CONTRIBUTION OF IONIC CORRELATIONS TO EXCESS FREE-ENERGY AND DISJOINING PRESSURE OF THIN LIQUID-FILMS .1. ELECTRIC DOUBLE-LAYER INSIDE THE FILM

An approach to the calculation of the ionic-correlation free energy per unit area of thin liquid films is developed. The Poisson-Boltzmann ion charge distribution inside the film is replaced by an equivalent stepwise model distribution. In this way comparatively simple expressions for calculating the correlation free energy and disjoining pressure are derived. The results are in good agreement with numerical data of other authors. The dependence of the ionic-correlation free energy on various factors (dielectric permittivities, width of the Stem layer, electrolyte concentration, ionic charge, surface charge density, temperature) is examined. In all cases the ionic correlations give rise to an attractive contribution to the disjoining pressure, which often exceeds the contribution due to the van der Waals forces. In the case of 2 : 2 electrolytes the combined action of ionic correlations and van der Waals attraction can prevail over the electrostatic repulsion and a net attraction between the two film surfaces takes place.