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

Abstract

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 ≈ 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 . Our simulations indicate that θ _c varies weakly with temperature and pressure: Δθ _c ≈ 10–20° for 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.