Solubility and Phase Behaviors of AOT Analogue Surfactants in 1,1,1,2-Tetrafluoroethane and Supercritical Carbon Dioxide

A series of AOT (aerosol-OT) analogue surfactants (sodium salt of dibutyl-2-sulfosuccinate, sodium salt of dipentyl-2-sulfosuccinate, sodium salt of dihexyl-2-sulfosuccinate, and sodium salt of dioctyl-2-sulfosuccinate) were synthesized and characterized by 1 H NMR and elemental analysis. A static method coupled with gravimetric analysis is developed to measure the solubility of the surfactants in 1,1,1,2-tetrafluoroethane (HFC-134a) and supercritical CO 2 (scCO 2 ). The solubilities of the surfactants in HFC-134a and scCO 2 are affected by the temperature, pressure, and carbon atom number of the surfactant. The solubility of the same surfactant in HFC-134a solvent is approximately two times that in the most commonly used supercritical solvent CO 2 . The pressure−temperature phase diagrams for water/HFC-134a microemulsions stabilized by the surfactants were determined using cloud-point measurements for a concentration range of the surfactant from (1.85 × 10 -3 to 5.60 × 10 -3 ) M, temperature up to 338 K, and pressure up to 40 MPa in a high-pressure vessel. At a fixed temperature, the cloud-point pressure increased with increasing water-to-surfactant molar ratio ( W o ). At a fixed W o, the cloud-point pressure decreased with increasing temperature. The surfactant with the longest hydrocarbon chain has the highest cloud-point pressure even at lower surfactant concentrations.