Screening of a macroion by multivalent ions: Correlation induced inversion of charge

Abstract

Screening of a strongly charged macroion by multivalent counterions is considered. It is shown that counterions form a strongly correlated liquid at the surface of the macroion. Cohesive energy of this liquid leads to additional attraction of counterions to the surface which is absent in conventional solutions of Poisson-Boltzmann equation. Away from the surface this attraction can be taken into account by a new boundary condition for the concentration of counterions near the surface. Poisson-Boltzmann equation is solved with this boundary condition for a charged flat surface, a cylinder and a sphere. In all three cases, screening is much stronger than in the conventional approach. At some critical exponentially small concentration of multivalent counterions in the solution they totally neutralize the surface charge at small distances from the surface. At larger concentrations they invert the sign the net macroion charge. Absolute value of the inverted charge density can be as large as 20% of that of the bare one. In particular, for a cylindrical macroion, it is shown that for screening by multivalent counterions predictions of the Onsager-Manning theory are quantitatively incorrect. The net charge density of the cylinder is smaller than their theory predicts and inverts sign with growing concentration of counterions. Moreover the condensation looses its universality and the net charge linear density depends on the bare one.