Solvent Effects on the Properties of Electric Double Layers with Surface Charge Regulation

The thermodynamically consistent representation of the charge at the surface of an electric double layer is given by the chemical equilibrium between the groups attached at the interface and the potential determining ions in the solution (i.e., charge regulation). We report that this surface chemical equilibrium is strongly coupled to the precise molecular structure of the solution in near the charged interface that explicitly includes the solvent contribution, also known as a “civilized model”. Our analysis uses classical density functional theory to obtain the molecular and ionic structure of an electric double layer with surface charge regulation. The model suggests that the excluded volume of the solvent is the main factor that couples the solution structure to the surface chemistry, although attractive interactions may also play a role. The Figure shows how the surface charge density depends on the relative size of the solvent molecules in a Lennard-Jones type of electrolyte solution. The different curves (top to bottom) are for increasing values of the Lennard-Jones energy. The conclusion from our work is that an explicit account of the solvent effects is necessary for quantitative description of an electric double layer with surface charge regulation.