Specific Ion Effects at Calcite Surface Defects Impact Nanomaterial Adhesion

Motivated by applications in subsurface hydrocarbon reservoirs such as colloidal instability and nanomaterial adhesion to mineral surfaces, we report atomistic molecular dynamics simulations to study the adhesion of carboxylate functionalized atomic force microscopy (AFM) tips (as surrogates of nanomaterials) on calcite surface line and point defects in deionized water, seawater, and brine. The line defects include acute and obtuse steps, and the point defects include Ca²⁺ and CO₃^2– vacancies. The accumulations of specific ions on both functionalized AFM tips and calcite surface defects result in reduced adhesion. The specific ions in fluids played the dominant roles to modulate adhesion, yet the surface defects also had distinct characteristics. We believe the molecular insights put forward in this work may have important implications for stabilizing nanomaterials in harsh subsurface oil and gas reservoirs. In addition, the differential affinities of the calcite surface defects with the presence of specific ions observed in this work may further the understanding of the wettability alteration mechanism of calcium carbonates in high salinity geologic settings. To the best of our knowledge, this is the first study showing the interaction of nanomaterials on different calcite surface defects, mediated by specific ions.