Interfacial Tension and Wettability in Water–Carbon Dioxide Systems: Experiments and Self-consistent Field Modeling

This paper presents experimental and modeling results on water–CO2 interfacial tension (IFT) together with wettability studies of water on both hydrophilic and hydrophobic surfaces immersed in CO2. CO2–water interfacial tension (IFT) measurements showed that the IFT decreased with increasing pressure and the negative slopes of IFT–pressure isotherms decreased with increasing temperature. Water contact angle on a cellulose surface (hydrophilic) immersed in CO2 increased with pressure, whereas the water contact angle on a hydrophobic surface such as hexamethyl disilazane (HMDS) coated silicon surface was almost independent of pressure. These experimental findings were augmented by modeling using the self-consistent field theory. The theory applies the lattice discretization scheme of Scheutjens and Fleer, with a discretization length close to the size of the molecules. In line with this we have implemented a primitive molecular model, with just small variations in the molar volume. The theory makes use of the Bragg-Williams approximation and has binary Flory–Huggins interaction parameters (FH) between CO2, water, and free volume. Using this model, we generated the complete IFT–pressure isotherms at various temperatures, which coincided well with the trends reported in literature, that is, the water–CO2 interfacial tension decreased with increasing pressure for pressures ≤100 bar and became independent of pressure >100 bar. The transition point occurred at higher pressures with increasing temperature. At three-phase coexistence (water–CO2–free volume) and at the water–vapor interface (water–free volume), we always found the CO2 phase in between the water-rich and free volume-rich phases. This means that for the conditions studied, the water–vapor interface is always wet by CO2 and there are no signs of a nearby wetting transition. Calculation of the water contact angle on a solid surface was based on the computed adsorption isotherms of water from a vapor or from a pressurized CO2-rich phase and analysis of surface pressures at water–vapor or water–CO2 coexistence. The results matched reasonably well with the experimental contact angle data. Besides, we also computed the volume fraction profiles of the CO2, H2O, and the V phase, from which the preferential adsorption of CO2 near the hydrophilic surface was deduced.