Anomalous temperature behavior in clay swelling due to ion-ion correlations

We show, experimentally and theoretically, that swelling of both natural and refined clays has an anomalous temperature behavior depending on counterion valency. In an aqueous clay dispersion dominated by monovalent counterions the swelling pressure increases with temperature as expected from entropic arguments. In a clay with predominantly divalent counterions, the opposite behavior is found. The explanation is due to the fact that ion-ion correlations increase with temperature. Ion-ion correlations are important at strong electrostatic coupling and in an aqueous solution the dielectric permittivity, ϵ_r, drops with increased temperature, T, in such a way that the product ϵ_rT also decreases. Thus, the net effect is an increased coupling.