ELECTROSTATIC INTERACTIONS OF BODIES BEARING THIN DOUBLE-LAYERS

The modified Poisson-Boltzmann (PB) equation was formulated for a system of flat dissimilar double-layers by considering the excluded volume effect and the specific interaction potential of ions with interfaces. An explicit equation was derived for the pressure (force) prevailing in the gap between plates which contains the generalized osmotic pressure term depending on the excluded volume parameter-beta. Limiting solutions of the modified PB equation derived for two-plate system at large and small separations and arbitrary electrolyte composition were discussed. Results obtained for the parallel-plate geometry were generalized to regularly shaped bodies (either convex or concave around their contact point) bearing thin double-layers, by applying the surface integration procedure analogous to the Derjaguin summation method. The excluded volume effect proved to be important for the dimensionless volume blocking parameter-beta larger than 10(-2) to a slight increase of the interaction force between bodies at large separations. At small separations, however, the force remains finite due to the excluded volume effect (in the constant charge model) in contrast to previous results predicting that the force should tend to infinity in this limit. Our theoretical predictions were proved to be adequate for interpreting the results of direct measurements of electrostatic interaction force between quartz plates immersed in an electrolyte solution.