Use of Room Temperature Ionic Liquids as Supporting
Electrolytes for Voltammetric Measurements in Vegetable Oils
by Platinum Microelectrodes
M.Antonietta Baldo,Alvise Perosa,
Salvatore Daniele
University of Venice, S. Marta 2137,
30123 Venice, Italy, [email protected]
Paolo Oliveri
University of Genoa, via B. Salerno 13,
16147 Genoa, Italy, [email protected]
RTILs AS ELECTROLYTES
Room temperature ionic liquids (RTILs) have recently gained
WHY VEGETABLE OILS?
An important advantage of many RTILs:
increasing interest for electrochemical applications, as they
exhibit a range od advantages over organic solvents which are
traditionally employed in electrochemistry [1-3].
a
such media, whose analytical characterization is of great interest in
olive oil marketing, are normally inaccessible to direct voltammetric
measurements (unless they are mixed with high quantities of suitable
high level of solubility in organic solvents [1]
organic solvents, as acetonitrile, or aqueous solutions+surfactants to
provide emulsified media)
RTILs can provide a suitable level of conductivity
also in low dielectric organic solvents, where
standard electrolytes are often not very soluble
Advantageous General Properties of RTILs:
negligible vapor pressure,
pressure, high chemical and termal stability
wide electrochemical stability,
stability, high conductivity
Research focus
Preliminary investigation on the cyclic voltammetric behaviour at a Pt
microdisk electrode of the ionic liquids
IN THIS WORK
low toxicity
we propose the use of some tetraalkylphosphoniumand tetraalkylammonium-based ionic liquids
as supporting electrolytes to perform
voltammetric measurements at microelectrodes
in olive and other vegetable oils
ability
ability to dissolve a large variety of organic and inorganic
compounds,, either polar or nonpolar
compounds
A drawback of RTILs in electrochemistry:
high viscosity ( which strongly depends on the nature
of the anion and cation)
Evaluation of the most suitable oil/RTIL mixture composition
Investigation on the I/E responses obtained for the oil/RTIL mixtures
and evaluation of their usefulness as “fingerprint” of the samples
Application of Principal Component Analysis (PCA) chemometric method
to CV signals, to recognize and differentiate olive oils according to
their quality and geographical origin.
EXPERIMENTAL AND RESULTS
Samples
Measuring oil samples:: 0.5-1 mL of
MICROELECTRODE
oil/RTIL mixture, previously sonicated
for about 3 min, then settled for 15 min
before the voltammetric measurement SAMPLE
Voltammetric technique:: Cyclic
Voltammetry (CV) at low scan rate
CELL: glass holder,
holder, 2 mL
- 10
-150
[TOMP]+[Cl]-
I ( µΑ)
v= 10 mV/s
0
- 1.5
-1 .0
-0 .5
-150
[TBMA]+[Cl]-
-300
40
IFc values recorded in RTILs are
much lower than those in acetonitrile
(ACN), in agreement with a relevable
decrease of D in the RTILs (DRTIL)
Fc in ACN+0.2M LiClO4
5
400
0
0 .0
-0.2
200
E (V )/ v s A g P S R e
150
a
Ionic liquid in the presence of air
after bubbling N2, 5 min
after bubbling N2, 10 min
0.0
0.4
0.6
+
-
Fc in [TOMP] [Cl]
+
Fc in [TOMA] [Cl]
0
400
600
800
1000
E (V) / vs Ag PSRe
0
[TOMA]+[Cl]-
-20
-2
-1
0
1
v= 10 mV/s
From the experimental I, D values
in the different media are calculated
VOLTAMMETRIC BEHAVIOUR OF OIL/TETDP MIXTURES
the dinamic viscosity η of the
RTILs are determined, by applying:
CV responses recorded in 2 different vegetable oils:: MAIZE and OLIVE
E (V) / vs Ag PSRe
v= 50 mV/s
A widely accessible electrochemical window is
observed (in the presence of Cl- ions, the anodic limit
I/nA
RTIL
occurs at less positive potentials, E= +1.7, +1.8V)
(±0.002)
[TETDP]+[decan]-
Cathodic wave at -0.5≤E≤-1V: due to the reduction
process of O2 present in the air-saturated ionic liquids,
as verified by bubbling N2 for different times
D/cm2s-1
η/cP
0.694
1.6*10-7
57
[TOMP]+[Cl]-
0.279
6.6*10-8
142
[TBMA]+[Cl]-
0.148
3.5*10-8
268
ACN
8.70
2.4*10-5
D=
kT
6πηrh
0
0
-20
DACN ηRTIL
=
DRTIL ηACN
-4
-8
-12
-40
Pure maize oil
Maize oil+TETDP 0.1M
Pure TETDP
-60
-80
Pure olive oil
Olive oil+TETDP 0.1M
Pure TETDP
-16
-20
-24
-28
-32
-100
-2
-1
0
1
-2
2
E (V) / vs Ag PSRe
-1
0
1
2
E (V) / vs Ag PSRe
v =50 mV/s
0.39
As expected, pure oils do not give any evaluable voltammetric response
The addition of [TETDP]+ based ionic liquid to the oil matrices provides a
well-defined CV response, which differs from I/E profile of pure ionic liquid.
APPLICATION OF PCA TO CV SIGNALS
DIFFERENTIATION BETWEEN
MAIZE AND OLIVE OILS
DIFFERENTIATION AMONG EXTRAVIRGIN OLIVE OILS
Examples of CV responses obtained in 10 extravirgin
olive oils coming from Italy (Liguria, cultivar Taggiasca)
and Spain (cultivar Arbequina)
CV
………concluding
PCA
DATA MATRIX: 30 objects (3 replicates x 1 oilTETDP mixture x 10 oil samples) x 2622 variables
(currents points at different potentials)
DATA PREPROCESSING: column autoscaling
PC2 (24.0
(24.0 %)
%)
PC2
DATA MATRIX: 30 objects (5 replicates x 3 oilTETDP mixtures x 2 oil samples) x 1601 variables
(currents points at different potentials)
DATA PREPROCESSING: column autoscaling
8
4
CV
PCA applied to the CV responses obtained in
maize and olive olive oils (see above)
Pt microdisk, r= 11.0 µm
CV
20
Determination of dinamic viscosity η
of RTILs from voltammetric data
2
highest miscibility with oil
The optimal concentration is [TETDP]+[decan]- 0.1M in oil
E (V) /vs Ag PSRe
200
20
0.2
The most suitable RTIL is [TETDP]+[decan]- lowest dinamic viscosity ( 57 cP)
I (nA)
I (nA)
0
I (nA)
I (µΑ)
150
CV
-5
I (pA)
I (nA)
10
[TETDP]+[decan]-
-30
On the basis of the preliminary electrochemical characterization of the
RTILs, and of the miscibility tests and effects on the conductivity of the
Olive oil
different RTILs in oil:
wide electrochemical window (+2.5V; -2.9V)
Maize oil
0
0
-15
I (nA)
EVALUATION OF THE OPTIMAL OIL/ RTIL COMPOSITION
Examples of CV responses obtained for RTILs
containing Ferrocene 1 mM as probe molecule
Pt microdisk, r= 11.0 µm
the software V-PARVUS [4] (NIPALS algorithm)
This technique allows the visualization of most of
the information (explained variance) present in the
data set, on the least possible number of new variables
(the Principal Components).
Data preprocessing: performed by column autoscaling
Electrochemical cell:: a glass
holder, 2 mL volume; two
electrodes configuration: a Pt
disc microelectrode, 10-12.5
µm radius, as working; Ag wire,
as pseudoreference (PSRe)
I (nA)
CV responses obtained for pure RTILs
Principal Component Analysis (PCA), performed by
Ag WIRE
VOLTAMMETRIC CHARACTERIZATION OF RTILs AT Pt MICRODISK
CV
Chemometric analysis::
Electrochemical measurements in the oil/RTIL mixtures
Ionic liquids (properly synthetized and purified):
-trihexyl(tetradecyl)phosphonium decanoate [TETDP]+[decan]-trioctylmethylphosphonium chloride [TOMP]+[Cl]-tetrabutylmethylammonium chloride [TBMA]+[Cl]-trioctylmethylammonium chloride [TOMA]+[Cl]Oils: maize commercial oil; extravirgin olive oils
(PDO and directly sampled in oil mills), coming
from different Italian and Spanish regions
The addition of [TETDP]+ based ionic liquids
to vegetable oils makes them accessible to
voltammetry at microelectrodes
Voltammetric Pt microdisc + Chemometric analysis :
Simple and rapid analytical procedure
Adequate repeatability of the measurements
Effective differentiation among vegetable oils
according to their quality and geographical origin
v= 50 mV s-1
The proposed method may be of
merceological interest in detection of
commercial frauds and food adulteration
v= 50 mV s-1
Electrode:: Pt disc microelectrode, r= 12.5 µm
PCA – Score plot
PCA applied to the CVs is able to discriminate, on the basis
of Principal Component 1 (PC1), between maize and olive oils.
PC2 discriminates among different groups of replicates.
The CV responses depend on the origin of the oil.
PC1
PC1 (52.3
(52.3 %)
%)
PCA – Score plot
PCA, based on PC2, discriminates between Ligurian and Spanish oils.
References
[1] J. Zhang, A.M. Bond, Analyst, 130(2005)1132
[2]M. Galinski, A. Lewandowski, I. Stepniak, Electrochim.Acta, 51(2006)5567
[3] P. Hapiot, C. Lagrost, Chem. Rev, 108 (2008) 2238
[4] M. Forina, S. Lanteri, C. Armanino, C. Casolino, M. Casale, P. Oliveri,
V-PARVUS 2008, University of Genova.