ELECTROLYTE SPECIES-DEPENDENT HYDRATION FORCES BETWEEN SILICA SURFACES

Force measurements between two pyrogenic silica sheets immersed in a series of monovalent electrolytes (CsCl, KCl, NaCl, LiCl) were performed using a surface force apparatus (SFA). For each species, a shortrange repulsive hydration force prevented any adhesion of the surfaces. The data were fitted using a charge regulation model of the double-layer repulsion by solving the nonlinear Poisson-Boltzmann equation numerically, together with the full Lifshitz calculation of the van der Waals attraction. The hydration force was obtained by subtracting these calculated forces from the data. The results showed that the strength and the range of the hydration force decrease with increasing the degree of hydration of the counterion. This is opposite to the behavior of mica for which adsorbed counterions have been reported to generate a hydration repulsion. The effects of counterions on hydration forces, weakening for silica and enhancing for mica, show that the origin of the short-range interaction is not unique.