Correlation between Contact Angle and Water Film Energetics in Carbon Dioxide-Water-Clay Mineral Interfacial Systems: A Molecular Dynamics Study

The wettability of mineral surfaces is an important parameter influencing the fluid flow in geological formations intended for the storage of carbon dioxide (CO2). Hence, the effects of the layer charge of clay minerals on their wettability should shed light on the variation of the solid-fluid interaction and its impact on the contact angle and water film energetics in CO2-water-clay mineral systems. Given the nanoscale thickness of the films formed on mineral surfaces and the difficulty of directly evaluating the related energetics, we use molecular dynamics simulations to investigate the effects of layer charge on the contact angle (water vs CO2) and the energetics of water films in clay slit-pore systems. An increase in the negative layer charge is shown to decrease the contact angle and increase the thickness of the water film. Permeation free energy calculations indicate that the amount of work needed to make the CO2 molecules approach the surface increases with increasing negative layer charge. These findings allow the establishment of a correlation between the contact angle and film energetics and thus enable us to propose a novel approach to complement direct contact angle calculations. Moreover, the present analysis of the structure and dynamics of the adsorbed water provides us with new insights into the change in the film energetics from an atomic-scale perspective, which cannot be gained using the widely used thermodynamic model; the connectivity of the adsorbed water molecules on the surface explains the effects of the layer charge on the contact angle and film energetics.