Effect of aggregation on the simple ion transfer across oil|water interfaces

Abstract

The theory of cyclic voltammetry of simple ion transfer across a liquid|liquid (L|L) interface including the effect of aggregation is developed. We focus on two different situations: ion transfer followed by an aggregation process in the organic phase, and a disaggregation process in the aqueous phase followed by ion transfer. We use parameters typical of the areas of study of molecular aggregation and polymer formation, namely average aggregate size and polydispersity index. These parameters allow us to analyse, in a simple and rigorous way, the interconversion between monomer and aggregate species, both at the aqueous or organic side of the L|L interface and in the interfacial region during the potential scan. Our results show that aggregation occurring in the organic phase facilitates ion transfer across the L|L interface. In contrast, when the aggregates are present in the aqueous phase, they must be disaggregated so that the monomers can be transferred, hence the transfer energy increases. Both processes significantly alter the shape of voltammograms. The peak current and mid-peak potential are analysed as a function of initial concentrations, aggregation constants and maximum number of monomers forming each aggregate. These results are compared with approximate analytical equations, which can be used to guide further experiments and extract information from experimental responses.