Ionic size, permittivity, and viscosity-related effects on the electrophoretic mobility: A modified electrokinetic model

A modified electrokinetic model is presented in which hydrated ions are represented as charged dielectric spheres so that the electrolyte solution becomes a nonhomogeneous fluid characterized by permittivity and viscosity values that are both functions of the local ionic concentrations. This modifies both the Poisson-Nernst-Planck and the Navier-Stokes equations and also introduces additional Born and dielectrophoretic forces acting on the ions. The model is numerically solved for a plane charged interface both in equilibrium and under the action of an applied tangential DC electric field, which makes it possible to determine the electrophoretic mobility of a suspended particle in the Smoluchowski limit. The obtained results show that the strong increase of the electrophoretic mobility due to steric and permittivity related effects is countered by the viscosity dependence on the ionic concentrations leading to values that are moderately higher than those predicted by the standard electrokinetic model. This behavior suggests that the proposed model may be used to improve the interpretation of experimental electrophoretic mobility data.