Insights into the adsorption behavior of ions at the calcite/brine interface: Charge preferences and energetic analysis via DFT calculations

Abstract

A comprehensive investigation into the intricate dynamics at the interface of brines and calcite is imperative for effectively tackling challenges in enhanced oil recovery (EOR). Utilizing Density Functional Theory (DFT) calculations, we scrutinized the adsorption behavior of NaCl and Na2SO4 brines on the calcite surface, aiming to glean atomistic insights into their physicochemical characteristics. Our structural analysis reveals distinctive features: NaCl brine forms an electrical double layer atop the calcite surface, while Na2SO4 brine demonstrates a predilection for the wetting layer, facilitated by electrostatic interactions of sulfate ions through the first hydration layer to the calcite surface. This preference is ascribed to the enhanced electronic potential of sulfate ions, which aids in their penetration to the surface and concurrent hydration by water molecules through non-covalent interactions. Calculations of adsorption energy unveil a decrease in calcite hydrophilicity upon ion interaction, with Na+/Cl− ions forming a comparatively less stable interface in contrast to Na+/SO42− ions. Furthermore, analysis of charge redistribution highlights electron transfer from the calcite surface to interfacial water, with sulfate ions exhibiting a capability to interact with surface sites non-covalently and exchange charge. These findings underscore the significant influence of sulfate ions in altering the wetting characteristics of calcite, carrying implications for ion-modified waterflooding techniques in EOR strategies targeting carbonate reservoirs.