Dynamic and rheological properties of classic and macromolecular surfactant at the supercritical CO2–H2O interface

For the first time, rheological and dynamical properties of various interfacial layers separating an aqueous phase and a carbon dioxide phase under supercritical conditions have been measured by means of a drop tensiometer, applying either sinusoidal or ramp interfacial area perturbation. Those approaches have been largely developed on liquid–air and liquid–liquid interface but very few studies were performed in pressurized conditions [F. Tewes, F. Boury, Formation and rheological properties of the supercritical CO2–water pure interface, J. Phys. Chem. B 109 (2005) 3990–3997; F. Tewes, F. Boury, Effect of H2O–CO2 organization on ovalbumin adsorption at the supercritical CO2–water interface, J. Phys. Chem. B 109 (2005) 1874–1881].For small surfactants, such as Tween® molecules, same results for equilibrium elasticity (Ee) values were obtained whatever the perturbation mode. However, non-equilibrium elasticity values (Ene) were in some cases significantly influenced by the kind of perturbation. Rheological measurements evidenced the effect of the size of the alkyl tail upon the rheological properties of the interface. In particular, an alkyl chain composed of 16 carbon atoms facilitated the formation of a mixed interface constituted from Tween® molecules and a network of structured H2O–CO2 molecules.Polymeric molecules like human serum albumin (HSA), produced interfacial films with an important elasticity. For these systems, Ee and Ene varied with CO2 pressure and with the type of protein.