Electrochemical Processes at Liquid Interfaces Petr Vanysek Northern Illinois University Department of Chemistry DeKalb, IL 60115
More than 100 years ago, Nernst performed the first experiments that provide the theoretical basis for today's potentiometric and voltammetric studies of interfaces (1). As the significance of interfaces has become more widely recognized, new techniques to probe them have evolved. Today, electrochemical experiments provide a better understanding of the significance of the potential difference that occurs at liquid-liquid (L-L) interfaces. These studies may have applications in electroanalytical chemistry, in separation science, and in the electrochemistry of biological membranes. This article will describe some applications of electrochemical techniques for use at the L-L interface as analytical tools in ion determinations and outline some areas for future development. Equilibrium studies of two immiscible phases in contact are considered a standard part of separation science (e.g., extraction-based separations). Most often, the extracted species are molecular compounds, in which case no charge transport is involved. Any electric potential difference across t h e boundary between the two solvents can be justifiably ignored. Unfortunately, the practice of disregarding the interfacial potential difference is often extended to salt separations, where the potential effect is important. In salt ex0003-2700/90/0362-827A/$02.5070 © 1990 American Chemical Society
traction theory, the balance of charge is usually considered, but the more general theory that includes potential at or across the interface and the resulting ion repartitioning is often disregarded.
Interface between two immiscible electrolyte solutions (ITIES) The behavior of the ITIES first came to the attention of electrochemists when Koryta et al. (2) postulated that this interface should behave similarly to an electronically conductive electrode immersed in a solution. Previously, Blank and Feig (3) suggested that this interface also could serve, at least to a rough approximation, as a model for one-half of a biological membrane. This concept
nonaqueous solvent because of its low mutual solubility with water and its high relative permittivity, e (34.8 at 25 °C), which is required to support the dissociation of dissolved salts. To study charge transport across the interface both solutions must be conductive, and this is achieved by adding suitable supporting or base electrolytes. LiCl is a commonly used supporting electrolyte in water; the most commçnly used salt for the nonaqueous phase is tetrabutylammonium tetraphenylborate (TBATPB) because it dissolves in organic solvents. When the two immiscible solvents that contain supporting electrolytes are brought into contact, the supporting electrolyte
INSTRUMENTATION spurred the early work of French researchers, especially Gavach at Montpellier (4). The biological membrane is a selfassembled structure of phospholipids with polar heads facing the aqueous intracellular and extracellular solutions. The lipophilic chains of the phospholipids form the oil-like inner layer of the membrane. One can consider an ITIES (here, oil and water) as half of the cross section of the membrane. The similarity in behavior between t h e ITIES and an electrode-electrolyte system will be explained later. A typical solvent pair used for ITIES studies is water and nitrobenzene. Nitrobenzene is the most commonly used
salts stay virtually in their original phases. Although some repartitioning occurs, its extent is negligible. The interfacial thermodynamic equilibrium of all the ions present can be described by equating their electrochemical potentials (μ) in both phases. For each ion in the phases a and β we can write an equation of the form μ?(α) + RTln
0 i (a)
+ ηΡφ(α) =
μ?(/3)+ΛΤ1ηα;(/Ϊ) + Λίν(/3) (1) where μ° is the standard chemical po tential of the ion i in each phase, a; is the activity of the ion, η is its charge, R is the universal gas constant, F is the Faraday constant, Τ is absolute tem-
ANALYTICAL CHEMISTRY, VOL. 62, NO. 15, AUGUST 1, 1990 · 827 A
INSTRUMENTATION perature, and φ is the inner potential. If more than two ions are considered, we obtain a transcendental function for the interfacial potential drop Αφ = [φ(α) — φ(β)]. Despite the apparent complexity of Equation 1, it leads to derivation of the Nernst equation. In deed, when partitioning of only one ion is considered, the result is formally equivalent to the Nernst equation. As a first approximation, we can as sume that within a certain interfacial potential range, ions of supporting electrolytes do not participate appre ciably in the equilibrium. Thus the electrical potential of the interface with only supporting electrolytes is not defined. This does not mean that the potential on the interface does not ex ist. In principle, its value can be calcu lated, but calculation of the potentialdetermining ion concentrations is a for midable task. (A similar case is that of a mercury electrode immersed in a deaerated solution of KC1.) An important property of this interface is that if a potential is applied to it from an extra neous source, the interface will attain the potential of the source. The inter face is then "ideally polarizable," fol lowing the terminology that is tradi tionally applied to electrodes. A different situation occurs if a salt soluble in both phases is added to the system. For example, lithium picrate is soluble in water and picrate anion is also highly soluble in nitrobenzene. If the picrate anion is the only shared ion between water and nitrobenzene, the interfacial potential drop is A water _ AO water ι r nitrobenzene ^V nitrobenzene '
u,
RT/F In [ α ^ η ζ β η 7 < £ β ]
(2)
and the potential is determined by the partition ratio of picrate anion between the two phases. Δ ° ^ Γ ^ η Μ η 8 is the standard potential of transfer of the individual ion and, for ITIES studies, its meaning is similar to the standard reduction potential in redox electro chemistry. The known values for Δ° are tabulated in Reference 5 and, for picrate ion, t h e standard potential is 47 mV. The potential is defined here as the difference between the aqueous and the oil phase; thus the sign of the potential corresponds to the polarity of the water phase. This simple relation ship (2) is fulfilled simultaneously for all ions that participate in the phase equilibrium. The difficult part is deter mining the individual ion activities. Equation 2, t h e Nernst-Donnan equation, describes the basic principle used in potentiometric determinations of ions with ion-selective electrodes (ISEs). An important property of the interface follows from further analysis
of the relationship. If the interfacial potential is forced to a value given by the potential of an extraneous source, the ratio of the picrate ion concentra tion must change. If the water phase is made more positive, more picrate ion will be driven into the organic phase to fulfill the equilibrium prescribed by Equation 2. As a result, a net electric current flows through the system and can be measured by a galvanometer in series with the completed circuit. This current is the basis for the voltammetric characterization of the interface. Experimental considerations The electrochemical cell that is often used in ITIES polarization studies is shown in Figure 1. That portion of the cell where the interface under investi gation is confined in the central, nar rower tube is typically made from glass tubing. The center portion can be lined with a Teflon insert, which is some times used to define the exact position and flatten the meniscus of the inter face. The existence of a meniscus rath er than a flat interface is not a major problem, although it causes difficulties in calculations that rely on the area of the interface. A screw-driven plunger attached to the side arm of the cell ad justs the position of the interface. By adjusting the volume of the lower part of the cell, it is possible to adjust the interface position precisely. In any electrochemical experiment, the determination of the phase bound ary potential difference requires the use of reference electrodes. Because the aqueous phase usually contains chlo ride ions, a silver wire coated with AgCl is the simplest choice for a reference electrode. For more demanding potenReference electrodes
Refer ence interface
Counter electrode
Glass frit Η,Ο Adjusting plunger
Interface
Nitrobenzene Glass fri'
Counter electrode
Figure 1. Experimental apparatus for voltammetric or potentiometric studies of the interface between two immisci ble electrolyte solutions (ITIES).
828 A · ANALYTICAL CHEMISTRY, VOL. 62, NO. 15, AUGUST 1, 1990
tiometric measurements, it is possible to separate the reference electrode from the solution by a salt bridge. The reference potential for the nitroben zene phase is usually maintained by a secondary water-nitrobenzene inter face. This is a constant potential inter face, often called a reference interface. The reference interface is set in such a way that both phases share a common ion. Typically, because the nitroben zene already contains TBATPB, the aqueous phase contains tetrabutylammonium chloride. A silver wire coated with AgCl serves as a reference elec trode connecting the solution to the outer electric circuit. The potential of the reference interface is determined by the shared ions. If one can assume that the only shared ion is TBA+, the interfacial potential will be given by an equation similar to Equation 2, in which the picrate ion activity is re placed by that of TBA+. The potential of this interface, even if a current of small density passes across it, stays constant during the measurement. For equal concentrations of TBA + in both phases, the potential assigned to this interface is - 2 4 8 mV, a value that has been calculated from extraction mea surements. Because the potential com pares activities of an ion in two dissimi lar phases it is not a true thermody namic constant, but it is useful as a constant potential reference point. The total potential of the electrochemical cell that includes the reference inter face contribution is denoted U in this article. The two reference electrodes and the reference interface are sufficient for potentiometric work. In voltammetry, however, it is desirable to use a potentiostat. Because compensation of two resistive solutions is necessary, a spe cial four-electrode potentiostat that re quires two additional electrodes in the cell is used. These counter electrodes are typically platinum flags or wires. To keep the current density low, a large surface is desirable. However, one must remember that redox processes are tak ing place on these electrodes and high overpotential would needlessly in crease demand on the compliance volt age of the potentiostat. The redox pro cesses on the counter electrodes in volve decomposition of the supporting electrolytes and, sometimes, of the sol vents as well. A glass frit separates the electrodes from the rest of the cell to avoid contamination of the interfacial area by the reaction products. Difference between ITIES chargetransfer and electrode reactions The similarities between the processes at the ITIES and on metal electrodes
have been emphasized so often (2) that we will concentrate on the differences instead. Current flow through an elec trochemical cell is usually associated with a redox process occurring in the cell. However, when the ITIES is polar ized by an external potential source, the net current flow does not result from a redox process at the interface. In fact, with a few rare exceptions, there is no redox reaction taking place at the L-L interface. Figure 2 compares a current flow on an electrode and at the ITIES. If a suf ficiently negative potential is applied to the electrode (Figure 2a), the system will have high enough energy to cause the reduction of some reducible species (here, Fe3+) in the solvent. An electron will leave the electrode and cause re duction, the reducible species will dis appear from the solution, and a re duced species (Fe2+) will appear on the surface of the electrode. Application of a positive potential would reverse the process, causing oxidation of the re duced form. The magnitude of the cur rent depends on the rate at which the reducible species crosses the electrode surface, the rate of the charge-transfer reaction, and the rate of diffusion of the reduced species away from the elec trode. If the solution is unstirred and contains enough supporting electro lytes so that the analyte ions move only by diffusion, and if the charge-transfer reaction is faster than the diffusion it-
(a) Aqueous solution ηί.
*e-
* Figure 8. Interfaciai potential depen dence as a function of the concentra tion of picrate ions in aqueous and nitrobenzene solutions. The nitrobenzene phase contains dissolved tetra butylammonium picrate and 0.01 mol L - 1 TBATPB; the aqueous phase contains picric acid and 0.01 mol L _1 LiCI. The calculations are for Τ = 25 °C.
trobenzene and picric acid in the aque ous phase. This system is, in essence, a picrate-sensitive electrode. The threedimensional perspective allows illus tration of the effect of varying picrate concentration in both water and nitro benzene. The graph clearly shows the region of the low aqueous picrate con centration in which the potential is no longer a function of [Pi - ]. The poten tial in that region is governed by the equilibrium of the supporting electro lytes. At high concentrations the level ing of the dependence indicates the Donnan failure (16,17). In the Donnan failure region only a single picrate salt participates in the equilibrium, and the potential is no longer dependent on the total picrate concentration. The linear part is sometimes described by Donnan exclusion, a process in which only one ion (here, picrate) determines primari ly the net potential. As the picrate con centration goes down, the contribution of the supporting electrolytes to the overall potential becomes significant and, eventually, the only potential-de termining factor. The potential profile in Figure 8 corresponds to those ob served for ISEs. With an appropriate modification, Equation 5 can be rewrit ten, for example, in the form of the Nicholsky-Eisenman equation for ISEs and other two-phase ion-contain ing systems. Ions that can be determined The analytical usefulness of the ITIES polarization relies on determining as many different ions as possible. There fore, for a given electrolyte pair it is usually desirable to choose a solvent pair that has the broadest polarization
potential window. For this purpose wa ter-nitrobenzene is the best choice, fol lowed by water-l,2-dichloroethane. Besides these solvent pairs, the use of water-solvent mixtures such as nitrobenzene-benzonitrile and nitroben zene-benzene also has been reported (18,19). Although these mixtures pro vide narrower potential windows, they often exhibit selective solvation prop erties, which influence rates of chemi cal reactions, solubilities, and so on. It is expected that choices of solvents could help the understanding of ion transfer or electron transfer mecha nisms and could lead to the design of systems of preferential or even selec tive response. Generally it is possible to determine ions that are less hydrophilic than the ions used as supporting electrolytes in water and, at the same time, that are less hydrophobic than the ions of the salt used as the nonaqueous supporting electrolyte. It is also obvious that the ions to be determined must not react with the supporting electrolytes or the solvents. From this perspective the most serious obstacle is the low solubil ity product of certain combinations of ions. A salt can precipitate during a measurement on the interface either as a result of natural repartitioning or as a result of ion transport caused by cur rent flow from an outside source. The precipitate adsorbs on the interface and modifies its properties. As a solid phase, it also functions as a sink for the transported ions, so reaching the equi librium condition prescribed by Equa tion 2 may be impossible. Many of the ions that can be deter mined by the techniques described for L-L interfaces are listed in the box be low. An extensive list can be found in Reference 5 and in subsequent reviews (20,21). Lithium and sodium ions can not be determined directly, and they cannot be determined if LiCl is used as a supporting electrolyte. Na+ and, to
some extent, K + have characteristics too close to Li+ to exhibit peaks that are distinguishable from the support ing electrolyte background. It is still possible to detect transport of these ions if proper steps are taken to form a more hydrophobic ion complex and, thus, to shift the ion potential of trans fer within the working window of the supporting electrolytes. An example of this process is the facilitated transport of sodium ion in the presence of 18-dibenzo-crown-6 (22) or the case of potassium ion and valinomycin (23). If a specific acceptor is found, the tech nique even becomes selective to the ligand ion. The appropriate complexing agent can then be used to expand the number of ions determined. Converse ly, the technique can be modified for determination of the complexing agents, as in an assay of monensin, which is based on the complexing reac tion with sodium (24). More recent work describes determination of Pb 2+ using complexing properties of poly ethylene glycol 400 (25). Microinterface between two immiscible solutions
The purpose of using a small orifice to restrict the area of the interface is to take advantage of recent progress in the development of ultramicroelectrodes. Ultramicroelectrodes help to overcome complications stemming from a potential shift arising from an iR drop. As the interfacial area be comes smaller, the diffusion geometry takes on the character of a spherically symmetric process. This means that the ratio of faradaic current versus so lution resistance is increasing and, in the end, rendering the contribution of the iR drop minimal. ITIES, which uses solutions of low conductivity, can benefit from this phenomenon. Re stricting the interfacial area and using a current amplifier is an alternative to a four-electrode potentiostat.
20
10
0
~ -10
-20
-30 200
300 400 U(mV)
500
Figure 9. Voltammetric determination of lauryl sulfate (0.4 mmol L_1) on a L-L microinterface. Scan rates (mV s~1): (1) 10, (2) 20, (3) 50, (4) 100, (5) 200, (6) 500, and (7) 1000. Supporting electrolytes: 0.02 mol L _ 1 LiCl in water and 0.02 mol L"1 TBATPB in nitrobenzene. Diameter of the hole is 130 μπ\.
Small L-L interfaces have been used by Girault and co-workers (26-28) and by Senda et al. (29). We have also ex plored a small interface formed in a hole of a thin glass wall (30,31). Figure 9 illustrates voltammetric curves ob tained on such an interface. One com plication of the microinterface com pared with the solid microelectrode is that it is difficult to keep the interface small. Its size is defined by a small win dow in a thin plate. Interfacial tension, capillary pressure, uneven hydrostatic pressure, thermal expansion, and even vibrations in the laboratory cause the interface to move away from this areadefining region. We have tried to solve this problem by immobilizing the electrolyte solution inside a tube holder by adding an immobilizing agent. Future directions
Selected ions that can be determined by L-L interface polarization Inorganic cations K+ Rb+ Cs + Pb2+ (as a complex) Na+ (only as a complex) Li + (only as a complex) Inorganic anions SCN~
cio; BFJ
r
Organic cations Quaternary ammonium cations (various alkyl chains shorter than butyl) Choline Acetylcholine Q
|c an|ons
Octanoate Picrate Lauryl sulfate
Ongoing investigations of the electrical behavior of the ITIES follow two main directions. The first, and so far pre dominant, is research in theoretical electrochemistry. Here, for example, the recent effort by Berube and Buck (32) to elucidate anomalies in theories of liquid membrane time response to a concentration step should be men tioned. Even though the work is theo retical, the results have an impact on applied work. An extension of results (32) can be applied to understand the observed difference between electrodes and the ITIES, such as the high sup porting electrolyte current in Figure 5b.
ANALYTICAL CHEMISTRY, VOL. 62, NO. 15, AUGUST 1, 1990 · 833 A
INSTRUMENTATION Some experimental techniques applicable to the ITIES are also of a more fundamental nature. Fast scan voltammetry, in particular on microinterfaces, can be used for determination of charge-transfer rate constants. Impedance analysis, which is an extension to ac voltammetry, can be used not only to detect analytes, but also to obtain a better understanding of surface phenomena (33) and adsorption (34). A special case of impedance analysis is derived from measurement of noise generated by the electrochemical system (35). This is especially useful for work on microinterfaces that have large absolute resistance. The second direction of interest is in application to new or improved analytical sensors. These can be potentiometric, voltammetric, or based on other appropriate techniques. The L-L interface is actually a component of an ionselective electrode with a liquid ion exchanger. Recent analytical applications have resulted in construction and systematic studies of microinterfaces solidified by gels. The advantage of such a modification is ease of handling (36-39). The immobilization can be extended further to studies of frozen in-
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terfaces, or even to use of solid electrolytes. Significantly, ITIES theory also applies to interfaces that are encountered in ion-doped, conductive, polymer-coated electrodes. This work and the author's research work described herein were supported in part by the Office of Naval Research. I thank Elizabeth A. Burton for review of the draft version.
References (1) Nernst, W. Z. Phys. Chem. 1888,2,61337. (2) Koryta, J.; Vanysek, P.; Bfezina, M. J. Electroanal. Chem. 1976,67, 263-66. (3) Blank, M.; Feig, S. Science 1963, 95, 561-72. (4) Gavach, C. C. R. Acad. Sci. (Paris) 1969, 269,1356-59. (5) Vanysek, P. Electrochemistry on Liquid/Liquid Interfaces; Springer: Berlin, 1985. (6) Koryta, J.; Vanysek, P.; Bfezina, M. J. Electroanal. Chem. 1977, 75, 211-28. (7) Vanysek, P.; Buck, R. P. J. Electrochem. Soc. 1984,131,1792-96. (8) Sinru, L.; Freiser, H. Anal. Chem. 1987, 59,2834-38. (9) Freiser, H. Abstracts of Papers, 40th Pittsburgh Conference and Exposition, Atlanta, GA; Pittsburgh Conference and Exposition: Pittsburgh, PA, 1989; Abstract 217. (10) Yoshida, Z.; Freiser, H. J. Electroanal. Chem. 1984,162, 307-19.
(11) Yoshida, Z.; Freiser, H. Inorg. Chem. 1984,23,3931-35. (12) Kihara, S.; Yoshida, Z.; Fujinaga, T. BunsekiKagaku 1982,31, E297-E300. (13) Hundhammer, B.; Solomon, T.; Alemu, H. J. Electroanal. Chem. 1983, 149,179-83. (14) Marecek, V.; Samec, Z. Anal. Lett. 1981,14,1241-53. (15) Hung, L. Q. J. Electroanal. Chem. 1980,115,159-74. (16) Melroy, 0. R.; Buck, R. P. J. Electroanal. Chem. 1983,143, 23-36. (17) Cosofret, V. V.; Buck, R. P. In Funda mentals and Applications of Drug Sensors; Schuetzle, D.; Haemmerli, R., Eds. ACS Symposium Series 309; American Chemical Society: Washington, DC, 1986 pp. 362-72. (18) Koczorowski, Z.; Paleska, I. Geblewicz, G. J. Electroanal. Chem. 1984, 164, 201-4. (19) Solomon, T.; Alemu, H.; Hundhammer, B. J. Electroanal. Chem. 1984,169, 311-14. (20) Koryta, J. In The Interface Structure and Electrochemical Processes at the Boundary between Two Immiscible Liquids; Kazarinov, V. E., Ed.; Springer: Berlin, 1987. (21) Girault, H.H.J.; Schiffrin, D.J. In Electroanalytical Chemistry; Bard, A. J., Ed.; Marcel Dekker: New York, 1989; Vol. 15. (22) Hofmanovâ, Α.; Hung, L. Q.; Khalil, M. W. J. Electroanal. Chem. 1982, 135, 257-64. (23) Homolka, D.; Hung, L. Q.; Hofmanovâ, Α.; Khalil, M. W.; Koryta, J.; Marecek, V.;
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Samec, Z.; Sen, S. K.; Vanysek, P.; Weber, J.; Bfezina, M.; Janda, M.; Stibor, I, Anal. Chem. 1980,52,1606-10. (24) Koryta, J.; Ruth, W.; Vanysek, P.; Hofmanovâ, A. Anal. Lett. 1982, 25,1685-92. (25) Sun, Z.; Vanysek, P. Anal. Chim. Acta 1990 228 241-49. (26) Taylor, G.; Girault, H.H.J. J. Electroanal. Chem. 1986, 208, 179-83. (27) Campbell, J. Α.; Stewart, Α. Α.; Gir ault, H.H.J. J. Chem. Soc, Faraday Trans. 11989,85, 843-53. (28) Campbell, J. Α.; Girault, H.H. J. Electroanal. Chem. 1989, 266, 465-69. (29) Senda, M.; Kakutani, T.; Osakai, T.; Ohkouchi, T. Proceedings of the 1st Bioelectrochemical Symposium, Mâtrafûred; Akademiai Kiado: Budapest, 1986; pp. 353-64. (30) Vanysek, P.; Hernandez, I. C. Anal. Lett., in press. (31) Vanysek, P.; Hernandez, I. C ; Xu, J. Microchem. J. 1990,41, 327-39. (32) Berube, T. R ; Buck, R. P. Anal. Lett. 1989,22,1221-35. (33) Buck, R. P. Ion Select. Electr. Reu. 1982,4, 3-74. (34) Vanysek, P.; Sun, Z. Bioelectrochem. Bioenerget. 1990,23, 177-94. (35) Bezegh, Α.; Janata, J. Anal. Chem. 1987,59,494 A-508 A. (36) Kakutani, T.; Ohkouchi, T.; Osakai, T.; Kakiuchi, T. Anal. Sci. 1985, 1, 21925. (37) Marecek, V.; Jànchenovâ, H.; Colombini, M. P.; Papoff, P. J. Electroanal. Chem. 1987,227,213-19. (38) Baum, J. Anal. Lett. 1970, 3,105-11. (39) Osakai, T.; Kakutani, T.; Senda, M. Bunseki Kagaku 1984,33, E371-E377.
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