THEORY OF DISSOCIATIVE ELECTRICAL DOUBLE-LAYERS - THE LIMIT OF CLOSE SEPARATIONS AND HYDRATION FORCES

The premises of the theory of dissociative electrical double layers (DEDL), i.e., the requirement of electrostatic self-consistency and the assumption of partial double-layer dissociation, are reformulated and discussed in detail. The DEDL theory is shown to yield a new limiting law that predicts large repulsive forces in the limit of very close separations of rigid charged planar surfaces. This limiting law is contrasted with Langmuir's expression for the limit of infinite potentials, and it is also discussed in relation to "hydration" forces. These large electrostatic forces arise from compressing the diffuse counterion charge into a smaller volume, when the charged surfaces are pushed together, while the electroneutrality of the overlapping double layers is being maintained. These forces depend only weakly on bulk ionic strength. Their magnitude is determined by the ease of dissociation of the Stern layer, which in turn is determined by the hydration of the Stern and diffuse ions. In this sense these electrostatic forces can be termed hydration forces.