Molecular Insights into the Wettability and Adsorption of Acid Gas-Water Mixture

Sequestration of acid gas in geological formations is a disposal method with potential economic and environmental benefits. The process is governed by variables such as gas-water interfacial tension, wetting transition, and gas adsorption into water, among other things. However, the influence of the pressure and temperature on these parameters is poorly understood. This study investigates these parameters using coarse-grained molecular dynamics (CG-MD) simulations and density gradient theory (DGT). Simulations were carried out at 313.15 K and a pressure range of 0-15 MPa. A comparison was made against H₂S-water systems to clarify the effects of adsorption on interfacial tension due to vapor-liquid-liquid equilibrium. The predicted H₂S-water interfacial tension and phase densities by CG-MD and DGT matched the experimental values well. The adsorption can be quantified via the Gibbs Adsorption function Γ₁₂, which correlated well with the three-phase transition. On the one hand, pressure increments below the three-phase transition revealed a significant adsorption of H₂S. On the other hand, above the three-phase transition, the Gibbs Adsorption capacity remained constant, which indicated a saturation of H₂S at the water surface due to liquid-liquid equilibrium. Finally, H₂S behaves markedly differently in wetting transition, rather than the involved for CO₂ to different molecular layers beneath the surface of aqueous solutions. In this respect, H₂S is represented by a first-order wetting transition while CO₂ presents a critical wetting. Finally, it has also been found that the preferential adsorption of H₂S over the H₂O interface is greater if compared to that of CO₂, due to its strong interaction with water. In fact, we have also demonstrated that CO₂ under triphasic conditions strongly influences the wetting of the ternary system.