Nonlocal Free Energy Density Functional Approximations for the Electrical Double Layer; Comparison with Monte Carlo Results for 2:1 Salt
MINNESOTA UNIV MINNEAPOLIS DEPT OF CHEMICAL ENGINEERING AND MATERIALS SCIENCE
Pagination or Media Count:
This paper reports density profiles and mean electrostatic potentials of a restricted primitive model double layer. Results for an asymmetric 21 salt, predicted by three different nonlocal free energy density functional approximations, are compared with Monte Carlo results and with those of Gouy-Chapman theory. Results for the diffuse layer potential are also compared with those of some recent theories of the double layer. The hard-sphere contribution to the free-energy functional is based on a nonlocal generic model functional proposed by Percus. We choose the Carnahan-Starling equation of state to calculate the free energy of the homogeneous hard-sphere mixture which enters in the hard-sphere functional. The mean spherical approximation for a neutral bulk electrolyte is used to model the electrostatic part of the non-uniform ion-ion correlations present in the interface. For singly charged counterions the agreement between the density functional approximations applied here and the Monte Carlo data is excellent. For doubly charged counterions the agreement is very good at high concentrations, but differs from the MC results in several aspects at low concentrations. The density functional theories are successful in predicting the extremum of the diffuse-layer potential as a function of surface charge present in the simulations.
- Physical Chemistry