Water Contact Angle Dependence with Hydroxyl Functional Groups on Silica Surfaces under CO₂ Sequestration Conditions

Functional groups on silica surfaces under CO₂ sequestration conditions are complex due to reactions among supercritical CO₂, brine and silica. Molecular dynamics simulations have been performed to investigate the effects of hydroxyl functional groups on wettability. It has been found that wettability shows a strong dependence on functional groups on silica surfaces: silanol number density, space distribution, and deprotonation/protonation degree. For neutral silica surfaces with crystalline structure (Q³, Q³/Q⁴, Q⁴), as silanol number density decreases, contact angle increases from 33.5° to 146.7° at 10.5 MPa and 318 K. When Q³ surface changes to an amorphous structure, water contact angle increases 20°. Water contact angle decreases about 12° when 9% of silanol groups on Q³ surface are deprotonated. When the deprotonation degree increases to 50%, water contact angle decreases to 0. The dependence of wettability on silica surface functional groups was used to analyze contact angle measurement ambiguity in literature. The composition of silica surfaces is complicated under CO₂ sequestration conditions, the results found in this study may help to better understand wettability of CO₂/brine/silica system