Estimations of Interfacial Tensions Between Liquid CO2 and Water from the Sessile-Drop Observations

Abstract

Greenhouse gases, such as CO2, have become a serious problem for mankind. To offset the CO2 emissions into the atmosphere, sequestration of CO2 in the deep ocean has been proposed. The behavior of CO2 injected into seawater can be predicted from the CO2-H2O phase diagram and a depth profile of the seawater temperature. In addition, the interfacial tension between liquid CO2 and water (or sea water) is an important factor for understanding the behavior of the injected CO2 droplets into deep water. However, CO2 hydrate formation on the droplets at depths deeper than 400 m makes the prediction of CO2 dissolution difficult due to insufficient knowledge of the relevant physical parameters. CO2 hydrate is an ice-like clathrate compound formed from CO2 and water under suitable conditions of low temperature T and high pressure P. This crystalline compound will form at the interface between the injected liquid CO2 and seawater and can reduce the dissolution rate of CO2 into seawater. The interfacial tension between liquid CO2 and water or NaCl solution was measured by a simple sessile-drop method at pressures up to 25 MPa and at temperatures of 278 and 288 K. The interfacial tension between liquid CO2 and pure water was approximately 38 mN m-1at 288 K and 5 MPa with a small pressure dependence, whereas the values between liquid CO2 and 3 wt% NaCl solution were more than 10 % larger than those between liquid CO2 and pure water. At 278 K, CO2 hydrate is stable and the interfacial tension has larger pressure dependence. This might be related to the supersaturation prior to hydrate formation.