Probing interaction forces associated with calcite scaling in aqueous solutions by atomic force microscopy

The prevention of calcite aggregation and scaling remains a challenging problem in aqueous based systems and environmental science. Decades of research studies have proposed microscopic mechanisms of aggregation control, but experiments at the nanoscale and molecular level are rarely conducted. Here we show that the nanoscale topographic features of calcite during its aggregation depend significantly on the intermolecular and surface forces involved in this process. By measuring the forces between a calcite or silica particle and a calcite surface in aqueous solutions using atomic force microscopy, we found that higher solution pH and inhibitor concentration and lower salinity resulted in a system of stronger repulsion and weaker adhesion, which is favorable for reducing the possibility of calcite aggregation and surface deposition. Conflicting roles of Mg²⁺ in calcite aggregation prevention, being positive in acidic pH and negative in alkaline pH, were also observed. The nanoscale structural changes of calcite, visualized by atomic force microscopy or scanning electron microscopy, indicated a size dependence of aggregated and deposited calcite crystals on the calcite-calcite and calcite-silica interactions, respectively. The generalized framework of the calcite aggregation mechanism achieved in this work can be extended to other types of systems and provides a basis for investigating the anti-aggregation strategy of calcite from industrial and environmental perspectives.