Formation and rheological properties of the supercritical CO2-water pure interface.

From the interfacial tension (gamma) measurement, we have analyzed the interfacial organization that occurs between pure H2O and pure CO2 from a kinetical and rheological point of view. This article is the followup to a previous one, where we showed that this equilibrated interface is composed of small H2O-CO2 cluster blocks [Tewes, F.; Boury, F. J. Phys. Chem. B 2004, 108, 2405]. By analyzing the variation of gamma with the square root of time, we found that the organization of the H2O-CO2 interface is, in the initial times, controlled by the diffusion of the CO2 molecules into the water. We compared the frictional coefficient determined from the measured CO2 diffusion coefficient with the frictional coefficient calculated from the Stokes equation (frictional ratio). From that, we concluded that it is a hydrated form of CO2 that diffuses and that the degree of hydration decreases with pressure. Rheological properties of the equilibrated interface vary with CO2 pressure, in the range of 50-90 bar, from a viscoelastic comportment to a purely elastic behavior, showing a change in the interfacial organization. The high equilibrium part of the elasticity (110 mN/m) obtained at 90 bar suggests a highly structured interface. Two phenomena could explain the interfacial rheological behavior: (i) an increase and a growth of the blocks H2O-CO2 cluster with the CO2 pressure or (ii) an increase in the interfacial capacity to form stable clusters under interfacial area compression.