The coupling between the hydration and double layer interactions

The electric potential and the polarization between two charged, flat surfaces immersed in water are calculated without the usual assumption that the polarization is proportional to the electric field. The new constitutive equation accounts for an additional interaction, due to the orientational correlation of the water dipoles, which is a result of the mutual interaction between neighboring dipoles. The surfaces should be characterized not only by their charges or potentials but also by their dipole densities. Because both the double layer and the hydration forces are dependent in the present model on the polarization, the repulsion cannot be separated into two additive terms, one being the traditional double layer repulsion (DLVO theory) and the other the structural repulsion (hydration). In the absence of surface dipoles, the repulsion between charged surfaces becomes stronger than that predicted by the DLVO theory, particularly at high ionic strengths. The total repulsion can be increased or even decreased by the presence of dipoles on the surfaces, which contradicts the additivity of the repulsions. The repulsion between uncharged surfaces that possess dipoles was found to depend on the electrolyte concentration, and to be extended over a much longer distance than the conventional exponential decay, particularly at high ionic strengths. As a consequence of the coupling between the double layer and hydration, the decay length of the repulsive force becomes larger than those of the two conventional repulsions and at high ionic strength the difference becomes increasingly larger.