ELECTROPHORETIC PROPERTIES OF COLLOIDAL DISPERSIONS OF MONODISPERSE ZINC-SULFIDE - EFFECTS OF POTENTIAL-DETERMINING LENS AND SURFACE OXIDATION

In this work we analyze the electrophoretic mobility of monodisperse c olloidal spheres of zinc sulfide as a function of pH for different con centrations of solutions of Zn(NO3)(2) and (NH4)(2)S. The effect of su rface oxidation on the electrokinetic properties of ZnS is also analyz ed by comparing the pH dependence of the mobility of the synthetic par ticles with that found for commercial ZnS (both pure and oxidized by d ispersion in 3 and 15% H2O2 solutions) and ZnO. Our results show that, although both Zn2+ and S2- (actually HS-) considerably affect the ele ctrokinetic behavior of ZnS, a more significant effect is found for Zn 2+ on the mobility of the colloidal particles. Oxidation of the commer cial ZnS colloids considerably changes the mobility-pH trend of variat ion, increasing the pH corresponding to the i.e.p. and modifying the s hape of the mobility-pH curve as compared to unoxidized samples. On th e basis of electrokinetic and X-ray photoelectron spectroscopy measure ments, it is suggested that the oxidation leads to the formation of su rface elemental sulfur in the case of our synthetic spheres, and of zi nc oxide and/or sulfur oxide species in the case of commercial powder. (C) 1995 Academic Press, Inc.