Determination of formal ion transfer potentials for alkali and select transition metal cations at a micro water/α,α,α-trifluorotoluene interface

Simple ion transfer is one of the fundamental charge transfer reactions that can occur at an immiscible liquid|liquid interface. Strongly hydrophilic metal cations often have high formal ion transfer potentials (ΔwTFTϕ°') that limit the size of the polarizable potential window (PPW). ΔwTFTϕ°' values are unique to each immiscible liquid|liquid biphasic system, e.g., water|1,2-dichloroethane (w|DCE) or water|α,α,α-trifluorotoluene (w|TFT). These constants are valuable physicochemical parameters; thus, their characterization is critical. Herein, we report the measurement of ΔwTFTϕ°' for the alkali metal cations as well as NH4+, Ba2+, Ca2+, Co2+, Cu2+, Mg2+, Ni2+, and Zn2+ at a micro w|TFT (25 μm in diameter) interface between two immiscible electrolyte solutions (ITIES). These were performed using differential pulse voltammetry (DPV) at conventional supporting electrolyte concentrations (5 mM). A multi-peak fitting approach using Igor Pro software was applied to isolate and identify peak signals within DPVs. The transition metals were selected based on their importance in electrocatalysis where they have been electrodeposited onto electrode surfaces as nanoparticles via water-in-oil emulsions generating medium- and high-entropy alloys. Gaining physical insight into their hydrophilicity through this method is key to predicting emulsion stability in such systems. The trend in alkali metal cation ΔwTFTϕ°' agrees well with the previously established trend at the w|DCE interface of decreasing hydrophilicity of Li+>Na+>K+>Rb+>Cs+.