Electrophoretic mobility without charge driven by spontaneous polarization of the nanoparticle/water interface

TL;DR

This paper explores how spontaneous polarization at water-hydrophobic interfaces induces electrophoretic mobility in particles without net charge, emphasizing the dominant role of dipolar polarization over quadrupolar effects.

Contribution

It introduces a model explaining particle mobility driven by interfacial polarization effects, highlighting the significance of dipolar order in zero-charge electrophoresis.

Findings

Quadrupolar polarization has negligible effect on particles larger than a few nanometers.

Dipolar interfacial order governs the mobility of sub-micron particles.

Surface charge density is inversely proportional to shear surface area.

Abstract

Polarization 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 polarization 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 polarization to the particle mobility becomes negligible compared to experimentally reported values for particles exceeding a few nanometers in size. The dipolar order of the interface dominates the zero-charge mobility of sub-micron particles. The corresponding electrokinetic charge is determined by the preferential orientation of interfacial dipoles relative to the surface normal.