Donnan potential of dilute colloidal dispersions: Monte Carlo simulations

Donnan equilibrium between a salt-free colloidal dispersion and an electrolyte solution has been investigated by Monte Carlo simulations. The Donnan potential is directly calculated by considering two compartments separated by a semipermeable membrane. In order to understand the role played by colloid-ion interactions, the influences of colloidal characteristics, including particle size R, intrinsic particle charge Z, counterion valency z(c), and concentration c(p), on Donnan potential Psi(D) and effective charge Z(eff) are examined. Our simulations show that the electroneutrality condition is not followed in both compartments and the Donnan potential arises due to the net charge density. The Donnan potential grows by increasing c(p) and Z(eff) and by decreasing dielectric constant epsilon(r), i.e., Psi(D) approximately Z(eff)c(p)/epsilon(r) approximately. Note that the effective charge varies with R,Z,c(p),epsilon(r) and z(c) as well. When the salt concentration is increased, the net charge density is lowered and thus the Donnan potential decays accordingly. The validity of the classical theory based on the Nernst equation and the electroneutrality assumption is also examined. In general, the simulation results at high-salt condition can be well represented by such mean-field theory.