Ion-dependent adhesion between calcite surfaces

Disjoining pressure that operates between mineral surfaces in fluid-filled granular rocks is often strongly influenced by the ionic composition of pore solutions. Can various ionic species exhibit a remarkably different influence on the mineral adhesion and thus the cohesion within granular rocks? We explore this question in atomic force microscopy (AFM) experiments using two brittle calcite surfaces in a symmetrical surface configuration. Our AFM results show a robust difference between the adhesion in the presence of Na⁺ and Ca²⁺ ions. The adhesion is significantly higher for monovalent Na⁺ at a given ionic strength in comparison to more hydrated divalent Ca²⁺ cations. In addition, the adhesive forces are weakly modulated by the varying Ca²⁺ concentration. We thus infer that for weakly charged minerals such as calcite, Ca²⁺ can sustain relatively high positive disjoining pressures and thus thicker water films between the contacting mineral grains.