Kinetics of Heterogeneous Electron Transfer Reactions at the Externally Polarized Water/ o-Nitrophenyl Octyl Ether Interface Shubao Xie, Xin Meng, Zhongwei Liang, Bo Li, Zhong Chen, Zhiwei Zhu, Meixian Li, and Yuanhua Shao* Beijing National Laboratory for Molecular Sciences, Institute of Analytical Chemistry, College of Chemistry and Molecular Engineering, Peking UniVersity, Beijing 100871, China ReceiVed: March 19, 2008; ReVised Manuscript ReceiVed: August 26, 2008 The heterogeneous electron transfer (ET) reactions between dimethylferrocenium (DiMFc + ) or ferrocenium (Fc + ) produced in situ in o-nitrophenyl octyl ether (NPOE) and hexacyanoferrite [Fe(CN) 6 4- ] in aqueous phase have been investigated by scanning electrochemical microscopy (SECM). The potential difference across the water/NPOE (W/NPOE) interface was controlled externally to maintain a certain driving force for the ET reactions. The standard kinetic rate constants (k 12 0 ) for the above bimolecular ET reactions were measured to be 0.075 and 0.050 cm s -1 M -1 when DiMFc and Fc were employed as the respective organic reactants. The transfer coefficients (R) were calculated to be 1.09 and 1.62, respectively. Those abnormal values of R were analyzed and ascribed to the Frumkin effect (or diffuse layer effect) at both sides of the interfaces. Some of the previous reports are summarized and also analyzed based on the Frumkin effect. The possible effect of ion transfer on the ET process is discussed in detail. Introduction There have been a number of investigations of heterogeneous electron transfer (ET) processes at liquid/liquid (L/L) interfaces (or interfaces between two immiscible electrolyte solutions, ITIES) in the past more than two decades because such studies can offer the possibility of elucidating the mechanisms of interfacial processes and the fundamentals of interfacial struc- tures. 1 Some electrochemical techniques, including cyclic vol- tammetry, 2 ac impedance, 3 thin layer cell voltammetry (TLCV), 4 microelectrochemical measurements at expanding droplets (MEMED), 5 spectroscopic and photoelectrochemical tech- niques, 6 and scanning electrochemical microscopy (SECM), 7 have been employed to study the ET processes. Among these techniques, SECM has played a key role in the investigation of the potential dependence of the ET kinetics, since the pioneering work by Bard et al., 8 for the reason that SECM measurements are essentially free of complications caused by iR drop and the charging current due to the application of ultramicroelectrodes (UMEs) as the SECM tip. One of the focuses in these SECM studies is the dependence of the ET kinetic rate constant on the potential difference across the interface. It has been demon- strated that the kinetic behavior of most of the ET reactions taking place at a L/L interface can be described in terms of the Butler-Volmer equation at lower driving forces, and the Marcus theory 9 of ET, at higher driving forces. The potential- independent ET rate constants were also reported by Liu and Mirkin 10 using SECM and by Shi and Anson using TLCV. 4a To clarify the potential dependence of the ET reaction, Unwin et al. carried out experimental studies for many systems, and their efforts in SECM methodology extended the range of processes and conditions that might be studied by SECM. 11,12 They also explained that the results obtained by Shi and Anson 4a were due to the diffusion limitation. 4b In the previous SECM kinetic investigations, the nonpolarized liquid/liquid interfaces have often been employed, and the interfacial potential drop was controlled by variation of the concentration of a common ion in one of the two phases. In this case, SECM measurements are free of complications caused by an iR drop, since there is no externally applied potential across the interface. However, the range of the controllable potential is limited, and the change in concentration of common ions in solutions sometimes makes the data analysis difficult. Shao’s group has combined the conventional electrochemistry at L/L interfaces with SECM, introduced the SECM experiment at externally polarized liquid/liquid interface, and observed the Marcus inverted region with a single redox pair, mainly because of the wide range of available potential difference. 13 Further- more, Quinn et al. designed a new cell configuration and facilitated the SECM measurements at the polarized L/L interfaces. 14 Although after many years of investigations concerning this aspect, the dependence of the heterogeneous ET reactions on the potential difference is still confusing due to the lack of exact knowledge of the interfacial structure, especially the potential distribution within the interfacial region, which determines whether the observed relationship of potential dependence should be ascribed to kinetic or to thermodynamic aspects, the former leading to the Butler-Volmer-type or the Marcus-type interpretation and the latter to applications of diffuse layer effect. 15 At present, one of the most popular models of a L/L interface was presented by Girault and Schiffrin, 16 in which the interface is supposed to be composed of two diffuse layers and an inner layer (also called mixed solvent layer), and the potential drop develops mainly at the two diffuse layers. A piece of excellent work has been conducted by Girault’s group, 6a in which the potential dependence of an ET reaction was demon- strated to be involved in both of the two aspects, that is, the diffuse layer effect and Butler-Volmer kinetics, and the ratio of the potential drop in the inner layer was estimated to be nearly * To whom the correspondence should be addressed. E-mail: yhshao@ pku.edu.cn. J. Phys. Chem. C 2008, 112, 18117–18124 18117 10.1021/jp802435m CCC: $40.75  2008 American Chemical Society Published on Web 10/25/2008