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 H 2 S-water systems to clarify the effects of adsorption on interfacial tension due to vapor-liquid-liquid equilibrium. The predicted H 2 S-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 Γ 12 , which correlated well with the three-phase transition. On the one hand, pressure increments below the three-phase transition revealed a significant adsorption of H 2 S. On the other hand, above the three-phase transition, the Gibbs Adsorption capacity remained constant, which indicated a saturation of H 2 S at the water surface due to liquid-liquid equilibrium. Finally, H 2 S behaves markedly differently in wetting transition, rather than the involved for CO 2 to different molecular layers beneath the surface of aqueous solutions. In this respect, H 2 S is represented by a first-order wetting transition while CO 2 presents a critical wetting. Finally, it has also been found that the preferential adsorption of H 2 S over the H 2 O interface is greater if compared to that of CO 2 , due to its strong interaction with water. In fact, we have also demonstrated that CO 2 under triphasic conditions strongly influences the wetting of the ternary system.