Electrophoretic mobility without charge driven by polarisation of the nanoparticle-water interface

Polarisation of the interface, spontaneously occurring when water is in contact with hydrophobic solutes or air, couples with the uniform external field to produce a non-zero force acting on a suspended particle. This force exists even in the absence of a net particle charge, and its direction is affected by the first-order, dipolar and the second-order, qudrupolar orientational order parameters of the interfacial water. The quadrupolar polarisation gives rise to an effectively negative charge. The corresponding surface charge density is inversely proportional to the area of the shear surface. As a result, the overall contribution from the quadrupolar polarisation to the particle mobility becomes negligible compared to experimentally reported values for particles exceeding a few nanometres in size. In contrast, the contribution of the dipolar order of the interface to the effective surface charge scales inversely with the particle size and dominates the zero-charge mobility of submicron particles. The corresponding electrokinetic charge is determined by the preferential orientation of interfacial dipoles relative to the surface normal.