ABSTRACT

Using computer simulations, the electrophoretic motion of a positively charged colloid (macroion) in an electrolyte solution is studied in the framework of the primitive model. In this model, the electrolyte is considered as a system of negatively and positively charged microions (counterions and coions, respectively) that are immersed into a structureless medium. Hydrodynamic interactions are fully taken into account by applying a hybrid simulation scheme, where the charged ions (i.e., ~ macroion and electrolyte), propagated via molecular dynamics (MD), are coupled to a Lattice Boltzmann (LB) fluid. In a recent electrophoretic experiment by Martin-Molina et al. [J. Phys. Chem. B 106, 6881 (2002)], it was shown that, for multivalent salt ions, the mobility µ initially increases with charge density  reaches a maximum and then decreases with further increase of . The aim of the present work is to elucidate the behavior of µ at high values of . Even for the case of monovalent microions, we find a decrease of µ with . A dynamic Stern layer is defined that includes all the counterions that move with the macroion while subject to an external electrical field. We find that the number of counterions in the Stern layer, q0, is a crucial parameter for the behavior of µ at high values of . The previous contention that the increase in the distortion of the electric double layer (EDL) with increasing  leads to the lowering of µ does not hold for high . In fact, we show that the deformation of the EDL decreases with increase of . The role of hydrodynamic interactions is inferred from direct comparisons to Langevin simulations where the coupling to the LB fluid is switched off. Moreover, systems with divalent counterions are considered. In this case, at high values of  the phenomenon of charge inversion is found.

January 19, 2007 2:00 PM (Refreshments will be served at 1:45 PM) SSL 150

**ALL FIRST YEAR MATERIALS SCIENCE MAJORS ARE REQUIRED TO ATTEND**