APPROXIMATE ANALYTICAL EXPRESSIONS FOR THE PROPERTIES OF AN ELECTRICAL DOUBLE-LAYER WITH ASYMMETRIC ELECTROLYTES - CYLINDRICAL AND SPHERICAL GEOMETRIES

Approximate analytical expressions for the electrical potential distri bution, surface excess of co-ion, and double-layer free energy and ent ropy for both cylindrical and spherical surfaces are derived. The pres ent analysis extends previous results in that asymmetric electrolytes are considered. The mathematical treatment of the Poisson-Boltzmann eq uation is based on a semi-empirical fourth-order polynomial function, along with a parameter reflecting the degree of asymmetry of electroly tes and a parameter measuring the effect of curvature of a surface. Th e result of numerical simulation reveals that the deviation of the pre sent approximate result from the corresponding exact value for the dou ble-layer properties under consideration is about 4%. The performance of the expression obtained by following a perturbation algorithm is al so examined. It is found that for a relative thin to medium-thick doub le layer, the performance of two approaches are comparable; for a rela tive thick double layer, the present analysis yields a much better res ult. (C) 1994 Academic Press, Inc.