Effect of compressed CO2 on the critical micelle concentration and aggregation number of AOT reverse micelles in isooctane

The effect of compressed CO, on the critical micelle concentration (cmc) and aggregation number of sodium bis-2-ethylhexylsulfosuccinate (AOT) reverse micelles in isooctane solution was studied by UV/Vis and florescence spectroscopy methods in the temperature range of 303.2-318.2 K and at different pressures or mole fractions of CO2 (X-CO2). The capacity of the reverse micelles to solubilize water was also determined by direct observation. The standard Gibbs free energy (AG(degrees)(m)), standard enthalpy (Delta H-degrees(m)), and standard entropy (Delta S-degrees(m)) for the formation of the reverse micelles were calculated by using the cmc data determined. It was discovered that the cmc versus X-CO2 curve and the Delta G(degrees)(m) versus X-CO2 curve for a fixed temperature have a minimum, and the aggregation number and water-solubilization capacity of the reverse micelles reach a maximum at the X-CO2 value corresponding to that minimum. These results indicate that CO2 at a suitable concentration favors the formation of and can stabilize AOT reverse micelles. A detailed thermodynamic study showed that the driving force for the formation of the reverse micelles is entropy.