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.