Water and Carbon Dioxide Capillary Bridges in Nanoscale Slit Pores: Effects of Temperature, Pressure, and Salt Concentration on the Water Contact Angle.

We perform molecular dynamics (MD) simulations of a nanoscale water capillary bridge (WCB) surrounded by carbon dioxide over a wide range of temperatures and pressures ( T = 280-400 K and carbon dioxide pressures P CO 2 ≈ 0-80 MPa). The water-carbon dioxide system is confined by two parallel silica-based surfaces (hydroxylated β-cristobalite) separated by h = 5 nm. The aim of this work is to study the WCB contact angle (θ c ) as a function of T and P CO 2 . Our simulations indicate that θ c varies weakly with temperature and pressure: Δθ c ≈ 10-20° for P CO 2 increasing from ≈0 to 80 MPa ( T = 320 K); Δθ c ≈ -10° for T increasing from 320 to 360 K (with a fixed amount of carbon dioxide). Interestingly, at all conditions studied, a thin film of water (1-2 water layers-thick) forms under the carbon dioxide volume. Our MD simulations suggest that this is due to the enhanced ability of water, relative to carbon dioxide, to form hydrogen-bonds with the walls. We also study the effects of adding salt (NaCl) to the WCB and corresponding θ c . It is found that at the salt concentrations studied (mole fractions x Na = x Cl = 3.50, 9.81%), the NaCl forms a large crystallite within the WCB with the ions avoiding the water-carbon dioxide interface and the walls surface. This results in θ c being insensitive to the presence of NaCl.