Theory of the electrical double layer: ion size effects in a molecular solvent

Recent application of the reference hypernetted-chain theory to the calculation of the structure of electrical double layers in wholly molecular models of aqueous electrolytes [J. Chem. Phys. 90, 4513 (1989)] is extended to NaCl solutions. The strong interaction of the small Na+ ion with the solvent molecules leads to a number of novel features, notably a very high adsorption of Na+ into a region about 4 Å from the surface next to the first solvent layer. This is sufficient to produce a positive potential of zero charge of about 29 mV and to cause the effective surface charge experienced by the diffuse layer to be positive even when the surface carries a substantial negative charge. In most cases, however, the surface potential is found to be remarkably insensitive to the details of the structure of the double layer for a given surface charge.