Theoretical treatment of ion transfers in two polarisable interface systems when the analyte has access to both interfaces.

Abstract

A new theory is presented to tackle the study of transfer processes of hydrophilic ions in two polarisable interface systems when the analyte is initially present in both aqueous phases. The treatment is applied to macrointerfaces (linear diffusion) and microholes (highly convergent diffusion), obtaining analytical equations for the current response in any voltammetric technique. The novel equations predict two signals in the current-potential curves that are symmetric when the compositions of the aqueous phases are identical while asymmetries appear otherwise. The theoretical results show good agreement with the experimental behaviour of the “double transfer voltammograms” reported by Dryfe et al. in cyclic voltammetry (CV) (Anal. Chem. 2013, 86, 435-442) as well as with cyclic square wave voltammetry (cSWV) experiments here performed. The theoretical treatment is also extended to the situation where the target ion is lipophylic and it is initially present in the organic phase. The theory predicts an opposite effect of the lipophilicity of the ion on the shape of the voltammograms, which is validated experimentally via both CV and cSWV. For the above two cases, simple and manageable expressions and diagnosis criteria are derived for the qualitative and quantitative study of ions lipophilicity. The formal ion-transfer potentials can be easily quantified from the separation between the two signals making use of explicit analytical equations.