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Study of Ion Transfer Across the Liquid-Liquid Interface Coupled to Electrochemical Redox Reaction at the Pt Electrode with a Three-Electrode Potentiostat Xiu Hui Liu, Jun Yang, Guo Fang Zuo, Kai Zhang, Cun Wu Dong, and Xiao Quan Lu* College of Chemistry & Chemical Engineering, Northwest Normal UniVersity, Lanzhou 730070 ReceiVed: July 23, 2007; In Final Form: October 3, 2007
The ion transfer across the liquid-liquid interface coupled to electrochemical redox reaction at the Pt electrode was investigated simultaneously with a three-electrode potentiostat in combination with cyclic voltammetry. The approach involves using a small aqueous droplet containing a redox couple supported on a platinum electrode and immersed into an organic electrolyte solution. Both electron and ion transfer processes can be observed at separate interfaces for the first time. The effects of concentrations of the redox couple on ion transfer indicate that the overall electrochemical process, proceeding as a coupled electron-ion transfer reaction, is controlled by the electron transfer across the Pt/W interface and the electron-transfer reaction is the drive force of ion transfer across the L/L interface. In addition, for the first time, a new oscillation phenomenon has been observed directly, which is related to the L/L interface, to concentration of ferrocyanide and to potential scan range. The oscillation is caused by the formation of an ion pair at the interface between Fe(CN)64in the aqueous phase and TAB+ in the DCE phase.
2. Experimental Section
1. Introduction Charge-transfer processes across the interface between two immiscible liquids represent a simplified model to understand complex electron and ion transfers across biomembranes, which play a key role in the functioning of living cells.1 Much attention has been given to the study of ion-transfer reactions and electron-transfer reactions because of their importance in the understanding of heterogeneous kinetics and potential applications in mimicking biological membranes over the past few decades.2-5 In the past decade, a new approach to performing an electrochemically driven phase-transfer of ions has been developed, which uses a three-electrode potentiostat on droplets,6-13 or on thin liquid films.14-19 The main advantage of this method of studying charge-transfer reactions at a L/L interface stems from the fact that a classical three-electrode potentiostat can be used instead of the four-electrode potentiostat specially designed for the electrochemical studies at the interface between two immiscible electrolyte solutions (ITIEs).10-21 Recently, Ulmeanu6 and Shao7 studied IT and ET reactions across the interface using an aqueous droplet containing a redox couple adhered to a Pt electrode. The specific composition of the aqueous droplet makes the Pt electrode function like a reference electrode. Thus, the interfacial potential can be controlled externally. Hitherto, most studies considered only ion transfer or facilitated ion-transfer processes between an aqueous solution and an organic phase. In this paper, we employed a similar idea to investigate ion-transfer (IT) reactions across a W/DCE interface coupled to electron-transfer reactions (ET) at the Pt electrode simultaneously with a three-electrode potentiostat. Such an approach has the advantages of extending the potential window significantly and allowing investigation of the chargetransfer process at separate interfaces. Besides, some interesting phenomena have been investigated experimentally in detail. * Corresponding author. Tel: +86 0931 7971276; fax: +86 0931 7971276; e-mail:
[email protected].
2.1. Materials and Solutions. Potassium chloride (KCl, AR), lithium chloride (LiCl, AR), potassium ferrocyanide (K4Fe(CN)6, AR), potassium ferricyanide (K3Fe(CN)6, AR), and 1,2-dichloroethane (DCE, 99.8%, AR) were purchased from Beijing Chemical Co., China. Tetrabutylammonium tetraphenylborate (TBATPB, 99%, Aldrich) and tetramethyammonium chloride (TMACl, 99%, Aldrich) were used without purification. The aqueous and organic phases were prepared with deionized water (Milli-Q, Millipore Corp.) and DCE, respectively. DCE was washed several times with deionized water before use. Special precautions were taken for dealing with DCE and other hazardous chemicals. 2.2. Apparatus and Electrochemical Measurements. A CHI-660A electrochemical workstation (CH Instruments Inc.,) with an IR compensation connected to a personal computer was employed to record cyclic voltammograms. The aqueous and organic solutions were mutually saturated prior to each experiment. A drop of a certain volume of aqueous solution containing an equal molar ratio of the K3Fe(CN)6/K4Fe(CN)6 redox couple was transferred to the surface of a freshly polished platinum disk electrode (d ) 2 mm) with a small syringe. The aqueous solution spreads spontaneously across the surface of this platinum electrode and covers it completely. Then the electrode was turned over and immersed immediately into a DCE solution (of a certain volume) containing TBATPB salt. A Pt wire (d ) 1 mm) was used as the counter electrode, and an Ag wire coated with AgTPBCl (Ag/AgTPBCl) was used as the organic phase reference electrode; this was made according to the literature.22 Prior to each experiment, the platinum electrode was polished sequentially with 0.3 and 0.05 µm alumina. Then it was cleaned ultrasonically with ethanol and deionized water for 2 min, respectively. All experiments were performed at room temperature (20 ( 2 °C).
10.1021/jp0757560 CCC: $40.75 © 2008 American Chemical Society Published on Web 12/12/2007
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3. Results and Discussion 3.1. Observed ET and IT Coupling Processes in the Same Potential Window. Figure 1A is the potential window of a 0.1 M LiCl aqueous droplet immersed in DCE solution containing 10 mM TBATPB. The negative side of the potential window is limited by the transfer of Cl- from the aqueous to the DCE phase or by the transfer of TBA+ from the DCE to the aqueous phase. Alternatively, the positive side is limited by the transfer of TPB- from the DCE to the aqueous phase or by the transfer of Li+ from the aqueous to the DCE phase. The potential window is about 1200 mV and is larger than that obtained with a four-electrode setup.23 The following electrochemical cell (I) was employed to observe the transfer of ions across the W/DCE interface.
Pt/x mM K3Fe(CN)6 + x mM K4Fe(CN)6 + 0.1 M LiCl/10 mM TBATPB/AgTPB/Ag (Cell I) There are two peaks appearing within the potential window (as shown in Figure 1C). Compared with Figure 1B, which is a typical cyclic voltammogram obtained of the redox couple in the aqueous phase, the right peak (1) belongs to the redox peak of K3Fe(CN)6/K4Fe(CN)6 at the Pt electrode, and the left peak (2) is the transfer peak of ion across the W/DCE interface. The peak-to-peak separation of peak 2 is about 75 mV; the peak 2 current is proportional to the square root of the sweep rate, and the ratio between the forward and the reverse peak current intensity is close to 1. These observations indicate that peak 2 in the cyclic voltammogram is a reversible, diffusion-controlled transfer process of a single charged species. On the basis of our results and refs 7 and 8, we confirm that the pair of peak 2 is a transfer peak of potassium ion across the W/DCE interface. Hitherto, there are no reports, to our knowledge, of observed ion transfer across a liquid-liquid interface coupled to electrochemical redox reaction at the Pt electrode in the same potential window simultaneously with a three-electrode setup directly. The electrochemical process couples electron-transfer I at the Pt/W interface with ion transfer II at the W/O interface:
Fe(CN)64- f Fe(CN)63- + e- (pt/w)
(I)
K+(w) f K+(o) (o/w)
(II)
Both processes proceed simultaneously, coupled by the electroneutrality requirement, hence appearing as a single overall process.
Fe(CN)64- + K+(w) f Fe(CN)63- + K+(o) + e-
(III)
The formal potential of the overall reaction III is
Ept/o ) Ept/w + Ew/o ) Ept/w + ∆owφion 0 EPt/w ) ∆Pt w φFe(CN)6 4-/Fe(CN)63- +
(1)
4RT C(Fe(CN)6 ) ln ZF C(Fe(CN) 3-) 6
Eo/w ) ∆owφK0 + +
w RT CK+ ln o ZF C +
(2) (3)
K
EPt/o is the potential between the working electrode and the organic phase that is potentiostatically controlled. EPt/w and Ew/o
Figure 1. (A) Cyclic voltammogram of a 0.1 M LiCl aqueous droplet immersed in DCE solution containing 10 mM TBATPB. Scan rate was 50 mV/s. (B) Cyclic voltammogram obtained with 1 mM Fe(CN)63-/1 mM Fe(CN)64- aqueous, 0.1 M LiCl. Scan rate was 50 mV/s. (C) Cyclic voltammogram obtained with cell 1. x ) 0.5. Scan rate was 50 mV/s.
are potential differences at the Pt/W and W/O interface, respectively. Equation 1 shows that the variation of EPt/o results in simultaneous variations of both EPt/w and Ew/o, which, in a general case, makes the situation rather complex. Hence, it is particularly recommended to simplify the situation by fixing the potential difference at one of the interfaces. This can be achieved in our experiments by using an aqueous droplet containing a redox couple with equal molar ratio adhered to a Pt electrode. The specific composition in the aqueous droplet makes the Pt electrode function like a reference electrode as long as the current does not alter significantly and the concentration ratio of K3Fe(CN)6/K4Fe(CN)6 remains constant during the ion-transfer process. Thus, the interfacial potential can be controlled externally. Equations 2 and 3 are the Nernst equation for the redox reaction on the platinum electrode and the Nernst equation for the ion transfer at W/O interface, 0 respectively. Here ∆Pt w φFe(CN)64-/Fe(CN)63- is the standard po-
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Figure 2. Effect of the scan circles on the peak of K+ ion transfer across the interface and on the redox peak of Fe(CN)63-/Fe(CN)64aqueous, using cell 1 (x ) 0.5). Scan rate was 50 mV/s. 1a was the first scan, and 1a (20th) was 20th scan. 0 tential of the redox couple, ∆ow φK+ is the standard transfer potential of the potassium ion, and CKw+ and CKo + are the concentrations of the potassium ion in the aqueous and organic phase, respectively. In addition, we also employed tetramethyammonium (TMA+) as an internal reference ion to estimate the standard transfer potential of the potassium ion in this case. According to the TATB assumption, the formal Galvani potential difference and the formal Gibbs transfer energy for the direct transfer of ions from water to DCE can be evaluated based on the relationships in the following equations:
w o w 1/2 ∆wo φoi ′ - ∆wo φ1/2 i ) ∆o φTMA+′ - ∆o φTMA+
(4)
o ∆wo Gtr,i,wfo ′ ) zF∆wo φoi ′
(5)
where i is the transferred ion, F is the Faraday constant, ∆wo φ1/2 is the half-wave potential of the transferred ion, ∆wo φo′ is the formal Galvani potential difference for the direct transfer of ions o from water to DCE, and ∆wo Gtri,wfo ′ is the formal Gibbs transfer energy of the transfer ion. The experimental result is that the standard transfer potential of the potassium ion is about 440 mV and the formal Gibbs transfer energy of the potassium ion is about 44.4 kJ/mol, which are in good agreement with reported data.7,8 We also investigated the effect of the potential scan cycles on both the ion-transfer peak and the electron-transfer peak. As can be seen in Figure 2, the K+ ion-transfer peak current decreased, whereas the electron-transfer peak increased with potential cycling. The possible reason, we consider, may be the influence of formation of an electric double layer at the W/O interface. In aqueous solution, if K+ ions absorb at the W/O interface, then the negatively charged ions (such as Cl-) in the aqueous phase will stay near the K+ ion and form the electric double layer. The negatively charged ions will embarrass Fe(CN)64- and Fe(CN)63- diffusion from the electrode to bulk solution, which equals to the increase in the concentrations of Fe(CN)64- and Fe(CN)63- around the electrode. Then, the electron-transfer peak increased with potential cycling. The reason that the peak current of K+ transfer decreases markedly after continuous potential scanning may be ascribed to the diffusion of the K+ ion in the organic phase. 3.2. Driving Force to Potassium Ion Transfer at the L/L Interface. An interesting phenomenon was observed in our
Figure 3. (A) Cyclic voltammograms obtained with different concentrations of K+ ion transfer across the water/1,2 DCE interface. Concentration of K+: 1, 0.052 M; 2, 0.102 M; 3, 0.502 M. Support electrolyte: KCl. Other conditions were the same as cell 1 (x ) 1.0). Scan rate was 50 mV/s. (B) Cyclic voltammograms obtained with K+ ion in different aqueous. 1, x ) 0.5; 2, x ) 1.0; 3, x ) 2.5. KCl: 0.1 M. Other conditions were the same as cell 1. Scan rate was 50 mV/s. (C) Cyclic voltammograms obtained with different concentrations of TMA+ ion transfer across the water/1,2 DCE interface, using cell 1. Scan rate was 50 mV/s. 1-3 were 5, 10, and 20 mM of TMA+ ion, respectively.
experiments that the transfer peak current of the potassium ion increases with increasing concentrations of the redox couple (K3Fe(CN)6/K4Fe(CN)6) in the aqueous phase. To understand this phenomenon clearly, we carried out two other experiments by employing KCl as supporting electrolyte. First, different concentrations of KCl (0.05, 0.1, 0.5 M) were added into the 1.0 mM K3Fe(CN)6/1.0 mM K4Fe(CN)6 solution, respectively, to ensure the same concentrations of Fe(CN)63- and Fe(CN)64-, and different concentrations of potassium ion in these solutions. As shown in Figure 3A, there are very slight differences in the
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Figure 4. (A) Cyclic voltammogram obtained with 5 mM Fe(CN)63-/5 mM Fe(CN)64- aqueous, 0.1 M LiCl. Scan rate was 50 mV/s. (B) Cyclic voltammogram obtained with 5 mM K+ ion transfer across the water/1,2 DCE interface, using cell 1 (x ) 2.5). Scan rate was 50 mV/s. S is the starting point of the potential scan. 1-3 represent the scan cycles. (C) Cyclic voltammogram obtained with 5 mM K+ ion transfer across the water/1,2 DCE interface, using cell 1 (x ) 2.5). Scan rate was 50 mV/s. S is the starting point of the potential scan. 1-3 represent the scan cycles. (D) Cyclic voltammogram obtained with 10 mM K+ ion transfer across the water/1,2 DCE interface, using cell 1 (x ) 5.0). Scan rate was 50 mV/s. 1-3 represent the scan cycles. (E) Cyclic voltammogram obtained with 5 mM K+ ion transfer across the water/1,2 DCE interface, using cell 1 (x ) 2.5). Scan rate was 10 mV/s. 1-3 represent the scan cycles. (F) Cyclic voltammogram obtained with 20 mM TMA+ ion transfer across the water/1,2 DCE interface, using cell 1 (x ) 5.0). Scan rate was 50 mV/s. 1-3 represent the scan cycles.
potassium ion transfer peak current of the three solutions (-3.407 µA (1), -3.466 µA (2), -4.194 µA (3)), which were caused mainly by the different concentrations of supporting electrolyte in the droplets. The second is to observe the potassium ion transfer peak currents in different concentrations of redox couple (1, 0.5 mM/0.5 mM; 2, 1.0 mM/1.0 mM; 3, 2.5 mM/2.5 mM). Figure 3B shows that there are huge differences in the transfer peak current (-1.577 µA (1), -3.466 µA (2), -8.409 µA (3)) although the K+ concentrations are almost the same in the three solutions (0.101, 0.102, 0.105 M). Moreover, Figure 3C shows that different concentrations of TMA+ have almost the same transfer peak currents in 1.0 mM/ 1.0 mM K3Fe(CN)6/K4Fe(CN)6 solution, although concentrations of TMA+ were 5, 10, and 20 mM, respectively. All of these results confirm that the transfer peak current of the ion strongly depended upon the concentration of the redox couple in droplets. In our case, K+ transfer across the W/DCE interface from the aqueous to the DCE phase is coupled with the oxidation of ferrocyanide at the Pt electrode. When the overall reaction III is kinetically controlled, both charge-transfer processes ought to proceed with the same rate. The rate of the overall process is dictated by the slower step, which can be either the electron transfer or the ion transfer. If the electron exchange is the slower step, then the kinetics is described by the following equation18
I ) ket exp(βetφPt/w)[(cRed)x)0 - exp(-φPt/w)(cOx)x)0] FS
(6)
where F is the Faraday constant, S is the electrode surface area, ket is the standard rate constant of the electron exchange, and βet is the anodic electron-transfer coefficient. (cRed)x)0 and (cOx)x)0 are concentrations of Red and Ox at the electrode surface, respectively. Just as discussed above, because the potential difference at the Pt/W interface, φPt/w, is constant during process of ion transfer, the variation of current I was caused by the corresponding variation of ket, (cRed)x)0, and (cOx)x)0 only. Therefore, the variation of ion transfer peak currents was caused by the corresponding variation of concentration of the redox couple. The results also demonstrate that the electron-transfer reaction is the rate-determining step in series couple reactions and is the driving force of the ion transfer across the L/L interface. Hence, Shao called it a process of ion transfer limited by an ET process.7 3.3. Oscillation Phenomena at the L/L Interface. We observed a new oscillation phenomenon when the concentration of the redox couple in aqueous solution is larger than 2.5 mM. To understand it better, we carried out some interesting experiments and the results are as follows: First, a finding from Figure 4A and B is that the oscillation is related to the interface. No oscillation is observed even if the concentration of Fe(CN)63-/Fe(CN)64- is larger than 5.0 mM in the aqueous phase. Second, the oscillation is related to the point of starting potential of the scan. When the concentration of the redox couple was 2.5 mM, oscillations did not appear in the first scan in the positive scan (Figure 4B, curve 1); they occurred in the second cycle after the reduction peak appeared, whereas, oscillation
152 J. Phys. Chem. C, Vol. 112, No. 1, 2008 appeared in the first scan (Figure 4C, curve 1) in the negative scan. We also performed the experiment by decreasing the potential scan range from -0.4-1.0 to 0.15-1.0 V. Figure 4D exhibits no oscillation even when the concentration of ferrocyanide has arrived at 10 mM. The above results demonstrate that the concentration of ferrocyanide plays a key role in inducing oscillations. Third, comparing Figure 4B and E, we find that the rate of scan also affects the oscillation. The more rapid the scan rate is, the easier the oscillation appears, which indicates that the oscillation is related to the ion diffusion at the interface. Fourth, Figure 4F suggested that oscillation has no relationship with K+ because oscillation can also be found at 20 mM TMA+ aqueous solution. All of these results demonstrate that the necessary conditions for the oscillation appearing are the L/L interface, the concentration of ferrocyanide, and the potential scan range. Therefore, oscillation is caused by the forced ion-pair formation at the interface between the negatively charged ferrocyanide ion in the aqueous phase and the positively charged TBA+ in the organic phase. Our results are very different from the report of Shao.7,8 In addition, our result demonstrates that the L/L interface is a mixed solution layer in which formation of an ion pair results in the decrease of the conductivity of the organic phase because of the decrease of the TBA+ ion suddenly. The experimental information is not enough to set up a model to explain the properties of the interesting nonlinear oscillation patterns. Further research is needed in this respect in the future. 4. Conclusions It is the first time that potassium ion transfer across a W/DCE interface and the electron transfer across an electrode/W interface in the potential window was observed simultaneously using a conventional three-electrode system. The effects of concentrations of the redox couple on ion transfer indicate that the overall electrochemical process, proceeding as a coupled electron-ion transfer reaction, is controlled by the electron transfer across the Pt/W interface and the driving force of ion transfer across W/DCE interface is an ET process occurring on the Pt surface simultaneously. Furthermore, a new oscillation phenomenon is observed, for the first time, for higher concentrations of Fe(CN)64- at the L/L interface because of the forced ion-pair formation at the interface between Fe(CN)64- in the aqueous phase and TAB+ in the DCE phase. All of these results are significant for us to understand the ion-transfer mechanism
Liu et al. better and to realize mechanism of potassium ion transfer across biomembranes in physiological processes. Acknowledgment. This work was supported by the Natural Science Foundation of China (nos. 20335030 and 20775060), the Teaching and Research Award Program for Outstanding Young Teachers in Higher Education Institutions of MOE P.R.C., the Natural Science Foundation of Gansu (no. 3ZS051A25-023), and Key Laboratory of Ploymer Materials of Gansu Province. References and Notes (1) Liquid Interfaces in Chemical, Biological, and Pharmaceutical Applications; Volkov, A. G., Ed.; Marcel Dekker, Inc.: New York, 2001. (2) Taylor, G.; Girault, H. H. J. Electroanal. Chem. 1986, 208, 179. (3) Campbell, J. A.; Stewart, A. A.; Girault, H. H. J. Chem. Soc., Faraday Trans. 1989, 85, 843. (4) Stewart, A. A.; Shao, Y.; Pereira, C. M.; Girault, H. H. J. Electroanal. Chem. 1991, 305, 135. (5) Stewart, A. A.; Taylor, G.; Girault, H. H. J. Electroanal. Chem. 1990, 296, 491. (6) Ulmeanu, S.; Lee, H.; Fermin, D.; Girault, H. H.; Shao, Y. Electrochem. Commun. 2001, 3, 219. (7) Yuan, Y.; Gao, Z.; Guo, J.; Shao, Y. J. Electroanal. Chem. 2002, 526, 85. (8) Yuan, Y.; Gao, Z.; Zhang, M.; Zhang, Z.; Shao, Y. Sci. China, Ser. B 2002, 32, 271. (9) Mircˇeski, V.; Gulaboski, R.; Scholz, F. Electrochem. Commun. 2002, 4, 814. (10) Scholz, F.; Komorsky-Lovric, S.; Lovric, M. Electrochem. Commun. 2002, 2, 112. (11) Lovric, M. Electrochem. Commun. 1999, 1, 207. (12) Komorsky-Lovric, S.; Lovric, M.; Scholz, F. J. Electroanal. Chem. 2001, 508, 129. (13) Schro¨der, U.; Wadhawan, J.; Evans, R. G.; Compton, R. G.; Wood, B.; Walton, D. J.; France, R. R.; Marken, F.; Bulman Page, P. C.; Hayman. C. M. J. Phys. Chem. B 2002, 106, 8697. (14) Shi, C. N.; Anson, F. C. J. Phys. Chem. B 1998, 102, 9850. (15) Shi, C. N.; Anson, F. C. J. Phys. Chem. B 1999, 103, 6283. (16) Shi, C. N.; Anson, F. C. Anal. Chem. 1998, 70, 3114. (17) Karyakin, A.; Vagin, M.; Ozkan, S.; Karpachova, G. J. Phys. Chem. B 2004, 108, 11591. (18) Mircˇeski, V.; Quentel, F.; L’Her, M.; Pondaven, A. Electrochem. Commun. 2005, 7, 1122. (19) Shul, G.; Opallo, M. Electrochem. Commun. 2005, 7, 194. (20) Samec, Z.; Marecek, V.; Koryta, J.; Khalil, M. W. J. Electroanal. Chem. 1977, 83, 393. (21) Samec, Z.; Marecek, V.; Weber, J. J. Electroanal. Chem. 1979, 100, 841. (22) Clarke, D. J.; Schiffrin, D. J.; Wiles, M. Electrochim. Acta 1989, 34, 767. (23) Shao, Y.; Weber, S. G. J. Phys. Chem. 1996, 100, 14714.
