Modelling probe beam deflection experiments in binary bathing electrolytes

Abstract

Optical probe beam deflection allows monitoring of concentration gradients within a mass-transfer boundary layer next to an electrode, and the data are used to infer chemical fluxes. A binary electrolyte is often used since electroneutrality requires the charge density due to anion and cation to be equal throughout the system, resulting in a single concentration gradient and a direct proportionality between the beam deflection signal and the diffusive flux. As migration contributes significantly to the flux of each ion in a binary electrolyte, this flux contribution must be considered to determine the ionic fluxes using the beam deflection signal. In this paper, we develop a model that accounts for flux by diffusion and migration in a binary electrolyte, uses measured time-dependent current and beam deflection responses as the model input, and computes the cationic and anionic fluxes through the system. The model assumes one-dimensional semi-infinite geometry and allows both cations and anions to transport through an interface into or out of an adjacent phase. The model is useful for monitoring exchange of ions between a liquid bathing electrolyte and a film containing redox sites, an exchange required to maintain film electroneutrality as the redox sites are oxidized or reduced. The use and value of the model are illustrated with previously published data from a conducting polymer film, poly(1-hydroxyphenazine).