Effects of ion hydration in double layer interaction

We consider highly charged surfaces, typical of many minerals, in monovalent salt solutions. Counterion separation in the vicinity of a surface is then small and short-range structure in ion-ion interaction potentials can no longer be neglected. We used the oscillatory potentials of mean force obtained in recent simulations of interacting ion pairs in water in order to explore the effects of aqueous structure on the double layer repulsion. Near each surface counterions preferentially assume separations corresponding to the minima of their interaction potential, the screening of surface charge by the Stern layer becomes stronger and the effective surface charge is greatly reduced. When surface separation is decreased to about 15 angstroms, regions of increased counterion density come into contact and the repulsion returns to higher values expected on the basis of simpler models. The change to a stronger short-range repulsion gives an impression of a distinct new approximately exponential force, as reported earlier in many experimental investigations.