Hydration forces between mica surfaces in aqueous electrolyte solutions

Surface forces between molecularly smooth mica sheets were measured in Na+ and K+ aqueous salt solutions (at ∼10−3 M) both at 21°C and, for the case of Na+, at 65°C. Additional short-range repulsive forces were found to be the same at both temperatures. In very low concentrations of electrolyte solution (∼5 × 10−6M) and in hydrochloric acid solutions (up to 1.2 × 10−3M) these forces were completely absent-giving force curves in very good agreement with DLVO theory. Analysis of these force-interaction results appears to prove conclusively the existence of counterion hydration forces on mica. These forces apparently also prevent both bubble coalescence and coagulation of latex colloids at high concentrations of electrolyte. The total interaction between charged colloidal surfaces should involve, as apparently in the case of mica, both short-range repulsive forces due to bound partially hydrated ions and a longer range repulsion due to hydrated counterions in the compressed double-layer. These “hydration” forces should be present in many colloidal systems and be dominant in those with high negative charge densities and little possibility of hydrogen bonding to adjacent water layers. Surfaces consisting of ionic species (e.g., Zwitterionic lecithin bilayers) should give rise to repulsive forces due to the surface ion hydration effects only.