(432g) Water Thin Films on Kaolinite Gibbsite and Edge Surfaces and Their Effects on Surface Wettability in Relation to Geological Carbon Sequestration

CO₂, as a major source of greenhouse gases (GHG), has increasingly drawn public attention in the recent decade. Geological CO₂ sequestration (GCS) is a viable option to mitigate this problem. GCS relies on various mechanisms, among which structural trapping plays an important role. Within the framework of structural trapping, the storage capacity is strongly dependent on CO₂-water-rock contact angle. Kaolinite is one of the common rocks in the caprock where structural trapping takes place. Even though CO₂-water-rock contact angle on kaolinite basal surfaces have been studied, the knowledge about kaolinite edge surface wettability remains limited.

In this work, we use molecular dynamics (MD) simulations to study CO₂-water-kaolinite contact angle on kaolinite gibbsite and edge surfaces under a typical GCS condition (330 K and 200 bar). The common belief is that surface wettability in the presence of water is dependent on the surface hydroxyl (-OH) group density. However, we find that surface -OH group density is not the only pivotal factor determining surface wettability. While the surface -OH group density of kaolinite edge surface is much lower than that of gibbsite surface, both kaolinite gibbsite and edge surfaces are strongly water-wet. Edge surface is better hydrated than gibbsite surface as both silanol and aluminol groups can form hydrogen bonding with water molecules thanks to large effective water accessible volume around them, and pocket water can further anchor water film. Our study provides important insights for CO₂ structural trapping as it illustrates that surface heterogeneity is crucial to wettability.