Europhysics Letters PREPRINT Charge reversal of colloidal particles

Abstract. – A theory is presented for the effective charge of colloidal particles in suspensions containing multivalent counterions. It is shown that if colloids are sufficiently strongly charged, the number of condensed multivalent counterion can exceed the bare colloidal charge leading to charge reversal. Charge renormalization in suspensions with multivalent counterions depends on a subtle interplay between the solvation energies of the multivalent counterions in the bulk and near the colloidal surface. We find that the effective charge is not a monotonically decreasing function of the multivalent salt concentration. Furthermore, contrary to the previous theories, it is found that except at very low concentrations, monovalent salt hinders the charge reversal. This conclusion is in agreement with the recent experiments and simulations. Introduction. – When a colloidal particle is placed inside a suspension containing multi-valent ions its electrophoretic mobility can become reversed [1,2]. If this happens, an applied electric field will produce a drift of a colloid in the direction opposite to the one expected based purely on its chemical charge [3–5]. Somehow an excessive number of counterions must become associated with the colloid forming an overcharged (charge reversed) complex [6–11]