Water
Contact Angle Dependence with Hydroxyl Functional
Groups on Silica Surfaces under CO<sub>2</sub> Sequestration Conditions
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Abstract
Functional
groups on silica surfaces under CO<sub>2</sub> sequestration
conditions are complex due to reactions among supercritical CO<sub>2</sub>, 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 (Q<sup>3</sup>, Q<sup>3</sup>/Q<sup>4</sup>, Q<sup>4</sup>), as silanol number
density decreases, contact angle increases from 33.5° to 146.7°
at 10.5 MPa and 318 K. When Q<sup>3</sup> surface changes to an amorphous
structure, water contact angle increases 20°. Water contact angle
decreases about 12° when 9% of silanol groups on Q<sup>3</sup> 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 CO<sub>2</sub> sequestration conditions, the
results found in this study may help to better understand wettability
of CO<sub>2</sub>/brine/silica system