Dependence of the Helmholtz layer capacitance on interfacial charge density at polarisable liquid|liquid interfaces

Abstract

The physical structure of a polarisable interface between two immiscible electrolyte solutions (ITIES) is modelled using an extended Gouy–Chapman–Stern framework that incorporates asymmetric ion charges. The interface is treated as three capacitors in series: two Poisson–Boltzmann diffuse layers on each side of the aqueous|organic boundary and a Helmholtz layer that includes the dielectric saturation of solvent molecules under an intense local electric field, which reduces the Helmholtz capacitance away from the potential of zero charge (PZC). The model is evaluated against AC voltammetric data for ITIES formed between various aqueous electrolytes and an α,α,α-trifluorotoluene phase containing bis(triphenylphosphoranylidene)ammonium tetrakis(pentafluorophenyl)borate. The results support the presence of back-to-back diffuse layers within the electric double layer (EDL) and show that Helmholtz capacitance varies with interfacial charge density, which depends on the aqueous ions' ability to penetrate the mixed layer, their kosmotropic/chaotropic character, electrolyte concentrations in both phases, and the applied interfacial Galvani potential difference (Δwoϕ). These insights advance understanding of the EDL at polarisable liquid|liquid interfaces.