Effect of ion species and particle surface properties on electric-field-dependent particle electrophoresis

The effects of ionic species (Li + , Cs + , and guanidinium (Gdm + )), particle surface modification (alkyl-OH, SO 3 H, and acrylate), and applied electric field strength on the zeta potential, fraction of charge in the slipping layer, and thickness of the slipping layer of polystyrene spheres were investigated using microscopic electrophoresis. Focusing on the hydrophilicity/hydrophobicity of ions and particle surfaces, we found that, regardless of surface modification, the zeta potentials of all particles were negative in electrolytes containing Li + , Cs + , or Gdm + . The absolute values of the zeta potential increased with increasing applied electric field strength up to 2 V cm −1 and subsequently plateaued. For hydrophilic particles (alkyl-OH and SO 3 H), electrolytes containing Li + and Cs + yielded smaller absolute zeta potential values than those observed in Gdm + -containing electrolytes. In contrast, for hydrophobically modified particles (acrylate), the presence of Gdm + resulted in smaller absolute zeta potential values than those of Li + and Cs + . Measurements under a weak applied electric field (0.5 V cm −1 ) revealed that the slipping layer thickness in LiCl solutions decreased with increasing particle hydrophobicity, whereas that in GdmCl solutions increased. These results indicate that zeta potential measurements under weak applied electric fields, which are less likely to delaminate the ion-accumulated layer, enable evaluation of the slipping layer thickness and interfacial ion-accumulated layer structure. In addition, stability ratio measurements suggest that characteristic interfacial states reflected in the slipping layer are related to aggregation behavior. The results provide insight into the electrokinetic origins of ion effects and offer a basis for evaluating particle surface hydrophilicity/hydrophobicity. • Electric-field-dependent electrophoretic mobility of particles is investigated. • Ion-specific accumulation in the slipping layer is evaluated. • The absolute electrophoretic mobility increases with applied electric field. • The slipping layer thickness decreases with increasing electric field. • Ion hydration and particle surface hydrophilicity affect ion accumulation.