Charge reversal of sulfate latex induced by hydrophobic counterion: effects of surface charge density

We studied experimentally and theoretically the charge reversal of sulfate latex colloid in the presence of monovalent hydrophobic counterion TPP⁺ (tetraphenylphosphonium). The intrinsic or chemical energy of adsorption of TPP⁺ on the latex was evaluated from the concentration at charge reversal. The isoelectric 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.