Molecular Dynamics Investigation of Wettability Control in Mixed H2–CO2 Gas Systems at Theoretical Calcite Interfaces

Abstract

Understanding wettability in subsurface gas–water–rock systems is essential for applications such as geological hydrogen storage, carbon sequestration, and reactive transport in porous media. In this study, molecular dynamics simulations were performed to investigate the wettability behavior of mixed H2-CO2 gas bubbles on a mineral surface under aqueous conditions. The focus was placed on disentangling the roles of gas composition and gas–rock interaction strength in controlling contact angle behavior. Systematic scaling of gas–solid interaction parameters revealed that wettability can be governed by the adsorption affinity, rather than gas fraction alone. Therefore, by increasing the CO2–rock interaction, a significant rise in contact angles can be observed, whereas scaling H2–rock interactions produced weaker effects. These findings indicate that CO2 acts as the dominant wettability controlling species due to its stronger dispersion interactions and quadrupolar character, which promote preferential adsorption at the mineral interface. Additionally, simulations varying the CO2 fraction demonstrated two distinct regimes depending on which gas dominated the interfacial adsorption layer. When CO2 formed the primary adsorbed layer, increasing its fraction enhanced surface hydrophobicity. In contrast, when H2 dominated the interface, changes in composition produced a different wettability response. This highlights the importance of interfacial structuring over bulk composition. The results provide a mechanistic framework for understanding competitive gas adsorption and its influence on wettability in mixed-gas systems. These insights are relevant for predicting multiphase behavior in subsurface energy storage and carbon management applications, where interfacial phenomena critically impact gas trapping, mobility, and long-term stability.