Thermodynamic Mechanisms of Water Molecule Adsorption and Hydration Stability at Bentonite Interfaces under High-Temperature and Saline Conditions

Abstract Solid–liquid interfacial interactions between montmorillonite (MMT) and aqueous fluids are critical in petroleum engineering. Molecular dynamics simulations were conducted to investigate the MMT surface hydration at elevated temperatures (403–583 K) and varying salinities (0.112/0.448 mol/L NaCl, KCl, and CaCl2). The results reveal a competition between the surface electrostatic interactions and entropy-driven effects. The first hydration layer remains stable due to strong surface electrostatic attraction, whereas the second hydration layer is governed by its hydrogen-bond (HB) network. The HB network collapses with increasing temperature, leading to structural reconstruction of water molecules and structural instability. Meanwhile, the free-energy barriers for water migration between adjacent hydration layers increase with temperature. Na+ and K+ weakly perturb the hydration structure, whereas Ca2+ forms a stable inner Helmholtz plane and alters the water orientation in the first hydration layer. These findings provide molecular-level insight into the temperature- and salinity-driven destabilization of MMT–water interfaces under harsh subsurface conditions.