Interfacial tension and the behavior of microemulsions and macroemulsions of water and carbon dioxide with a branched hydrocarbon nonionic surfactant

Abstract

Measurements of interfacial tensions for 2-ethyl-hexanol-(propylene oxide)∼ 4.5–(ethylene oxide)∼ 8 (2EH-PO 4.5-EO 8) at the planar water–CO 2 interface and the surfactant distribution coefficient are utilized to explain microemulsion and macroemulsion phase behavior from 24 to 60 °C and 6.9 to 27.6 MPa. A CO 2 captive bubble technique has been developed to measure the interfacial tension γ at a known surfactant concentration in the aqueous phase, with rapid equilibration at the water–CO 2 interface. The surface pressure ( γ o − γ) decreases modestly with density at constant temperature as CO 2 solvates the surfactant tails more effectively, but changes little with temperature at constant density. The area per surfactant at the CO 2–water interface determined from the Gibbs adsorption equation decreases from 250 A 2/molecule at 24 °C and 6.9 MPa, to 200 A 2/molecule at 27.6 MPa. It was approximately twofold larger than that at the water–air interface, given the much smaller γ o driving force for surfactant adsorption. For systems with added NaCl, γ decreases with salinity at low CO 2 densities as the surfactant partitions from water towards the W–C interface. At high densities, salt drives the surfactant from the W–C interface to CO 2 and raises γ. Compared with most hydrocarbon surfactants, this dual tail surfactant is unusually CO 2-philic in that it partitions primarily into the CO 2 phase versus the water phase at CO 2 densities above 0.8 g/ml, and produces γ values below 1 mN/m. With this small γ, a middle phase microemulsion and a C/W microemulsion were formed at low temperatures and high CO 2 densities, whereas macroemulsions were formed at other conditions.