Theory of normal-pulse voltammetric current of electron transfer reaction at liquid/liquid interface

Abstract

Theory of normal-pulse voltammetric current of electron transfer (ET) reaction at organic solvent(O)/water(W) interface between a redox couple of A species, A RED and A OX , in O phase and another redox couple of B species, B OX and B RED , in W phase has been discussed. A RED is supposed to be a neutral molecule and A OX a cation, whereas both B RED and B OX are supposed to be highly hydrophilic. Then, two kinds of the reaction mechanism, that is, (1) the heterogeneous ET reaction: A RED (O) + B OX (W) = A OX (O) + B RED (W) at the O/W interface producing the voltammetric current, and (2) the transfer of A RED (O) molecule from O to W phase across the interface, then the homogeneous ET reaction: A RED (W) + B OX (W) = A OX (W) + B RED (W) in W phase, followed by the transfer of A OX (W) cation from W to O phase producing the voltammetric current, are considered. Theoretical equations of normal-pulse voltammetric current of the ET reaction at O/W interface are derived under the assumptions (i) that the ET reaction is pseudo-monomolecular with respect to A species, (ii) that the partition of A RED molecule between O and W phases is highly biased to O phase, and (iii) that the rate constants of homogeneous ET reaction in W phase are sufficiently large. Application of the theoretical equations to investigate the mechanism of the ET reaction at O/W interface has been discussed.