ION-ION CORRELATIONS IN POLYELECTROLYTE SOLUTIONS - HARD-SPHERE COUNTERIONS

The intimately related mechanisms of macroion screening and ion-ion correlations in electrolyte solutions are investigated with the aim to explore the simplest theoretical estimations of the latter on the former. The Poisson-Boltzmann theory of macroion screening in electrolyte solutions was developed under two important assumptions. Specifically, the correlated motion of the mobile ions is ignored by identifying the mean field electrostatic potential with the potential of average force and, moreover, only point charges are considered so that finite size effects are not accounted for. Utilizing the generalized van der Waals free energy density functional formulation for inhomogeneous fluids, we obtain simple corrections to the Poisson-Boltzmann result where both these mechanisms are treated in local and nonlocal approximations. By incorporating a somewhat more realistic short range repulsive interaction, the asymptotic instability encountered in the point particle case is avoided and self-consistent density profiles can be computed. Nonlocal approximations are shown to be thermodynamically consistent in terms of the contact theorem for the osmotic pressure, and excellent agreement with Monte Carlo simulation is achieved even at high electrostatic coupling and with large particles.