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

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, ARED and AOX, in O phase and another redox couple of B species, BOX and BRED, in W phase has been discussed. ARED is supposed to be a neutral molecule and AOX a cation, whereas both BRED and BOX are supposed to be highly hydrophilic. Then, two kinds of the reaction mechanism, that is, (1) the heterogeneous ET reaction: ARED(O) + BOX(W) = AOX(O) + BRED(W) at the O/W interface producing the voltammetric current, and (2) the transfer of ARED(O) molecule from O to W phase across the interface, then the homogeneous ET reaction: ARED(W) + BOX(W) = AOX(W) + BRED(W) in W phase, followed by the transfer of AOX(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 ARED 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.