SURFACE FORCES BETWEEN ZINC-SULFIDE AND SILICA IN AQUEOUS-ELECTROLYTE

The asymmetric interaction between single ZnS spheres and silica in aq ueous electrolyte has been studied by using colloid probe atomic force microscopy. At pH 5.8 the interaction was found to be monotonically r epulsive on approach, with no evidence of adhesion on separation. The experimental force data as a function of separation were fitted well b y a numerical solution to the Poisson-Boltzmann equation for dissimila r electrical double-layer interaction. The silica diffuse layer potent ial was the only free parameter in the fitting procedure, with the zin c 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. T he sign of the ZnS potential, which had been assumed previously to be positive at pH 5.8 on the basis of electrokinetic measurements, is sho wn to be negative under the low-volume-fraction conditions of surface force measurement. This simply confirms that the isoelectric point obs erved at pH 7.2 in electrokinetic studies is due to adsorption of zinc hydrolysis products rather than to the intrinsic properties of the Zn S surface. (C) 1998 Elsevier Science B.V.