Nonlinear counterion screening in colloidal suspensions

A new ‘‘ab initio’’ method is presented which is designed to simulate highly asymmetric systems of charged particles such as micellar solutions and charge-stabilized colloidal suspensions. The hybrid description considers the macroion degrees of freedom explicitly, while the microscopic counterions are treated within the framework of density functional theory. The counterion density profile is treated as a dynamical variable which is coupled to the macroion positions; the corresponding equation of motions are derived from a Lagrangian which contains a fictitious kinetic energy term associated with the inhomogeneous counterion density, with a fictitious mass chosen so that the counterions stay as close as possible to the surface of lowest free energy (adiabatic condition). The discontinuous behavior of the counterion density profile at the macroion surfaces is suppressed by the use of a classical pseudopotential scheme without spoiling the rapid variation of the counterion density profile outside the macroion cores. The ab initio method is implemented in Molecular and Brownian Dynamics simulations of concentrated colloidal suspensions, and the results are compared to the predictions of much simpler simulations based on the pairwise additive effective Derjaguin–Landau–Verwey–Overbeek (DLVO) potential between macroions. The density profiles calculated from the DLVO model differ considerably from the predictions of the ab initio simulations, but the macroion pair structures are in reasonable agreement. Recent ‘‘improvements’’ of the standard DLVO theory are found to overestimate or underestimate the pair structure considerably. The density functional formalism may be used to derive systematic many-body corrections to the effective DLVO pair potential. The extension of the ab initio method to treat colloidal suspensions in the presence of added salt is briefly sketched.