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We studied experimentally and theoretically the charge reversal of sulfate latex colloid in the presence of monovalent hydrophobic counter-ion TPP + (tetraphenylphosponium). The intrinsic or chemical energy of adsorption of TPP + on the latex was evaluated from the concentration at charge reversal. The iso-electric point (IEP) increases with increasing the surface or electrokinetic charge density of sulfate latex spheres. That is, at low surface or electrokinetic charge density, the charge inversion concentration is low, and IEP shifts to higher values with the increase of surface or electrokinetic charge density. The intrinsic energy of adsorption decreases with increasing the surface or electrokinetic charge density. Finally our experimental and theoretical results suggest that the hydrophobicity is a determining factor for the charge inversion of hydrophobic colloids, and the intrinsic energy of adsorption also varies with the variations of surface or electrokinetic charge density.

To investigate the effect of hydrophobicity on the charge reversal of colloidal particles, we measured and analyzed the electrophoretic mobility (EPM) of carboxyl latex particles in mixed electrolytes solutions containing potassium chloride KCl and tetraphenylphosphonium chloride TPPCl. Tetraphenylphosphonium (TPP +) ion strongly adsorbs on the particle surface due to its hydrophobicity, and thus causes the charge reversal/overcharging. Measurements of EPM were carried out as functions of pH, ionic strength, and the mixed molar ratio of X=[TPP + ]/[K + ] to unveil the influence of surface charge on hydrophobic interaction. Experimental EPM was analyzed by using 1-pK H Stern Gouy Chapmann model with the Ohshima equation including the relaxation effect or the Smoluchowski equation neglecting the relaxation effect for calculating theoretical EPM values. Our results demonstrate that carboxyl latex particles show charge reversal indicated by positive EPMs at low pH due to the accumulation of TPP + ions on the surface and the reversed EPM values at low pH are augmented with increasing the mixed molar ratio of X=[TPP + ]/[K + ]. Also, we observed that charge re-reversal at higher pH as the deprotonation of surface carboxyl groups proceeded. The pH at which the charge re-reversal occurred increased with increasing the mixed molar ratio. From the comparison between our experiments and theoretical analysis, we found that the intrinsic energy of adsorption decreases with increasing the surface charge density to describe the observed charge re-reversal. These results indicate that the intrinsic adsorption energy of TPP + , which is probably due to hydrophobic interaction, decreases with increasing the surface charge density.

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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 pHindependent 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.