Surface forces between zinc sulfide and silica in aqueous electrolyte

The asymmetric interaction between single ZnS spheres and silica in aqueous electrolyte has been studied by using colloid probe atomic force microscopy. At pH 5.8 the interaction was found to be monotonically repulsive on approach, with no evidence of adhesion on separation. The experimental force data as a function of separation were fitted well by a numerical solution to the Poisson-Boltzmann equation for dissimilar electrical double-layer interaction. The silica diffuse layer potential was the only free parameter in the fitting procedure, with the zinc sulfide potential fixed at the value obtained from symmetric sphere-sphere interaction. The diffuse layer potential of silica was found to be reduced, indicating some adsorption of zinc hydrolysis products. The sign of the ZnS potential, which had been assumed previously to be positive at pH 5.8 on the basis of electrokinetic measurements, is shown to be negative under the low-volume-fraction conditions of surface force measurement. This simply confirms that the isoelectric point observed at pH 7.2 in electrokinetic studies is due to adsorption of zinc hydrolysis products rather than to the intrinsic properties of the ZnS surface. (C) 1998 Elsevier Science B.V.