Wettability of Fully Hydroxylated and Alkylated (001) α-Quartz Surface in Carbon Dioxide Atmosphere

Wettability of alkylated quartz surfaces is of primary importance in several technological applications, including the development of oil and gas reservoirs and carbon geo-sequestration. It is intuitively understood and experimentally confirmed that hydroxylated quartz surfaces are hydrophilic. By gradually saturating a hydroxylated (001) a quartz surface with pentyl groups, we show using molecular dynamics simulations that the surface can also exhibit extreme hydrophobicity. Within a range of surface pentyl group density from 0.29 to 3.18/nm(2), the contact angle of a water droplet under 10 MPa pressure of carbon dioxide at 300 K changes from 10-20 to 180 degrees. This study has shown that a complete description of wettability of alkylated quartz surfaces requires three contact angles-one at the tip level of pentyl groups and two at the level of the quartz surface. The latter two are the contact angle of the spherical droplet and the hidden contact angle of a water skirt formed between the tip level of pentyl groups and the quartz surface. Analysis of the hidden contact angle unveils a binary wettability, where the surface relatively abruptly transforms from hydrophilic (the contact angle is less than 90) to hydrophobic (the contact angle is 180 degrees) with an increase in surface pentyl group concentration.