Facile determination of formal transfer potentials for hydrophilic alkali metal ions at water|ionic liquid microinterfaces

Abstract

The formal transfer potentials of hydrophilic alkali metal ions Li⁺, Na⁺, K⁺, Rb⁺, and Cs⁺ were determined at water|room temperature ionic liquid (w|RTIL) interfaces. A working curve for an interface held at the tip of a micropipette (25 μm in diameter) was developed through simulated cyclic voltammograms (CVs) via finite element analysis with Comsol Multiphysics software. This methodology takes advantage of the symmetric diffusion regime experienced at the w|RTIL micropipette interface between two immiscible electrolytic solutions (micro-ITIES) which generates peak-shaped waves in the forward and reverse scans similar to those in CVs obtained at large (centimeter scale) ITIES. Through the simulation a profile of IT was generated in order to construct the working curve from which, in conjunction with experimentally obtained CVs, the formal transfer potentials were extrapolated. The unique characteristics of diffusion at an interface utilizing a pulled capillary make this approach possible. Additionally, within the simulation the geometry can be tailored to approximate closely the actual physical and experimental conditions. In this way the formal transfer potentials of Li⁺, Na⁺, K⁺, Rb⁺, and Cs⁺ were found to be 0.565, 0.548, 0.521, 0.531, and 518 V, respectively, at the interface between water and our extremely hydrophobic ionic liquid, trihexyltetradecylphosphonium tetrakis(pentafluorophenyl)borate. The implications of these constants towards the evaluation of metal ion extractions will also be discussed.