Effect of Compressed CO₂ on the Critical Micelle Concentration and Aggregation Number of AOT Reverse Micelles in Isooctane

Abstract

The effect of compressed CO2 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 fluorescence 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 (DeltaGo(m)), standard enthalpy (DeltaHo(m)), and standard entropy (DeltaSo(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 DeltaGo(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.

Surfactant Aggregation in CO2/Heptane Solvent Mixtures

“…Because of the low dielectic constant of these CO 2 -heptane mixtures, it can be assumed that the AOT aggregates to generate reversed micelles, and this is supported by the absence of any strong repulsive electrostatic S(Q) peaks in the recorded SANS profiles. The level of alkane required to trigger aggregation obtained previously 33 is similar to that seen here (30 vol% heptane), and the aggregate size observed by SANS is smaller than that reported in ref 33. This points to the particular strength of HP-SANS 5,35 for studying small aggregate structures in unconventional solvent systems, as compared to indirect and invasive methods such as fluorescence quenching, which may be strongly influenced by local environmental factors.…”

“…34 On the basis of fluorescence probe 33 and small-angle X-ray scattering (SAXS) 34 data, extensive aggregation was detected in the mixed systems. (Note, in those studies 33,34 low CO 2 pressures up to ∼60 bar, and temperatures between 15 and 45 °C were employed, so that the majority of systems investigated contained compressed gas CO 2 , different from this work where pumped CO 2 is in the liquid phase. )…”

l-Tryptophan transport through a hydrophobic liquid membrane using AOT micelles: Dynamics of the process as revealed by small angle X-ray scattering

“…The enthalpy of micellization is endothermic for reverse micelles. Consequently, formation of reverse micelles is found to be entropy driven [41]. The formation of smaller reverse micelles is also favored by entropy since they have higher degrees of freedom that increase the system disorder.…”

“…6A, the L-tryptophan cation interacts electrostatic with the surfactant headgroup. These interactions are strong enough to make the surfactant layer more rigid [31,41], stabilizing the reverse micelles, which then inhibit significant changes in their droplet size as the temperature is increased. However, in situation B (Fig.…”

Switching Micellization of Pluronics in Water by CO2

“…[10][11][12] In addition, compressed CO 2 is quite soluble in many liquids, and the dissolution of CO 2 in liquids can change the properties of the liquid solvents considerably. [13] Up to now, compressed CO 2 has been successfully applied in colloid science, such as tuning the properties of reverse micelles, [14][15][16] enhancing the stability of vesicles, [17] and switching the transition between different surfactant aggregates. [18,19] Pluronics (PEO-PPO-PEO; triblock copolymers in which PEO represents poly(ethylene oxide) and PPO represents poly(propylene oxide)) are an important class of surfactants with wide industrial applications, mostly notable in detergency, dispersion stabilization, foaming, emulsification, and lubrication.…”