Probing the Coupling of Charge-Transfer Processes Across Liquid/Liquid Interfaces by the Scanning Electrochemical Microscope Yoram Selzer and Daniel Mandler* Department of Inorganic and Analytical Chemistry, The Hebrew UniVersity of Jerusalem, Jerusalem 91904, Israel ReceiVed: October 27, 1999; In Final Form: February 10, 2000 The feedback mode of the scanning electrochemical microscope (SECM) is used to induce and measure charge-transfer coupling (CTC) between two ion-transfer (IT) processes across liquid/liquid interfaces. Results are interpreted using a general model for coupling processes under steady-state conditions that is also applicable to other modes of coupling such as IT-ET and ET-ET (ET ) electron transfer). The measured feedback curves demonstrate the effect of mass-transfer limitations in these processes on the potential across the interface, shedding new light on the dynamics of bioenergetics and phase-transfer catalysis systems as well as on previous SECM studies of liquid/liquid interfaces. Introduction By charge-transfer coupling (CTC) we define the mutual effect that two (or more) charge-transfer processes have, when they take place simultaneously across an interface. More specifically, we are interested in systems where the potential across the interface is not controlled by an extraneous source but by the distribution of the charge carriers across the interface. The mutual effect is manifested by the dependence of the kinetics of the various charge-transfer processes on the resulting potential across the interface. CTC processes play a key role in many biological systems 1 (such as the chemiosmotic proton circuit and electroneutral symport and antiport processes) as well as in much simpler systems, such as phase-transfer catalysis. 2-4 This paper concerns CTC across liquid/liquid interfaces. Since these interfaces had been proposed in the past as simple models for biological membranes, 5,6 our discussion and in particular our approach might be useful for treating charge transfer in biological systems. Although significant progress has been achieved in understand- ing these interfaces 7 and the kinetics of charge transfer across them, 8,9 the understanding of charge-transfer coupling mecha- nisms is still rather poor. Charge can be transferred across liquid/liquid interfaces by means of ion transfer (IT) or electron transfer (ET). Historically, charge-transfer studies initiated with the so-called conventional methods, i.e., interface polarization using appropriate reference electrodes on both sides of the interface. 9 These methods suffer from several experimental drawbacks: lack of ability to discriminate between IT and ET, distortions due to charging current, iR-drop in nonaqueous solutions, and limited potential window dictated by the nature of the electrolyte. The conven- tional methods also lack the ability to investigate CTC processes that can be either ET-IT or IT-IT. The introduction of the scanning electrochemical microscope (SECM) as an experimental tool for investigating liquid/liquid interfaces circumvented some of the above experimental prob- lems. 10 The SECM has been used to study ET 10-14 as well as IT processes. 15,16 The main objective of these studies was to elucidate the dependence of various charge-transfer processes on the potential across the interface. The potential was controlled by using potential-determining ions. In these experiments charge was forced to cross a liquid/liquid interface not by polarizing it but as a result of generating an oxidizing/reducing agent near the interface (in the case of ET) or upon inducing a change in ion distribution across the interface (in the case of IT). As a result the steady-state charge-transfer processes under investiga- tion were coupled to compensating IT processes that maintain electroneutrality. Because potential-determining ions in these studies also serve as charge-compensating ions, their concentra- tion must be sufficiently high to ensure that compensating processes will not become the rate-limiting steps. Previous studies avoided the complex situation involving low concentrations of compensating ions. We continue from this point and explore the effect of low concentration of potential- determining/compensating ions on the kinetics of charge transfer. Experiments under such conditions focus on the effect of mass- transfer limitations of the compensating ions on the overall processes. Our model system was first studied by Mirkin et al. 17 using the SECM and is schematically depicted in Figure 1. It is based on the process of facilitated K + transfer by dibenzo-18-crown-6 (DB18C6) from water into 1,2-dichloroethane (DCE), a process which has been investigated extensively in the past: 8,18-22 In essence, a glass-pulled micropipet consisting of K + ions in an aqueous phase was polarized vs a reference electrode located in DCE that also contained the K + -complexing agent DB18C6. The potential inside the micropipet was set to a positive value for which the extraction of K + was diffusion controlled. DB18C6 carrying the K + ion diffuses away from the micropipet to the lower DCE/W (W ) water) interface. By releasing K + ion across that interface into the aqueous phase, DB18C6 is regenerated to its neutral form, and by diffusing back to the micropipet establishes the feedback effect. The kinetics of K + transfer across the lower interface depends on the potential Δ o w , which in turn depends on the ratio of concentrations of a potential-determining ion (in our case tetrabutylammonium, TBA + ) in the two liquid phases according to the Nernst- K + (w) + DB18C6(DCE) S [K + DB18C6](DCE) (1) 4903 J. Phys. Chem. B 2000, 104, 4903-4910 10.1021/jp993808i CCC: $19.00 © 2000 American Chemical Society Published on Web 04/29/2000