Study on phase behaviors of supercritical CO2 including surfactant and water

Phase behaviors for the supercritical CO2 (scCO2) + surfactant binary system and the scCO2 + surfactant + water ternary system were investigated at temperatures from 308.2 to 343.2 K by using a synthetic-type apparatus. Polyethylene oxide-2,6,8-trimethyl-4-nonyl ethers (TMN) was used as the surfactant. For the binary system, vapor–liquid phase-transition pressures increased with increasing temperature and the critical points (maximum pressures on the phase-boundary curves) were observed at TMN composition around 1.5 wt%. On the other hand, for the ternary system including 1 wt% of TMN, two phase-transition pressures were obtained at each temperature and composition. A transparent phase was observed above the upper phase-transition pressure and a cloudy phase including emulsions appeared between the upper and the lower phase-transition pressures. Vapor and liquid phases were clearly separated below the lower phase-transition pressure. The upper phase-boundary curves had a minimum pressure around water composition about 1 wt% on P–w phase diagram. On the other hand, the lower phase-transition pressures decreased monotonously with increasing water composition.