THE ELECTROSTATIC GIBBS ENERGY OF FINITE-SIZE IONS NEAR A PLANAR BOUNDARY BETWEEN 2 DIELECTRIC MEDIA

We have calculated analytically the electrostatic Gibbs energy profile for transfer of an ion between two bulk dielectric phases separated by a planar boundary. The profile incorporates the finite ionic size and dielectric image interactions. There is no discontinuity as the ion traverses the boundary and the model is well represented by a simple point charge when the ionic centre is beyond about half an ionic radius from the boundary. The profile is in line with observations based on interfacial inner-layer capacitances that ions can partly penetrate the boundary between two immiscible liquids at equilibrium. It represents an inner-layer capacitance contribution when the distance of closest approach is different for cations and anions.