An analysis on electrophoretic mobility of hydrophobic polystyrene particles with low surface charge density: effect of hydrodynamic slip

The electrophoretic mobility of hydrophobic polystyrene particles in aqueous monovalent electrolyte solutions is analyzed using standard electrokinetic equations with a hydrodynamic boundary condition called the Navier slip condition, in which a fictitious slip length is a parameter for a slip velocity, at the particle surface. The standard electrokinetic equations underestimate the mobility of polystyrene particles bearing lower surface charge density when using the values of surface charge density from titration and a no-slip boundary condition. The introduction of the slip length of a few nanometer increases the magnitude of electrophoretic mobility and leads to the agreement between experimental mobility and theoretical mobility. The slip length increases with increasing the distance between chargeable groups on the particle surface. This result indicates the increase in hydrophobicity for the surface of polystyrene particle with the lower number of chargeable sites.