Selectivity in the Coextraction of Cation and Anion by

Anal. Chem. 2006, 78, 2717-2725

Selectivity in the Coextraction of Cation and Anion by Electrochemically Modulated Liquid-Liquid Extraction Alfonso Berduque and Damien W. M. Arrigan*

Tyndall National Institute, Lee Maltings, University College, Cork, Ireland

Electrochemistry at the interface between two immiscible electrolyte solutions has been presented as a method of electrochemically modulated liquid-liquid extraction, where ions in a mixture can be selectively partitioned as a function of the applied interfacial potential difference. In this study, a mixture comprising 4-octylbenzenesulfonate (4-OBSA-) and tetraethylammonium (TEA+) ions was evaluated. The application of negative potential differences enabled the selective extraction of 4-OBSAinto the organic phase, and more positive potential differences enabled the selective extraction of TEA+. However, intermediate potentials lead to the coextraction of both ions into the organic phase, with apparent selectivity for TEA+ over 4-OBSA-. An increased concentration of either ion in the mixture inhibited the extraction response of the other ion, but the order of the extraction at these intermediate potentials was always TEA+ followed by 4-OBSA-. The reasons for the selectivity for the cation over the anion are discussed. The development of new analyte extraction methods and technologies receives major attention from the analytical research community around the world. The benefits of innovations in this area include overcoming sample matrix effects (sample cleanup) and analyte preconcentration. Traditional manual methods of liquid-liquid extraction (LLE) suffer from drawbacks such as the large volumes of sample and reagents employed,1,2 solvent toxicity,3 multistage and time-consuming operations,3 or sample loss (adsorption) and contamination.4 Examples of recent approaches to overcome these problems include LLE based on affinity two-phase partitioning in acoustically levitated drops (also known as the airborne analytical system),5-9 liquid-phase micro* To whom correspondence should be addressed. Phone +353-21-4904079. Fax +353-21-4270271. E-mail: [email protected]. (1) Fang, Z.; Zhu, Z.; Zhang, S.; Xu, S.; Guo, L.; Sun, L. Anal. Chim. Acta 1988, 214, 41-55. (2) Chao, J.-B.; Liu, J.-F.; Wen, M.-J.; Kiu, J.-M.; Cai, Y.-Q.; Jiang, G.-B. J. Chromatogr., A 2002, 955, 183-189. (3) Psillakis, E.; Kalogerakis, N. Trends Anal. Chem. 2003, 22, 565-574. (4) Cladera, A.; Miro´, M.; Estela, J. M.; Cerda´, V. Anal. Chim. Acta 2002, 421, 155-166. (5) Santesson, S.; Barinaga-Rementerı´a Ramirez, I.; Viberg, P.; Jergil, B.; Nilsson, S. Anal. Chem. 2004, 76, 303-308. (6) Santesson, S.; Johansson, J.; Taylor, L. S.; Levander, I.; Fox, S.; Sepaniak, M.; Nilsson, S. Anal. Chem. 2003, 75, 2177-2180. (7) Santesson, S.; Andersson, M.; Degerman, E.; Johansson, T.; Nilsson, J.; Nilsson, S. Anal. Chem. 2000, 72, 3412-3418. 10.1021/ac0521192 CCC: $33.50 Published on Web 03/16/2006

© 2006 American Chemical Society

extraction,10,11 and micro liquid-liquid extraction12 which use microliter or submicroliter volumes (100 nL-2 µL)5 and offer ways of avoiding unspecific adsorption5,7,13 and speeding up the operation (fast LLE).14-16 Another approach to manipulate the extraction or coextraction of ions from an aqueous sample phase into an organic solvent phase is via electrochemistry. Liquid-liquid electrochemistry, or electrochemistry at the interface between two immiscible electrolyte solutions (ITIES),17-22 can be used for voltammetric and amperometric determination of ions,23-25 including the analysis of non-redox-active species,22,25,26 where detector selectivity is a function of the potential difference imposed between the immiscible phases.23 Liquid-liquid electrochemical detection may be employed as a detection method in flow injection analysis (FIA)23,27-32 and in ion chromatography,25,26 (8) Santesson, S.; Degerman, E.; Johansson, T.; Nilsson, J.; Nilsson, S. Am. Lab. 2001, 33, 13-18. (9) Santesson, S.; Cedergren-Zeppezauer, E. S.; Johansson, T.; Laurell, T.; Nilsson, J.; Nilsson, S. Anal. Chem. 2003, 75, 1733-1740. (10) Ho, T. S.; Reubsaet, J. L. E.; Anthonsen, H. S.; Pedersen-Bjergaard, S.; Rasmussen, K. E. J. Chromatogr., A 2005, 1072, 29-36. (11) Halvorsen, T. G.; Pedersen-Bjergaard, S.; Reubsaet, J. L. E.; Rasmussen, K. E. J. Sep. Sci. 2003, 26, 1520-1526. (12) Carlsson, K.; Karlberg, B. Anal. Chim. Acta 2000, 415, 1-7. (13) Welter, E.; Neidhart, B. Fresenius J. Anal. Chem. 1997, 357, 345-350. (14) Lucchetti, D.; Fabrizi, L.; Esposito, A.; Guandalini, E.; Di Pasquale, M.; Coni, E. J. Agric. Food Chem. 2005, 53, 9689-9694. (15) Scherer, C.; Wachter, U.; Wudy, S. A. Analyst 1998, 123, 2661-2663. (16) Terje´ki E.; Kapa´s M. J. Pharm. Biomed. Anal. 2001, 24, 913-920. (17) Samec, Z.; Marecˇek, V.; Weber, J. J. Electroanal. Chem. 1979, 100, 841852. (18) Vany´sek, P. Anal. Chem. 1990, 62, 827A-835A. (19) Girault, H. H. Modern Aspects of Electrochemistry; Plenum Press: New York, 1993; Vol. 25, pp 1-62. (20) Vany´sek, P.; Buck, R. P. J. Electrochem. Soc. 1984, 131, 1792-1796. (21) Vany´sek, P. Trends Anal. Chem. 1993, 12, 357-363. (22) Ortun ˜o, J. A.; Herna´ndez, J.; Sa´nchez-Pedren ˜o, C. Electroanalysis 2004, 16, 827-831. (23) Wilke, S.; Franzke, H.; Mu ¨ ller, H. Anal. Chim. Acta 1992, 268, 285-292. (24) Reymond, F.; Fermı´n, D.; Lee, H. J.; Girault, H. H. Electrochim. Acta 2000, 45, 2647-2662. (25) Lee, H. J.; Girault, H. H. Anal. Chem. 1998, 70, 4280-4285. (26) Lee, H. J.; Pereira, C. M.; Silva, A. F.; Girault, H. H. Anal. Chem. 2000, 72, 5562-5566. (27) Sa´nchez-Pedren ˜o, C.; Ortun ˜o, J. A.; Herna´ndez, J. Anal. Chim. Acta 2002, 459, 11-17. (28) Marecek, V.; Janchenova, H.; Colombini, M. P.; Papoff, P. J. Electroanal. Chem. 1987, 217, 213-219. (29) Hundhammer, B.; Solomon, T.; Zerihun, T.; Abegaz, M.; Bekele, A.; Graichen, K. J. Electroanal. Chem. 1994, 371, 1-11. (30) Wilke, S. Anal. Chim. Acta 1994, 295, 165-172. (31) Sawada, S.; Toril, H.; Osakai, T.; Kimoto, T. Anal. Chem. 1998, 70, 42864290. (32) Sawada, S.; Taguma, M.; Kimoto, T.; Hotta, H.; Osakai, T. Anal. Chem. 2002, 74, 1177-1181.

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in which the selectivity can be tuned by choice of appropriate ionophores and the interfacial applied potential difference, thus overcoming the lack of selectivity of conductometric detectors. Several reports discuss the use of water|gel interfaces24,27,28 or a combination of water|gel interfaces with microporous membranes between the two phases (referred to as micro-ITIES)33 for interface stabilization. Chronocoulometric detection of transferring ions has also be implemented in a FIA system.27 Kihara and co-workers34 have used coulometric ion transfer at the oil|water interface for the determination of ions. They employed a flow cell based on a porous poly(tetrafluoroethylene) (PTFE) tube with aqueous phase flowing through it, immersed in an organic electrolyte solution. The coulometric efficiency was dependent on the length of the tube, the electrode diameter, and the aqueous-phase flow rate. All of these examples show clearly that liquid-liquid electrochemistry has an important role to play in the development of new detection methods for ions. Recently,35 we reported a mode of liquid-liquid extraction based on the electrochemical control of partition at the ITIES. The advantage of such a system is that the extraction process is both controlled and monitored by the electrochemical properties of the system: the applied interfacial potential difference and the ion transfer current, respectively. In a typical setup, the organic phase acts as the receiving phase and can be stationary as well as provided with additional mechanical strength by jellification. In this case, the aqueous phase (the donor phase) is flowed over the surface of the organic phase. The extraction characteristics can be manipulated by changes in the applied interfacial potential difference. As the interfacial potential difference is made more positive, the aqueous phase becomes positive with respect to the organic phase and positively charged analytes will be induced to move from the aqueous to the organic phase. Conversely, when the interfacial potential difference is made more negative, the aqueous phase becomes negative with respect to the organic phase and negatively charged analytes will be induced to transfer from the aqueous phase to the organic phase. Furthermore, different ions have different transfer potentials, based on their Gibbs free energies of transfer across the ITIES. Hence, the application of a potential difference can be tailored to the extraction of a particular ion in the presence of other ions, thus imparting a degree of selectivity to the procedure.35 Therefore, selective extraction or separation of different ionic species is possible. In this paper, we report on further studies into the application of electrochemistry at ITIES for the extraction or separation of ionic compounds as opposed to their detection. We focus on extraction of ions of opposite charge and, in particular, the coextraction of these ions at applied potentials at which both ions transfer. The aim was to examine whether it would be possible to coextract ions of opposite charge that transfer at the same applied potential. Under such circumstances, it was thought possible to extract both ions but to observe no transfer current, due to the currents being of opposite sign, thus canceling each other. As shown below, it was observed that, at extremes of applied (33) Wilke, S.; Osborne, M. D.; Girault, H. H. J. Electroanal. Chem. 1997, 436, 53-64. (34) Yoshimizu, A.; Uehara, A.; Kasuno, M.; Kitatsuji, Y.; Yoshida, Z.; Kihara. S. J. Electroanal. Chem. 2005, 581, 275-283. (35) Berduque, A.; Sherburn, A.; Ghita, M.; Dryfe, R. A. W.; Arrigan, D. W. M. Anal. Chem. 2005, 77, 7310-7318.

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potential difference of either polarity, the selective extraction of the cation or anion of the target analyte pair was achieved, as expected. However, at intermediate applied potentials, it was observed that dual extraction occurred, with the cation always transferring before the anion, as shown by a positive current (for the cation extraction) followed by a negative current (for the anion extraction). This implies selectivity in the extraction process in favor of the cation. EXPERIMENTAL SECTION Chemicals. The aqueous-phase electrolyte was lithium chloride (10 mM), prepared in purified water (18 MΩ cm purity). The ionic analyte species studied were 4-octylbenzenesulfonate (4OBSA-) and tetraethylammonium (TEA+), obtained as the sodium and chloride salts, respectively. The organic phase was nitrobenzene (NB) containing bis(triphenylphosphoranylidine)ammonium tetrakis(4-chlorophenylborate) (BTPPATPBCl; 10 mM), stabilized by jellification with low molecular weight poly(vinyl chloride). The BTPPATPBCl was prepared by metathesis25,35,36 of bis(triphenylphosphoranylidene)ammonium chloride and potassium tetrakis(4-chlorophenyl-borate). Solutions of 4-OBSA- and TEA+ were prepared in aqueous LiCl (10 mM). Apparatus. A CHI660B electrochemical analyzer (CH Instruments) was used for all electrochemical investigations. A fourelectrode electrochemical flow cell was employed, with two platinum mesh counter electrodes (one for each phase), a Ag|AgCl reference electrode for the aqueous phase, and a Ag|AgCl pseudoreference electrode for the organic gel phase.35 All potentials reported in this paper were corrected to the Galvani scale 0 (∆wo φ) by assuming a standard ion transfer potential (∆wo φTEA +) of -0.059 V for TEA+ between water and NB.37,38 The flow cell was prepared from PTFE, with the aqueous phase flowing over the jellified organic phase.35 The cross-sectional area of the interface was 1.13 cm2. The aqueous phase was delivered to the cell by a syringe pump (KD Scientific KDS200 series syringe pump), and a six-port valve (with a 100-µL injection loop) was used to inject the sample. Methodology. Investigations were initiated by performing cyclic voltammetry (CV) of the background LiCl (10 mM) aqueous electrolyte under stationary solution conditions.35 The CVs were followed by hydrodynamic constant-potential extractions with a subsequent linear sweep voltammetric (LSV) scan for confirmation of the extracted ions in the organogel phase. This potentiostatic extraction was applied to blank aqueous-phase electrolyte as well as to injections of the two target analytes (4-OBSA- and TEA+) in the flowing aqueous phase. These potentiostatic extractions were performed at various applied potential differences. All analyses are based on triplicate injections of 100 µL of the analyte (1 mM) in background aqueous-phase electrolyte (LiCl (10 mM)). The solution flow rate was 1 mL min-1.35 Regeneration of the organogel phase was achieved by the potentiostatic back-extraction of the analyte into the aqueous phase at applied potential differences extreme enough to remove the analyte from the organic phase. The application of sufficiently positive potentials, in the case of the anionic analyte, or sufficiently negative potentials, (36) Ulmeanu, S.; Lee, H. J.; Fermin, D. J.; Girault, H. H. Electrochem. Commun. 2001, 3, 219-223. (37) Samec, Z. Pure Appl. Chem. 2004, 76, 2147-2180. (38) Tatsumi, H.; Katano, H. Anal. Sci. 2004, 20, 1613-1615.

Figure 1. Potentiostatic extraction of 4-OBSA- at ∆wo φ ) -0.263 V (3) and of TEA+ ∆wo φ ) 0.113 V (1), and potentiostatic regeneration of the gel after the extraction of 4-OBSA- (4) and of TEA+ (2) using ∆wo φ ) 0.113 V and ∆wo φ ) -0.238 V, respectively. Flow rate: 1 mL min-1. Triplicate sample injections: 100 µL of 1 mM 4-OBSA- or 1 mM TEA+.

Figure 2. (A) LSV of blank LiCl (10 mM) (3), LSV of the 4-OBSA- extracted (1), and LSV after the potentiostatic regeneration of the gel (2). (B) LSV after cleaning of the gel or LSV of the blank (3) using a negative sweep, LSV of TEA+ extracted (1), and LSV after the potentiostatic regeneration of the gel (2).

in the case of the cationic analyte, together with higher flow rates (i.e., 2 mL min-1) than used during experimental studies increased the efficiency of this gel regeneration procedure. RESULTS AND DISCUSSION Initial Studies. The eventual aim of this work is a simple electrochemically manipulated liquid-liquid extraction system, which may be used as a cleanup or preconcentration procedure prior to, for example, instrumental analysis. In such a scenario, the formal ion-transfer potentials of the analyte ions of interest will be known or can be measured. By application of a potential positive of the formal transfer potential for a cationic analyte, it will be extracted into the organic phase; conversely, application of a potential negative of the formal potential for this ion will result in it remaining in the aqueous phase (if already there) or its backextraction from the organic phase into the aqueous phase. Similarly, for an anionic analyte, potentials negative of its formal potential will allow its extraction into the organic phase and potentials positive of its formal potential will enable its back-

extraction into the aqueous phase. The ionic species studied here, 4-OBSA- and TEA+, were initially investigated individually to compare their extraction performance with that obtained when they are interacting with each other in a mixture. Figure 1 shows the current signals for the potentiostatic extraction into the organogel phase of both 4-OBSA- and TEA+, injected as single analytes into the flowing aqueous phase electrolyte. Also shown in Figure 1 are the amperometric signals obtained during organogel-phase regeneration by application of an interfacial potential difference sufficient to back-extract the ion into the aqueous phase. Figure 2 demonstrates the back-extraction of either analyte from the organogel phase back into the aqueous phase. By comparison of the voltammograms performed after the organogel regeneration step (Figure 2A(2) and (B)2) with that of blank aqueous-phase electrolyte (Figure 2A(3) and B(3)), it is clear that the gel-phase regeneration works well. These postregeneration voltammograms show no peaks corresponding to 4-OBSAor TEA+. In fact, there is absolutely no difference between the LSV of the blank and the LSV performed after the regeneration Analytical Chemistry, Vol. 78, No. 8, April 15, 2006

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Figure 3. (A) Potentiostatic extraction response as a function of ∆wo φ. Flow rate, 1 mL min-1; sample injection, 100 µL of mixture (1mM 4-OBSA- and 1 mM TEA+). (B) Kinetic selectivity of the system for TEA+ over 4-OBSA-. Note: the peaks were normalized on the time axis relative to the time of injection.

step. This proves the suitable regeneration of the gel so that it can be used to extract other ions or for other analyses. Electrochemical Extraction and Coextraction of 4-OBSAand TEA+. The extraction of 4-OBSA- (1 mM) and TEA+ (1 mM) from a mixture was studied as a function of the extraction potential. The formal transfer potentials of both ions were determined by CV to be -0.059 V, identical under the conditions of this work. Potentials positive of this should favor extraction of the cation into the organogel phase, while potentials negative of this should favor extraction of the anion. First, a potential ∆wo φ ) 0.113 V was used, resulting in positive extraction amperometric peaks due to the transfer of TEA+ (Figure 3A). For successively decreasing applied potentials, only positive current peaks were obtained, indicating TEA+ extraction at ∆wo φ ) -0.062, -0.038, 0.013, 0.063, and 0.113 V. These experiments were followed by extractions at an applied potential ∆wo φ ) -0.263 V. This produced negative amperometric currents, as 4-OBSA- is selectively extracted into the organogel at sufficiently negative potentials. A further series of less negative applied potentials produced only negative current peaks, for ∆wo φ ) -0.263, -0.238, -0.188, and -0.138 V (Figure 3A). This is as expected: extremes of the 2720 Analytical Chemistry, Vol. 78, No. 8, April 15, 2006

potential enable selective extraction of the desired ion, in this case allowing selection of either the cation or anion from a mixture. However, it was found that subsequent extractions at intermediate applied potentials produced unusual behavior. These intermediate applied extraction potentials were those at which both ions were extractable when injected into the system as single analyte solutions. In a mixture, it was thought that, since currents of opposite sign would be produced (cation, positive current; anion, negative current), simultaneous injection and transfers across the interface might lead to zero net current. But this was found not to be the case. For example, using an extraction potential ∆wo φ ) -0.088 V, two peaks of low intensity were observed (Figure 3B), a positive peak (due to TEA+ extraction) followed by a negative peak (corresponding to 4-OBSA- extraction). Subsequent CV analysis produced a voltammogram of the mixture of extracted ions, which was similar to that obtained for either TEA+ or 4-OBSA- alone (Figure 4). This indicates that the dual-extract has electrochemical parameters identical to both of the single-species extracts carried out at either extreme of the applied potential scale in this work; i.e., that the formal transfer

Figure 4. Cyclic voltammograms of (1) 4-OBSA- extracted using the potential ∆wo φ ) -0.263 V (3 injections of 100 µL of 1 mM 4-OBSA-), (2) TEA+ extracted using the potential ∆wo φ ) 0.113 V (3 injections of 100 µL of 1 mM TEA+), and (3) mixture of 4-OBSA- and TEA+ coextracted using the potential ∆wo φ ) -0.088 V (3 injections of 100 µL of 1 mM 4-OBSA- and 1 mM TEA+).

potentials of both 4-OBSA- and TEA+ are identical and the CV of their mixture is not different from the CV of either ion. Other intermediate potentials close to -0.088 V, i.e., ∆wo φ ) -0.082 and -0.077, also lead to the coextraction of both ionic species (Figure 3B). These potentials are located within the potential range between the peak potentials in the CV (Figure 4), which means that both cationic and anionic species are extracted into the organogel phase. From the above, it can be seen that the ability to selectively extract the cation, by choice of more positive potentials, or the anion, by choice of more negative potentials, is clear and obvious. However, the interesting behavior observed at intermediate potentials offers scope for the coextraction of cations and anions. Notably, for this coextraction, the cation always transfers across the interface first. This selectivity may be termed a kinetic selectivity, achieved at intermediate values of ∆wo φ, that is, potentials between -0.138 and -0.038 V (approximately) lead to the extraction of both 4-OBSA- and TEA+, and the amperometric response not only shows this, but also indicates the kinetic selectivity for TEA+ over 4-OBSA-. Mixture introduction using these extraction potentials causes an initial positive peak (due to TEA+) that decreases and forms a second, negative, peak (the transfer of 4-OBSA-). The initial positive peak (transfer of the cation) increases and the negative peak (transfer of the anion) decreases when ∆wo φ goes toward -0.038 V, as more positive potentials induce the transfer of cations and hinder the transfer of anions. In contrast, the initial positive peak decreases and the negative peak increases when ∆wo φ goes toward -0.138 V, as more negative potentials cause the transfer of anions and hinder the transfer of cations. However, the cation-transfer peak (TEA+) was always observed to appear first, even for extraction potentials that favor the extraction of 4-OBSA- with respect to the transfer of TEA+. This can be seen in Figure 3B, which is an enlargement of part of Figure 3A (from -0.138 to -0.062 V). Effect of Concentration Ratio on the Electrochemical Coextraction of 4-OBSA- and TEA+. The potentiostatic coextraction of 4-OBSA- and TEA+ was studied as a function of the 4-OBSA-/TEA+ concentration ratio to see if the concentration ratio

influenced the order of ion transfer. Figures 5 and 6 show the potentiostatic extraction response of a series of mixtures of 4-OBSA- and TEA+ of different concentration ratios. The amperometric response was observed to increase (more positive peaks) in the case of TEA+ when ∆wo φ was increased. However, these positive amperometric peaks decreased considerably when 4-OBSAwas present in the sample (i.e., 1 mM 4-OBSA- and 1 mM TEA+); see comparison between the extraction of TEA+ (curve 2 in Figure 5A) and TEA+ in the mixture (curve 4 in Figure 5A). The effect of the anions on the extraction of TEA+ increased when the extraction potential was more negative (compare curves 2 and 4 in Figure 5A-D and curves 1 and 4 in Figure 5E), while more positive extraction potentials did not affect the TEA+ transfer to the same extent. When the effect of increasing TEA+ concentration on the extraction of 4-OBSA- was studied (i.e., 1 mM 4-OBSA- and 2 mM TEA+; 1 mM 4-OBSA- and 4 mM TEA+) (see curves 1 and 3 in Figure 5A-D, and curves 2 and 3 in Figure 5E), a compromise situation was obtained. In this case, the positive TEA+ amperometric extraction peaks increased when the extraction potential was positive enough so that 4-OBSA- transfer did not affect the TEA+ transfer. However, when the extraction potentials were made more negative (e.g., ∆wo φ ) -0.088 V), the higher concentrations of TEA+ still produced smaller positive current peaks with a concomitant increase of a negative peak due to 4-OBSA-. Increases in concentration of 4-OBSA- caused a decrease of the TEA+ peak at any extraction potentials (compare curve 2 and curves 4 and 5 in Figure 5A-D and curve 1 and curves 4-6 in Figure 5E), although the effect of 4-OBSA- was radically magnified when ∆wo φ was more negative. Similar potentiostatic extraction responses were obtained from the extraction peaks for 4-OBSA- transfer. In this case, the amperometric peak intensities became more negative when ∆wo φ was made more negative (see curve 6 in Figure 5A-D and curve 7 in Figure 5E). These amperometric peaks decreased in magnitude when TEA+ was added to the sample (compare curves 6 and 4 in Figure 5A-D and curves 7 and 4 in Figure 5E), as TEA+ Analytical Chemistry, Vol. 78, No. 8, April 15, 2006

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Figure 5. Potentiostatic extraction response as a function of ∆wo φ and the 4-OBSA-/TEA+ concentration ratio. Flow rate, 1 mL min-1; sample injection, 100 µL of sample. Note: the peaks were normalized on the time axis relative to the time of injection. Potential difference ∆wo φ ) -0.037 (A), -0.052 (B), -0.057 (C), -0.062 (D), and -0.088 V (E).

transfers across the interface at similar extraction potentials. The effect of TEA+ in the extraction of 4-OBSA- was dramatically increased by the application of more positive extraction potentials, as this supports the transfer of the cation, and by the increase of TEA+ concentration (compare curve 6 with curves 1, 3, and 4 in Figure 5A-D and curve 7 with curves 2-4 in Figure 5E). The increase in concentration of either 4-OBSA- or TEA+ followed the same general trend described for the positive TEA+ transfer peaks (above), but in this case, with the opposite sign: the extraction of 4-OBSA- was greater at more negative extraction potentials and at decreased TEA+ concentration in the mixture. These experiments (Figure 5) also demonstrate the kinetic selectivity of the system for TEA+ over 4-OBSA-; that is, situations involving the transfer of both ions always lead to an initial positive peak followed by a negative peak, with the peak magnitudes depending on the extraction potential and the concentration ratio. 2722 Analytical Chemistry, Vol. 78, No. 8, April 15, 2006

Figure 5 also shows that TEA+ has a greater effect on the extraction of 4-OBSA- than that of 4-OBSA- on the extraction of TEA+. This is possibly due to the ion-transfer kinetics of both species: sufficiently positive extraction potentials (∆wo φ ) -0.062, -0.057, -0.052, and -0.037 V) caused greater amperometric peaks for TEA+ extraction when the concentration of TEA+ was increased with 4-OBSA- present in the mixture (i.e., 1 mM 4-OBSA- and 4 mM TEA+; compare curves 1-4 in Figure 5), but the peaks due to 4-OBSA- did not increase at any extraction potential ∆wo φ when the sample contained TEA+, even when the concentration of 4-OBSA- was quadrupled (compare curves 4-6 in Figure 5A-D and curves 4-7 in Figure 5E). Figure 6 summarizes all the experimental values extracted from Figure 5, showing the effect of the potential difference and of the concentration ratio on the extraction of both species. Figure 7 clearly demonstrates that the signal as a function of the applied potential

Figure 6. Effect of ∆wo φ and 4-OBSA- to TEA+ concentration ratio in the potentiostatic extraction peak response. Single points indicate the amperometric peak response at each ∆wo φ in the absence of the opposite ionic species. (A) Peak currents as a function of the concentration of 4-OBSA-. (B) Peak currents as a function of the concentration of TEA+.

for a single-ion analyte solution (cation or anion) is compromised by the addition of the other analyte (anion or cation, respectively): current decreases on addition of the other ion, at all applied potentials in the intermediate potential zone where both anions transfer. From the above, the increase of concentration of an ion in the mixture is observed to inhibit the amperometric peak response due to the extraction of the oppositely charged ionic species. This inhibition increases when the extraction potentials applied are opposite to the extracted ion charge (positive potentials in the case of anions and negative potentials in the case of cations). General Discussion. The selectivity in the transfer process might be due to the different solvation energies of these two ions or to a kinetic effect. However, differences in solvation energies

should be seen also in differences in the formal transfer potentials; in this work, the transfer potentials were observed to be identical. A kinetic effect could occur due to a combination of simple ion transfer (for TEA+) and adsorption plus simple ion transfer (for 4-OBSA-). 4-OBSA- contains a highly hydrophilic moiety, the sulfonate group, which is difficult to transfer into the organic phase in comparison to alkylammonium cations of the same carbon atom number. Furthermore, the potentials at which both ions transfer are in all cases at the foot of the voltammetric waves, where the charge transfer is under electrochemical kinetic control rather than mass transport control. In this case, differences in kinetics of the ion transfer would be observable, in comparison to a situation in which the applied potentials were in a diffusioncontrolled ion-transfer region. Kinetic studies of ion transfer at Analytical Chemistry, Vol. 78, No. 8, April 15, 2006

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Figure 7. Effect of ∆wo φ and 4-OBSA- to TEA+ concentration ratio in the potentiostatic extraction peak response.

ITIES have been interpreted in terms of an activation barrier to the change of solvation as the ions cross the interface or to a slower diffusion of the ion through the interfacial mixed-solvent zone.39 The possible adsorption of 4-OBSA- at the interface before it transfers into the organic phase could be due to its slower diffusion through this zone. If the 4-OBSA- transfer is preceded by an interfacial adsorption, but the TEA+ transfer is not, then a more rapid transfer of the cation should result in its transfer first even when both ions arrive at the interface from the flowing aqueous phase at the same time. A number of studies have reported the adsorption of aromatic sulfonates at liquid-liquid interfaces.40-43 The adsorption and transfer of 1-pyrenesulfonate (PSA-) at the water|1,2-dichloroethane interface was studied by voltammetry and potential modulated fluorescence spectroscopy.40 Despite the reversible transfer behavior of PSA- across the interface observed by voltammetry, the dynamic spectroscopic measurements revealed adsorption and dimerization processes at potentials near the transfer region.40 These studies suggest that PSA- was adsorbed

at the interface prior to the transfer step.40 Although 4-OBSAwas used in our studies, the observations described for PSA- are relevant, as both compounds are aromatic sulfonates. Other aromatic sulfonates whose adsorption at electrified liquid-liquid interfaces has been studied are dodecylbenzene sulfonate,41,42 and 2-(n-octadecylamino)naphthalene-6-sulfonate.43 The interaction of these compounds at such interfaces can have complex effects. For example, Kakiuchi44-46 studied the potential-dependent adsorption of ionic surfactants, showing electrochemical instability near the standard ion-transfer potential of the surfactant.45,46 Girault’s group showed47 the effect of the adsorption of ionic species on the interfacial potential distribution, where the potential difference splits into three different parts. Thus, adsorption effects of aromatic sulfonate compounds at liquid-liquid interfaces are well known. In contrast, although preliminary studies of the transfer of tetraalkyammonium cations across the ITIES suggested a possible adsorption at the interface,48 more recent investigations49 based on interfacial tension measurements suggest a change of the inner layer thickness associated with a change of

(39) Cai, C.; Tong, Y.; Mirkin, M. V. J. Phys. Chem. B 2004, 108, 17872-17878, and references therein. (40) Nakatani, K.; Nagatani, H.; Fermı´n, D. J.; Girault, H. H. J. Electroanal. Chem. 2002, 518, 1-5. (41) Li, Y.; Zhang, P.; Dong, F.-L.; Cao, X.-L.; Song, X.-W.; Cui, X.-H. J. Colloid Interface Sci. 2005, 290, 275-280. (42) Watry, M. R.; Richmond, G. L. J. Am. Chem. Soc. 2000, 122, 875-883. (43) Higgins, D. A.; Corn, R. M. J. Phys. Chem. 1993, 97, 489-493.

(44) Kakiuchi, T. J. Electroanal. Chem. 2001, 496, 137-142. (45) Kakiuchi, T. J. Electroanal. Chem. 2002, 536, 63-69. (46) Kakiuchi, T.; Chiba, M.; Sezaki, N.; Nakagawa, M. Electrochem. Commun. 2002, 4, 701-704. (47) Su, B.; Eugster, N.; Girault, H. H. J. Electroanal. Chem. 2005, 577, 187196. (48) Martins, M. C.; Pereira, C. M.; Girault, H. H.; Silva, F. Electrochim. Acta 2004, 50, 135-139.

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the radius of the ion, rather than adsorption of the ions at the interface.49 CONCLUSION The studies reported here demonstrate that electrochemistry at the ITIES can be used as a route for the selective separation of ions in mixtures. Application of potentials positive of the formal potential of a cation favors its extraction into the organogel phase, whereas application of potentials negative of the formal transfer potential of an anion favors its extraction into the organogel phase. However, it has been found that, at intermediate potentials, at which both cation and anion extract into the organogel phase, there is unusual behavior, which always results in the cation transferring before the anion. This is suggested as a kinetic selectivity for the extraction of TEA+ over OBSA- at the potentials where both ions extract across the interface. This temporal resolution of cation and anion species implies an inherent selectivity within the flow extraction system developed here and merits further study as to the fundamental reasons for the observed selectivity. This phenomenon may be explained by the adsorption of anionic surfactants (like 4-OBSA-) at the oil|water interface40-47 and the nonadsorption of tetraalkylammonium spe(49) Lhotsky´, A.; Marecˇek, V.; Za´lisˇ, S.; Samec, Z. J. Electroanal. Chem. 2005, 585, 269-274.

cies, such as TEA+.49 At intermediate potentials (where both TEA+ and 4-OBSA- transfer across the interface), the initial transfer of TEA+ is followed by the 4-OBSA- transfer, as the latter ions adsorb at the interface first and then transfer. Furthermore, the potentiostatic extraction peaks of TEA+ in mixture with 4-OBSA- are of lower magnitude than those attained in the absence of ionic surfactant, as the surfactant adsorption hinders the transfer of TEA+, as the transfer interfacial area is partially occupied by adsorbed ions of surfactant. Conversely, the potentiostatic extraction peaks of 4-OBSA- in the mixture are also of lower magnitude than those obtained in the absence of the tetraalkyammonium. This may be an artifact associated with positive and negative transfer currents simply partially canceling each other out. Nevertheless, the results presented here illustrate the rich possibilities for molecular extractions and separations to be achieved by electrochemically controlled liquid-liquid extraction processes. ACKNOWLEDGMENT The financial support of Science Foundation Ireland (02/IN.1/ B84) is acknowledged. Received for review December 2, 2005. Accepted February 15, 2006. AC0521192

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