Molecular Insights into the Impact of Surface Chemistry and Pressure on Quartz Wettability: Resolving Discrepancies for Hydrogen Geo-storage

Subsurface formations are promising for large-scale H2 storage, balancing the energy demand and supply. Wettability is vital in ensuring storage safety, efficiency, and capacity, whereas noticeable discrepancies exist in the literature. This work reconciles these discrepancies by revealing the mechanisms of quartz wettability alteration with surface chemistry and pressure using classical molecular dynamics simulation. We find that the fully rigid quartz substrate results in much lower hydrophilicity than the fully flexible and the hydroxyl group flexible quartz substrates due to the lower probability of hydrogen bond formation between water and hydroxyl groups. Also, quartz wettability relies on not only the area density but also the arrangement of the surface hydroxyl groups. Monolayer water adsorption on both hydrophilic and hydrophobic quartz surfaces is observed, whereas the structure of the adsorbed water film is different. Dissolved H2 prefers to move to the quartz surface rather than staying in bulk water. The water contact angle on the fully hydroxylated quartz fluctuates between 30.7 and 37.1(degrees) with pressure ranging from 1-30 MPa, without a monotonic trend. We reveal that the dominant mechanism of wettability alteration within this pressure range is due to the pinning effect induced by the microstructures on the quartz surface.