Theoretical Study Of Thin-Layer Cyclic Voltammetry For Measuring The Rates Of Electron Transfer At Liquid/Liquid Interfaces

The study of electron transfer across liquid/liquid interface has important significance for understanding complex physiological change process of organisms, because liquid/liquid interface could be treated as simple models of biologic or artificial membrane. The thin-layer cyclic voltammetry proposed by Anson et al. in 1998 is a new method of studying electron transfer at liquid/liquid interface. The method becomes a powerful measure of studying cross-phase electron transfer, due to its simplicity and convenience. Thus, study of thin-layer cyclic voltammetry for measuring electron transfer rate at liquid/liquid interface is very important.There are four parts in this paper, main contents are as follows:1. This paper reviewed summarily the historical background of the thin-layer cyclic voltammetry for measuring electron transfer rate at liquid/liquid interface. Moreover, the research progress on theory and experiment of the thin-layer cyclic voltammetry for measuring the rates of cross-phase electron transfer were summarized in detail. Besides, all kinds of possible influencing factors were outlined briefly and analyzed. All contents are: (1) The historical background of thin-layer cyclic voltammetry; (2) The experimental principle of thin-layer cyclic voltammetry; (3) The influence factors for measuring rate of electron transfer across liquid/liquid interface; (4) The application of thin-layer cyclic voltammetry in the liquid/liquid interface electroanalytical chemistry.2. Anson et al. has proposed the theory of thin-layer cyclic voltammetry for measuring the rates of electron transfer at liquid/liquid interface. However, the theory has not caused enough attention because of the absence of boundary conditions. This paper gave some new theoretical conditions to remedy the defects. That is, we gave the selection criteria to concentrations of coreactants, which not only provided the selective ranges of reactants concentration contained in the organic phase, but also was precondition for properly selecting concentrations of reactants contained in the aqueous phase. We also demonstrated the utility of the theory by experimental tests. In addition, the effect of thin layer thickness on the electron transfer rate was analyzed in detail. The result showed that rate constants of electron transfer of rapid kinetics systems would be expediently determined by increasing the thickness of the thin layer. This can avoid the existing difficulties that larger electron transfer rate constants were determined by meaning of adding concentrations of reactants contained in the aqueous phase.3. A simple method of measuring the rates of multi-step electron transfer across liquid/liquid interfaces is described. Based on a theory, the rate constants of multi-step electron transfer reaction can be obtained expediently by the thin-layer cyclic voltammetry. The bimolecular rate constants for the two-step electron transfer reactions between ZnTPP in the organic phase and K₄Fe(CN)₆ located in the adjacent aqueous phase were obtained simultaneously by the method as the rate constant of the 1st step is 0.12 cm s^–1 M^–1, and the rate constant of the 2nd step is 0.15 cm s^–1 M^–1. In addition, numerical simulation according to the treatment was used to model the voltammetric responses of electrodes to study facters. Meanwhile, we also have done a further validation by experiments. Results obtained not only gave information regarding the effect of the concentration ratio of the reactants in two phases and the thin layer thickness on multi-step electron transfer, but also the agreement between simulations and experiments proved the validity of the theory.4. Thin-layer cyclic voltammetry with its unique advantages has become a powerful means of measuring the rate of electron transfer at liquid/liquid interface. In this paper, numerical simulation was employed to comparative study the thin-layer cyclic voltammetry for determination of multi-step electron transfer and single-step electron transfer at liquid/liquid interface. The effects of the concentration ratio of two-phase coreactants, the thin layer thickness and the diffusion coefficient of reactants on the multi-step electron transfer and single-step electron transfer were investigated and analyzed respectively. The results showed that the results obtained for first step electron transfer reaction of multi-step electron transfer was consistent with single-step electron transfer process, which suggested that the first step electron transfer of multi-step electron transfer can be approximated to deal with as the single-step electron transfer process. In addition, the results also showed that the change for the second step electron transfer of multi-step electron transfer was different and even opposite from the first step electron transfer. This implied the electron transfer process of that starting from the second step electron transfer of multi-step electron transfer had more complex mechanism, more various factors and more diverse changes laws.