Electrophoresis of neutral oil in water

Negative electrophoretic mobilities of oil in water are widely interpreted in terms of adsorption of hydroxide leading to negative surface charge. Challenging this traditional view, an increasing body of evidence suggests surface depletion of hydroxide and surface accumulation of hydronium leading to a positive surface charge. We present results from molecular dynamics (MD) simulations showing electrophoretic mobilities of oil in water with the same sign and magnitude as in experiment but in the absence of ions. The underlying mechanism involves interfacial roughness leading to gradients in dielectric permittivity in field direction and, thus, local elevation of the applied electric field. Although all molecules have zero net charge, their partial charges are distributed non-uniformly such that oil exhibits negative and water positive excess charge in regions of high field intensities; this induces a net force on the oil or the water against or in field direction, respectively. Our results indicate that deducing net charges from electrophoretic mobilities as widely done can be misleading. Our findings suggest that pH dependent electrophoretic mobilities in experiment being negative above and positive below pH 2.5 arise from a competition between the negative mobility of the ion-free interface and the positive mobility from adsorbed hydronium ions.