FORCES BETWEEN MICA SURFACES IN THE PRESENCE OF ROD-SHAPED DIVALENT COUNTERIONS

Recent theoretical and experimental studies have shown that the swelling of some common clay minerals (e.g., montmorillonite) is limited on addition of Ca2+ salts because of ion-ion correlation forces. This effect gives rise to strong, short-range attractive forces completely unexpected from classical DLVO theory. In this study, we have investigated the effect of a rod-shaped divalent ion on the long-range double-layer and short-range solvation forces between mica surfaces in aqueous solution. At a concentration of 3 mM this ion both reduces the diffuse double-layer repulsion and creates a strong adhesive minimum at a mica-mica separation of about 0.8-0.9 nm, probably because the ion acts as a bridging agent between the facing surfaces. The force measurements are compared with the coagulation behavior of Na+-montmorillonite.