Electrophoretic Mobility of Nanoparticles in Water.

Classical equations for colloidal mobility anticipate linear proportionality between the nanoparticle mobility and zeta potential caused by the combined electrostatics of free charges at the nanoparticle and screening bound charges of the polar solvent. Polarization of the interfacial liquid, either spontaneous due to molecular asymmetry of the solvent (water) or induced by nonelectrostatic (e.g., charge-transfer) interactions, is responsible for a static interface charge adding to the overall electrokinetic charge of the nanoparticle. The particle mobility gains a constant offset term that is formally unrelated to the zeta potential. The static charge is multiplied by the static dielectric constant of the solvent in the expression for the electrokinetic charge and is sufficiently large in magnitude to cause electrophoretic mobility of even neutral particles. At a larger scale, nonlinear electrophoresis linked to the interface quadrupole moment can potentially contribute a sufficiently negative charge to a micrometer-size nanoparticle.