7.A TGA curves of poly(4HBA-oca), poly(4HBA-dea), and 37
poly(4HBA-doa).
2.7.B FTIR spectra of the condensate of the evolved gases at the 37
peak rate degradation of poly(4HBA-oca) and octylamine.
xi
2.8.A Surface tension at different weight concentrations.
41
2.8.B Surface tension at different molar concentrations.
41
2.9 Variation of cmc and γcmc with alkyl chain length.
42
2.10.A The effect of salinity on the surface tension of different 43
poly(4HBA-oca- Na+) solutions.
2.10.B The effect of salinity on the surface tension of different 44
poly(4HBA-dea- Na+) solutions.
2.10.C The effect of salinity on the surface tension of different 44
poly(4HBA-doa- Na+) solutions.
2.11 The effect of salinity on the cmc and the surface tension at 45
cmc (γcmc) of the solutions of poly(4HBA-oca- Na+),
poly(4HBA-dea- Na+) and poly(4HBA-doa- Na+).
3.1 A schematic representation of micelles formation by a 61
polymeric surfactant in aqueous media; unimer-to-aggregate
transition.
3.2 Representative structures of the three anionic polymeric 62
surfactants.
3.3 Surface tension variation over time for the poly(4HBA-doa- 63
Na+) solutions with various viscosities.
3.4 The effect of poly(4HBA-oca- Na+) concentration on both the 64
lamella height and the maximum force at 21+0.1 oC
3.5 The viscosities of poly(4HBA-oca- Na+), poly(4HBA-dea- 65
Na+), and poly(4HBA-doa- Na+) solutions at different low
concentrations, and constant temperature, 30±0.1 oC.
+
3.6 Variation of reduced viscosity of 0.1g/L poly(4HBA-doa Na ) 67
solution with added NaCl.
plot
(intrinsic
viscosity
=(ln
relative 67
3.7 Kraemer
viscosity)/concentration vs. concentration) for poly(4HBAoca- Na+), poly(4HBA-dec- Na+) and poly(4HBA-doa- Na+)
xii
solutions at different concentrations and 30±0.1 oC.
3.8 The effect of salinity on the cmc and the surface tension at cmc 69
(γcmc) of the solutions of poly(4HBA-oca- Na+) (star),
poly(4HBA-dea- Na+) (square), and poly(4HBA-doa- Na+)
(triangle).
3.9 The variation of surface tension as a function of poly(4HBA- 70
oca- Na+) concentration in aqueous solution at different
temperatures (21, 38, and 48 oC).
3.10 The variation of cmc and γcmc as a function of temperature for 71
poly(4HBA-oca- Na+).
3.11 Thermodynamic parameters of micellization for poly(4HBA- 75
oca- Na+).
3.12 ΔmicH° vs. ΔmicS° plot for poly(4HBA-oca- Na+).
75
3.13 Hydrodynamic diameter intensity of poly(4HBA-oca- Na+) 77
micelles in aqueous solution at 25 °C measured by DLS at
concentration of 0.5 g/L, and scattering angles of 60 o (top),
90 o (middle) and 130 o (bottom).
3.14 Hydrodynamic diameter distributions f(Dh) of poly(4HBA- 79
oca- Na+) micelles in aqueous solution at 25 °C measured by
DLS at concentration of 0.5 g/L, and scattering angle of 90
o
C.
3.15 The dependence of the foam volume ratios on time, 150 ppm 81
(left) and 75 ppm (right).
3.16 The decay rate plot (top) and the surfactants solution foams 82
(bottom) after 1 min, a-poly(4HBA-oca- Na+), b- poly(4HBAdea- Na+) and c- poly(4HBA-doa- Na+).
3.17 Temperature dependence of κ in the systems of polymeric 84
surfactant solutions, 0.05 wt% (poly(4HBA-oca- Na+),
poly(4HBA-dea- Na+) and poly(4HBA-doa- Na+)). A sharp
rise of κ with temperature is due to the Krafft phenomenon,
and shows the Krafft point of the polymeric surfactant
species.
xiii
4.1 FTIR spectra of 4HBA-oca+HBZ-COOH (1:1) at 25 oC/0 hr, 99
140 oC/1 hr and 140 oC/ 3 hrs.
4.2 Dynamic DSC plots for (4HBA-oca) at 25oC, and for (4HBA- 101
oca + HBZ_COOH) at 140oC after heating for 1 hr and 3 hrs.
4.3 TGA curves of di(4CaP-oca).
102
4.4 Representative structure of the anionic dimeric surfactant, 103
represented by CPK model using VMD.
4.5 Surface tension at different weight concentrations.
104
4.6 Spontaneous aggregation of di(4CaP-oca-Na+) into a micelle; 106
snapshots of the simulation at the start (t=0 ns), intermediate
(t=3- 10 ns), micelle and single molecule stage (t=10- 16 ns),
and micelle (16- 20 ns) are shown. Water molecules are
omitted for clarity and the black points represents the Na ions.
4.7 Radial distribution functions of water relative to the polar 108
heads (W-Co), spacer (W-N), and carboxylic groups (W-OH)
calculated from the MD simulations were carried out at 298K.
4.8 The RDFs between the charged head groups represented by 109
the carbon atoms in the carboxylate groups [Co-Co] and
between the spacers [N-N] (a). The RDFs between
neighboring head groups [Co-Co] for each di(4CaP-oca-Na+)
in aqueous media and in gas phase (b).
4.9.a Schematic definitions of the dihedral angles for the spacer.
111
4.9.b Distribution of the gauche and trans dihedral-angles for the 113
spacer at vaccum system (left) and in aqueous media (right)
that prodeuced by the various dehidral angle sampling
methods.
4.10 Distribution of the dihedral angle for the alkyl chain produced 114
by the various dihedral angle sampling methods.
4.11 Density profiles along the Z axis.
115
4.12 The average values of the tilt angles and the spacer bend 117
angle (a) that characteristic the geometric shape of the
surfactant at air/water interface (b). The RDF between N and
H of the hydroxyl groups in each individual di(4CaP-oca-Na+)
xiv
molecule (C). Equilibrium MD simulation snapshot (d)
showing a dimeric benzoxazine molecule at the air/water
interface, Color legend: dark, di(4CaP-oca-Na+) molecule;
gray, water molecule.
5.1 Snapshots represent the spontaneous aggregation of the 130
amphiphilic polybenzoxazines (at a concentration of 49.8×103
M) into spherical and cylindrical micelles. Water molecules
are omitted for clarity and the black points represents the Na
ions.
.
5.2.a iBnXz-iBnXz (Atom_Atom)
interactions (Coul-SR).
electrostatic
short-range 132
5.2.b iBnXz-iBnXz (Atom_Atom) van der Waals short-range 132
interactions (LJ-SR).
(Atom_Atom)
5.3.a iBnXz-H2O
interactions (Coul-SR).
electrostatic
short-range 133
5.3.b iBnXz-H2O (Atom_Atom) van der Waals short-range 133
interactions (LJ-SR).
5.4 The number of clusters as a function of time (a), and the 135
relative clusters sizes during the simulations for all the iBnXz
molecules based on 3BnXz cluster (b). The analysis was
performed by using 1.2 nm cutoffs to define the interaction
between iBnXz molecules.
5.5 Shows the eccentricity of the micelle over the last 5 ns of the 137
simulation.
5.6 Overview of simulations performed: The snapshots at t= 20 139
ns represent the effect of both the molecule size and the
molecules concentration on the micellization morphology.
The letters in the first row define the systems. The numbers
between the brackets represent the following: 1- millimolarity (mM), 2- iBnXz molecules, 3- water molecules, 4Na ions, 5- wt% of amphiphile. The micelles in the center of
the cell represented as van der Waals spheres; Red spots
represent the oxygen atoms, cyan color represents the carbon
and hydrogen atoms, and the nitrogen atoms are blue.
xv
5.7 Plots of repeating units, i, in the iBnXz molecules vs. iBnXz 140
concentration (mM)(a) and iBnXz wt% (b). The snapshots of
micellar shape in the iBnXz system after 20 ns represent the
spherical and the cylindrical regions. The dashed blue line
represents the micelles shape transfer zone of iBnXz
molecules and indicates the maximum available iBnXz
concentrations for spherical micellar shape. The color legend
is as described before.
5.8 (a) the end-to-end distances of the iBnXz backbones (left). 142
The measured distances represented by the black lines (right).
(b) The end-to-end distance distribution.
xvi
ACKNOWLEDGMENTS
My deepest gratitude goes to all my family members, parents, wife, kids, brothers
and sisters for their support and encouragement during my time at Case Western Reserve
University. Special recognition goes out to my little boys, Ahmed and Omar, and little
daughter, Saviya, who missed out on a lot of daddy time.
I would like to express my sincerest thanks and appreciation to my advisors
Professor Syed Qutubuddin and Professor Hatsuo Ishida for their guidance, advice and
motivation throughout my Ph.D. studies. Their continued supports led me to the right
way.
I would also like to extend my appreciation to the committee members, Professor
Daniel Lacks and Professor Mohan Sankaran. I also want to thank Dr. Tarek Agag for
introducing me to the polybenzoxazine field.
Finally,
I
would
also
like
to
thank
my
colleagues, who
have
supported me over the past several years. Special thanks to the research groups of Prof.
Qutubuddin and Prof. Ishida for their advice and mentorship. I want to thank Professor J.
Adin Mann for giving me access to his lab and equipments.
xvii
Synthesis, Evaluation and Molecular Dynamic Simulation of
Novel Anionic Polymeric Surfactants Based on
Polybenzoxazines
Abstract
by
Riyad A. S. Mahfud
Polymeric surfactants attracted considerable attention in recent years for
applications such as personal care product and stabilization of emulsions and
suspensions. The characteristic properties of polymeric surfactants originate from the
formation of aggregates through the association of the hydrophobic alkyl chains in
aqueous solution, within a narrow concentration range. The aggregates are called
micelles, and the narrow concentration range above which micelles are formed in the
solutions is called the critical micelle concentration (cmc),. The characteristics of
micelles are easily controlled by changing the solution conditions such as temperature,
concentration and ionic strength, and by changing the surfactant properties such as chain
length, hydrophobic volume and head group area.
In present work, a novel platform of anionic polymeric surfactants, poly(4HBAoca-Na+), poly(4HBA-dea-Na+), and poly(4HBA-doa-Na+), has been synthesized by
polymerizing benzoxazine monomers that are synthesized by reacting an aliphatic amine
xviii
of variable chain length (C8, C10 and C12), with 4-hydroxybenzoic acid and
paraformaldehyde. The structures of the monomers and polymeric surfactants are
confirmed by NMR and FTIR. The ring-opening polymerization and thermal behavior of
the benzoxazine monomers are studied by DSC and TGA. Size exclusion
chromatography (SEC) coupled with Viscotek triple detection technique is used to
characterize the molecular weight distribution of polybenzoxazine surfactants. The
influence of the structure on the surface activity is investigated by measuring the surface
tension of aqueous solutions of the polymeric surfactants using the Wilhelmy plate
method. The tensiometry results indicate that the adsorption at the air/water interface is
similar for the octylamine, decylamine and dodecylamine-based surfactants. Increasing
the alkyl chain length from C8 to C12 does not significantly affect the surface tension at
the critical micelle concentration (γcmc), while the critical micelle concentration (cmc)
gradually increases due to increasing hydrophobic effect. The influences of salt addition
on the surface tension at the cmc (γcmc) along with the thermodynamic properties of
micellization in aqueous solutions surfactants are studied. Upon salt (NaCl) addition, the
cmc initially decreases slightly and remains constant at 3wt% NaCl. For poly(4HBA-ocaNa+), micellization is affected by temperature as the hydrophobic and head group
interactions change . As temperature increases the cmc
of poly(4HBA-oca- Na+)
decreases in the studied temperature range.
The synthesis of anionic gemini surfactant based on 4-carboxylphenol
benzoxazine dimer, abbreviated as
di(4CaP-oca) is described. Molecular dynamics
simulations are employed to gain a fundamental understanding of the self-assembly
of amphiphilic di(4CaP-oca-Na+); particularly the morphology and dynamics of the
xix
aggregates. Starting with a randomly distributed surfactant molecules in water, the
mechanism of micelle formation is observed at a molecular level. Simulation results
show that this surfactant forms spherical micelles at concentrations higher than the cmc,
and the conformation of the di(4CaP-oca-Na+) shows that the spacer is more flexible than
the tail. The radial distribution functions (RDF) and the effect of electrostatic interactions
between the head groups are investigated. Furthermore, fully atomistic molecular
dynamic simulations are performed to address the self-assembly of amphiphilic and
comb-like polybenzoxazines (iBnXz) in water, with i=3 (trimer), i=4 (tetramer); i=6
(hexamer), i=8 (octamer), and i=10 (decamer). Spontaneous aggregation of the comb-like
polybenzoxazine molecules into a single micelle occurs in the simulations. The
simulations show that the molecular size and concentration play important roles in
micellar morphology. The micellar morphology is spherical at low concentrations, but
undergoes a transition to cylindrical shape as the concentration increases. The transition
point depends on the molecular size – both the true size as indicated by molecular weight,
as well as an additional effective size dependent on molecular flexibility.
xx
CHAPTER 1
GENERAL INTRODUCTION
1.1 BACKGROUND
Polymeric surfactants have become increasingly important for preparation of
many dispersion systems such as oil-in-water (O/W) and water-in-oil (W/O) emulsions
[1]. Interest in polymeric surfactants arises mainly from the fact that these materials have
very low critical micelle concentration (cmc) values and low diffusion coefficient as
compared to low molecular weight surfactants. The low cmc of the polymeric surfactants
is attributed to their relatively high molecular weights [2], and it can be precisely
measured for
polymeric surfactant
solutions in water [2]. At cmc or at higher
concentrations the solution contains both micellar aggregates and single molecules [3].
The self- assembly of polymeric surfactants depends on the molecular structure of the
amphiphile and on the solution conditions including concentration, temperature, pH and
salinity [4]. Some studies on the synthesis and characterization of the surface activity of
polymeric surfactants have been carried out via in the literature. For example, nonionic
polymeric surfactants based on carboxymethylcellulose and alkyl poly(etheroxy) acrylate
were synthesized by using ultrasonic irradiation to produce macroradicals and the
solutions show low surface tension [5]. Further, nonionic polymeric surfactants show
good interfacial properties, for example, poly(N-acylethylenimines), which is synthesized
based on cationic polymerization of 2-alkyl-2-oxazolines [6]. A general overview of the
polymeric surfactants types including classifications and the synthesis methods will be
introduced in a later section. The high molecular weight polymeric surfactants is a result
1
of the large number of the repeating molecular units, hence it is called polymeric.
Further, the phrase oligomeric is used to describe the polymeric surfactants with a few
repeating units or of low molecular weights (< 15,000) [7].
The most prominent advantage of the polymeric surfactants is the wide variability
of the chemical structure of the polymer. Because of their structural variables such as
backbone length, branch length, and branch spacing, they generally have great potential
to realize desirable properties such as micelles formation and surface tension reduction of
liquids. The external factors that influence the micellization process, aggregation and the
morphology of micelles include surfactant structure [8], concentration of the surfactant
[9], temperature [9], and the surfactant-solvent interactions [9]. The critical micelle
concentration (cmc) is known as the onset of intermolecular chain association [10], where
the onset of these associations occurs at very dilute concentrations of polymeric
surfactants [2].
The value of the cmc can be determined by measuring the change in the physical
properties of the surfactant solution as the surfactant concentration increases [11]; any
physical parameter can be used to register that change. Many techniques are used to
determine the cmc such as UV-absorption spectroscopy [12], fluorescence spectroscopy
[12], electrical conductivity for ionic surfactant [12], capillary electrophoresis [13],
surface tension [14], static and dynamic light scattering [14, 15], self-diffusion
measurements [16], viscosity and cryo-electron microscopy (Cryo-TEM) [17]. The most
common technique for determining the cmc is surface tension measurements, based on
either Wihelmy plate or du Nouy ring. The measurements show a break at the cmc after
2
which the surface tension remains constant with further increase in surfactant
concentration.
1.2 Classifications and synthesis of polymeric surfactants
Classical surfactants are classified into four primary groups according to the composition
of their head group: (a) anionic, containing a negatively charged head group, such as
carboxylic acids and salts, and alkyl benzene sulfonates; (b) cationic, containing a
positively charged head group, such as quaternary ammonium salts and amines; (c)
zwitterionic, electrically neutral compounds have both cationic and anionic centers
attached to the same molecule and separated by intervening atoms, such as octadecyl
dimethyl betaine (C18DMB); and (d) non-ionic, having a hydrophilic head group that is
not charged, such as ethers, alcohol ethoxylates, and carboxylic acid esters. Anionic
polymeric surfactants, which are most relevant to the present study, have been
synthesized by many different methods such as polymerization of monomeric surfactant
with a sulfate head group, namely poly(sodium undecylenic sulfate) [18], polymerization
of sulfonated styrene with allyl fatty ester [19], based on sodium poly(oxyethylene) lauryl
ether sulfate[20], chemical modification of dextran with epoxy group containing phenoxy
resin and then with sodium sulfopropyl groups [21], polymerization of carboxymethyl
cellulose and alkyl poly(etheroxy) acrylate under ultrasonic irradiation [22], etc.
Block and graft copolymers are the most common types of polymeric surfactants
and the most efficient for stabilization of emulsions and suspensions [1]. Hence, the
polymeric surfactants are designed along two main routes as described below.
3
1.2.1
Block copolymers
Block copolymers consist of alternating blocks of hydrophobic groups (B) and
blocks of hydrophilic groups (A) connected in a wide variety of ways such as A-B
diblock copolymers, A-B-A triblock copolymers and (A-B)n multiblock copolymers, see
Figure 1.1. The poly(ethylene oxide)/poly(propylene oxide) (PEO/PPO) copolymers are
examples of this type and are well known as Pluronic (PEO/PPO/PEO) or inverse
Pluronic (PPO/PEO/PPO). These nonionic polymeric surfactant form micelles in aqueous
solution when the concentration is higher than the cmc for a wide range of molecular
weights (2900-14600) [23, 24]. In general, the most convenient approach to the synthesis
of block copolymers is by creating an active site on the end of one polymer to initiate the
polymerization of another monomer. The active site can be created by free radicals,
anions, Ziegler catalyst, or cations [2]. Niwa and coworkers [25] synthesized
polyoxyethylene-block-polystyrene (POE-b-PS) copolymers by using organometallic
catalyst to generate macroradicals. The disadvantage of the polymerization by free radical
is in the simultaneous formation of homopolymers, which is formed from the same
repeating units with little surface activity at interface [1]. Khan and coworkers [26]
prepared polysteren-block-polyoxyethylene (PS-b-POE) by anionic polymerization using
cumyl potassium as the styrene block initiator. The main problem with anionic
polymerization lies in difficulty of required conditions such as high vacuum, inert
atmosphere, low temperatures, and high purity of the reactants [1]. Thus, anionic
polymerization is not typically used industrially.
4
B
A
A-B diblock
B
A
A-B-A triblock
A
A
B
(A-B)n multiblock
Figure 1.1 Schematic represents the different types of block copolymers containing
alternating hydrophilic and hydrophobic blocks.
1.2.2
Graft copolymers
Graft copolymers consist of either hydrophilic chains grafted to a hydrophobic
backbone, or hydrophobic chains grafted to a hydrophilic backbone, see Figure 1.2.a.
The polymeric surfactants of the graft type are designed to produce molecules suitable for
use as emulsifiers or dispersants under extreme conditions such as high salinity, low or
high pH, and different temperatures [2]. In general, three methods are used to synthesize
graft copolymers [2], see Figure 1.2.b: (1) grafting-from approach, where the monomer is
grafted from the backbone, such as the hydrophilic poly(2-(dimethylamino)ethyl acrylate)
(PDMAEA) side chains grafted from the hydrophobic poly(6-methyl-1,2-heptadiene-4ol) (PMHDO) backbone [27]; (2) grafting-onto approach, where the functional end
groups of one kind of polymer react with other reactive groups, of the other polymer
(backbone) that are distributed randomly on the main chain, such as the hydrophilic
poly(ethylene oxide) side chains linked to α-C of carbonyl of polyacrylate-based
backbone using CuBr as catalyst [28]; (3) grafting-through or macromonomer approach,
where a monomer is copolymerized with a low molecular weight prepolymer containing
5
n
a polymerizable double bond, such as the free radical copolymerization of distilled
methyl methacrylate with the poly(oxyethylene) macromonomer (PEO-MA), 2,2azobisisobutyronitrile (AIBN) was used as initiator and toluene as solvent [29].
Figure 1.2.a Schematic represents the different types of graft copolymers containing a
hydrophilic backbone and hydrophobic side chains (left), and hydrophobic backbone and
hydrophilic side chains (right).
Grafting-from
Monomer
Macroinitiator
Polymeric backbone
Grafting-onto
Functional
polymer
Reactive
Polymer
Graft copolymer
Grafting-through
Initiator
Monomer
Figure 1.2.b Synthetic methods for preparing Graft copolymers.
6
Figure 1.3 presents a schematic representation of mechanism of micelle formation
for polymeric surfactants in aqueous media. The micellar core consists of the
hydrophobic segments and the shell region consists of the hydrophilic segments. The size
of the segments plays a critical role in controlling the micellization process; for example,
if the hydrophobic segments are slightly shorter than the hydrophilic segments, then
spherical micelles are formed in aqueous solution. Further, if the hydrophilic segments
are too long, polymeric surfactant molecules exist as unimer (individual molecules),
while polymeric surfactants with very long hydrophobic segments form non-spherical
structures such as rods and lamellae [11]. The cmc plays the main role in determining the
polymeric surfactants concentration above which micelles are formed; the micelles
become more stable at concentrations higher than the cmc. Thus the micelles are more
stable at a given concentration for surfactants with low cmc [11].
This thesis presents new and easy approach to synthesize the amphiphilic graft
copolymers that show surface activity better than the classical surfactant. In particular,
this study illustrates the novel application of benzoxazine chemistry to make polymeric
surfactants.
7
Figure 1.3 Formation of polymeric micelles from different types of amphiphilic copolymers (Extracted from Torchillin, 2001).
1.3 Polybenzoxazines
In 1944, Holly and Cope reported the synthesis of 1,3-benzoxazines by combining
a primary amine, a phenolic derivate, and formaldehyde [30]. The synthesis and
characterization of polybenzoxazines was first reported by Ning and Ishida in 1994 [31].
Since then, extensive studies of the synthesis, characterization, and applications of
benzoxazine monomers and polymers have been reported [32]. Benzoxazines exhibit
various unusual properties, including near-zero shrinkage upon polymerization [33], fast
property development at low conversion [34], high char yield [35], very low surface
energy [36, 37], and low water absorption despite having hydrophilic chemical repeat
8
units [38]. Of particular interest in the current study is the extremely versatile molecular
design of benzoxazines [33]. While the majority of polybenzoxazines are hydrophobic,
some show potential for applications in hydrophilic environment. Benzoxazines may
have hydrophilic functionalities such as carboxylic [39], amine [40], and hydroxyl groups
[41], and a comonomer in the main chain, including polyether chain [42].
Advantages of polymeric surfactants based on benzoxazine chemistry have been
reported using a Jeffamine family with long hydrophilic chain based on polyethylene
oxide [43, 44]. Sawaryn et al. [43] synthesized nonionic polymerizable benzoxazine
surfactants that were used to stabilize miniemulsions. High molecular weight nonionic
benzoxazine surfactants, with hydrophilic polymeric blocks and benzoxazine moieties in
the polymer backbone, have also been synthesized and used as protective colloids to
stabilize o/w miniemulsions of benzoxazine resins [44]. Ishida et al. [45] synthesized a
water soluble phenolic-type polybenzoxazine that was obtained by cationic ring-opening
polymerization of monofunctional benzoxazine monomers. This Mannich base phenolictype polybenzoxazine has methylene groups in the molecule repeating unit as shown in
Figure 1.4.
OH
CH2
N
CH2
CH3
CH3
n
Figure 1.4 Scheme showing the phenolic-type polybenzoxaizne
9
The polybenzoxazine structure obtained via thermal polymerization can be
thought of as the phenolic-type [45, 46]. Thermal polymerization proceeds through an
autocatalytic mechanism where the formed phenol groups at the beginning of the
polymerization promote the benzoxazine ring-opening and accelerate the process due to
their acid character [47, 48]. Further, the polymerization temperature can be decreased by
adding acidic catalysts such as carboxylic acids or phenols [49-51]. In earlier studies [39,
46], the curing temperature of benzoxazine was improved by using monomers contain
both carboxylic groups and benzoxazine ring. These carboxylic groups acted as catalyst
that reacts with the amine moieties formed during the curing process. The existence of
the carboxylic groups in the phenolic-type polybenzoxazine increases the hydrophilic
property of the polymer. However, the inter/intra-molecular hydrogen bonding of the
hydroxyl groups that exist in the phenolic-type polybenzoxazine increases its
hydrophobicity
[52,
53].
Water
molecules might break intermolecular
hydrogen
bonds among the hydroxyl groups, resulting in the unusual hydrophilic property of this
polymer [45]. Originally, the phenolic-type polybenzoxazine is hydrophobic, and must be
converted to amphiphilic nature in order to make it compatible with the amphiphilic
graft-type copolymers. Normally, this can be done via neutralization of ion exchange
resins of the inorganic cations by adding equivalent amount of sodium hydroxide. The
neutralization depends on the type of functional groups such as strongly acidic
(sulphonate -SO3H), and weakly acidic (carboxylate –COOH). The carboxylic functional
groups reach the maximum hydrolysis at pH> 7.0 [54].
The polybenzoxazine-based amphiphilic graft copolymers possess methylene
groups and tertiary amine along the backbone, alkyl side chains and the head which
10
consists of benzene groups that hold a hydroxyl and a carboxylate ion. They all selfassemble in aqueous media to form spherical micelles at concentrations above critical
micelle concentrations [55]. The amphiphilic graft copolymer can aggregate in water to
form micelles with non-spherical morphologies [56, 57]. The molecular dynamic (MD)
simulation technique can provide microscopic level information which is used to study
amphiphile aggregates, such as the determination of geometrical characteristics of
aggregates and the concentration of free surfactant that may supplement experimental and
theoretical studies [58].
MD simulation allows one to obtain the structure of aggregates and conformations
of amphiphilic molecules by using appropriate force fields and equations of motion.
Thus, MD simulations were conducted as part of this thesis. A model system is built at
the atomic level with prescribed potentials (the force field) acting between the atoms.
These interactions may consist of site-site type interactions, such as van der Waals
dispersion and Coulombic forces, as well as intramolecular forces such as chemical
bonds, angle bending and dihedral torsional barriers. The intramolecular forces are often
treated to be a good approximation with simple harmonic or periodic functions [59], but
recently more detailed OPLS (Optimized Potential for Liquid Simulations) is a set of
force fields introduced to increase accuracy [60]. A surfactant molecule can be modeled
as a collection of atoms in the presence of solvent such as water. These approaches
provide an elegant way to predicate the polymeric surfactant structure and how it affects
the micellization process. Thus a new simulation approach which incorporates all
relevant parameters is presented for the analysis of surfactant micellization in aqueous
media. Another important application of MD simulations in this field is the designing of
11
new amphiphiles in order to obtain desirable aggregate properties. Since MD simulations
can also provide information on dynamics, the amphiphile self-assembly and other
dynamical processes can be studied in the future.
1.4 Scope of present work
Many industrial processes rely on surfactants to decrease the surface tension of
aqueous solutions at a relatively low surfactant concentration. Polymeric surfactants
gained increasing interest due to their low cmc values. The raw materials cost and the
design of the molecular structure are regarded as influential factors in selection of
methodology for polymeric surfactant synthesis. The hypothesis of this work is that
benzoxazine chemistry can be used to synthesize new polymeric surfactants with a
variety of molecular structures based on relatively inexpensive raw materials and the
molecular design flexibility for desired performance properties. While the majority of
polybenzoxazines are hydrophobic, some show potential for applications in hydrophilic
environment. The synthesis of carboxylic acid-functionalized benzoxazine monomers of
various hydrophobe chain lengths, the polymerization of these monomers to obtain
anionic polymeric surfactants, and the ability of these polymeric surfactants to reduce the
water/air surface tension are the main experimental objectives. The results justify that
these polybenzoxazines offer a superior alternative to conventional surfactants. The
specific objectives of this study are the following:
1. Synthesize and characterize carboxylic acid-functionalized benzoxazine
monomers of various chain lengths, and obtain anionic polymeric surfactants
via polymerization of the monomers.
12
2. Demonstrate the ability of these surfactants to reduce the water/air surface
tension, as a surface active agent, by measuring the critical micelle
concentration, cmc, and the surface tension at cmc, γcmc.
3. Evaluate the effects of solution conditions such as concentration, temperature
and salinity on micellization, and the significance of the enthalpy and entropy
of micellization of anionic polymeric surfactants.
4. Apply molecular dynamic (MD) simulations to predict or model the
micellization behavior of introduced surfactants and to improve the
understanding at the molecular level.
Chapter 2 describes the synthesis of three anionic polymeric surfactants,
poly(4HBA-oca-Na+), poly(4HBA-dea-Na+), and poly(4HBA-doa-Na+) via polymerizing
benzoxazine monomers by reacting an aliphatic amine of variable chain length (C8, C10
and C12), with 4-hydroxybenzoic acid and paraformaldehyde. In chapter 3, the
physiochemical properties, such as critical micelle concentration (cmc), surface tension at
cmc (γcmc), and surface activity parameters of the solutions of three anionic polymeric
surfactants have been studied. The influences of salt addition, temperature change, chain
length on the surface tension at the critical micelles concentration (γcmc) are discussed.
Chapter 4 describes the synthesis of anionic dimeric surfactants based on dimeric
benzoxazine, 3,3’-(octylazanediyl)bis(methylene)bis(4-hydroxybenzoic acid), or, 4carboxylphenol-based benzoxazine dimer abbreviated as di(4CaP-oca), which contains
only one hydrophobic alkyl tail and two hydrophilic carboxyl groups is described. MD
simulations are employed to gain a molecular-level understanding of the self-assembly
of amphiphilic di(4CaP-oca-Na+), particularly the morphology and dynamics of the
13
aggregates. The results of MD simulations of the aggregation behavior of five
amphiphilic anionic polybenzoxazines are presented in chapter 5. The effect of the
Mannich-bridge backbone length on the surfactant micellization is discussed, and the
transition from spherical to cylindrical shape is predicted as a function of concentration
and molecular size. Finally, Chapter 6 summarizes the overall conclusions and
suggestions for future work.
14
1.5
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18
Chapter 2
2. Synthesis and Evaluation of Novel Anionic Polymeric Surfactants
Based on Polybenzoxazines
19
2.1 Introduction
Surfactants constitute one of the most versatile and powerful class of materials
used in the chemical industry. Their surface activity and self-assembly behavior make
surfactants useful
for many applications including detergents, coatings, inks,
pharmaceuticals, personal care products, and advanced materials such as nanocomposites,
for the preparation of oil-in-water (O/W) and water-in-oil (W/O) emulsions and
microemulsions, as well as solid/liquid dispersions [1, 2]. Polymeric surfactants may be
used as an alternative to classical surfactants in most of the above applications, and based
on the stability criteria, perform the best in dispersions [3, 4]. The characteristic
properties of polymeric surfactants originate from the formation of micellar aggregates in
aqueous solution through the association of hydrophobic segments. The polymeric
surfactants may form monomolecular-layer micelles or aggregate to form multimolecular
structures of various shapes [5]. The aggregation behavior of polymeric surfactants
depends on the molecular structure of the amphiphile and on the solution conditions
including concentration, temperature, pH and salinity [6-8]. Some studies on the
synthesis and characterization of the surface activity of polymeric surfactants have been
carried out via designing the molecular structure. For example, block copolymers were
synthesized via ring opening reaction of cyclic imide [9], and comb-like copolymers were
obtained using ethoxylated alkyl-phenol and formaldehyde for oil recovery applications
[10]. Polymerizable surfactants have reactive functionalities that can exist in the
hydrophobic tail or the polar headgroup. The most widely-used reactive group is a vinyl
that can polymerize via thermal or photolytic initiation [11, 12]. It is important to
20
distinguish between polymeric surfactants from polymerizable surfactants such as used in
microemulsion polymerization [13] and nanocomposites [14]. Polymerizable cationic
surfactants such as vinylbenzyldimethyldodecylammonium chloride (VDAC) can
homopolymerize as well as copolymerize with monomers like styrene [13].
This study illustrates the novel application of benzoxazine chemistry to make
polymeric surfactants.
In 1944, Holly and Cope reported the synthesis of 1,3-
benzoxazines by combining a primary amine, a phenolic derivate, and formaldehyde [15].
The synthesis and characterization of polybenzoxazines was first reported by Ning and
Ishida in 1994 [16]. Since then, extensive studies of the synthesis, characterization, and
applications of benzoxazine monomers and polymers have been reported [17].
Benzoxazines exhibit various unusual properties, including near-zero shrinkage upon
polymerization [18], fast property development at low conversion [19], high char yield
[20], very low surface energy [21, 22], and low water absorption despite having
hydrophilic chemical repeat unit [23]. Of particular interest in the current study is the
extremely versatile molecular design of benzoxazines [18].
While the majority of
polybenzoxazines are hydrophobic, some show potential for applications in hydrophilic
environment. Benzoxazines may have hydrophilic functionality such as carboxylic [2427], amine [28-30], and hydroxyl groups [31, 32], and a comonomer in the main chain,
including polyether chain [33-35]. Recently, Sawaryn et al. [36] synthesized nonionic
benzoxazine polymerizable surfactants that were used to stabilize miniemulsions. High
molecular weight nonionic benzoxazine surfactants, with hydrophilic polymeric blocks
and benzoxazine moieties in the polymer backbone, have also been synthesized and used
as protective colloids to stabilize o/w miniemulsions of benzoxazine resins [37].
21
However, the synthesis of anionic surfactants based on benzoxazine has not been
described in the literature. This paper is the first to report anionic polybenzoxazine
surfactants. The goals of this research are twofold: (a) the synthesis and characterization
of carboxylic acid-functionalized benzoxazine monomers of various hydrophobe chain
lengths, and (b) polymerization of these monomers to obtain anionic polymeric
surfactants. The ability of the synthesized polymeric surfactants to reduce the water/air
surface tension and form micelles is evaluated by measuring the critical micelle
concentration, cmc, and the surface tension at cmc, γcmc. Comparison of the cmc and γcmc
of the new anionic polymeric surfactants with literature values for both low and high
molecular weight surfactants are made to justify that these polybenzoxazines offer a
superior alternative to conventional surfactants. Furthermore, the polymeric surfactants
reported herein are thermally stable up to about 170 oC and do not contain sulfur, and
therefore are more ecofriendly than many commercial surfactants.
2.2 EXPERIMENTAL
Paraformaldehyde (96%) was used as purchased from Acros Organics USA. 4Hydroxybenzoic acid (99%), octylamine (99%), decylamine (95%), and dodecylamine
(98%) were used as received from Sigma-Aldrich. 1,4-Dioxane was purchased from
Fisher Scientific.
The benzoxazine monomers were prepared from 4-hydroxybenzoic acid,
paraformaldehyde, and primary amines, including octylamine, decylamine and
dodecylamine by using a modified solvent method reported in the literature [15].
22
2.2.1 Preparation of 3-octyl-3, 4-dihydro-2H-benzo[e][1,3]oxazine-6-carboxylic acid
(abbreviated as 4HBA-oca)
In a 100 mL flask were mixed together octylamine (5 mmol, 0.646 g), 4hydroxybenzoic acid (5 mmol, 0.69 g), and paraformaldehyde (12.5 mmol, 0.3877 g) and
heated at 90 oC with magnetic stirring in dioxane (10 mL) for 24 h. The mixture was
allowed to cool to room temperature and poured into 100 mL deionized water in a 200
mL flask to give a yellowish precipitate. The product was filtered and washed three times
with deionized water and dried at 60 oC. The monomer was dissolved in chloroform and
then filtered by using filter paper to remove the dispersed material, followed by drying
under vacuum in a rotary evaporator. (Yield: 72%).
1
H NMR (DMSO-d6, frequency: 300 MHz, ppm: δ, 298K): 1.31-1.48 (10H,
CH2─CH2─CH2), 3.96 (2H, Ar─CH2─N), 4.89 (2H, O─CH2─N), 6.76-7.66(3H, Ar─H).
FTIR (KBr, cm-1):
1700 (the C═O stretching of the carboxylic group), 1240 (the
stretching of Ar─O─C), 938 (out-of-plane vibration, benzene ring to which oxazine is
attached).
2.2.2 Preparation of 3-decyl-3, 4-dihydro-2H-benzo[e][1,3]oxazine-6-carboxylic acid
(abbreviated as 4HBA-dea)
4HBA-dea was prepared from decylamine (5 mmol, 0.786 g), 4-Hydroxybenzoic
acid (5 mmol, 0.690 g), and paraformaldehyde (12.5 mmol, 0.387 g) as previously
described for 4HBA-oca, (Yield: 73%).
1
H NMR (DMSO-d6, frequency: 300 MHz, ppm: δ, 298K): 1.31-1.47 (10H,
CH2─CH2─CH2), 3.96 (2H, Ar─CH2─N), 4.87 (2H, O─CH2─N), 6.77-7.68(3H, Ar─H).
23
FTIR (KBr, cm-1):
1700 (the C═O stretching of the carboxylic group), 1240 (the
stretching of Ar─O─C), 938 (out-of-plane vibration, benzene ring to which oxazine is
attached).
2.2.3 Preparation of 3-dodecyl-3,4-dihydro-2H-benzo[e][1,3]oxazine-6-carboxylic
acid
(abbreviated as 4HBA-doa)
4HBA-doa was prepared from dodecylamine (5 mmol, 0.945 g), 4Hydroxybenzoic acid (5 mmol, 0.690 g), and paraformaldehyde (12.5 mmol, 0.387 g) as
previously described for 4HBA-oca, (Yield: 78%)
1
H NMR (DMSO-d6, frequency: 300 MHz, ppm: δ, 298K): 1.31-1.47 (10H,
CH2─CH2─CH2), 3.95 (2H, Ar─CH2─N), 4.87 (2H, O─CH2─N), 6.77-7.68(3H, Ar─H).
FTIR (KBr, cm-1):
1700 (the C═O stretching of the carboxylic group), 1240 (the
stretching of Ar─O─C), 938 (out-of-plane vibration, benzene ring to which oxazine is
attached).
2.2.4 Polymerization of benzoxazine monomers and ionization of polymeric
benzoxazines
1.0 gram of each monomeric benzoxazine, 4HBA-oca, 4HBA-dea, and 4HBAdoa, was polymerized on separate glass plates by following the same heating procedure in
an air circulating oven at 160 oC for 30 minutes.
1.0 gram of each polymerized benzoxazine monomer, abbreviated as poly(4HBAoca), poly(4HBA-dea) and poly(4HBA-doa), was weighed separately into a 50 mL
beaker. Since the starting monomers are monofunctional benzoxazines, the polymers
24
formed are at best small oligomers with molecular weight in the range of 2000-6000 [38].
The designated amount of NaOH (to neutralize all carboxylic acid groups) was dissolved
in 20 mL of deionized water, and then added to the polybenzoxazine oligomer. The
beaker containing polybenzoxazine oligomer sample and base solution was placed in an
ultrasonic bath until the solid was dissolved. The solution was then filtered with a filter
paper and cooled to room temperature. The resulting polymer was dried overnight at 60
o
C in an air circulating oven to a constant weight.
2.3 Measurements
Proton nuclear magnetic resonance (1H NMR) spectra were taken on a Varian
Gemini 2000 NMR operating at a proton frequency of 300 MHz. All samples were
dissolved in deuterated dimethylsulfoxide (DMSO-d6).
Fourier transform infrared (FTIR) spectroscopic analysis was carried out on a
Bomem Michelson MB100 Spectrophotometer with a deuterated triglycine sulfate
detector. After casting a thin film onto a KBr plate and purging with dry air, coadded
spectra of 64 scans were recorded at a spectral resolution of 4 cm-1.
Differential scanning calorimetry (DSC) was performed with a TA Instruments
DSC Model 2920 at a heating rate of 10 oC/min from 25 to 300 oC and nitrogen flow rate
of 62 mL/min; 2mg samples were sealed between aluminum hermetic pans and lids for
all tests.
Thermogravimetric analysis (TGA) was performed with a TA Instruments High
Resolution 2950 Thermogravimetric Analyzer at a heating rate of 10 oC/min from 25 to
25
800 oC and nitrogen purge at a flow rate of 40 mL/min; 5 mg samples were placed in an
open platinum crucible for all tests.
Size exclusion chromatography (SEC), also known as gel permeation
chromatography (GPC), was performed with a triple detector array from Viscotek
GPCmax
instrument (Malvern instruments, Worcestershire, UK). The Viscotek SEC apparatus
equipped with three–column set-up with pore size of 10, 50 and 1000 nm with common
particle size of 5 mm using THF as an eluent, was pumped through the columns at a rate
of 1.0 ml/min. Volume of injection was 75µL. The Viscotek system contains the
following detectors in order: a 90o angle light scattering detector (LS), a refractive index
detector (RI, concentration detector), and a four-capillary differential viscometer. The
wavelength of the light scattering laser used was 670 nm. OmniSEC software was used
for data analysis and acquisition. The number average molecular weights (Mn) and
polydispersity index (Mw/Mn) were calculated relative to polystyrene standards.
Surface tension measurements of aqueous solutions were carried out with KRÜSS
Tensiometer (K100) using the Wilhelmy platinum plate method. All measurements were
carried out at 23±0.1 oC. Reproducibility was checked by frequent determination of the
surface tension of de-ionized distilled water (72–73 mN/m).
26
2.4 Results and Discussion
2.4.1 The molecular structures of synthesized monomers and oligomers
Benzoxazines are typically hydrophobic materials and show limited solubility in
water. The interfacial and association behavior of anionic polymeric surfactants based on
benzoxazine chemistry are reported for the first time by observing the surface tension
change of water versus surfactant concentration. The polymerized benzoxazines contain
alkyl chain as hydrophobic segment, carboxylic moieties attached to phenolic rings as
hydrophilic segment, and the rings connected by Mannich bridge as the backbone.
Formation of the Mannich bridge structure is due to the ring-opening of benzoxazine in
acidic media [39]. The segments are distributed throughout the whole polymer backbone
in the form of a comb-like polymer. Figure 2.1 shows the hydrophilic and hydrophobic
segments of the proposed structures for the three surfactants. The anionic polymeric
surfactants introduced here have quite unusual properties in terms of high affinity for the
air/water interface at low concentration, low critical micelle concentration, cmc, and high
water-solubility. The hydrophobicity of the synthesized polymeric surfactants was varied
by using three different chain lengths of the primary amine, C8, C10, and C12. The three
benzoxazine monomers were synthesized following the same procedure as shown in
Scheme 2.1. Scheme 2.2 shows the ionization of the oligomers which correspond to
Figure 2.1.
27
CH 3
CH3
CH3
Hydrophobic
segment
OH
OH
OH
N
N
N
COO- Na +
COO- Na +
COO - Na+
Hydrophilic
segment
i
i
i
poly(4HBA-oca- Na+)
poly(4HBA-dea - Na +)
poly(4HBA-doa - Na +)
Figure 2.1 Representative structures of the three anionic polymeric surfactants, the
symbol i is the degree of polymerization.
H2
C CH
N
n 3
O
OH
+ Amine + (CH2 O) m
90 oC, 24h
dioxane
COOH
COOH
Monomer
4HBA-oca
4HBA-dea
4HBA-doa
Amine
n+1
CH 3(CH 2) 7NH 2
CH3 (CH 2) 9NH2
CH3 (CH 2) 11 NH 2
8
10
12
MW
243
271
299
Scheme 2.1 The synthesis of benzoxazine monomers 4HBA-oca, 4HBA-dea and 4HBAdoa.
28
O
H2
C CH
N
n 3
OH
160 oC, 30 min
COOH
CH 3
CH 3
N
COOH
OH
CH 2 n
NaOH
i
CH 2 n
N
COO- Na + i
Scheme 2.2 The proposed oligomer structures of poly(4HBA-oca), poly(4HBA-dea) and
poly(4HBA-doa) and the ionization of these polymers into
poly(4HBA-oca- Na+),
poly(4HBA-dea- Na+) and poly(4HBA-doa- Na+), respectively; n in Scheme 1, and the
symbol i is the degree of polymerization.
2.4.2 NMR analysis
The 1H NMR spectra shown in Figure 2.2 clearly reveal the benzoxazine ring
formation for 4HBA-oca, 4HBA-dea, and 4HBA-doa. The characteristic resonances
attributed to benzoxazine structure are observed at 3.95-3.96 ppm (s, Ar─CH2─N) and
4.87-4.89 ppm (s, N─CH2─O─), which are consistent with the formation of benzoxazine
ring [15].
29
Figure 2.2 The 1H NMR spectra of benzoxazine monomers 4HBA-oca, 4HBA-dea, and
4HBA-doa.
2.4.3 FTIR analysis
Moreover, the FTIR spectra of 4HBA-oca, 4HBA-dea, and 4HBA-doa as
illustrated in Figure 2.3 show the characteristic absorption bands of benzoxazine structure
at 1240 cm-1 due to the stretching of C─O─C and at 940 cm-1 due to the out-of plane
bending vibration of the benzene ring attached to the oxazine [40]. The IR spectrum
taken at room temperature shown in Figure 2.4 indicates the weakening of the
characteristic bands of benzoxazine at 1460, 1240, and 940 cm-1. Disappearance of these
bands after polymerizing at 80 oC, 120 oC and 160 oC confirms ring-opening of
30
benzoxazine moieties and the formation of polybenzoxazine. Similar thermal behavior
was also observed for 4HBA-oca and 4HBA-dea.
Figure 2.3 FTIR spectra of benzoxazine monomers 4HBA-oca, 4HBA-dea, and 4HBAdoa.
31
Figure 2.4 The FTIR spectra of 4HBA-doa after polymerization at 80 oC, 120 oC and 160
o
C for 30 min.
2.4.4 DSC and TGA studies
Thermally accelerated ring-opening polymerization of 1,3-benzoxazines is an
autocatalytic, exothermic process having a maximum around 200- 270 oC depending on
the functionalities of the benzoxazines [17]. The polymerization behavior of the
monomers was examined by DSC. The melting point, onset temperature, maximum
temperature and the amount of exotherm for 4HBA-oca, 4HBA-dea and 4HBA-doa are
illustrated in Figure 2.5. The exotherm corresponding to the ring opening polymerization
is observed for all three monomers. 4HBA-oca has an exotherm with an onset at 130 oC
and a maximum peak at 184 oC, corresponding to the polymerization of benzoxazine with
32
a heat of polymerization, ΔH, of 75 J/g. For 4HBA-dea, the exotherm starts at 135 oC
with a maximum at 187 oC and ΔH of 67 J/g. Finally, for 4HBA-doa, the exotherm starts
at 142 oC with a maximum at 190 oC and ΔH of 61 J/g. These exotherm temperatures are
unusually low in comparison to ordinary benzoxazine monomers [17] due to the effective
catalytic role of carboxylic acid in the polymerization [41]. Benzoxazine monomers
containing carboxylic acid groups show similar acceleration of the rate of polymerization
[24, 42]. The melting points for 4HBA-oca, 4HBA-dea and 4HBA-doa are sharp
indicating good purity of the monomers used. 4HBA-oca with chain-length C8 has a
melting point of 92 oC and 4HBA-doa with chain-length C12 has a melting point of 99
o
C. This increase in melting point with chain length is attributed to the increase of the
hydrophobic interactions between the nonpolar alkyl groups. These benzoxazine
monomers have low melting points compared to other benzoxazine monomers with
aromatic functional groups [24].
33
Figure 2.5 The DSC thermograms of benzoxazine monomers 4HBA-oca, 4HBA-dea,
and 4HBA-doa.
Figure 2.6 shows the DSC thermograms of 4HBA-oca at various thermal
treatments. After each polymerization cycle for 4HBA-oca, the exotherm decreases with
increasing temperature and almost disappears after polymerization at 120 oC for 30
minutes. The heat of polymerization decreases from 75 J/g to 0 J/g as the temperature is
increased from 80 oC to 160 oC, indicating the disappearance of benzoxazine structure.
The endotherm around 218 oC is due to the degradative evaporation of the amine moiety,
as discussed later.
The overlap of this endothermic peak with the polymerization
exotherm makes the determination of the heat of polymerization slightly inaccurate,
especially at high conversion.
34
The ring opening behavior was also monitored by the decreasing intensity and
final disappearance of C-O-C band at 1240 cm-1 in sequential FTIR spectra in Figure 2.4.
Moreover, Figure 2.6 reveals that ring-opening of benzoxazine moieties at 160 oC is
almost complete after 30 minutes, and these cycles were also investigated by FTIR.
Figure 2.6 Dynamic DSC plots for 4HBA-oca polymerization at various temperatures for
30 minutes.
The thermal stability of benzoxazine polymers was analyzed by TGA and the
results are shown in Figure 2.7.A. The first weight loss of about 45% that was observed
around 200 oC for the three polymers is attributed to the degradation of linear aliphatic
amine. Figure 2.7.B shows the FTIR spectrum of the condensate from poly(4HBA-oca)
after heating at 200 oC for 20 min and collecting the vapor phase. The FTIR spectrum of
35
octylamine used to produce poly(4HBA-oca) is included in Figure 2.7.B for comparison.
Comparison of the two spectra indicates the cleavage of the alkyl chain in the polymer
due to thermal cracking. Expectedly, the cleaved species did not show the NH stretching
mode at 3330 cm-1 due to the lack of the primary amine structure. Detailed molecular
mechanisms of fragmentation and the structure of the fragmented species have been
reported in the literature [43- 47].
Furthermore, the DSC endotherms of benzoxazine monomers shown in Figure 2.5
in the temperature range from 205 oC to 230 oC (Tmax=218 oC) are mainly attributed to
the cracking reactions that lead to the cleavage of aliphatic side chain: These endotherms
are consistent with the TGA weight loss from 200 oC to 230 oC. The second weight loss
of about 15% observed between 300 oC and 400 oC can be ascribed to decarboxylation of
acid groups on the polybenzoxazines. The temperatures with 5% and 10% weight loss
under nitrogen environment are: 176 and 192 °C for poly(4HBA-oca), 177 and 193 °C
for poly(4HBA-dea), 179 and 200 °C for poly(4HBA-doa), respectively. These polymers
show a low char yield of 20-25% when the residual weight is examined under nitrogen at
800 °C. This char yield is low because of the existence of aliphatic chains.
36
Figure 2.7.A TGA curves of poly(4HBA-oca), poly(4HBA-dea), and poly(4HBA-doa).
Figure 2.7.B FTIR spectra of the condensate of the evolved gases at the peak rate
degradation of poly(4HBA-oca) and octylamine.
37
2.4.5 Determination of molecular weights by SEC
Size exclusion chromatography (SEC) coupled with Viscotek triple detection
technique (light scattering, viscometry and refractometry) was used to determine the
molecular weight of each peak coming off the column and simultaneously measure the
hydrodynamic radius and polydispersity of polymeric benzoxazines. Table 2.1 shows the
analytical results for molecular weight distributions and related properties. The
experimental results demonstrate that the polydispersity is large probably because of the
presence of long-chain branched polymers. The branching leads to poor separation in
SEC, and significantly changes the hydrodynamic volume [48].
To know the shape of the polybenzoxazines in the eluting solution (THF) the radius of
gyration (Rg) was calculated by using the Flory-Fox and Ptitsyn-Eisner equations [49]:
1�
2
1
𝑅𝑔 = � �
6
𝑀
�[𝜂] � ��
𝐹
1�
3
… … … … … … … … … … … … … … … … … … … … … … … … . (1)
Where M is the molecular weight, [η] is the intrinsic viscosity, and F is obtained from
𝐹 = 2.86 × 1021 (1 − 2.63𝜀 + 2.86𝜀 2 ) … … … … … … … … … … … … … … . … … … … (2)
and
𝜀=
(2𝑎−1)
3
… … … … … … … … … … … … … … … … … … … … … … … … … … … … … … … . (3)
Here 𝑎 is the exponent of the Mark-Houwink-Sakurada equation:
[𝜂] = 𝐾 𝑀𝑎 … … … … … … … … … … … … … … … … … … … … … … … … … … … … … … (4)
𝑅𝑔
The shape ratio (𝑅 ) can be used as a qualitative measure in judging what architectural
ℎ
𝑅𝑔
structure may be present [50]. The ratio ( 𝑅 ≈ 0.77 ) represents a sphere of uniform
38
ℎ
density [51- 53]. The data obtained are fairly close to 0.77 and suggest spherical shape of
uniform density for all three polybenzoxazines.
Table 2.1
Summary of analytical results for molecular weight distributions and related properties
determined from SEC with triple detector.
𝑀𝑤 𝑏
𝑀𝑛
𝑅ℎ 𝑐
(nm)
[𝜂]𝑑
(dl/g)
Sample
Poly(4HBA-oca)
2248
3.319
1.364
0.0283
0.135
0.942
0.690
Poly(4HBA-dea)
6021
1.328
1.378
0.0242
0.090
1.215
0.881
Poly(4HBA-doa)
4936
1.232
1.334
0.0251
0.057
1.133
0.849
𝑎
𝑒
𝑅𝑔 𝑓
(nm)
𝑅𝑔 𝑔
𝑅ℎ
𝑀𝑛 𝑎
(Da)
a
Number average molecular weight
Polydispersity
c
Hydrodynamic radius
d
Intrinsic viscosity
e
Mark-Houwink constant
f
Radius of gyration
g
Shape ratio
b
The molecular weights of the polybenzoxazines depend on the polymerization
mechanism as well as interchain hydrogen bonding and the interactions between the alkyl
chains. The ring opening initiation of benzoxazine polymerization produces a carbocation
and an iminium ion in equilibrium [54], and the propagation rate is controlled by the
carbocation. If the iminium ion is stable the propagation rate is low. However, if the
iminium ion is unstable the propagation rate is high [54]. The length of the alkyl chain
attached to the iminium ion obviously affects its stability. In this work, the results
indicate a wide range of polybenzoxazines molecular weights (2200 to 6000 Da), where
the poly(4HBA-dea) and poly(4HBA-doa) exhibit a narrow range of molecular weights
39
(5000 to 6000 Da) and the poly(4HBA-oct) shows a lower molecular weight (2200 Da).
This variation is attributed to the short chain length of poly(4HBA-oct) that results in low
hydrophobicity in the polar eluent. These observations in the molecular weights are
consistent with the values of the radius of gyration presented in Table 2.1.
The values of the degree of polymerization for poly(4HBA-oca), poly(4HBA-dea)
and poly(4HBA-doa) are 9, 22 and 16, respectively. The carboxylic acid group of the
three polybenzoxazines were neutralized by adding required amount of NaOH as shown
in Scheme 2.2, and converted to polymeric surfactants, poly(4HBA-oca-Na+),
poly(4HBA-dea-Na+) and poly(4HBA-doa-Na+), respectively.
2.4.6 Surface tension measurements
The variation of surface tension with the concentration of the anionic polymeric
surfactants in water is shown in Figure 2.8.A and B. The reduction of the surface tension
by the polymeric surfactants due to adsorption at the air–water interface is observed at
very low concentrations. In addition, Figure 2.8.B shows the surface tension as a function
of the molar concentration of the polymeric surfactants. The cmc decreases with
increasing molecular weight of the polybenzoxazine surfactants. Least square regression
analysis was performed to find the best equation for each of the linear portion below the
cmc (the pre-cmc line) and the portion above the cmc (the post-cmc line). The surface
tension plots exhibit a shallow minimum which indicates the effect of polydispersity as
observed in SEC analysis. Figure 2.9 shows that the cmc increases quite significantly
40
from 0.12 g/L to 0.17 g/L with increase in alkyl chain length from C8 to C12 due to the
hydrophobic effect. However, varying alkyl chain length does not affect the cmc.
Figure 2.8.A Surface tension at different weight concentrations.
Figure 2.8.B Surface tension at different molar concentrations
41
Figure 2.9 Variation of cmc and γcmc with alkyl chain length.
The presence of NaCl at a fixed temperature lowered the cmc’s of the
polybenzoxazine surfactant solutions compared to those evaluated for the salt-free
polybenzoxazine surfactant solutions at 23±0.1oC. Figures 2.10.A, B and C show a
progressive increase in surface activity for rising polybenzoxazine surfactant
concentrations, with the salt containing systems showing a slight reduction in surface
tension compared to the salt free systems. Figure 2.11 shows the presence of salt reducing
the surface tension and the cmc of the polybenzoxazine surfactant solutions compared to
salt free polybenzoxazine surfactant solutions. The minimum values of the γcmc
approached at the 1 wt% of NaCl for the polybenzoxazine surfactant solution. For
example, the γcmc of the poly(4HBA-oca- Na+) solution was gradually decreased by
increasing salinity from 38 mN/m at 0 wt% NaCl to 27 mN/m at 1 wt% NaCl, and then
42
gradually increased by increasing salinity to 30 mN/m at 3 wt% NaCl. Only slight
decreases in cmc values were observed with increasing salinity from 0 to 3 wt% of NaCl
for the polybenzoxazine surfactant solution, Table 2.2. The mechanism of surface tension
reduction is due to the increase in diffusion of surfactant from bulk to the air-liquid
interface by the electrolyte [55]. The effect can be observed at very low polymeric
surfactant concentrations, where the pure polymeric surfactant cannot adsorb at the air–
water interface. Further, adding salts tend to screen electrostatic repulsions between the
head groups of the amphiphiles, and make the amphiphiles effectively more hydrophobic.
It then increases hydrophobic interactions among the surfactant monomers and cause
them to aggregate at lower concentration, thus the cmc decreases [56].
Figure 2.10.A The effect of salinity on the surface tension of different poly(4HBA-ocaNa+) solutions.
43
Figure 2.10.B The effect of salinity on the surface tension of different poly(4HBA-deaNa+) solutions.
Figure 2.10.C The effect of salinity on the surface tension of different poly(4HBA-doaNa+) solutions
44
Figure 2.11 The effect of salinity on the cmc and the surface tension at cmc (γcmc) of the
solutions of poly(4HBA-oca- Na+), poly(4HBA-dea- Na+) and poly(4HBA-doa- Na+)
The maximum surface excess concentration (Γmax) and minimum surface area per
surfactant headgroup (ɑs) were calculated, respectively, according to:
𝛤𝑚𝑎𝑥 = −
𝑎𝑠 =
1
𝜕𝛾
�
�
2𝑅𝑇 𝜕 ln(𝑚�𝑚𝑜 )
𝑃,𝑇
… … … … … … … … … … … … … … … … … … … … … … … (5)
1
… … … … … … … … … … … … … … … … . … … … … … … … … … … … … … … . (6)
𝑁𝐴 𝛤𝑚𝑎𝑥
45
where R is the gas constant, T is the absolute temperature, NA is the Avogadro constant
and m is the surfactant molal concentration (m°=1 mol kg−1).
𝛤𝑚𝑎𝑥 is a useful measure of the effectiveness of adsorption of the surfactant at the air-
water interface, and ɑs provides information on the degree of packing and the orientation
of the adsorbed surfactant molecule when compared with the dimensions of the molecule.
The cmc, γcmc, 𝛤𝑚𝑎𝑥 and ɑs data as a function of the ionic strength are collected in Table
2.2. With increasing salinity from 0 to 1 wt%; the 𝛤𝑚𝑎𝑥 and the ɑs of poly(4HBA-ocaNa+) and poly(4HBA-doa-Na+) slightly increased and slightly decreased, respectively.
The 𝛤𝑚𝑎𝑥 and the ɑs of poly(4HBA-dea-Na+) showed rapid increase and rapid decrease,
respectively. The 𝛤𝑚𝑎𝑥 difference in response to increasing salinity is possibly due to the
different molecular weights of (4HBA-oca-Na+), poly(4HBA-doa-Na+) and poly(4HBA-
dea-Na+). The poly(4HBA-dea-Na+) with the highest molecular weight showed rapid
increase in the 𝛤𝑚𝑎𝑥 . With increasing salinity from 1 to 3 wt%; the 𝛤𝑚𝑎𝑥 and the ɑs tends
to decrease and increase, respectively, for (4HBA-oca-Na+), poly(4HBA-doa-Na+) and
poly(4HBA-dea-Na+) with different response that may related to their different molecular
weights. This trend accompanying the salinity change means that, with increasing
salinity, more surfactant molecules are adsorbed when the surface is saturated, resulting
in a higher packing density and lower γcmc. Certainly, at high salt concentrations above 1
wt% the salt ions reduce the electrostatic repulsion between the intermolecular head
groups, and the electrostatic repulsion become invariable leads the cmc values to become
constant [55, 56]. As the concentration of NaCl increases, the electrical double-layer
thickness (1/κ), as measured by the Debye length, decreases sharply. At 1 wt% the 1/κ
was 0.734 nm, and at 3 wt% the 1/κ was 0.424 nm. The 1/κ and the forces involving
46
water structures play a significant role in influencing salinity on Γmax. The ions in NaCl
affect the structure of water where the Na+ ions (structure-making ions) promote
hydrogen bonding of neighboring waters, and the Cl¯ ions (structure-breaking ions)
promote electrostatic interaction with the neighboring waters [57]. The structure-making
ions tend to flee the air/water interface because they can better organize the water dipoles
in bulk water than at the interface. The structure-breaking ions pushed toward the
air/water interface by the bulk water because the bulk water molecules can better
organize its hydrogen-bond network without the structure breaking ions in order to
minimize the system free energy to the lowest values [58]. This trend observed in our
experiments when increasing the salinity from 1 to 3 wt%. The diffusion of Cl¯ ions to the
air/water interface leads the head groups to become more hydrophilic and decrease the
surface excess concentration (Γmax) resulting increase in the minimum surface area per
surfactant headgroup (ɑs).
47
Table 2.2
Influence of NaCl concentration on the cmc, γcmc, 𝛤𝑚𝑎𝑥 and ɑs of poly(4HBA-oca-Na+),
poly(4HBA-dea-Na+) and poly(4HBA-doa-Na+)
Surfactant
poly(4HBA-oca-Na+)
-
+
poly(4HBA-dea Na )
poly(4HBA-doa-Na+)
NaCl
(wt%)
0
cmc
γcmc
(g/L)
(mN/m)
0.120±0.002 38.04±0.24
𝛤𝑚𝑎𝑥
(µmol.m-2)
2.368
ɑs
(nm2)
0.701
1
0.118±0.020 27.15±3.46
2.691
0.616
3
0.116±0.04
30.56±1.73
1.419
1.169
0
0.123±0.021 38.07±0.85
1.782
0.931
1
0.121±0.001 28.70±0.43
2.353
0.705
3
0.116±0.011 31.56±1.73
1.669
0.994
0
0.173±0.004 39.11±0.56
2.439
0.680
1
0.164±0.005 34.45±0.39
2.457
0.675
3
0.148±0.004 35.83±0.87
1.640
1.012
The cmc values at 23±0.1 °C obtained in this work are comparable with the
values reported for both the low molecular weight surfactants such as sodium
dodecylsulfate (SDS) [59], and the high molecular weight surfactants such as carboxy
methyl cellulose-based polymeric surfactant (CMC-polymeric surfactant) [60]. Addition
of NaCl did not change the surface tension of the CMC-polymeric surfactant solutions,
though they have anionic group –COO-. The influence of NaCl on the surface tensions of
the polybenzoxazine surfactant solutions was investigated and was reported minima at 1
wt% NaCl. A further comparison between the polybenzoxazine surfactants and some
polymeric surfactant reported in literature showed that polybenzoxazine surfactants
provided slightly lower γcmc [8-10, 60].
48
2.5 Conclusions
The synthesis of novel anionic polymeric surfactants from benzoxazine
monomers is reported. The structure of these compounds was determined via FTIR and
1
H NMR spectroscopy. Ring-opened structures were also identified by using FTIR and
DSC. The 4HBA-oca, 4HBA-dea and 4HBA-doa showed single exothermic peaks at 184
o
C, 187 oC and 190 oC, respectively, which are relatively low due to the effective
catalytic nature of the carboxylic acid. TGA showed 45% weight loss around 200 oC for
the three benzoxazine polymers, which is attributed to the degradation of linear aliphatic
amine. The branched polybenzoxazines have low number average molecular weight (Mn
~ 2200-6000) and high polydispersity (Mw/Mn ~ 1.2-3.3).
Surface tension measurements at the air/water interface clearly show the high
surface activity of the anionic polybenzoxazines surfactants. The cmc increases from 0.12
g/L to 0.17 g/L with change in alkyl chain length from C8 to C12. The cmc values at
23±0.1 °C are comparable with literature values reported for polymeric surfactants [8-10,
60]. While surface tension measurements of nonionic polymerizable benzoxazine
surfactants have been reported, their cmc values were not determined [36, 37].
The raw materials of these new surfactants are available at low-cost, and the
simple purification process ensures high yield and purity. These surfactants have
potential applications in many fields including detergency, personal care products such as
shampoo, coatings, inks, and for the preparation of oil-in-water (O/W) and water-in-oil
(W/O) emulsions. A detailed study of the micellization thermodynamics of these
surfactants in aqueous media, and the properties and stability of (O/W) emulsion systems
made by these surfactants will be the subject of a forthcoming paper.
49
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53
Chapter 3
3. Anionic Surfactants Based on Comb-like Polybenzoxazine Oligomers:
Effect of Salinity and Temperature on Critical Micelle Concentration
54
3.1 Introduction
Polymeric surfactants attracted considerable attention in recent years for
application in personal care product and stabilization of emulsions and suspensions [1].
The characteristic properties of polymeric surfactants originate from the formation of
micelle-like aggregates through the association of the hydrophobic alkyl chains in
aqueous solution, within a narrow concentration range [2]. Polymeric surfactant
micellization process in aqueous media has attracted only little attention as compared to
the vast number of extensive studies reported in the literature dealing with monomeric
surfactant micellization in aqueous media. The polymeric surfactants may form
monomolecular-layer micelles or aggregates of multimolecular-layer micelles of various
shapes. As the aggregates are formed, most of the physical and rheological properties of
the polymeric surfactant solutions change abruptly. Over a narrow concentration range
the critical micelle concentration (cmc), above which micelles are formed in the
solutions. The characteristics of these aggregates are easily controlled by the change in
the solution conditions such as temperature, concentration and ionic strength, and by the
change in the surfactant properties such as chain length, hydrophobic volume and head
group area [3-6]. Micellar aggregation can be demonstrated by measurements of physical
properties, such as surface tension, against surfactant concentration. As surfactant
concentration is increased, surface tension falls to a minimum at the cmc [7]. An
important factor that influences cmc is the temperature. The study of cmc versus
temperature is reported in the literature for ionic surfactants to probe hydrophobic and
head group interactions [8].The available data indicate that the cmc decreases with
55
the increase in temperature of the system [9]. The thermodynamic parameters of
micellization in aqueous solutions; the standard Gibbs free energy of micellization,
ΔmicG°, the enthalpy of micellization, ΔmicH° and the entropy of micellization, ΔmicS°,
can be derived from the temperature dependence of the cmc. They quantify the relative
importance of hydrophobic interactions, surfactant-water contact and head-group
repulsion. ΔmicG° is also the free energy of transfer of one surfactant from the aqueous
phase to the micellar pseudophase.
In previous paper [10] we introduced a series of novel anionic polymeric surfactants
based on comb-like polybenzoxazine oligomers. In this study experimental values of
enthalpy and entropy of micellization in aqueous solutions of these surfactants are
calculated and the effects of solution conditions such as concentration, temperature and
salinity in the micellization process are reported. The significance of the enthalpy and
entropy of micellization of anionic polymeric surfactants and their relation to the theory
of micelle formation is considered. These anionic surfactants with univalent counterion
are considered 1-1 electrolytes.
3.2 EXPERIMENTAL
3.2.1 Chemicals
Three anionic polymeric surfactants, based on Comb-like Polybenzoxazine
Oligomers, named; poly(4HBA-oca- Na+), poly(4HBA-dea- Na+), and poly(4HBA-doaNa+) used during this study were reported somewhere else in the literature [10]. Ions free
56
water was used as solvent. The salt used in this study was NaCl and this salt was
purchased from Fisher Scientific.
3.2.2 Preparation of solutions
Stock solution of poly(4HBA-oca- Na+), poly(4HBA-dea- Na+), and poly(4HBA-doaNa+) were prepared by dissolving a known amount of these surfactants, either in pure
(ion free) water or in the solution of known NaCl concentration. Solutions were kept in
identical glass containers and the container wall was kept at a minimum to reduce the
effects due to adsorption on the container. Solutions so prepared were used for viscosity,
conductance and surface tension measurements.
3.3 Measurements
3.3.1 Surface tension measurements
The surface tension of solutions was determined by means of Wilhelmy platinum
plate method and Du Nouy ring platinum (diameter: 19.6 mm, thickness: 0.1 mm) on
KRÜSS Tensiometer (K100). The maximum force (Fmax) exerted on the surface of the
lamella is measured as it is removed from the solution just before it breaks. The unit of
measurement is given in milliNewton/m (mN/m). Temperature was controlled with a
jacket linked to a water circulating system (± 0.1 oC). Reproducibility was checked by
frequent determination of the surface tension of de-ionized distilled water. The results
show accuracy within ±0.1 mNm−1. The results were the average of three measurements.
The vessel covered with a hole only allowing a small ring (Du Nouy ring) to go through,
57
to prevent the contamination of the solution from dust in the air during the operation. The
whole vessel was placed inside a closed sample chamber of the surface tensiometer.
The maximum surface excess concentration, Γmax, and the minimum surface area per
surfactant headgroup, 𝑎𝑠 , were calculated, respectively, according to:
𝛤𝑚𝑎𝑥 = −
𝑎𝑠 =
1
𝜕𝛾
�
�
2𝑅𝑇 𝜕 ln(𝑚�𝑚𝑜 )
1
𝑁𝐴 𝛤𝑚𝑎𝑥
𝑃,𝑇
(1)
(2)
where R is the gas constant, T is the absolute temperature, NA is the Avogadro constant
and m is the surfactant molal concentration (m°=1 mol kg−1).
3.3.2 Viscosity measurements
The viscosity measurements were carried out using an Ubbelohde suspendedlevel capillary viscometer. The viscometer was suspended vertically in a thermostat used
to provide a stable constant temperature of 30±0.1◦C. The viscosity of polymeric
surfactant solutions at different concentrations (0.01-1 wt %) was measured. The
viscometer was carefully washed, rinsed and dried before use. The flow times for a
constant volume of solution through the capillary were measured with a calibrated
stopwatch. The viscometer was so selected that the solvent flow time was more than 100
seconds, in order to minimize the contribution of kinematic energy.
3.3.3 Dynamic light scattering
Dynamic light scattering (DLS) measurement performed on a laser light scattering
spectrometer with ALV–5000E (ALV–GmbH, Langen, Germany), using He–Ne laser
58
with wavelength at 632.8 nm. Rayleigh ratio was calibrated with the value for toluene
(1.3522 ×10-5 cm-1). The sample solution at 0.5 g/L poly(4HBA-oca- Na+) content was
subjected to measurements after optical clarification with a 0.45-μm Millipore filter into a
clean scintillation vial, and the sample was characterized by DLS at 25 °C and at the
measurement angles 60°, 90° and 130o.
3.3.4 Foaming power measurements
The foam height was measured following a method reported in this paper for the
first time as follows; the method uses a 100 ml graduated cylinder with polypropylene
stopper. The cylinder manufactured from thermally-stable borosilicate glass with the
specifications of; readability: 1 ml, height: 25 cm, diameter of cylinder: 3.1 cm. An
amount of 40 ml from a solution of surfactant (75 ppm or 150 ppm) contained in the
cylinder at a given temperature (25oC). The samples were shaken for 30 seconds by hand
on a standard way, hold the cylinder from the middle and shake it fast by raising your
hand up and down with frequency of one cycle each second. The foam height produced in
the cylindrical was read immediately after all the solution shake had run out of the
cylinder (initial foam height) and again after a given amount of time (generally 1min and
5 min). The experiment repeated 4 times for reliability.
3.3.5 Conductance measurements
Measurements of the electric conductivity (k) were carried out by using a
conductivity meter (Fisher Scientific accumet basic AP65 Portable Conductivity Meter).
59
The measurements were achieved when the sample solution was carefully adapted to a
definite temperature to avoid any disturbance coming from nonequilibrium states that
arise from temperature shifts.
3.4 Results and Discussion
The use of polymeric surfactant is wide spread both in industry such as stabilizer
of suspensions, and for domestic or personal care purpose [1, 3]. However, their activity
as surfactants starts after micellization. Figure 3.1 shows the mechanism of micelle
formation as an aggregate. The molecules aggregate to form micelles when the surfactant
concentration is above a certain concentration called the critical micelle concentration
(cmc). The non-polar chains come close to each other to form a hydrophopic core in such
a way that the polar ends point towards the aqueous medium. Due to this aggregation
almost all the physical properties change. To investigate the aggregation phenomena, we
measured the viscosity and surface tension for the cmc determination.
60
Non polar chain
Polar group
Aggregate- polymeric surfactant
Unimer- polymeric surfactant
Figure 3.1 A schematic representation of micelles formation by a polymeric surfactant in
aqueous media; unimer-to-aggregate transition.
Figure 3.2 shows the hydrophilic and hydrophobic segments of the structures of
the new anionic polymeric surfactants, based on comb-like polybenzoxazine oligomers.
The olefin backbone with straight chain alkane is the hydrophobic segments and the
carboxylic moieties act as hydrophilic segments distributed throughout the copolymer
backbone, both in the form of a comb-polymer. The hydrophobicity of these surfactants
depends on the alkyl-chain-length.
61
CH 3
CH3
CH3
Hydrophopic
segment
OH
OH
OH
N
N
N
COO- Na +
COO- Na +
COO - Na+
n
poly(4HBA-oca-
Na+
n
poly(4HBA-dea -
)
Na +
)
Hydrophilic
segment
n
poly(4HBA-doa -
Na +
)
Figure 3.2 Representative structures of the three anionic polymeric surfactants
The degree of polymerization for the poly(4HBA-oca- Na+), poly(4HBA-deaNa+) and poly(4HBA-doa- Na+) are: 9, 22, and16, respectively. The surface tension
variation with the concentrations of the anionic polymeric surfactants for the solutions of
poly(4HBA-oca- Na+), poly(4HBA-dea- Na+) and poly(4HBA-doa- Na+) was reported
recently [10]. A summary of their surface activity properties are presented in Table 1.
Table 3.1 Summary of the surface activity properties of poly(4HBA-oca- Na+),
poly(4HBA-dea- Na+) and poly(4HBA-doa- Na+)
Property
poly(4HBA-ocaNa+)
poly(4HBA-dea-
poly(4HBA-doa-
Na+)
Na+)
cmc (g/L)
0.120±0.002
0.123±0.021
0.173±0.004
γcmc (mN/m)
38.04±0.24
38.07±0.85
39.11±0.56
62
Surface tension variations over time measured with the Wilhelmy Plate are shown
in Figure 3.3 for three concentrations of poly(4HBA-doa-Na+). The surface tension
measuring time period is 60 s. At low viscosity with The 0.01 wt% solution has the
lowest viscosity, the liquid film above the meniscus drains fast, and the surface tension is
essentially constant over time. On the contrary, the 0.067 wt% solution has higher liquid
viscosity drains slowly and reaches equilibrium contact angle after a short period thus the
time dependence of the surface tension correlates with the viscosity of the surfactant
solution.
Figure 3.3 Surface tension variation over time for the poly(4HBA-doa- Na+) solutions
with various viscosities.
Figure 3.4 shows the maximum force (Fmax) exerted on the surface of the lamella as
it is removed from the poly(4HBA-oca- Na+) solution just before it breaks is measured for
different concentrations. However, the higher the poly(4HBA-oca- Na+) concentration in
63
the solution the lower the maximum force ( Fmax) by the liquid on the ring. Fmax is directly
proportional to the surface tension.
Figure 3.4 The effect of poly(4HBA-oca- Na+) concentration on both the lamella height
and the maximum force at 21+0.1 oC
Viscosity measurements were made using a capillary glass viscometer or
Ubbelohde viscometer at 30±0.1oC. Viscosity measurements provide valuable information on
the association and conformation behavior of the polymeric surfactant. Figure 3.5 shows the
increase in solution viscosity at different surfactant concentrations chosen to represent values
below the critical micelle concentration (cmc), in the cmc region, and above the cmc. The
variation in viscosity is attributed to the concentration and structural changes in the
micelles. The variation of viscosity with concentration changes in the cmc region. The
solution viscosity depends upon on the number of polymeric surfactant molecules as well as the
64
size and number of micelles. The number of monomers is essentially constant while the micelles
size and number increase above the cmc. During the viscosity measurement, the micelles may
have changed to separated individual molecules and hence it becomes difficult to find the cmc
minima. The drastic increase in viscosity with concentration below the cmc may be due to the
electroviscous effect rather than the hydrophobic segment interaction of the polymeric
surfactants.
0.885
cmc region based on surface tension
Viscosity (mPa.s)
0.875
0.865
0.855
poly(4HBA-doa- Na+)
poly(4HBA-dea- Na+)
0.845
poly(4HBA-oca- Na+)
0.835
0
0.2
0.4
0.6
Concentration (g/L)
0.8
1
1.2
Figure 3.5 The viscosities of poly(4HBA-oca- Na+), poly(4HBA-dea- Na+), and
poly(4HBA-doa- Na+) solutions at different low concentrations, and constant temperature,
30±0.1 oC.
Figure 3.6 shows the effect of the addition of NaCl on the reduced viscosity (ηred) of
poly(4HBA-doa- Na+) solution at a concentration of 0.1g/L. The ηred of poly(4HBA-doa- Na+)
decreases from 0.012 L/g at 0.5 wt% to 0.002 L/g at 4 wt% salt concentration. As expected, the
presence of NaCl affects the ηred of the anionic polymeric surfactant solution because of the
65
reduction in electroviscous effect resulting from the intramolecular repulsive interactions between
ionized groups of the anionic polymeric surfactant molecules [11]. The intrinsic viscosity, [η],
was determined from Huggin’s equation by extrapolation to infinite dilution [12];
𝜂𝑠𝑝
�𝐶 = [𝜂] + 𝑘[𝜂]2 𝐶
𝜂𝑠𝑝 = 𝜂𝑟 − 1
(6)
(7)
Where; ηsp and ηr are the specific viscosity and the relative viscosity of the
polybenzoxazine surfactrant solution, k is the Huggins’ constant, and C is the polybenzoxazine
surfactrant solution concentration in g/L. The experiments were performed in concentrations
of polymeric surfactant solutions lower than the cmc, and the flow time used in all
subsequent calculations of [η] was the average of at least four readings which agreed to
within ±0.5 second. The intrinsic viscosities were found to increase slightly with
increasing the alkyl chain lengths of the polybenzoxazine surfactants; [η] of poly(4HBAoca- Na+) =2.8 L/g, [η] of poly(4HBA-dec- Na+) =3.1 L/g, and [η] of poly(4HBA-doaNa+) =3.2 L/g, Figure 3.7. Hence, the increase in [η] with increasing chain length can be
attributed mainly to longer hydrophobic segment due to change in the polymeric
surfactant structure. This explains the modest increase of viscosity with the chain length,
particularly at high surfactant concentration.
66
ηred (L/g)
0.02
0.01
0.00
0
1
2
3
4
NaCl wt%
Figure 3.6 Variation of reduced viscosity of 0.1g/L poly(4HBA-doa- Na+) solution with added
NaCl.
35
poly(4HBA-oca-Na+)
Intrinsic viscosity (dL/gm)
30
poly(4HBA-dec-Na+)
25
poly(4HBA-doa-Na+)
20
15
10
5
0
0
0.002
0.004
0.006
0.008
Concentration (gm/dL)
0.01
0.012
Figure 3.7 Kraemer plot (intrinsic viscosity =(ln relative viscosity)/concentration vs.
concentration) for poly(4HBA-oca- Na+), poly(4HBA-dec- Na+) and poly(4HBA-doaNa+) solutions at different concentrations and 30±0.1 oC.
67
The presence of NaCl at a fixed temperature lowered the cmc of the polybenzoxazine
surfactant solutions compared to the salt-free solutions at 23±0.1oC. Figure 3.8 illustrates that the
presence of salt reducing the surface tension and the cmc of the polybenzoxazine surfactant
solutions. The γcmc approached an asymptotic value 1 wt% in case of poly(4HBA-doa- Na+)
solution. However, the γcmc of the poly(4HBA-oca Na ) solution gradually decreased from 37
-
+
mN/m without NaCl to 29 mN/m at 3 wt% of NaCl. The decrease in cmc value observed with
increasing salinity from zero to 3 wt% of NaCl. The mechanism of surface tension reduction is
due to the increase in diffusion of surfactant from bulk to the air-liquid interface by the electrolyte
[13]. The effect can be observed at very low polymeric surfactant concentrations. Adding salt
tends to screen the electrostatic repulsion between the ionic head groups of the
amphiphiles, and make the amphiphiles effectively more hydrophobic. It then increases
hydrophobic interactions among the surfactant monomers and cause them to aggregate at
lower concentration, thus the cmc decreases [14,15].
68
Figure 3.8 The effect of salinity on the cmc and the surface tension at cmc (γcmc) of the solutions
of poly(4HBA-oca- Na+) (star), poly(4HBA-dea- Na+) (square), and poly(4HBA-doa- Na+)
(triangle).
69
To investigate the effect of temperature, the surface tension was measured as a
function of surfactant concentration at three different temperatures, 21, 38, and 48 oC, as
shown in Figure 3.9. The cmc values and corresponding surface tension γcmc are plotted
in Figure 3.10. The cmc and γcmc decreases with increasing temperature for poly(4HBAoca- Na+). This decrease in cmc due to the reason that the hydrophobic effect increases in
strength as the temperature is raised [16], resulting attractive interaction of the nonpolar
segments which aggregate to form a hydrophobic core. The same behavior is expected
from the other surfactant, poly(4HBA-dea- Na+) and poly(4HBA-doa- Na+).
Figure 3.9 The variation of surface tension as a function of poly(4HBA-oca- Na+)
concentration in aqueous solution at different temperatures (21, 38, and 48 oC).
70
Figure 3.10 The variation of cmc and γcmc as a function of temperature for poly(4HBAoca- Na+).
The Gibbs energy of micellization for univalent ionic surfactants can be calculated from
the mass action law model [17] according to:
∆𝑚𝑖𝑐 𝐺 𝑜 = (1 + 𝛽)𝑅𝑇 ln 𝑋𝑐𝑚𝑐
(3)
Where β is the fraction of charges of micellized univalent surfactant ions neutralized by
micelle-bound univalent counterions and Xcmc is the surfactant cmc expressed in mol/dm3
or surfactant molar fraction units.
The variation of the cmc with temperature allows for the determination of the enthalpy of
micellization according to:
∆𝑚𝑖𝑐 𝐻 𝑜 = (1 + 𝛽)𝑅 �
𝑑 ln 𝑋𝑐𝑚𝑐
�
𝑑(1�𝑇)
𝑃
(4)
and thus the entropy of micellization,
∆𝑚𝑖𝑐 𝐻 𝑜 − ∆𝑚𝑖𝑐 𝐺 𝑜
∆𝑚𝑖𝑐 𝑆 =
𝑇
𝑜
71
(5)
The mass action law model assumes that micelles comprised of n surfactant molecules
are formed via the reaction
nS
+
surfactant ions
mG
= (SnGm)z =
counterion
Mz
micelle
in which z is the charge or the valence of the micelles.
However,
𝑚
1
𝛽 = � 𝑛 − 𝑛� [18]
In this study, the anionic surfactants ions with univalent counterion are considered 1-1
electrolytes. So, m=n then β=0. Therefore, the values of ΔmicG° have been calculated
using these cmc’s and β = 0.
The thermodynamic parameters of micellization ΔmicG°, ΔmicH° and ΔmicS° were
determined using Eqs. (3)–(5), and are shown in Figure 3.11. The negative values of
standard Gibbs energy change indicate spontaneous micellization; ΔmicG° remains
approximately constant over the studied temperature range. The observed ΔmicS° values are
positive, meaning that the entropy change is favorable to the formation of the micelles. The
positive ΔmicS° values are due to the destruction of ordered hydrogen bonded water structure in
the vicinity of the hydrophobic chain [19]. So, the large gain in entropy occurs when water
molecules in hydration shells around the hydrophobic parts are released during micellization. In
addition, Figure 3.12 shows the micellization of anionic surfactants is endothermic, and in
the temperature range of 21- 48°C the ΔmicH°> 0. This endothermic behavior is a result
from the sum of the enthalpy of association between the hydrocarbon tails (ΔHass ≤ 0),
their dehydration, (ΔHdesolvation ≥ 0), and the repulsion between the hydrophilic
headgroups, (ΔHhead ≥ 0), which is of particular importance in the case of charged
amphiphiles [20]. Since the sum is positive, the repulsion between the headgroups and
72
the desolvation of the hydrophobic tails outweigh the exothermic enthalpy of association
between the hydrocarbon tails due to the favourable chain-chain attraction. Moreover, the
enthalpic (ΔmicH°) and entropic (−TΔmicS°) terms, Figure 3.11, indicate that the
micellization is entropically-driven, and the entropy term plays the dominant role in the
negative free energy ΔmicG°. The entropy change of micellization process is always
positive over the whole temperature range. The increase in entropy of micellization in an
aqueous medium can be attributed to the increase in entropy of the hydrophobic chain of
surfactant molecules when the surfactant molecules are removed from the aqueous
medium to the micelle. The driving force is the tendency of the hydrophobic group of the
surfactant to transfer from aqueous bulk phase to non-aqueous micellar interior.
According to equation (5) the ΔmicH° and ΔmicS° has an opposite effect on ΔmicG°. Figure
11 shows enthalpy-entropy compensation in the micellization process at the experimental
temperatures. It is interesting to observe a good linearity in the entropy and enthalpy
compensation plot for thermodynamic measurements [21]. The compensation
phenomenon between ΔmicH° and ΔmicS° can be described usually as follows:
∆𝑚𝑖𝑐 𝐻 𝑜 = ∆𝑚𝑖𝑐 𝐻 ∗ − 𝑇𝑐 ∆𝑚𝑖𝑐 𝑆 𝑜
(9)
Where Tc is the slope of ΔmicH° vs ΔmicS° plot and has a dimension of temperature
which is referred to as the compensation temperature. The compensation temperature is
proposed to be characteristic of solute-solvent interaction [22], which is considered as a
measure of the ‘desolvation’ part, i.e., the dehydration of the hydrocarbon tail of
surfactant molecules, [23]. Figure 12 yields the compensation temperature (Tc) of
73
poly(4HBA-oca- Na+) is ~ 310 K, which is quite consistent with the compensation
temperature ~ 315 K for ionic surfactants in aqueous solution [24]. The intercept ∆𝑚𝑖𝑐 𝐻 ∗
is the measure of solute-solute interaction [24]. From the previous data, a series of
valuable surface properties can be evaluated, such as the surface excess concentration at
saturation, 𝛤𝑚𝑎𝑥 , and surface area per molecule, ɑs, at the air-liquid interface (Table 3.2).
𝛤𝑚𝑎𝑥 , and ɑs were calculated by use of Equations 1 and 2. 𝛤𝑚𝑎𝑥 is a useful measure of the
effectiveness of adsorption of the surfactant at the air-water interface, and ɑs provides
information on the degree of packing and the orientation of the adsorbed surfactant
molecule when compared with the dimensions of the molecule. Table 3.2, shows that
𝛤𝑚𝑎𝑥 increases with temperature increase while ɑs decreases. This trend indicates that
with increasing temperature, more surfactant molecules are adsorbed on the saturated
surface, resulting in a higher packing density and lower γcmc.
74
30
ΔmicG˚, ΔmicH˚, -TΔmicS˚/KJ.mol-1
ΔmicG˚
ΔmicH˚
R² = 0.9923
-TΔmicS˚
10
-10
R² = 0.9937
-30
R² = 0.983
-50
290
295
300
305
310
315
320
325
T/K
Figure 3.11 Thermodynamic parameters of micellization for poly(4HBA-oca- Na+).
ΔmicH°/KJ.mol-1
20
21 oC
R² = 1
15
38 oC
10
5
48 oC
0
0.08
0.1
0.12
ΔmicS°/KJ.K-1.mol-1
0.14
Figure 3.12 ΔmicH° vs. ΔmicS° plot for poly(4HBA-oca- Na+).
75
0.16
Table 3.2 The cmc, 𝛤𝑚𝑎𝑥 , minimum surface area per molecule, ∆𝑚𝑖𝑐 𝐺 𝑜 for poly(4HBA-
oca- Na+) at different temperatures.
ɑs/nm2
5.7
𝛤𝑚𝑎𝑥 /µmol.m-2
2.05
0.81
∆𝑚𝑖𝑐 𝐺 𝑜 /KJ.mol-1
38
4.0
2.17
0.76
-26.2
48
3.7
2.57
0.64
-27.2
T/oC
cmc×105/(mol/L)
21
76
-23.8
Figure 3.13 Hydrodynamic diameter intensity of poly(4HBA-oca- Na+) micelles in
aqueous solution at 25 °C measured by DLS at concentration of 0.5 g/L, and scattering
angles of 60 o (top), 90 o (middle) and 130 o (bottom).
77
Dynamic light scattering (DLS) techniques are powerful tools for obtaining
information on conformation of polymer morphology in a dilute solution [23].To confirm
the spherical shape of poly(4HBA-oca- Na+) micelles in aqueous solution, the DLS was
carried out at different angles to measure the hydrodynamic diameter (Dh). Figure 3.13
shows the Dh values of 161.2 nm, 146.0 nm and 161.0 nm at scattering angles of 60o, 90o
and 130o, respectively. The micelles are generally spherical and their average diameters
are around 156 nm, the average value of the Dh values. The width of the Dh distribution
indicates that more than one oligomer species is present in solution. Figure 3.14 shows
the hydrodynamic diameter distribution f(Dh) of poly(4HBA-oca- Na+) micelles at 25oC
measured by DLS at concentration of 0.5 g/L and scattering angle of 90° in the Dh range
from 50 nm to 500 nm. The Dh of poly(4HBA-oca- Na+) micelles detected at two Dh
values (116 nm and 289 nm). These two peaks with different intensity indicate the
present of two assemblies. Here, the polydispersity index of the poly(4HBA-oca- Na+) is
slightly high, 3.3, reported somewhere else [10]. However, the presence of poly(4HBAoca- Na+) oligomers with different degree of polymerization confirmed by the surface
tension plots that exhibit a shallow minimum attributed to the effect of polydispersity
(Figure 2.8). The poly(4HBA-oca-Na+) of high degree of polymerization aggregated and
produce micelles larger in size.
78
Figure 3.14 Hydrodynamic diameter distributions f(Dh) of poly(4HBA-oca- Na+)
micelles in aqueous solution at 25 °C measured by DLS at concentration of 0.5 g/L, and
scattering angle of 90 oC.
The surfactants present in the aqueous solution adsorbed at the gas/liquid
interface and lower the surface tension. The presence of the surfactant facilitates the
dispersion of gas bubbles in a surfactant solution and reduces the bubble size [24]. The
experimental systems used to study the stability of foams made with aqueous surfactant
solutions against coalescence and characterized by the variation of the foam height with
time or the rate of foam decay. The foam produced with the anionic surfactant sodium
dodecylsulfate (SDS) depends on the SDS concentration, the maximum foam produced
when the SDS micelles are less stable [25]. The stability of foams made with 75 ppm
(0.075 g/L) and 150 ppm (0.15 g/L) anionic polymeric surfactants solutions were studied
at 25 oC by measuring of foam volume decrease during 1min and 5min. The dependence
of the foam volume ratios on time shown by:
79
𝐹𝑜𝑎𝑚 𝑣𝑜𝑙𝑢𝑚𝑒 𝑟𝑎𝑡𝑖𝑜 =
𝐹𝑜𝑎𝑚 𝑣𝑜𝑙𝑢𝑚𝑒 𝑎𝑡 𝑡𝑖𝑚𝑒=0
𝐹𝑜𝑎𝑚 𝑣𝑜𝑙𝑢𝑚𝑒 𝑎𝑡 𝑡𝑖𝑚𝑒=𝑡
........................................ (10)
Figure 3.15 shows the foam volume ratio as measured by the ratio of the height of the
foam initially produced to the foam height measured after 1 min and 5 min. The foam
volume ratios increase with an increase in the chain length. The rate of foam decay
calculated from the following equation:
𝐷𝑒𝑐𝑎𝑦 𝑟𝑎𝑡𝑒 =
Where,
𝑑𝐼
… … … … … … … … … … … … … … … … … … … … … … … … . . (11)
𝑑𝑡
I= Property (Foam volume ratio) and
t=time (minute)
Figure 3.16 shows the rate of foam decay increases as the surfactants chain increase, and
shows that the foam stability measurements are consistent with that of cmc measurement.
For example, the rate of foam decay increases from 0.19, 0.21 and 0.25 at concentrations
75 ppm to 0.21, 0.40 and 0.46 at concentration 150 ppm for poly(4HBA-oca- Na+),
poly(4HBA-dea- Na+) and poly(4HBA-doa- Na+), respectively.
80
Figure 3.15 The dependence of the foam volume ratios on time, 150 ppm (left) and 75
ppm (right).
81
Figure 3.16 The decay rate plot (top) and the surfactants solution foams (bottom) after 1
min, a-poly(4HBA-oca- Na+), b- poly(4HBA-dea- Na+) and c- poly(4HBA-doa- Na+).
It can be concluded that, the surfactant system of low surface tension is more
efficient against coalescence of bubbles and foam collapse, which has the highest foam
height and the lowest foam decay rate.
Determination of the Krafft point, K.P., otherwise known as critical micelle
temperature relies on the measurement of electrical conductivity (κ). The concentration of
82
polybenzoxazine surfactant solutions were 0.05 wt % (0.5 g/L) which is above the cmc of
all the polybenzoxazine surfactants, and in this system, all components are different only
in hydrocarbon chain length and are completely soluble in water. The temperature
dependences of κ is shown in Figure 3.17, which shows that the Kraft Point increases
with alkyl chain length of the polymeric surfactant, as reported in the literature [28].
Thermodynamically, at the Kraft Point, the hydrated solid surfactants, monomers, and
liquid micelles are in equilibrium and the hydrated solid surfactants dissolve into
monomers and transform into liquid micelles. Then, the Kraft Point is known as the
melting temperature of a hydrated solid surfactant [29]. From Figure 3.17, at lowtemperature region, rather low values of κ are observed due to partial dissolution of the
surfactant species. At a certain temperature, illustrated by an arrow in Figure 3.17, the
amount of the dissolved species increases resulting in a sudden change in κ, as shown by
the abrupt rise of the curves. This region of sharp rise in κ designates the Kraft Point of
the surfactant. The values of the Kraft Point for poly(4HBA-oca- Na+), poly(4HBA-deaNa+) and poly(4HBA-doa- Na+) are 4°, 6° and 8°C respectively. Above this point the
polymeric surfactant associates to form liquid micelles, and the conductivity of the
micellar solution increases with temperature depending on the chain length which affects
the size, charge and mobility of the micelles [30].
83
Conductivity (µs/cm)
600
500
400
300
200
poly(4HBA-oca- Na+)
100
poly(4HBA-dea- Na+)
poly(4HBA-doa- Na+)
0
0
5
10
15
20
25
o
Temperature ( C)
30
35
40
Figure 3.17 Temperature dependence of κ in the systems of polymeric surfactant
solutions, 0.05 wt% (poly(4HBA-oca- Na+), poly(4HBA-dea- Na+) and poly(4HBA-doaNa+)). A sharp rise of κ with temperature is due to the Krafft phenomenon, and shows the
Krafft point of the polymeric surfactant species.
84
3.5 Conclusion
In this work, we have investigated the aqueous micellization behavior of the
solutions of three anionic polymeric surfactants based on comb-like polybenzoxaizne
oligomers; poly(4HBA-oca- Na+), poly(4HBA-dea- Na+), and poly(4HBA-doa- Na+).
Upon salt (NaCl) addition, the cmc initially decreases slightly and remains constant at
3wt% NaCl. The chain expansion due to changes in polymeric surfactant structure was
mainly affected on the values of the intrinsic viscosities. However, the electroviscous
effect was investigated by addition of an electrolyte, NaCl. The existence of electrolyte
can suppress the electroviscous effect. The temperature-dependence studies have shown a
linear fall in cmc i.e. as temperature increases cmc decreases of poly(4HBA-oca- Na+), in
the studied temperature range.
poly(4HBA-oca- Na+) shows an enthalpy–entropy
compensation, and the micellization is entropically-driven. The Krafft temperature
increases with alkyl chain length of the polymeric surfactant molecule.
85
3.6 References
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147–148 (2009) 281–299.
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emulsions using polymeric surfactants based on inulin. Adv. Colloid Interface Sci.
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[4]
P. Anton, P. Köberle and A. Laschewsky, Structure and properties of zwitterionic
polysoaps: functionalization by redox-switchable moieties. Prog. Colloid Polym.
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[5]
S. Qutubuddin, C.A. Miller, W.J. Benton and T. Fort, Effects of Polymers,
Electrolyte, and pH on Microemulsion Phase Behavior, Macro- and Microemulsions, ,in: D.O. Shah, Editor, American Chemical Society, Washington, DC
(1985).
[6]
Y. Cao and H. Li, Effect of temperature and pH values on aggregation behavior
of polymeric surfactants in aqueous solution. J. Appl. Polym. Sci. 98 (2005) 945949
[7]
Sinnko J, Martin’s “Physical Pharmacy and Pharmaceutical Sciences,” 5th Ed.,
Lippincott Williams & Wilkins, Baltimore, 2006, Chapter 9.
[8]
Y Chevalier and T Zemb, The structure of micelles and microemulsions. Rep.
Prog. Phys. 53 (1990) 279-371.
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[9]
D.D. Miller, L.J.
Magid and D.F. Evans, Fluorescence quenching in double-
chained surfactants. 2. Experimental results. J. Phys. Chem. 94 (1990) 5921-5930.
[10] R. Mahfud, T. Agag, H. Ishida, S. Shaikh and S. Qutubuddin, Synthesis and
evaluation of novel anionic polymeric surfactants based on polybenzoxazines.
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[11] R. Paker and S.P. Wasik, On the Electroviscous Effect in Dilute Aqueous Solutions
of Ionic Detergents. The Journal of Physical Chemistry 62 (1958) 967-969.
[12] M.L. Huggins, The viscosity of dilute solutions of long-chain molecules. IV.
Dependence on concentration. Journal of the American Chemical Society 64 (1942)
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[13] S. G. Woolfrey, G. M. Banzon and M. J. Groves, The Effect of Sodium Chloride on
the Dynamic Surface Tension of Sodium Dodecyl Sulfate Solutions. J. Colloid
Interface Sci. 112 (1986) 583–587.
[14] R. J. Hunter, Foundations of Colloid Science I Oxford, New York, 1992.
[15] K.R. Lange, 1999. Surfactant: A Practical Handbook. Hanser Gardner Publications,
Inc., Cincinnati, Ohio.
[16] J A Schellman, Temperature, stability, and the hydrophobic interaction. Biophys. J
6(1997) 2960-2964.
[17] R. Zana, Critical Micellization Concentration of Surfactants in Aqueous Solution
and Free Energy of Micellization. Langmuir 12 (1996) 1208-1211.
[18] Hong-Un Kim and Kymng-Hee Lim, Description of Temperature Dependence of
Critical Micelle Concentration, Bull. Korean Chem. Soc. 24 (2003) 1449-1454.
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[19] T. Liu, L.Z. Liu, B. Chu, in: P. Alexandridis, B. Lindman (Eds.), Amphiphilic
Block
Copolymers, Elsevier Science, Amsterdam, 2000, p. 115.
[20] Vincent Ball and Clarisse Maechling, Isothermal Microcalorimetry to Investigate
Non Specific Interactions in Biophysical chemistry. Int. J. Mol. Sci.10 ( 2009)
3283-3315.
[21] S.B. Sulthara, S.G.T. Bhat and A.K. Rakshit, Thermodynamics of micellization of
a non-ionic surfactant Myrj 45: effect of additives. Colloids Surf. 111 (1996) 57-65.
[22] C.V. Krishnan and H.L. Friedman, Studies of hydrophobic bonding in aqueous
alcohols: Enthalpy measurements and model calculations. J Soln Chem 2 (1973)
119-140.
[23] G. Sugihara and M. Hisatomi, Enthalpy–Entropy Compensation Phenomenon
Observed for Different Surfactants in Aqueous Solution. J. Colloid Interface Sci.
219 (1999) 31-36.
[24] Chen L, Lin S Y and Huang C, Temperature dependence of critical micelle
concentration of polyoxyethylenated non-ionic surfactants. Colloids Surf A
Physicochem Eng Asp., 135 (1998) 175-181.
[25] W. Burchard, Static and dynamic light scattering from branched polymers and
biopolymers. Adv. Polym. Sci. 48 (1983) 1–124.
[26] J.C. Lim, E.K. Kang and B.M. Lee, Syntheses and surface active properties of
cationic surfactants having multi ammonium and hydroxyl groups. J Ind Eng
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88
[27] S.-G. Oh and D.O. Shah, Relationship between Micellar Lifetime and Foamability
of Sodium Dodecyl Sulfate and Sodium Dodecyl Sulfate/1-Hexanol Mixtures.
Langmuir 7 (1991) 1316–1318.
[28] T. Tadors, 2005. Applied Surfactants. Principles and. Applications. Wiley-VCH,
Weinheim.
[29] K. Shinoda, S. Hiruta and K. Amava, The heat of solution and of wetting of ionic
surfactants close to the Krafft point. J. Colloid Interface Sci. 21 (1966) 102-106.
[30] H. Hirata, A. Ohira and N. Iimura, Measurements of the Krafft Point of Surfactant
Molecular Complexes: Insights into the Intricacies of “Solubilization”. Langmuir
12 (1996) 6044-6052.
89
Chapter 4
4.
Gemini
(dimeric)
benzoxazine
surfactants:
Synthesis,
characterizations and molecular dynamics simulation of self assembly
90
4.1 Introduction
Dimeric surfactants, also known as gemini surfactants consist of two hydrophilic
segments connected by a spacer and two hydrophobic tails [1,2]. The properties of
dimeric surfactants differ from conventional surfactants that consist of a hydrophilic head
group and a hydrophobic tail. The dimeric surfactants have lower critical micelle
concentration, cmc, and are more to reduce the surface tension of water to values lower
than conventional surfactant. In addition, they exhibit interesting rheological properties
[3]. However, it is believed that the spacer plays an important role in the micellization
behavior. The spacer length and flexibility are essential parameters for determining the
shape of the surfactant aggregate, where the spacer reduces the intramolecular
electrostatic repulsion between head groups, and this leads to micelle formation at low
cmc values in gemini surfactants [4], in this work a new class of anionic dimeric
surfactants based on benzoxazine chemistry developed. The synthesized anionic dimeric
surfactants consist of a linear hydrophobic tail opposing two hydrophilic polar headgroups. The two polar head groups that containing carboxylic show unique feature when
compared to regular dimeric surfactants. The polar head groups are benzene rings with a
negatively charged carboxylate group and a hydroxyl group, connected to amine group
via methylene bridge. The hydrophobic tail connected to the nitrogen atom results in a
symmetric dimer structure [scheme 1]. The spacer consists of the amine and the
methylene bridge known as Mannich bridge. Formation of the Mannich bridge structure
is due to the cationic ring opening polymerization of benzoxazine [5]. Recently,
benzoxazine chemistry has gained immense interest because of the capability of the
benzoxazine to have a great deal of molecular design flexibility compared with ordinary
91
phenolic resins [6]. Hence, it is our interest to originally propose the inclusion property of
an original structure of anionic dimeric surfactant derived from benzoxazine is proposed
and validated in this work. Though several researchers have synthesized amphiphiles
with unique head group topology (e.g., two carboxylates rigidly held on a
dibenzobarrelane skeleton) [7] and examined the relationship between amphiphile
structure and aggregate morphology [8], not enough studies in the literature provide a
molecular level picture of the surfactant aggregates and the self-assembly of these
molecules. It is important to understand the role of the spacer on the dimer benzoxazine
flexibility. The dynamics of polymer backbones can be investigated in great detail on
nanosecond time scales theoretically by all-atom molecular dynamics (MD) simulations
[9]. MD is a powerful technique for studying the micellization behavior [10] and surface
phenomena [11] at the molecular level. In the present study, experimental and
computational studies are done to investigate the amphiphilic nature of the anionic
dimeric benzoxazine surfactant, di(4CaP-oca-Na+). Experimentally, surface tension
measurements are used to investigate the surface activity of di(4CaP-oca-Na+) at the
air/water interface, and to show its ability to form micellar aggregates. Computationally,
atomistic level molecular dynamics (MD) simulations are performed to study the
di(4CaP-oca-Na+) adsorption at air/water interface and to investigate the aggregation
behavior of the di(4CaP-oca-Na+) in aqueous media. So, this paper describes the synthesis
and characterization of di(4CaP-oca-Na+) its surface activity investigated by experiments
and MD simulations.
92
4.2 Experimental method
4.2.1 Materials
3-octyl-3, 4-dihydro-2H-benzo[e][1,3]oxazine-6-carboxylic acid (abbreviated as
4HBA-oca) was prepared as described in the literature [12], and 4-hydroxybenzoic acid
(abbreviated as HBA-COOH) (99%) was used as received from Sigma-Aldrich.
4.2.2Preparation of 3,3’-(octylazanediyl)bis(methylene)bis(4-hydroxybenzoic acid),
or, [4-carboxylphenol-based benzoxazine dimer] [abbreviated as di(4CaP-oca) ]
The model dimer for benzoxazine was reported elsewhere in the literature [13]
and the synthesis was done by following the same procedure. The 4HBA-oca and 4Hydroxybenzoic acid were used as starting monomers. Briefly, in a 100 mL flask were
mixed together 4HBA-oca (10 mmol, 2.43 g) and 4-hydroxybenzoic acid (10 mmol, 1.38
g), 1:1 mol ratio, and heated at 140 oC and solvent-less reaction for 3 hrs. The yellowish
product was removed and used without any further purification.
4.2.3 Ionization of di(4CaP-oca) into di(4CaP-oca- Na+) “Salt formation”
1.0 Gram of dimeric benzoxazine, abbreviated as di(4CaP-oca), was weighed into
50 mL beaker. The designated amount of NaOH (to neutralize all carboxylic acid groups)
was dissolved in 20 mL of deionized water, and then added into the 50 mL beaker of
dimeric benzoxazine. The beaker containing dimeric benzoxazine sample and base
solution was placed in an ultrasonic bath until the solids were dissolved. The solution was
then filtered using a filter paper and cooled to room temperature. The resulting dimer salt
was dried overnight at 60oC in an air circulating oven to a constant weight.
93
4.3 Measurements.
Fourier transform infrared (FTIR) spectroscopic analysis was carried out on a Bomem
Michelson MB100 Spectrophotometer with a deuterated triglycine sulfate detector. After
casting a thin film onto a KBr plate and purging with dry air, coadded spectra of 64 scans
were recorded at a spectral resolution of 4 cm-1. Differential scanning calorimetry (DSC)
was performed with a TA Instruments DSC Model 2920 at a heating rate of 10 oC/min
from 25 to 300 oC and nitrogen flow rate of 62 mL/min; 2mg samples were sealed
between aluminum hermetic pans and lids for all tests. Thermogravimetric analysis
(TGA) was performed with a TA Instruments High Resolution 2950 Thermogravimetric
Analyzer at a heating rate of 10 oC/min from 25 to 800 oC and with nitrogen purge at a
flow rate of 40 mL/min; 5 mg samples were placed in an open platinum crucible for all
tests. Surface tension measurements of aqueous solutions were carried out using a
KRÜSS Tensiometer (K100) using the Wilhelmy platinum plate method. All
measurements were performed at 24±0.1 oC. Reproducibility was checked by frequent
determination of the surface tension of de-ionized distilled water (72–73 mN/m).
4.4 Computational Methods
Molecular dynamics (MD) simulation track the motion of atoms by estimating the
atomic trajectory over time in response to the inter-intra molecular forces. It was used to
elucidate the behavior of dimeric benzoxazine in aqueous media. MD simulations can
provide us with detailed information about a system, such as the structure and dynamic
94
properties. In particular, every atom was explicitly. The simulations were carried out with
the Gromacs software package [14]. The input topology file was generated by the server
TopolGen version 1.1 [15], and was carefully adjusted before using for MD. The
TopolGen is a reliable tool for quickly obtaining topologies using the empirical all-atom
optimized potentials for liquid simulations, OPLS_AA, force field [16]. The SPC/E water
model [17] and the OPLS_AA force field were used to describe the water and the organic
molecules, respectively. Energy minimizations were carried out using steepest descent
algorithm until the maximum force in the system becomes less than 0.001 N/mol (1000
kJ/mol/nm). Using the NVT ensemble (constant number of particles, N, and constant
volume of the system, V, at well defined temperature, T) and the NPT ensemble (constant
number of particles, N, and constant pressure of the system, P, at well defined
temperature, T) the MD simulations were sequentially carried out to equilibrate the
system. First, the simulations were carried out at constant temperature (T=298 K) and constant
volume where the MD trajectories were generated in the NVT ensemble using the Berendsen
thermostat [18]. The NVT MD simulation was performed for 200 ps at 298 K with time
step of 1 fs. Then the pressure was coupled to a semi-isotropic Parinello-Rahman pressure
coupling [19] in other continuous simulations conducted under the NPT ensemble at a
constant pressure of 1 bar. The NPT MD simulations were performed for 200 ps at 1 bar
with time step of 1 fs. Finally, the MD simulations were carried out under conditions that
best mimic the experimental conditions. During the MD simulations; the energies and
other statistical data (coordinates, velocities and forces) are stored every 2000 steps. In
addition, the time step of 1 fs is taken to be constant for all the simulations of this study.
Periodic boundary conditions were applied, in all three directions for all the trajectories,
95
to generate a quasi-infinite solution. A 1.0 nm-cutoff was used for the short-range
interactions. Long-range electrostatic interactions were treated using the particle mesh
Ewald (PME) summation method with a spaced grid of 0.12 nm, and fourth-order Bspline interpolation. For the organic molecules, the bonds containing hydrogen atoms
were constrained by the LINCS algorithm [20] and for water molecules, they were kept
rigid by the SETTLE algorithm [21]. The equations of motion were integrated with a
time step of 1 fs using the Verlet (leapfrog) algorithm. The VMD 1.9.1 [22] viewer was
used to analyze the MD trajectories and inspect the arrangement of surfactant molecules,
both in the bulk water and on the water surface, by capturing images throughout the
trajectories.
4.5 Simulation details:
To simulate the self-assembly of Gemini benzoxazine surfactant an initial simulation box
structure was generated using the Gromacs utility “genbox”. Comparing the effect of the
hydrocarbon spacer length ((CH2)n) and the chain length (m) on the average aggregation
number (N) of a micelle of dimeric surfactant, the Mannich bridge is assumed to be
equivalent to a hydrocarbon spacer of n=4. Therefore, the number of di(4CaP-oca-Na+)
molecules to be simulated was chosen to be 31. This is compatible with the average
aggregation number N=30 for a micelle of dimeric surfactant of n=4 and m=10 [23]. 31
di(4CaP-oca-Na+) molecules were positioned or inserted randomly into a cubic simulation
box of 10×10×10 nm3, and then solvated by adding 32384 SPC/E water molecules into
the box. The simulated di(4CaP-oca-Na+) concentration, 22.006 g/L (0.046 M), is higher
96
than the predicted cmc, 0.270 g/L (5.71×10-4 M), in order to avoid simulations over large
domains that result by extending the box size 200 times to reach the predicted cmc. In
addition, 62 water molecules were randomly replaced by Na+ counterions for system
neutralization by using the Gromacs utility “genion”. In this case the spontaneous selfassembly into a single spherical micelle was observed after 10 ns of simulation. The
simulation was extended for another 10 ns, after the system had reached an equilibrium
state based on the self-assembly and the RDFs, to allow the micelle to relax toward its
equilibrium structure. The last 5 ns of simulation were chosen for analysis. To simulate
the behavior of di(4CaP-oca-Na+) molecules at the air/water interface, a thin water layer
(10×10×4 nm3) was sandwiched between two layers (10×10×2 nm3) containing 20
di(4CaP-oca-Na+) molecules in each layer. The surfactant molecules were initially placed
randomly at the sandwiched water. Further simulations were carried out to investigate the
nature of the hydrophilic segments affected by the electrostatic interactions between the
charged head groups of each di(4CaP-oca-Na+). For example, simulations carried out in
vacuum were compared with the simulations performed in aqueous media to study how
the electrostatic interactions might occur. In other words, the repulsion between the
charged head groups of each di(4CaP-oca-Na+) molecule may either increase the distance
or increase the plane angle between the carboxylate groups in each molecule.
97
4.6 Results and discussions
4.6.1 Synthesis and characterizations
Benzoxazines are typically hydrophobic materials and show limited water
solubility. In this work, the application of benzoxazine chemistry to the field of anionic
Gemini surfactant was studied by observing the surface tension change of water versus
surfactant concentration. The dimeric benzoxazine contain a hydrophobic olefin
backbone with straight chain alkane as the hydrophobic segments and carboxylic
moieties as hydrophilic segments distributed throughout the dimer backbone. These
anionic dimeric surfactant molecules introduced here have an affinity for interfaces, and
are able to reduce the water/air surface tension and form micelles. The benzoxazine dimer
is shown in Scheme 1, and its synthesis followed the published procedure [13].
CH3
CH3
O
N
OH
+
COOH
OH
T(oC), t (min)
COOH
OH
N
COOH
COOH
Scheme 4.1 Synthesis of dimeric benzoxazine 4CaP-oca.
The FTIR spectra of 4CaP-oca at room temperature as illustrated in Figure 1
shows the characteristic absorption bands of benzoxazine structure at 1240 cm-1 due to
the stretching of C─O─C and at 940 cm-1 due to the out-of plane bending vibration of the
98
benzene ring attached to the oxazine ring [24]. Figure 1 also shows the weakening of the
characteristic bands of benzoxazine at 1240, and 940 cm-1. Disappearance of these bands
after polymerizing at 140 oC for 1 hr to 3 hrs indicates ring-opening of benzoxazine
moieties and the formation of benzoxazine dimer.
Ar
COOH
Absorbance
140 oC, 3 hr
140 oC, 1 hr
25 oC, 0 min
3500
3000
2500
2000
1500
Wavenumber (cm-1)
1000
500
Figure 4.1 FTIR spectra of 4HBA-oca+HBZ-COOH (1:1) at 25 oC/0 hr, 140 oC/1 hr and
140 oC/ 3 hrs.
It is well-known that thermally accelerated ring-opening polymerization of 1,3benzoxazines is an autocatalytic, exothermic process having maximum around 200 to 270
o
C depending on the functionalities of the benzoxazines [25]. The ring-opening behavior
of the monomer was examined by DSC. Figure 2 shows the exotherm observed for the
99
di(4CaP-oca) corresponding to the ring opening polymerization. 4HBA-oca showed an
exotherm with an onset at 130 oC and a maximum peak at 184 oC, corresponding to the
polymerization of benzoxazine with a heat of polymerization, ΔH, of 75 J/g. This
exotherm temperature is unusually low in comparison to ordinary benzoxazine monomers
[25] due to the effective catalytic nature of carboxylic acid towards benzoxazine
polymerization [26]. After each heating cycle of 4HBA-oca+HBA-COOH, the exotherm
corresponding to benzoxazine polymerization decreased constantly and almost
disappeared after 3 hrs at 140 oC. Figure 2 shows the DSC thermograms of 4HBAoca+HBA-COOH at different thermal treatments; the heat of polymerization decreased
from 75 J/g to 0 J/g due to heating for 3 hrs at 140 oC, indicating the disappearance of
benzoxazine structure. The endotherm around 218oC is due to the degradative
evaporation of the amine moiety, as discussed in later.
100
Figure 4.2 Dynamic DSC plots for (4HBA-oca) at 25oC, and for (4HBA-oca +
HBZ_COOH) at 140oC after heating for 1 hr and 3 hrs.
The thermal stability of benzoxazine dimer di(4CaP-oca) was analyzed by TGA
as shown in Figure 3. The first weight loss of about 45% observed around 200oC for the
benzoxazine dimer, is attributed to the degradation of linear aliphatic amine as reported in
the literature [12]. The carboxylic acid group of the benzoxazine dimer was neutralized
by adding required amount of NaOH as shown in Scheme 2, and converted to anionic
dimeric benzoxazine surfactant di(4CaP-oca- Na+).
101
Figure 4.3 TGA curves of di(4CaP-oca)
4.6.2 Surface tension experiments
Scheme 4.2 shows the ionized form of the Gemini surfactant and Figure 4.4
illustarates the hydrophilic and the hydrophobic segments of the proposed structure. The
surface tension variation with the concentrations of the anionic dimeric surfactant for the
di(4CaP-oca- Na+) solutions is shown in Figure 4.5. The effect of the dimeric surfactant
on the surface tension can be observed at very low surfactant concentration, where
dimeric surfactant adsorbs at the air–water interface. Least squares regression analysis
was performed to find the best equation for each of the linear portion below the cmc
value (the pre-cmc line) and the portion above the cmc value (the post-cmc line). The
surface activity properties of this anionic dimeric surfactant are summarized in Table 4.1.
However, these values are shown for general comparison only; there are significant
102
differences in the surface tension values below the cmc. This anionic Gemini surfactant
shows better surface activity than sodium dodecylsulfate (SDS); the cmc is an order of
magnitude lower for the dimeric surfactant.
OH
OH
N
COOH
OH
NaOH
COOH
OH
N
+Na -O
C
O
+Na -O
C
O
Scheme 4.2 The proposed dimeric benzoxazine structure of di(4CaP-oca) and its
ionization into the ionized form, di(4CaP-oca- Na+ ).
Figure 4.4 Representative structure of the anionic Gemini surfactant, represented by
CPK model using VMD.
103
Table 4.1 Summary of the surface activity properties of di(4CaP-oca- Na+) compared
with values for anionic monomeric surfactant, SDS, from the literature.
Property
di(4CaP-oca-Na+)
SDSa
cmc (g/L)
0.27
2.36
γcmc (mN/m)
37
39.5
a
reference 27
75
Surface tension (mN/m)
70
65
60
55
50
45
40
35
30
0.000
0.100
0.200
0.300
0.400
0.500
0.600
0.700
Concentration (g/L)
Figure 4.5 Surface tension at different weight concentrations of di(4CaP-oca-Na+) in
water.
104
4.6.3
4.6.3.1
Simulation analysis
Bulk behavior of dimeric benzoxazine surfactant
4.6.3.1.1 Aggregation into spherical micelles Knowing the dynamics of self-assembled
surfactant leads to understanding the aggregation mechanism, and associated interactions
between the surfactant molecules. The spontaneous aggregation of the di(4CaP-oca-Na+)
molecules in aqueous media is illustrated in Figure 4.6. The micelle formation is
identified via direct visual examination using VMD software, which is used to animate
and analyze the trajectory of the MD simulation. Micelles are identified as clusters of
neighboring di(4CaP-oca-Na+) molecules. The simulations were initiated by placing the
system molecules randomly in the simulation box. To avoid the exposure of the
hydrophobic region to the aqueous surrounding, the molecules tend to rapidly aggregate
into unstable small micelles, observed in 2 ns that coalesce to form a large micelle,
observed in 10 ns. The hydrophobic segment of the di(4CaP-oca-Na+) molecules points
toward the center of the hydrophobic micellar interior, whereas its hydrophilic segments
faces the aqueous phase. The self-assembly is opposed by electrostatic repulsions
between the charged head groups as they come closer to each other [28]. The aggregation
process may be divided into three periods; first period (0-3 ns) for single surfactants and
small clusters, second period (3-10 ns) for intermediate and large loose clusters, and
finally the third period (10-16 ns) for compact micelles.
105
Figure 4.6 Spontaneous aggregation of di(4CaP-oca-Na+) into a micelle. Snapshots of the
simulation at the start (t=0 ns), intermediate (t=3- 10 ns), micelle and single molecule
stage (t=10- 16 ns), and micelle (16- 20 ns) are shown. Water molecules are omitted for
clarity and the black points represents the Na ions.
4.6.3.1.2
Radial distribution function (RDF) analysis The radial distribution
function (RDF) provides more intimate understanding of the packing as it measures how
the density of some material varies as the distance from another species increases
106
𝑔𝐴𝐵 (𝑟) =
〈𝜌𝐵 (𝑟)〉
�〈𝜌 〉
𝐵 𝑙𝑜𝑐𝑎𝑙
𝑔𝐴𝐵 (𝑟) represents the probability of finding particle B within the range r + dr around
particle A, where 〈𝜌𝐵 (𝑟)〉 is the density of B at a distance r around A, and 〈𝜌𝐵 〉𝑙𝑜𝑐𝑎𝑙 is
density of B averaged over all sphere around A with radius rmax.
Figure 4.7 shows the RDF between the water molecules (W) and each of the spacer (N),
the polar head-groups (COO) and the hydroxyl functional groups (OH) of the dimeric
benzoxazine surfactant at the simulation period from 15ns to 20ns. The system achieved
equilibrium after 15 ns, and the simulation results for analysis at the last 5 nm are
credible. However, the negatively charged polar heads are the carboxylate group,
essentially COO-. To demonstrate the possible artifacts due to truncating electrostatic
interactions between these charged groups, RDFs between the water molecules and the
carbon atoms in the carboxylate groups were examined [29]. From Figure 4.7, the
calculated RDFs show two peaks, representing two water shells around the spacer [W-N],
polar head [W-COO], and the carboxylate group [W-OH], at radial distances of (0.550
and 0.814 nm), (0.256 and 0.356 nm), and (0.186 and 0.286 nm), respectively. The strong
peaks around 0.256 and 0.356 nm for the [W-COO] correspond to the first and second
solvation shells of the head-groups composed of hydrogen bonded water molecules. Both
the [W-N] and the [W-OH] curves show two small but well-defined peaks, which are the
signature of the solvation shells for the head-groups. Further, the sharp peaks of [WCOO] and [W-OH] are attributed to a close interaction between water molecules and
each of polar heads, and carboxyl groups, Figure 4.7. The water distribution around the
polar head-groups is generally of low intensity and suggests that water molecules
107
penetrate the polar head-groups and tend to reduce the repulsion between hydrophilic
groups and lead to eliminate the polar heads interaction. From the Figures 4.6 and 4.7, it
can be concluded that the dimeric surfactants orient themselves with the polar headgroups spread out to form the surface facing the water shells.
W-COO
1.2
W-N
W-OH
1
RDF
0.8
0.6
0.4
0.2
0
0
0.5
1
1.5
2
2.5
3
3.5
r/nm
Figure 4.7 Radial distribution functions of water relative to the polar heads (W-Co),
spacer (W-N), and carboxylic groups (W-OH) calculated from the MD simulations were
carried out at 298K.
To obtain more insight into the structure of dimeric surfactants, the RDFs of a
polar head group with the surrounding polar head groups [COO-COO] and a spacer with
the surrounding spacers [N-N] were calculated and presented in Figure 4.8a. The [COOCOO] curve shows a small first peak located at 0.526 nm, a prominent second peak at
1.00 nm, and a broad third peak at 1.846 nm. Peaks can be specified knowing the RDF
108
between neighboring head groups [COO-COO] for each di(4CaP-oca-Na+). Figure 4.8b
shows The RDFs between neighboring head groups [COO-COO] for each di(4CaP-ocaNa+) in aqueous media and in gas phase. In case of aqueous media, the [COO-COO]
curve shows a small first peak located at 0.520 nm, and a prominent second peak at 1.00
nm, the peaks suggest two different conformations, as will be explained later.
60
a
[COO-COO]
[N-N]
RDF
40
20
0
0
80
1
2
b
r/nm
3
4
5
[COO-COO]aqueous
[COO-COO]vacuum
RDF
60
40
20
0
0
0.5
1
1.5
r/nm
2
2.5
3
Figure 4.8 The RDFs between the charged head groups represented by the carbon atoms
in the carboxylate groups [COO-COO] and between the spacers [N-N] (a). The RDFs
109
between neighboring head groups [COO-COO] for each di(4CaP-oca-Na+) in aqueous
media and in vacuum (b).
4.6.3.1.3
Conformational analysis: Conformational analysis and the surfactant
conformation give detailed information about the inner structure of surfactant. The spacer
of di(4CaP-oca-Na+) is treated as a chain, including the dihedral angles (θ1, θ2) between
C-C bonds, and the dihedral angles (β1, β2) between C-N bonds, that are arranged
symmetrically as shown in Figure 4.9a. Each bond may take one of three rotational states
(trans, gauche+, and gauche-). Figure 4.9b shows the analysis of the dihedral angle
distribution of spacer in di(4CaP-oca-Na+). In case of aqueous system, the analysis
showed the predominance of gauche conformation around both the C-C bonds and the CN bonds, and the results are summarized in Table 4.1. Table 4.1 presents the fraction of
the three conformers, calculated from the area of each Gaussian with respect to the total
area (Figure 4.9b). The gauche (+,-) conformer has an occurrence of 75.3 %, whereas
24.7 % trans is supposed to be present. This means that 23 di(4CaP-oca-Na+) out of the
total 31 di(4CaP-oca-Na+) in the system favor gauche conformation. Overall, this
suggests that, the dihedral angles β1 and β2 for C-N bonds play a major role in defining a
conformational propensity than the dihedral angles θ1 and θ2 for C-C bonds. This is
attributed to the considerable probability that the dihedral angles β1 and β2 exist in trans
conformation, and the change causes difference in energy, while the dihedral angles θ1
and θ2 are fixed at gauche conformation. The distribution of dihedral angles of the alkyl
chain shown in Figure 4.10 indicates that the tail has relatively larger trans fractions and
are more extended.
110
Figure. 4.9.a. Schematic definitions of the dihedral angles for the spacer.
In order to recognize the effects of intermolecular hydrogen bonding of water
molecules in the high percentage of gauche and electrostatic attraction or repulsion
between neighboring polar heads, MD simulations were carried out in vacuum,
comprising di(4CaP-oca-Na+), and compared with the simulation results of an aqueous
system, comprising di(4CaP-oca-Na+), as shown in Figure 4.9b. Comparison of the
dihedral angle distribution of the two systems gives the extent of match, and this
suggests, according to the [COO-COO] vacuum curve in Figure 4.8b, the existence of a
strong specific intramolecular interaction between the binary head-groups in vacuum. The
first small peak located at 0.774 nm confirms the existence of electrostatic repulsion in
case of the vacuum system, and the same peak shifted to 0.520 nm in case of aqueous
medium, and the low intensity of these peaks represent the low existence of trans
111
conformation. The same figure (Figure 4.8b) shows that water molecules can reduce the
electrostatic repulsion compared to vacuum. The hydration shell created around the polar
heads allows the polar groups to come closer to each other up to 0.520 nm, represented
by the first small peak of the [COO-COO] aqueous curve. In summary, the distribution of
dihedral angles (β1, β2, θ1, θ2) in the di(4CaP-oca-Na+) structure is well explained by
macroscopic analysis of data obtained from atomic molecular simulation. The observed
sharp peaks represent the dominant guache conformation of the spacer structure; the
sharpness is due to the high energy barriers between the trans and gauche conformations
that translates into the high peak found in the dihedral angle distribution. Thus there is no
need to invoke additional interactions to explain the spacer conformations of the dimeric
benzoxazine surfactants.
112
Vacuum system
aqueous media
gauche-
gauche+
trans
Figure 4.9.B Distribution of the gauche and trans dihedral-angles for the spacer at
vaccum system (left) and in aqueous media (right) obtained by various dehidral angle
sampling methods.
Table 4.2 The fractions of the guache and trans spacer conformers in aqueuos media.
gauche+
trans
gauche-
β1
5.84
42.73
51.44
β2
55.83
36.77
7.40
θ1
36.49
9.68
53.83
θ2
54.72
9.19
36.09
Average
38.14
24.67
37.19
113
trans
gauche+
gauche-
Figure 4.10 Distribution of the dihedral angle for the alkyl chain produced by the various
dihedral angle sampling methods
4.6.3.2
4.6.3.2.1
Behavior of dimeric benzoxazine surfactant at air/water interface
Density Profiles: Figure 4.11 shows the calculated mass density profiles,
along the Z axis of the box, for the system investigated by MD simulation. The density
distributions profile of the surfactant molecules (two layers) and water (middle layer) in
the system are symmetric, and the traces correspond to the center of mass position of
each group. The calculated bulk density of water is 1008 kg/m3 agree with the
experimental value of 997 kg/m3 for water [30]. The density profile of water shows a flat
region of bulk water in the middle, approximately from -1.00 nm to 1.00 nm. At the
water/air interfaces, the water density increased slightly up to a maximum value at around
Z ≈ ± 1.5 nm, before gradually decreased from its maximum value to zero at around Z ≈
± 2.5 nm. The small increase in density is attributed to water molecules attracted to the
charged head groups and their counterions [31]. The density distribution of the surfactant
114
molecules show a uniform distribution of surfactant at the interfaces that confirm the well
adsorption of surfactant molecules along the air/water interface resulting in a stable
monolayer [32].
Density (kg/m3)
1000
H2O
di(4CaP-oca-Na+)
500
0
-4
-3
-2
-1
0
nm
1
2
3
4
Figure 4.11 Density profiles of water and di(4CaP-oca-Na+) along the Z axis.
4.6.3.2.2 The geometric shape. In order to obtain a quantitative view on how the
anionic head groups and the alkyl chain of di(4CaP-oca-Na+) are arranged at the air/water
interface, the topology was analyzed during MD trajectories by reporting the average
values of the tilt angles and the spacer bend angle. Figure 4.12.a shows the time
dependencies of the tilt angles of one di(4CaP-oca-Na+) that is depicted in Figure 4.12.b
over a period of 10 ns. The tilt angles α1, α2 and α3 are defined as the angles between the
Z axis and the vectors that are connecting; the N atom and the methyl carbon atom in the
surfactant tail, the N atom and the carbon atom that connected one ring to the spacer, the
115
N atom and the carbon atom that connected the other ring to the spacer, respectively. In
addition, φ measures the degree to which spacer bend. Although tilt angles of the spacer
are observed to fluctuate, the tilt angle of the alkyl chain (α1) appears to show low
fluctuation and yield nearly constant angles around 46˚±5.5˚. The tilt angles of the spacer
(α2, α3) are around (84˚±8.4˚, 112˚±10.2˚), respectively. Their high fluctuation are
possibly resulted by breaking and forming intramolecular hydrogen bonds between the N
atom and the hydrogen atoms (H1, H2) of the hydroxyl groups as shown in Figure 4.12.b. φ is
around 118˚±1.2˚ and shows small fluctuation, this is probably due to the rigidity of the benzene
ring. Figure 4.12.c shows the RDF between N and H of the hydroxyl groups in each
individual di(4CaP-oca-Na+) molecule, the hydrogen atoms were observed at distance of
d(N…HO)= 0.22±0.01 nm from the spacer (N), and the existence of an intramolecular
hydrogen bond of type N…HO was considered. The observed N…HO distance is
significantly higher than the reported N…HO distance for crystal structure of benzoxazine
dimer which is about 0.098 nm by x-ray diffraction of a benzoxazine dimer [33],
although solid-state 1H-NMR analysis reported closer value of 0.196 ± 0.005 nm [34].
Overall, the knowledge obtained from density profile and tilt angle distributions provides
a clearer picture of the di(4CaP-oca-Na+) in the air/water interface, where the dimer polar
heads lie approximately parallel to each other in the planes of the benzene rings, as
shown in Figure 4.12.d.
In summary, the first measurements of the molecular structure; distance,
orientation, bending, and conformational dynamics of anionic dimeric benzoxazine
surfactant molecules are reported. The results provide a detailed picture of the
aggregation of these molecules in aqueous bulk media, the theoretical predictions of the
116
conformational order of the alkyl chain and the spacer in the polar headgroup, and the
orientation of these molecules at the air/water interface. This study demonstrates the
feasibility and value of future investigations in the design of benzoxazine based
surfactants for understanding their behavior in aqueous media and at air/water interface.
a
b
140
Angle (degree)
Z
Ф
α3
α2
α1
100
60
20
0
2
4
6
8
10
ns
c
d
Figure 4.12 The average values of the tilt angles and the spacer bend angle (a) that
characterizess the geometric shape of the surfactant at air/water interface (b). The RDF
between N and H of the hydroxyl groups in each individual di(4CaP-oca-Na+) molecule
(C). Equilibrium MD simulation snapshot (d) showing a dimeric benzoxazine molecule at
117
the air/water interface, Color legend: dark, di(4CaP-oca-Na+) molecule; gray, water
molecule.
118
4.7 Conclusion
In this study, the synthesis and the surface activity characterizations of Gemini
benzoxazine surfactants have been investigated by experiments and molecular dynamics
(MD) simulations. The chemical structure was determined via FT-IR. Ring-opened
structures were also identified. Surface tension measurements clearly show the surface
activity of the dimeric benzoxazine surfactant and micelle formation above the cmc. A
spontaneous self-assembly of the dimeric benzoxazine surfactant in aqueous media was
observed by using MD simulations. The micelles have a randomly distributed
configuration. Detailed results on the structures and dynamics of the aggregates are
analyzed. Two thin shells of water are detected around the hydrophilic segments. There
are mainly two conformations: gauche and trans, present in vacuum and aqueous phases;
the aqueous environment strongly favors the gauche conformation. This investigation
may provide a fundamental approach to predict the behavior of benzoxazine and other
surfactant molecules.
119
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122
Chapter 5
5
Molecular dynamic simulations of self-assembly of amphiphilic
comb-like anionic polybenzoxazines
123
5.1 Introduction
Amphiphilic macromolecules have been a recent focus of both applied science as
well as basic research. When polymeric amphiphiles are dissolved in water, the
molecules spontaneously self-assemble into micelle structures whose geometry and size
depend on the structure [1] and concentration [2] of the amphiphilic molecule, as well as
solution temperature [3], pH [4], ionic strength [4], and other physicochemical
parameters. Many studies have been devoted to the elucidation of micelle structures
important for diverse fundamental and technological applications [5-7]. Various
experimental techniques, such as surface tension, light scattering, and density
measurements, have been employed to characterize the structure, dynamic behavior, and
thermodynamic properties of micellar systems [8]. More recently, molecular dynamics
(MD) simulation studies have generated valuable information complementary to the
experimental results, at the Angstrom level and at time scales in the nanosecondmicrosecond range.
Many simulation studies have been carried out in relation to the self-assembly of
amphiphilic molecules [9- 19]. Smit et al. [9, 10] reported MD simulations of the
spontaneous aggregation of surfactants and analyzed the structure of a water/oil interface
in the presence of micelles. Maillet et al. [11] performed large-scale MD simulations to
investigate the structural and dynamical properties of self-assembled cationic surfactants
in aqueous solution. Goetz et al. [12, 13] carried out MD simulations of coarse-grained
amphiphilic molecules in aqueous solution to investigate the spontaneous self-assembly
into spherical micelles, cylindrical micelles, and bilayers. Khurana et al. [14] studied the
behavior of a series of Gemini surfactants at the air/water interface. Several recent
124
investigations have focused the dynamics of surfactants between micelles, and the fusion
and fission of small micelles [15-17], as well as the sphere-to-rod transition in micellar
structures [18, 19].
In this paper, amphiphilic polybenzoxazines are investigated via molecular dynamics
simulation. The amphiphilic polybenzoxazines contain alkyl chains as hydrophobic
segments, carboxylic moieties attached to phenolic rings as hydrophilic segments, with
the rings connected by Mannich bridges as the backbone. Formation of the Mannich
bridge structure is due to the ring-opening of benzoxazine [20]. Previously, the surface
tension of high-repeat-unit polybenzoxazines (with 9, 18 and 22 repeat units) was
measured by Mahfud et al. [21] as a function of the surfactant concentration. The
amphiphilic polybenzoxazines offer several interesting physiochemical properties, such
low surface tension and low critical micelle concentrations [21, 22]. Our particular
interest is to investigate the self-assembly processes of the amphiphilic polybenzoxazine
molecules and how the micellar shape that develops depends on the molecular size and
concentration.
5.2 Computational Methods
Molecular dynamics (MD) simulation, a method to track the atomic trajectory
over time in response to the inter- and intra-molecular forces, was used to study systems
that consist of amphiphilic polybenzoxazines in aqueous solution. In this study, the
amphiphilic polybenzoxazines are abbreviated as iBnXz, where i represents the number
of repeat units: trimer (i=3), tetramer (i=4), hexamer (i=6), octamer (i=8) and decamer
125
benzoxazine (i=10); these molecules are shown in Table 5.1. Different concentrations of
iBnXz were simulated to examine the effects of both the number of repeat units and the
molecular concentrations on the micellar morphology. The simulated concentrations are
higher than the critical micelle concentrations (cmc’s) (see [21] for the cmc of the i=9
molecule and [23] for the cmc of the i=2 molecule), to allow manageable system sizes to
be examined (for concentrations closer to the cmc, the box size would need to be 200
times larger). The MD simulations provide us with detailed information about a system,
such as the structure and dynamic properties.
Every atom was explicitly represented (including all hydrogen atoms). The
simulations used an all-atom force field for organic molecules, the OPLS_AA force field
[24]. The OPLS_AA force field describes the bonded (bond stretching, bond angle and
dihedral angle) and non-bonded (Lennard-Jones and Columbic) interactions in the
system. The SPC/E water model [25] was used to describe the water molecules.
The simulations were carried out with the Gromacs software package [26]. The
input topology files were generated by TopolGen version 1.1 [27], and carefully adjusted
before use for MD. A specific number (between 5 and 90) of iBnXz molecules were
positioned or inserted randomly into a cubic simulation box of 10×10×10 nm3, and then
solvated by adding specific SPC/E water molecules into the box; all simulations began
with a random distribution of the molecules in a cubic periodic box, generated with the
Gromacs utility “genbox”. In addition, a specific number of water molecules were
randomly replaced by Na+ counterions for system neutralization, using the Gromacs
utility “genion”. As a first step, energy minimizations were carried out using the steepest
126
descent algorithm until the maximum force in the system becomes less than 0.001 N/mol
(1000 kJ/mol/nm). Then, MD simulations in the NVT ensemble (constant number of
particles N, volume V, and temperature, T) and the NPT ensemble (constant N, pressure,
P, and T) were sequentially carried out for 200 ps each to equilibrate the system at T=298
K and P=1 bar. Finally, a 20 ns MD production run was carried out under T=298 K and
P=1 bar, and the last 5 ns of this run were used for analysis.
The simulation details are as follows.
The simulations use the Berendsen
thermostat [28] and the semi-isotropic Parinello-Rahman pressure coupling [29]. The
energies and other statistical data (coordinates, velocities and forces) were stored every
2000 steps during the simulation. The time step of 1 fs was used for all the simulations.
Periodic boundary conditions were applied in all three directions to generate a quasiinfinite solution. A 1.0 nm-cutoff was used for the short-range Lennard-Jones and nonbonded interactions. Long-range electrostatic interactions were treated using the particle
mesh Ewald (PME) summation method with a grid spacing of 0.12 nm, and a fourthorder B-spline interpolation. For the organic molecules, the bonds containing hydrogen
atoms were constrained by the LINCS algorithm [30] and the water molecules were kept
rigid by the SETTLE algorithm [31]. The equations of motion were integrated with the
Verlet (leapfrog) algorithm. The viewer VMD 1.9.1 [32] was used to analyze the MD
trajectories and inspect the arrangement of surfactant molecules in bulk water, by
capturing images throughout the trajectories. The simulations were performed in parallel
on an Ohio Supercomputer Center (OSC) achieving a rate of 60 CPU hours/ns using 8
nodes.
127
Table 5.1 The chemical structures and the snapshots of the used amphiphilic
polybenzoxazines: The first column shows the abbreviated names, the second column
shows the chemical structures using ChemDraw, and the third column shows the
snapshots of the molecules at the minimum energy using VMD molecular viewer.
Name
Chemical structure
3BnXz
OH
OH
OH
N
+
Na -O
4BnXz
C
O
+Na -
N
C
O
OH
O
6BnXz
C
O
+Na -O
C
O
8BnXz
C
O
+Na -O
OH
10BnXz
C
O
C
O
O
+
Na -O
C
+Na -O
C
O
Na -O
O
Na -O
+
Na -O
C
O
C
O
+Na -O
C
O
O
Na -O
+Na -O
C
C
O
Na -O
+Na -O
O
C
O
C
O
+Na -O
N
C
+Na -O
O
O
C
+Na -O
O
C
O
+Na -O
C
OH
N
N
C
+Na -O
N
OH
OH
OH
OH
OH
N
N
C
+Na -O
OH
OH
+
O
N
N
C
+
OH
OH
+Na -O
O
N
C
N
N
OH
+Na -O
OH
OH
OH
N
N
C
+
OH
OH
+
O
N
N
C
OH
N
OH
OH
+Na -O
O
N
N
+Na -O
C
OH
OH
N
Na -O
Na -O
N
OH
+
+
OH
N
+Na -O
VMD snapshot
+Na -O
128
O
OH
N
C
O
+Na -O
OH
N
C
O
+
Na -O
OH
N
C
O
+Na -O
C
O
5.3 Results and discussions
As reported in our previous work (Mahfud et al. 2013) the anionic dimeric
benzoxazines are surface-active molecules that aggregate in aqueous media. Simulations
were carried out at a concentration much higher than the experimental cmc, which is 0.27
g/L for i=2 and 0.12 g/L for i=9, so that micelles would be expected to form. The
simulations began from random initial structures and the spontaneous aggregation of the
amphiphilic polybenzoxazine molecules was observed, forming either spherical or
cylindrical micelles depending on the molecular concentration and size.
5.3.1 Simulations Analysis
Figure 5.1 shows a series of snapshots illustrating the spontaneous aggregation
process at various times (t=0, 1, 2, 4, 8, 16 and 20 ns). The number of the iBnXz
molecules was 30 in each case, corresponding to a concentration of 49.8×10-3 M. The
water molecules are not depicted in Figure 5.1 in order to more clearly show the micelles.
Initially, the configurations of the iBnXz molecules are random and almost all the iBnXz
molecules are isolated. At t=1 ns, small micelle-like clusters have formed in several
positions. As time continues, small micelle-like clusters coalesce into larger structures,
such as spherical micelles for 3BnXz and 4BnXz, and cylindrical micelles for 6BnXz,
8BnXz and 10BnXz. Figure 5.1 shows that the micellar shape depends on the molecule
size (number of repeat units) at constant molarity. This behavior is attributed to the
dependence of the micellar shape on the intensity of the molecular interactions [33], and
in particular the short-range forces [34].
129
time
3BnXz
4BnXz
6BnXz
0 ns
1 ns
2 ns
4 ns
8 ns
16 ns
20 ns
130
8BnXz
10BnXz
Figure 5.1 Snapshots represent the spontaneous aggregation of the amphiphilic
polybenzoxazines (at a concentration of 49.8×10-3 M) into spherical and cylindrical
micelles. Water molecules are omitted for clarity and the black points represents the Na
ions.
The changes in energy during the micellization process were probed. The iBnXziBnXz and the iBnXz-H2O intermolecular potentials are composed of the Coulomb
electrostatic and van der Waals (vdW) contributions. Figures 5.2.a and 5.2.b show the
changes in iBnXz-iBnXz interaction energy that accompany the micellization process.
The changes in the electrostatic interaction are much stronger than the vdW interaction,
and therefore electrostatic interactions play a dominant role in the micelle formation.
Figures 5.3.a and 5.3.b show the iBnXz-H2O interaction energy, where the electrostatic
interactions is again dominant. In addition, Figures 5.2 (a-b) and 5.3 (a-b) show that
electrostatic and vdW interactions are enhanced by increasing the molecule size, mainly
due to the increase of interactions between the hydrocarbon chains.
131
100.0k
10BnXz
Energy (kJ/mol)
80.0k
8BnXz
60.0k
6BnXz
4BnXz
40.0k
3BnXz
20.0k
0.0
0
5000
10000
time (ps)
15000
20000
Figure 5.2.a: iBnXz-iBnXz (Atom_Atom) electrostatic short-range interactions (CoulSR).
-2.0k
3BnXz
Energy (kJ/mol)
-4.0k
4BnXz
-6.0k
-8.0k
6BnXz
-10.0k
8BnXz
-12.0k
10BnXz
-14.0k
0
5000
10000
time (ps)
15000
20000
Figure 5.2.b: iBnXz-iBnXz (Atom_Atom) van der Waals short-range interactions (LJSR).
132
-60.0k
3BnXz
4BnXz
Energy (kJ/mol)
-80.0k
6BnXz
-100.0k
-120.0k
8BnXz
-140.0k
10BnXz
-160.0k
-180.0k
0
5000
10000
time (ps)
15000
20000
Figure 5.3.a: iBnXz-H2O (Atom_Atom) electrostatic short-range interactions (Coul-SR).
0.0
Energy (kJ/mol)
-2.0k
-4.0k
3BnXz
4BnXz
6BnXz
8BnXz
10BnXz
-6.0k
-8.0k
0
5000
10000
time (ps)
15000
20000
Figure 5.3.b: iBnXz-H2O (Atom_Atom) van der Waals short-range interactions (LJ-SR).
133
The cluster size distribution during the simulations was analyzed. Figure 5.4(a
and b) shows the number of clusters and the maximum cluster size through the entire
simulation. It is assumed that two molecules belong to the same cluster if the distance
between their centers of mass is shorter than a cutoff. The clustering algorithm applied to
the MD trajectories was developed by Daura et al. [35]. As previously mentioned, the
iBnXz molecules are initially randomly distributed in the cubic box and unaggregated,
(each molecule is a “cluster”). As the micellization proceeds, the number of clusters
within the simulation box decreases in time. The 6BnXz, 8BnXz and 10BnXz clusters
show high stability compared to the 3BnXz and 4BnXz clusters, as shown in Figure 5.4.a.
This higher stability may be attributed to the fact that molecular interaction increases as
the molecule size increased, as shown in Figures 5.2 and 5.3. In addition, the cluster size
increases with increase in molecule size. The cluster tripled in size when the molecule
size increased from 3BnXz to 10BnXz, as shown in Figure 5.4.b.
134
Figure 5.4: The number of clusters as a function of time (a), and the relative clusters
sizes during the simulations for all the iBnXz molecules based on 3BnXz cluster (b). The
analysis was performed by using 1.2 nm cutoffs to define the interaction between iBnXz
molecules.
The shape of the iBnXz micelles was investigated by the snapshots of typical
configurations, as shown in Figure 5.1. Furthermore, a quantitative analysis, reported by
Salinal et al [36], was used to describe the geometrical shape of the micelles. This
analysis is based on the principal moments of inertia of the micelles. Of particular
relevance is Imax/Imin and the eccentricity (𝐞) = 𝟏 −
𝐈𝐦𝐢𝐧
where Imin is the smallest
�𝐈
𝐚𝐯𝐠
moment of inertia, Imax is the largest moment of inertia, and Iavg is the average value over
135
all three axes. A micelle with Imax/Imin=1 and e=0 has a perfect spherical shape, whereas
Imax/Imin>>1 and e→1 for a cylindrical shape. In Table 5.2, the shape parameters for all
the simulated aggregates are reported. The 3BnXz and 4BnXz micelles are nearly
spherical with small fluctuations [36], and as Imax/Imin gets larger the micelles become less
spherical.
Table 5.2: Structural Properties of the iBnXz clusters formed in the 49.8 mM simulation
systems
iBnXz
Imax/Imin
Iavg /(a.m.u.)/(nm2)
e
3BnXz
1.66±0.24
5.7E104±4.8E103
0.23±0.06
4BnXz
1.38±0.16
2.8E105±7.7E103
0.17±0.06
6BnXz
2.69±0.19
1.2E106±4.3E104
0.48±0.03
8BnXz
4.47±0.57
2.0E106±8.0E104
0.68±0.05
10BnXz
8.76±0.73
2.3E106±1.3E104
0.83±0.01
Figure 5.5 shows the eccentricity as a function of simulation time for the final 5
ns of the production run. Increasing the molecule size from 3 to 10 repeating units leads
to an increase in the eccentricity from 0.23±0.06 to 0.83±0.01, corresponding to a change
from spherical to cylindrical shape. Visual inspections of snapshots, as shown in Figure
5.1, confirm this result.
136
Figure 5.5 Shows the eccentricity of the micelle over the last 5 ns of the simulation.
5.3.2 Surfactant Concentration and Micelle Structure
Micellar growth is influenced by many factors, including the alkyl chain length,
temperature, counter ion, and surfactant concentration [37]. To study the effect of the
iBnXz concentrations on micelle formation kinetics and structure, the concentration was
systematically varied from 8.3 mM to 149.4 mM for each of the molecules studied, as
shown in Figure 6. More specifically, simulations were performed for same size systems
(10 nm3) but with different numbers of molecules. Figure 5.6 shows snapshots from the
simulations. All pictures confirm that the hydrophobic alkyl chains point to the center of
the micelle and the hydrophilic groups arrange to coat the outside, shielding the
hydrophobic core from the surrounding water molecules. It is clear from these results that
larger molecules and higher concentrations act to make cylindrical micelles stable
137
relative to spherical micelles. Figure 5.7(a-b) summarizes the systematic dependence of
the micelle shape on the concentration of the systems. At concentrations below the
dashed line the most efficient packing geometry is a spherical shape. However, at
concentrations above this line the molecules arrange into cylindrical shaped micelles.
Note (from Figure 5.6) that the spherical micelles are not perfect spheres, but more or
less elliptical, and the cylindrical micelles are not perfectly straight, but have diameters
that vary significantly along their axes.
138
System
3BnxZ
4BnXz
6BnXz
A
1
(8.3)
2
(5)
---------------
--------------
-------------
B
1
(16.6)
2
(10)
----------------
8BnXz
10BnXz
(32595)3
(40)4, (1.80)5
(32425)3
(50)4, (2.28)5
(31923)3
(80)4, (3.60)5
(31616)3
(100)4, (4.56)5
(30255)
(186)4, (7.91)5
(29079)3
(248)4, (8.92)5
(28943)3
(260)4,
(11.70)5
----------------
-------------------
-------------------
-----------------
------------------
-----------------
--------------(32291)3
(60)4, (2.63)5
C
1
(49.8)
2
(30)
(31921)3
(90)4, (4.13)5
(31378)3
(120)4, (5.86)5
D
1
(99.6)
2
(60)
(30601)3
(180)4, (8.27)5
E
1
(149.4)
2
(90)
3
(29493)3
(240)4, (11.72)5
---------------(29324)3
(270)4,
(12.40)5
Figure 5.6 Overview of simulations performed: The snapshots at t= 20 ns represent the
effect of both the molecule size and the molecules concentration on the micellization
morphology. The letters in the first row define the systems. The numbers between the
brackets represent the following: 1- milli-molarity (mM), 2- iBnXz molecules, 3- water
molecules, 4- Na ions, 5- wt% of amphiphile. The micelles in the center of the cell
represented as van der Waals spheres; Red spots represent the oxygen atoms, cyan color
represents the carbon and hydrogen atoms, and the nitrogen atoms are blue.
139
(a)
(b)
Figure 5.7 Plots of repeating units, i, in the iBnXz molecules vs. iBnXz concentration
(mM)(a) and iBnXz wt% (b). The snapshots of micellar shape in the iBnXz system after
20 ns represent the spherical and the cylindrical regions. The dashed blue line represents
the micelles shape transfer zone of iBnXz molecules and indicates the maximum
available iBnXz concentrations for spherical micellar shape. The color legend is as
described before.
140
Figure 5.7.a shows that much lower concentrations are needed for the sphere-tocylinder transition as the number of repeat units in the molecule increase. For example,
the transition occurs at ≈ 130 mM for 3BnXz, but at less than 20 mM for 10BnXz. One
reason for this is that the molecules with more repeat units are larger in size. Therefore,
the concentration by weight percent would be a more effective quantity to describe this
transition than molarity (concentration by number of molecules). In Figure 5.7.b the
results are shown in terms of weight percent, and the variation is the spherical-tocylindrical transition point is indeed smaller: a factor of 3 difference between the
transition concentrations for 3BnXz and 10BnXz, rather than a factor of more than 6.
But even when weight percent is used for the concentration, there remains a
dependence of the transition concentration on the number of repeat units in the molecule.
We believe this dependence may be explained by the relative flexibility of the molecules.
The flexibility of the iBnXz molecules was assessed through the end-to-end distance. The
molecule is considered dynamically flexible if its end-to-end distance varies significantly
with time. Figure 5.8.a shows the end-to-end distance of each molecule over the
simulation time (20 ns). It is clear that the small molecules (3BnXz and 4BnXz) are less
flexible than the larger molecules (6BnXz, 8BnXz and 10BnXz). The distributions of the
end-to-end distances have one peak and are approximately Gaussian, as shown in figure
5.8.b. The distributions of the end-to-end distances of the large molecules are broader
than those of the small molecules, because the large molecules are much more flexible.
We believe these flexibility results can help explain the dependence of the
spherical-to-cylindrical transition on molecular size. Molecules with greater flexibility
have greater size fluctuations – the large fluctuations prevent other molecules from
141
coming close, and thus the more flexible molecules effectively take up more space. Thus
the molecules with more repeat units are thus effectively larger than indicated by their
molecular weight. For this reason a lower concentration is needed for the spherical-tocylindrical transition for larger molecules, as found in our results (Figure 5.7.b).
Figure 5.8: (a) the end-to-end distances of the iBnXz backbones (left). The measured
distances represented by the black lines (right). (b) The end-to-end distance distribution.
142
5.4 Conclusion:
Molecular dynamics simulations of amphiphilic polybenzoxazine molecules
(iBnXz) in water have been carried using the OPLS-AA force-field for the iBnXz
molecules and the SPC/E force field for water, where i is the number of repeat units in
the molecule. The study is carried out for trimers, tetramers, hexamers, octamers and
decamers.
Simulations begin with random iBnXz placement, but as the simulation
proceeds the iBnXz molecules self-assemble into micelles; the self-assembly process is
monitored through changes in the intermolecular energy. At a concentration of 49.8 mM,
increasing the molecular size causes the preference of a cylindrical shape over a spherical
shape for the micelles; the shape is identified visually as well as quantitatively based on
the eccentricity (e), which varies from e=0.23 for 3BnXz to e=0.83 for 10BnXz. The
effect of the iBnXz concentrations on micelle shape were studied by systematically
varying the concentration of iBnXz from 8.3 mM to 149.4 mM, and the phase diagram
was obtained that shows where the spherical and cylindrical shaped micelles are stable.
Spherical micelles are stable at lower concentrations, and cylindrical micelles are stable
at higher concentrations. The transition point depends on the molecular size – smaller
molecules have a higher transition concentration than larger molecules. The effective
molecule size depends not only on the molecule’s physical size (related to molecular
weight), but there is also an entropic component related to the flexibility of the molecule
(the more flexible molecule will require more space).
143
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146
Chapter 6
6.
Conclusions and Future Work
147
6. Conclusions and Future work
This chapter is split into three main sections. The first part focuses on the
synthesis of a novel series of anionic polymeric surfactants via benzoxazine ring opening
polymerization, and the influence of the structure, salinity and temperature change on the
surface activity. The second part summarizes the conclusions drawn from the MD
simulations, highlights their micellization morphology and reviews the predicted
behavior of a newly designed family of anionic polybenzoxazine surfactants. The third
part of this chapter suggests some promising directions for future research that this thesis
has opened.
6.1 Surface active anionic polybenzoxazines
This dissertation focused on the synthesis of novel anionic polymeric surfactants
from benzoxazine monomers and correlating the enhancements of surface activity to their
chain length change. In the case of poly(4HBA-oca-Na+) surfactant, octylamine (chain
length C8) reacted with 4-hydroxybenzoic acid and paraformaldehyde to produce 4HBAoca. The 4HBA-oca monomer was thermally polymerized via ring-opening of
benzoxazine to form poly(4HBA-oca). The poly(4HBA-oca) was ionized by using NaOH
to yield poly(4HBA-oca-Na+). The poly(4HBA-dea-Na+) and poly(4HBA-doa-Na+) were
synthesized by using decylamine (chain length C10) and dodecylamine (chain length
C12), respectively. The structure of these compounds was determined via FTIR and 1H
NMR spectroscopy. Ring-opened structures were also identified by using FTIR and DSC.
The 4HBA-oca, 4HBA-dea and 4HBA-doa showed single exothermic peaks at 184 oC,
187 oC and 190 oC, respectively, which are relatively low due to the effective catalytic
148
nature of the carboxylic acid. TGA showed 45% weight loss around 200 oC for the three
benzoxazine polymers, which is attributed to the degradation of linear aliphatic amine.
The branched polybenzoxazines have low number average molecular weight (Mn ~ 22006000) and high polydispersity (Mw/Mn ~ 1.2-3.3).The surfactant chain length affected the
surface activity of anionic polybenzoxazines in the aqueous media and the overall
micellization process. The surface tension measurements revealed transition points
corresponding to the critical micelle concentrations of the polymeric surfactants The cmc
increases from 0.12 g/L to 0.17 g/L with change in alkyl chain length from C8 to C12.
The cmc values at 23±0.1 °C are comparable with literature values reported for polymeric
surfactants.
The effects of electrolyte and temperature change on the properties of anionic
polybenzoxazine surfactants are important specially at low surfactant concentrations.
Upon salt (NaCl) addition, the cmc initially decreases slightly and becomes constant at
3wt% NaCl. The minimum values of the γcmc were obtained 1 wt% NaCl for the
polybenzoxazine surfactant solution. For example, the γcmc of the poly(4HBA-oca- Na+)
solution gradually decreased from 38 mN/m at 0 wt% NaCl to 27 mN/m at 1 wt% NaCl,
and then gradually increased to 30 mN/m at 3 wt% NaCl. Only slight decrease in cmc
value was observed with increasing salinity from 0 to 3 wt% of NaCl for the
polybenzoxazine surfactant. The mechanism of surface tension reduction is due to the
increase in diffusion of the surfactant from bulk to the air-liquid interface by the
electrolyte. The temperature-dependence studies of poly(4HBA-oca- Na+)
show a
linear fall in cmc, i.e. as temperature increases cmc decreases in the temperature range
21 to 48 °C. Poly(4HBA-oca- Na+) shows an enthalpy–entropy compensation, and the
149
micellization is entropically-driven. The Krafft temperature increases with alkyl chain
length of the polymeric surfactant molecule; the values
for poly(4HBA-oca- Na+)
poly(4HBA-dea- Na+) and poly(4HBA-doa- Na+) are 4o, 6oand 8oC, respectively
6.2 MD simulations of surfactants
This thesis also focuses on using MD simulation methods to investigate the
structural and dynamical behavior of the low and the high molecular weight anionic
benzoxazine surfactants in bulk water phase or at the water/air interface. All MD
simulations were carried out by using the OPLS-AA force-field. Due to the complexity of
simulating large size molecules, a small size molecule of an anionic dimeric benzoxazine
surfactant, di(4CaP-oca-Na+), was synthesized and studied by means of MD simulations
in order to understand the main forces and conformational transitions which govern their
self-assembly. Furthermore, five amphiphilic anionic polybenzoxazines (iBnXz; where
the repeating unit i=3, 4, 6, 8 and 10) were investigated via MD simulations. This study is
directly relevant to the experimental work on synthesized anionic amphiphilic
polybenzoxazines.
6.2.1 MD simulations of anionic dimeric benzoxazine
The synthesis and the surface activity characterization of dimeric anionic
benzoxazine surfactant were investigated experimentally and supplemented by MD
simulations. The chemical structure was determined via FT-IR. Ring-opened structures
were also identified. Surface tension measurements clearly show the surface activity of
the dimeric benzoxazine surfactant and micelle formation above the cmc. A spontaneous
150
aggregation of the dimeric benzoxazine surfactant in aqueous media was reported by
using MD simulations, which form a randomly distributed configuration. Detailed results
on the structures and dynamics of the aggregates are analyzed. Two thin shells of water
are detected around the hydrophilic segments. There are mainly two conformations:
gauche and trans, present in the gas and aqueous phases, the aqueous environment
strongly favors the gauche conformation. This investigation provides a theoretical
approach to predict the behavior of di(4CaP-oca-Na+) molecules. In summary, this thesis
presents the first analysis of the molecular structure: distance, orientation, bending, and
conformational dynamics of the anionic dimeric benzoxazine surfactant molecules. The
results provide a detailed picture of: the self-assembly of these molecules in aqueous bulk
media, the predictions on the conformational order of the alkyl chain, the spacer and the
polar head-groups, and the orientation of these molecules at the air/water interface. This
study demonstrates the feasibility and potential value of future investigations in the
design of polymeric surfactants for understanding their behavior in aqueous media and at
air/water interface.
6.2.2 MD simulations of anionic polybenzoxazines
MD simulations of the iBnXz molecules were begun with random iBnXz
placement. At a concentration of 49.8 mM, iBnXz simulation results show that increasing
the molecule size causes a spherical to cylindrical shape transition. The change in micelle
shape is related to an increase in the eccentricity (e); the value of e is equal to 0.23 and
0.83 for 3BnXz and 10BnXz, respectively. The iBnXz surfactants are shown to be
Gaussian with respect to their end-to-end distance, where the 6, 8 and 10BnXz are much
151
more flexible than the 3 and 4BnXz molecules. The 6, 8 and 10BnXz molecules tend to
exhibit larger van der Waals and electrostatic inter/intra-molecular interactions compared
to the 3, 4BnXz molecules; increasing molecule size enhances the hydrophobic
interactions between the hydrocarbon chains, and the larger molecules experience larger
attractive force. The effect of iBnXz concentration on micelle morphology was studied
by systematically varying the concentration of 3, 4, 6, 8 and 10BnXz from 8.3 mM to
149.4 mM, and the regions of spherical and cylindrical shape micelles were defined. The
theoretical physical radius of the 9BnXz micelle was 1.356 nm, complementary to the
experimental data, 1.364 nm.
6.3 Future Work
A comparison of the cmc and γcmc of the new anionic polymeric surfactants with
literature values for both low and high molecular weight surfactants are made to justify
that the polybenzoxazines offer a superior alternative to conventional surfactants.
Furthermore, the polymeric surfactants reported herein are thermally stable up to about
170 oC and do not contain sulfur, and therefore are more ecofriendly than many
commercial surfactants. So, the focus for future research should be on controlling the
repeating units of the amphiphilic anionic polybenzoxazines because it plays a crucial
role in surface activities. Moreover, the head groups can be anionic, cationic,
zwitterionic, or nonionic. The negatively charged group can be carboxylate, -CO2, sulfate, SO42− or sulfonate, -SO3-. The carboxylic head group can be replaced by sulfate
head group by using 4-Hydroxyphenyl hydrogen sulfate instead of 4-Hydroxybenzoic
acid, in order to examine the effect of increasing the hydrophilicity of the charged head
152
groups. If the produced surfactant is more efficient in decreasing the surface tension and
reducing the cmc, then the sulfate based surfactant provides another opportunity for
fundamental research. However, this work clearly demonstrates that variation in the alkyl
chain length of the anionic polybenzoxazine surfactants has a significant effect on
reducing the water/air surface tension and decreasing the cmc.
Potential avenues for future research based on this study are extensive. The
possible ideas presented here are greatly enriched by suggestion, but by no means are
fairly exhaustive. All-atom MD simulations can performed to explore many aspects of
self-assembly in detail. This work shows clearly the usefulness of using MD simulations
at an all-atom level, as it provided detailed information about the micelle morphology,
the structural and conformational analysis, the inter/intra-molecular interactions, etc.
Research points to possible new route to modify the surfactants based benzoxazine might
have some capability to suggest the surfactant structure. The illustration of the energetic
and structural determinants of amphiphilic polybenzoxazine aggregation poses an
essential challenge to synthesis and physical studies of macromolecular compounds and
still needs to rely on the study of simplified model systems. Although the OPLS-AA
model still remains simple, it presents a model that will lead to the determination of
reliable structural models for amphiphilic polybenzoxazines. Moreover, concepts and
insight from theoretical and simulation studies help to describe and understand polymeric
surfactant behavior at a molecular level.
153
Appendix
i.
The Molecular Dynamics Algorithm
The molecular systems are described by the Optimized Potentials for Liquid
Simulations- All Atom (OPLS-AA) force field [1]. This force field contains terms for
following interactions:
𝑉𝑡𝑜𝑡𝑎𝑙 = 𝑉𝑏𝑜𝑛𝑑𝑒𝑑 + 𝑉𝑛𝑜𝑛𝑏𝑜𝑛𝑑𝑒𝑑
𝑉𝑏𝑜𝑛𝑑𝑒𝑑 = 𝑉𝑏𝑜𝑛𝑑𝑠 + 𝑉𝑎𝑛𝑔𝑙𝑒𝑠 + 𝑉𝑑𝑖ℎ𝑒𝑑𝑟𝑎𝑙𝑠
𝑉𝑏𝑜𝑛𝑑𝑠 = � 𝐾𝑟 (𝑟 − 𝑟0 )2
𝑏𝑜𝑛𝑑𝑠
𝑉𝑎𝑛𝑔𝑙𝑒𝑠 = � 𝐾𝜃 (𝜃 − 𝜃0 )2
𝑎𝑛𝑔𝑙𝑒𝑠
Here the subscripts 0 are used to denote the equilibrium values of the bond length r and
angle𝜃.
𝑉𝑑𝑖ℎ𝑒𝑑𝑟𝑎𝑙𝑠
𝑉1𝑖
𝑉2 𝑖
[1 − 𝑐𝑜𝑠2(∅𝑖 − 𝑓2𝑖 )]
= � � [1 + cos(∅𝑖 − 𝑓1𝑖 )] +
2
2
𝑖
𝑉3 𝑖
[1 + 𝑐𝑜𝑠3(∅𝑖 − 𝑓3𝑖 )]�
+
2
Where ∅𝑖 is the dihedral angle, V1, V2, and V3 are the coefficients in the Fourier series,
and f1, f2, and f3 are phase angles.
𝑉𝑛𝑜𝑛𝑏𝑜𝑛𝑑𝑒𝑑
𝑎
𝑏
𝑖
𝑗
𝜎𝑖𝑗12 𝜎𝑖𝑗6
𝑞𝑖 𝑞𝑗 𝑒 2
= � �[
+ 4𝜖𝑖𝑗 � 12 − 6 �]𝑓𝑖𝑗
𝑟𝑖𝑗
𝑟𝑖𝑗
𝑟𝑖𝑗
154
The non bonded interactions between two molecules 𝑎 𝑎𝑛𝑑 𝑏 are represented by the
1/2
𝜎
Columbs and Lennard-Jones where: 𝑟𝑖𝑗 is atom i-atom j distance; 𝜎𝑖𝑗 = � 𝑖𝑖�𝜎𝑗𝑗 � and
1/2
𝜖
𝜖𝑖𝑗 = � 𝑖𝑖�𝜖𝑗𝑗 � , 𝜎𝑖𝑗 is the arithmetic mean for the unlike size parameter motivated by
collisions of hard spheres (atom i and atom j), and 𝜖𝑖𝑗 is the geometric mean for the
unlike energy parameter.
The same expression is used for intramolecular non bonded interactions between all pairs
of atoms (i < j) separated by three or more bonds; 1,4 interactions are scaled down by the
"fudge factor" 𝑓𝑖𝑗= 0.5 , otherwise 𝑓𝑖𝑗= 1.0. All the interaction sites are centered on the
atoms; there are no "lone pairs". The OPLS-AA parameters are supported by GROMACS
[2].
ii.
Integrating the equations of motion
The integration in GROMACS is performed using the leap-frog algorithm [3] and
can be summarized in the following scheme;
𝑉�𝑡 + ∆𝑡2� = 𝑉�𝑡 − ∆𝑡2� +
𝐹(𝑡)
𝑚
∆𝑡
𝑟�𝑡 + ∆𝑡2� = 𝑟(𝑡) + 𝑉(𝑡 + ∆𝑡2)∆𝑡
Here ∆𝑡 denotes time step, r(t) the particles coordinate vector, and V(t) respective
velocities.
155
iii.
Berendsen temperature and pressure coupling
To maintain the temperature, the system is coupled to an external heat bath with
fixed Temperature T0 [4]. The temperature of the system is corrected such that the
deviation exponentially decays with some time constant, 𝜏;
𝑑𝑇(𝑡) 1
= �𝑇𝑜 − 𝑇(𝑡)�
𝜏
𝑑𝑡
∆𝑇 =
𝛿𝑡
(𝑇 − 𝑇(𝑡))
𝜏 𝑜
λ2 = 1 +
𝛿𝑡
𝜏
�
𝑇𝑜
𝑇(𝑡−𝛿𝑡
)
2
− 1� , where 𝜆2 is the scaling factor for the velocities.
To maintain the pressure, the system is made to obey the equation of motion at the
beginning of each time step [4];
𝑑𝑃(𝑡)
𝑑𝑡
1
= 𝜏 �𝑃𝑜 − 𝑃(𝑡)�
Where P(t) is the instantaneous pressure, Po is the desired pressure, and τ is the barostat
relaxation time constant.
At each time step the MD cell volume and the cell vector are scaled by 𝜂 𝑎𝑛𝑑 𝜂1/3, respectively.
𝜂(𝑡) = 1 −
Δ𝑡
𝜏
𝛾(𝑃𝑜 − 𝑃(𝑡))
Where 𝛾 is the isothermal compressibility of the system.
156
iv.
Trajectory Analysis
The trajectory files were analyzed through g_rdf, g_angle, g_dist, g_gyrate, g_rms
and g_energy GROMACS utilities in order to obtain the radial distribution function
(RDF), dihedral angle analysis, atom-atom distance, radius of gyration, root mean square
deviation (RMSD) and interaction energies, respectively. Furthermore, the VMD (Visual
Molecular Dynamics) was used to animate and analyze the trajectory of MD simulations,
by displaying and animating the molecules undergoing simulation on a remote computer.
v.
References
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157
Chapter 7
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