Voltammetric and visual evidence of adsorption reactions at the liquid-liquid interfaces supported on a metallic electrode

Article abstract of DOI:10.1016/j.electacta.2011.12.037

A three electrode configuration consisting of an aqueous drop containing a redox couple of Fe²⁺/Fe³⁺ or IrCl₆ ^2-/IrCl₆^3- attached onto the surface of a platinum electrode and immersed into an organic electrolyte solution was apply to evaluate the adsorption-desorption processes associated with l-α-dipalmitoyl phosphatidylcholine (DPPC) interfacial complex formation. It has been observed that a significant deformation of the aqueous droplet immersed into the solution occurs in the presence of the phospholipid at potentials where the adsorption occurs. Nevertheless as the potential increases, the desorption of charged complexes of DPPC takes place, resulting in a potential-dependent drop shape phenomenon.

Full text of DOI:10.1016/j.electacta.2011.12.037

Electrochimica Acta 62 (2012) 290–295
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Electrochimica Acta
journal homepage: www.elsevier.com/locate/electacta
Voltammetric and visual evidence of adsorption reactions at the liquid–liquid
interfaces supported on a metallic electrode
Ibrahim Uyanik∗, Yunus Cengeloglu
Department of Chemistry, Faculty of Science, Selcuk University, Campus, 42075 Konya, Turkey
a r t i c l e i n f o
a b s t r a c t
A three electrode configuration consisting of an aqueous drop containing a redox couple of Fe2+/Fe3+ or
IrCl62−/IrCl63− attached onto the surface of a platinum electrode and immersed into an organic electrolyte
solution was apply to evaluate the adsorption–desorption processes associated with l-␣-dipalmitoyl
phosphatidylcholine (DPPC) interfacial complex formation. It has been observed that a significant defor-
mation of the aqueous droplet immersed into the solution occurs in the presence of the phospholipid at
potentials where the adsorption occurs. Nevertheless as the potential increases, the desorption of charged
complexes of DPPC takes place, resulting in a potential-dependent drop shape phenomenon.
© 2011 Elsevier Ltd. All rights reserved.
Article history:
Received 25 November 2011
Accepted 10 December 2011
Available online 19 December 2011
Keywords:
Liquid–liquid interface
Three-electrode configuration
Drop electrode
1. Introduction
It has also been proposed the addition of supporting electrolytes
in both phases, so that the required potential difference for deform-
Phospholipid monolayers adsorbed at the liquid–liquid inter-
faces formed between two immiscible electrolyte solutions (ITIES)
have been widely used to mimic the behavior of the lipid biomem-
branes [1], especially to investigate the stability [2–4] and ion
permeability [5–7] of the monolayers as a function of the well-
defined interfacial potential difference.
The electrochemical investigation of charge transfer reactions
across the liquid–liquid interface started when Samec et al. intro-
duced the four electrodes arrangement [8]. In this technique,
biphasic reactions can be controlled by varying the potential of the
two phases (oil and water). However, due to some a priori limita-
tions of such an experimental setup, three-phase electrodes have
been introduced in the last decade. For this purpose, many methods
have been used to modify the surface of electrodes with oil [9–13] or
water [14,15,6,16] droplets or arrays [17] of these. The main advan-
tage of using a small liquid volume on a solid electrode to study
charge transfer reactions at a liquid–liquid interfaces stems from
the fact that a classical three-electrode potentiostat can be used
instead of the four-electrode potentiostat specifically designed for
the electrochemical studies of ITIES [18,19].
ing in an appreciable extent the geometry, and consequently its
contact angle with the supporting surface, could be reduced [22].
In the present work, a three-electrode system is utilized for ana-
lyzing mainly the desorptive process of a phospholipid from the
water–DCE interface and significant variations of the liquid–liquid
interfacial tension and consequently of the contact angle is ana-
lyzed when adsorption processes are involved. We think that this
paper may be useful for electro-wetting applications since an
important variation in the contact angle can be obtained with a vari-
ation of only few hundred millivolts always that a surface-active
molecule, like DPPC, be present.
2. Experimental
2.1. Chemicals
The phospholipid (l-␣-dipalmitoyl phosphatidylcholine,
DPPC, M = 734.0 g/mol), K3IrCl6 and K4IrCl6 were from Sigma
(St. Louis, MO). Deionized water (18.2 Mꢀ cm) was obtained
using a Milli-Q system from Millipore (Bedford, MA). For the
electrochemical measurements, the organic salts used were
bis(triphenylphosphoranylidene) ammonium tetrakis(pentafluo-
rophenyl) borate (BTPPA+TPFB−) and tetraethylammonium
tetrakis(pentafluorophenyl) borate (TEA+TPFB−). These salts were
both obtained by metathesis of lithium tetrakis(pentafluorophenyl)
borate with bis(triphenylphosphoranyldiene) ammonium chloride
and tetraethylammonium chloride, respectively (all bought from
Fluka, Switzerland). The organic phase solvent used was DCE. All
In the literature, significant efforts have been directed towards
the obtention of tunable electro-wetting properties, specially for
oil–water systems [20,21].
∗
Corresponding author. Tel.: +90 3322233890; mobile: +90 5053101077;
fax: +90 3322412499.
E-mail address: ibrhm.uyanik@gmail.com (I. Uyanik).
0013-4686/$ – see front matter © 2011 Elsevier Ltd. All rights reserved.
doi:10.1016/j.electacta.2011.12.037

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291
the other employed compounds were analytical grade and used as
received, unless otherwise stated.
2.2. Electrochemical measurements
Before starting each experiment, a 0.06 0.01 cm2 Pt disc elec-
trode (Metrohm CH) was freshly polished sequentially with 1.0, 0.3
and 0.05 ␮m alumina. Then, water and acetone were used to rinse
the electrode. The aqueous and organic solvents were mutually sat-
urated prior to each experiment. A drop of a volume of 10 ␮L of
an aqueous solutions contains an equimolar mixture of oxidized
and reduced species, namely K2IrCl6/K3IrCl6 or FeSO4/Fe2(SO)3,
was deposited using a syringe onto the platinum disc electrode
mounted in a Teflon holder, ensuring that the entire electrode
surface is covered by the aqueous droplet. The electrode was
then immersed into a 1,2-DCE solution (3.0 mL) containing (BTP-
PATPFB) as supporting electrolyte. A Pt wire was used as the counter
electrode, and an Ag wire coated with AgCl was used as the ref-
erence electrode. Detailed description of the electrochemical cell
employed has been reported elsewhere [14]. Voltammetric mea-
surements were carried out using a programmable potentiostat
(AUTOLAB-PGSTAT 30, Metrohm, Switzerland) and a glass cell. The
cell was always first cleaned by chrom-sulfuric acid and rinsed
thoroughly with ultrapure water. The iR potential drop was com-
pensated using a positive-feedback method. The electrochemical
cells compositions were the following:
Fig. 1. Schematic representation of an aqueous droplet supported on a platinum
electrode immersed in DCE.
3. Results and discussion
Fig. 2 shows one of the cyclic voltammograms used to con-
vert our potential scale into the Galvani potential scale. Galvani
potential differences are estimated by referring the cell potential
applied between two reference electrodes to an internal reference
based on a certain extrathermodynamic assumption. The com-
monly used “TATB” assumption states that tetraphenylarsonium
Ag AgCl LiCl 10 mM
BTPPACl 1 mM
BTPPATPFB 10 mM
K 2IrCl6 10 mM Pt
K3IrCl6 10 mM
Cell I
HCl x mM
Ag AgCl LiCl 10 mM
BTPPACl 1 mM
BTPPATPFB 10 mM
FeSO 4 10 mM
Fe2(SO)4 5 mM
HCl x mM
Pt
Cell II
All the potentials for ion transfer are referred to the half-wave
potential of tetraethylammonium TEA+ obtained by adding at the
end of each experiment a predetermined amount of TEA+TPFB−
to the organic phase. The value of the standard transfer potential
of TEA+ has been taken equal to 0.048 V [23]. All the measure-
ments were carried out at room temperature of 22 1 ◦C. Videos of
the droplet during the voltammetric measurement were obtained
with a Hand-Held Microscope (Proscope HR, Bodelyn Technologies,
USA).
2.3. Contact angle determination
For the image analysis, the geometry of the droplet was assumed
to be that of a truncated oblate spheroid. In this sense, a, c and z0
were graphically determined from video snapshots taken during a
voltammetric measurement and sec(ꢁ) was calculated according to
[24]
ꢀ
a2z02
c2(c2 − z02)
sec(ꢁ) =
1 +
where, a is the equatorial radius, c is the vertical radius, and z0 is the
vertical distance from the center at which the spheroid is truncated
(see Fig. 1). Detailed description of the equation has been reported
elsewhere.
Fig. 2. Cyclic voltammogram for the transfer of TEA+ ion across the water|DCE inter-
face obtained with cell I.

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Fig. 3. Calculated sec(ꢁ) as a function of the potential in the presence of DPPC
(20 ␮M) in DCE and at an aqueous pH value of 1.
(TPAs+) and tetraphenylborate (TPB−) have an identical standard
transfer energy at the water|1,2-DCE interface [25]. On the basis of
this assumption, the formal ion transfer potential of tetraethylam-
monium cation (TEA+) at the water|1,2-DCE interface is estimated
as 0.048 V [23]. Accordingly, the potential where the faradaic cur-
rent wave of the TEA+ ion transfer centered is 0.048 V.
In presence of excess of an equimolar mixture of either Fe2+/Fe3+
or IrCl62−/IrCl63−, the current is limited by the linear diffusion asso-
ciated to the ion transfer reactions and the peak-to-peak separation
should be 60 mV for singly charged species. It is also worth not-
ing that the ion-pair excess condition used here ensures that the
electrode potential is constant during the voltammetry, and all the
variations of the applied potential difference between the reference
electrode and the platinum electrode occur solely at the water–DCE
interface.
3.1. K2IrCl6/K3IrCl6 system
In Fig. 5 the ion transfer voltammograms in absence and in pres-
ence of DPPC are shown. It can be clearly observed that in absence of
the phospholipid the voltammogram the potential window is lim-
ited at negative potentials by the transfer of the aqueous anions,
i.e. IrCl62−or IrCl63−, which is in turn associated with the reduction
Fig. 4. Cyclic voltammograms obtained at the interface formed between an aqueous
supporting electrolyte solution at different pH values (see figure) and DCE (cell I) in
presence of DPPC 20 ␮M.
3−
of IrCl62−. Analogously, the oxidation of IrCl6 is accompanied by
the transfer of K+ at the positive edge of the potential window. In
stark contrast, when DPPC is added to the organic phase a positive
current signal can be appreciated at ca. 0.4 V and corresponds to the
desorption of the complex formed between K+ (which is the only
cation present in the aqueous phase) and DPPC molecules previ-
ously adsorbed at the water–DCE interface, as it has been reported
at the classic water–DCE interfaces [26,27].
The thermodynamic analysis of the lipid adsorption at the inter-
face taking into account the interfacial complex formation and
facilitated ion transfer has been performed in the literature [26].
At the same time, the association constants between dioleoyl l-␣-
phosphatidylcholine and different metallic cations were estimated
by assuming that the desorption potential, evaluated as the onset
of the increase in the interfacial tension in electrocapillary curves,
is dictated by the half-wave potential for the facilitated ion transfer
by the phospholipid [27].
In consequence, it has been proposed that the onset of the
interfacial tension increase observed in the electrocapillary curves
obtained in presence of both, the lipid and the complexed cation,
allows the calculation of the association. In our particular case,
these significant variations of the interfacial tension stemming
from the lipid desorption cannot only be confirmed by classic
voltammetric experiments but also visually monitored. Accord-
ingly, in our case, a significant deformation of the aqueous
droplet immersed into the solution occurs when the surface-active
molecule is present, DPPC in this case. Furthermore, given that the
volume droplet is constant, the interfacial area will be strongly
affected by this process and visual evidence of its change can be
recorded as a function of the potential.
Additionally, voltammograms were found to be time-
dependent, that is to say that there is a marked influence of
the equilibration time on the magnitude of the total charge passed

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293
in the forward scan. Nonetheless, after 30 min a reproducible
voltammogram is always obtained, proving a steady-state is
reached after that period of time. The latter can be explained
by the nature of the adsorption process itself. As a matter of
fact, adsorption is diffusion-controlled process, meaning that the
absolute surface coverage will be proportional to the square root
of time. For that reason all the voltammograms herein presented
were recorded after 30 min of equilibration time given that repro-
ducible voltammetric signals were generally obtained after this
time lapse.
As mentioned above, if one assumes an adequate geometry for
the aqueous droplet, meaningful information can be extracted from
the videos acquired along with the voltammograms. Thus, the val-
ues obtained for sec(ꢁ) are plotted as a function of the potential
difference and the shape of the curve obtained (Fig. 3) corresponds
to that generally obtained for the case of DPPC [1,29,30].
The addition of protons was also investigated and the results
obtained are presented in Fig. 4.
Fig. 5. Cyclic voltammogram obtained at the interface formed between an aqueous
supporting electrolyte solution and DCE (cell I, x = 0) in absence (dashed line) and in
presence (solid line) of DPPC (20 ␮M). Embedded pictures correspond to the video
snapshots acquired in the forward scan.
Fig. 6. Cyclic voltammogram obtained at the interface formed between an aqueous supporting electrolyte solution in absence (A) and in presence of HCl 0.1 M (B), and DCE
(cell II) in absence (dashed line) and in presence (solid line) of DPPC (20 ␮M). Embedded greyscale pictures correspond to the video snapshots acquired in the forward scan.

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I. Uyanik, Y. Cengeloglu / Electrochimica Acta 62 (2012) 290–295
As it can be observed, at pH = 3 the voltammogram does not
show major variations from that obtained in absence of protons as
shown in Fig. 5. Nonetheless, at inferior pH values the occurrence
of a new voltammetric signal at approximately 0.15 V is evidenced.
The latter leads to the monotonic decrease of the signal originally
present at 0.4 V. This phenomenon can be accounted for by the
competition between the binding of DPPC to K+ and H+. Thus, as
the concentration of protons increases beyond a certain threshold,
the complex between H+ and DPPC will prevail over that formed
between K+ and DPPC.
k
1
K+(aq) + DPPC(ads) ꢀ (K − DPPC)+
ads
k
−1
2
k
H+(aq) + DPPC(ads) ꢀ (H − DPPC)+
ads
k
−2
From the facilitated ion transfer voltammetry theory it is also
expected that complexes with higher stability constants will appear
on the voltammetric scans at lower potentials. In other words, DPPC
plays the role of an ionophoric species that facilitates the transfer of
cations like K+ and H+, as previously reported [27]. Nevertheless, the
three-electrode setup employed for analyzing complexation and
adsorption processes combined allows the visual confirmation of
significant changes in the interfacial tension as well as the distinc-
tion between the two interfacial complexes. Simultaneously, it was
also confirmed the higher affinity of DPPC for protons compared to
that for potassium.
Fig. 7. Calculated sec(ꢁ) as a function of the potential in the presence of DPPC
(20 ␮M) in DCE in absence (black markers) and in presence (red markers) of HCl
0.1 M. (For interpretation of the references to color in this figure legend, the reader
is referred to the web version of the article.)
same geometry as that of the redox pair IrCl62−/IrCl63−, the volume,
area and contact angle of the supported droplet can be calculated
(Fig. 7).
3.2. Fe2(SO4)3/FeSO4 system
As it can be seen, from the calculated values of sec(ꢁ) the general
shape of a typical electrocapillary curve is obtained in both cases,
even when the ionic composition of the aqueous phase has been
changed. Nonetheless, the desorptive process that occurs in pres-
ence of protons alters in a much more significant way the interfacial
tension. In such a way, the magnitude of the current spike observed
in the cyclic voltammograms is closely related to the magnitude of
the variation of the interfacial tension.
In order to confirm the strong influence of the cations present in
the aqueous droplet, the redox pair IrCl62−/IrCl63− was substituted
by Fe2+/Fe3+. It can be seen from Fig. 6A the presence of a sharp
peak at approximately 0.18 V.
This signal stems from the formation and further desorption
from the ITIES of the complex between DPPC and either Fe2+ or
Fe3+. It has also to be pointed out that no clear distinction between
the two cases can be made. The desorption process is also char-
acterized for strongly altering the stability of the ITIES, giving rise
then to the current spike observed at the end of the voltammetric
signal. Snapshots of the video obtained during the voltammetric
measurement clearly show the deformation of the aqueous droplet
(embedded pictures Fig. 6) and also reveal, after determining the
geometrical parameters as a function of the potential difference,
the strong influence of the potential on the contact angle (Fig. 7,
blank line).
As it is expected for DPPC, clear distinction between a potential-
dependent zone and a potential-independent zone can be made
for the adsorption process. As it has been previously discussed,
the presence on these two zones supposes the formation of a
lipid–cation complexes at positive potentials.
It has to be highlighted also that the utilization of a three-
electrode setup allows one to obtain the characteristic signature
of either adsorptive or desorptive processes. Thus, triangular peaks
are obtained, as it has been predicted before [28], confirming the
analysis previously made.
Following the same reasoning, HCl was added into the aqueous
droplet, as previously carried out for the IrCl62−/IrCl63− redox pair,
and it was evident the spike intensity increases as well as that of
the voltammetric peak presented in Fig. 6B. As discussed in the
previous section, a strong affinity of DPPC for protons can induce the
occurrence of voltammetric signals at lower potentials than those
obtained in its absence. Nonetheless, no clear distinction could be
done for the complex formation of DPPC with neither H+, Fe2+ nor
Fe3+. Therefore, the variation of the aqueous pH was not considered
in this case. However, if one assumes the aqueous droplet has the
4. Conclusions
From the application of three-electrode system, visual and
voltammetric confirmation of the coupling between complexation
and desorption processes is presented. The results herein presented
are also in good agreement with interfacial tension measurements
performed a typical four-electrode cell at the water–DCE interface.
Additionally, the presence of triangular-shaped voltammetric sig-
nals was clear in the presence of DPPC, confirming the validity of
previous theoretical approaches to the adsorption phenomenon at
ITIES. Finally, this system could be of crucial interest for electro-
wetting applications since an important variation in the contact
angle can be obtained with a variation of only few hundred milli-
volts always that a surface-active molecule, like DPPC, be present.
Acknowledgements
We gratefully acknowledge the Scientific and Technological
Research Council of Turkey (TUBITAK) under the 2212-In-Out
of The Country Joint PhD Scholarship Program. We also thank
the Scientific Research Projects Foundation of Selcuk University
(SUBAP-Grant Number 09201143). This work was produced from
a part of I.U.’s Ph.D. Thesis.
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