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-CO2 approximate to 0-80 MPa). The water-carbon dioxide system is confined by two parallel silica-based surfaces (hydroxylated beta-cristobalite) separated by h = 5 nm. The aim of this work is to study the WCB contact angle (theta(c)) as a function of T and P-CO2. Our simulations indicate that theta(c) varies weakly with temperature and pressure: Delta theta(c) approximate to 10-20 degrees for P-CO2 increasing from approximate to 0 to 80 MPa (T = 320 K); Delta theta(c) approximate to -10 degrees 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 theta(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 theta(c) being insensitive to the presence of NaCl.