Charge reversal of sulfate latex particles in the presence of lanthanum ion

Abstract

Effects of multivalent cations and its hydrolyzed forms on the charge reversal are investigated by measuring electrophoretic mobilities of three different sulfate latex particles bearing pH-independent surface charge densities as a function of LaCl3 concentration and pH. The obtained experimental results are analyzed by using a simple adsorption model including an ion-ion correlation model, intrinsic energy of adsorption for hydrolyzed La ions, and the speciation calculation of La. From the experimental electrophoretic mobilities at pH = 4 without hydrolyzed La ions, we observed that the LaCl3 concentrations at charge reversal increased with decreasing the magnitude of the latex charge density. This experimental trend can be qualitatively captured by the ionic correlation model used here. While the sulfate latex particles bear pH-independent surface charge and the LaCl3 concentration was lower than the concentration where the charge reversal took place at pH 4, the increase in pH significantly gave rise to the charge reversal at pH above 7.8 where the hydrolyzed La ions begin to form. Therefore, this charge reversal at higher pH can be assigned to the stronger adsorption of hydrolyzed La ions. In addition, we demonstrate that the simple model can capture such experimental trends of charge reversal by introducing surmised values of its intrinsic energy of adsorption. Therefore, our results confirm that the ion correlation can be a prevailing mechanism on the charge reversal at low pH, while the stronger adsorption of hydrolyzed La ions predominates to induce the reversal with increasing pH.