IONIC SIZE EFFECTS ON THE FORCE BETWEEN PLANAR ELECTRICAL DOUBLE-LAYERS

The effect of varying ionic size on the force between planar electrical double layers is studied for several systems with different surface potentials and ionic concentrations, through the solution of the three-point extension hypernetted chain/mean spherical approximation theory. We include conditions modeling clay particle swelling pressure experiments that have known a strong dependence on the choice of counterion. Our results show that the pressure between charged surfaces does depend on the ionic size, though they do not explain the clay swelling experiments. The increased repulsive force at small separations for the finite sized ions over the point ions is so great that the addition of an attractive van der Waals component, as is common in the colloid literature, does not result in a nonmonotonic force vs distance curve even at small separations.