A modified Poisson-Boltzmaan equation II. Models and solutions

The properties of the charged interface between a dielectric particle and a surrounding aqueous electrolyte solution are calculated numerically over a wide range of surface charge densities for plane, cylindrical and spherical geometries. As a basis for the calculations, we present detailed models for the partial molar volumes, the dielectric permittivity and the activities of the components. These models are combined with a generalized set of local balance thermodynamic and electrostatic differential equations derived in the first part of this series. The influences of volume effects, dielectric saturation, polarization and self-atmosphere potentials on surface potential and electrostatic energy of a charged particle are investigated. Deviations from the ordinary Poisson-Boltzmann theory become very important at surface charge densities above 0.2 Cm-2. Quite generally, self-atmosphere potentials are of minor importance. The most important correction of the ordinary Poisson-Boltzmann equation is due to dielectric saturation in combination with the volume effect. It is found that the electrostatic potential, the electric field and the concentration of the counterion near a charged surface strongly depend on the excluded volume of the counterion. This leads to a distinct counterion sensitivity of the Gibbs energy of the system. Assuming a positively charged surface, competition between the counterion pairs Cl-/Br- and Cl-/SO42- is investigated. For sufficiently high surface charge densities it is found that, in the immediate vicinity of the surface, the smaller Br--ion displaces the larger Cl--ion and the Cl--ion, in turn, displaces the larger SO42--ion, although the latter is divalent. (C) 2000 Elsevier Science B.V. All rights reserved.