Characterization of water/sodium bis(2-ethylhexyl) sulfosuccinate/sodium bis(amyl) sulfosuccinate/n-heptane mixed reverse micelles and w/o microemulsion systems: The influence of water and sodium bis(amyl) sulfosuccinate content

The unique properties of reverse micelles, microemulsions, enable a variety of applications, from oil recovery to pharmaceuticals. For an effective process development for a targeted application, systems with specific ionic and molecular transport properties are needed. In this work, experiments were carried out to study microstructures of mixed reverse micelles, microemulsions (water/n-heptane), obtained with the use of anionic surfactants, sodium bis(2-ethylhexyl) sulfosuccinate (AOT) and the structural-related sodium bis(amyl) sulfosuccinate (DAS). Viscosity measurements, electrical conductivity, differential scanning calorimetry (DSC) and small-angle X-ray scattering (SAXS) were experimental techniques used for characterization of the prepared systems. The effect of DAS content on conductivity was that with increasing molar ratio of DAS/AOT (concentration of Na⁺ cations was kept at constant) the conductivity maximum shifted to lower water content, indicating an enlargement in reverse micelles size, which was also confirmed by SAXS analysis. It was further observed that at total surfactant concentrations of 0.45mol/L, at a DAS/AOT molar ratio of 10/90, and 0.35mol/L, at a DAS/AOT molar ratio of 15/85, this maximum almost vanished but an additional conductivity maximum at the composition with higher water content was detected. The effect of DAS on percolation phenomena was striking. The percolation threshold shifted towards smaller water content intensively with an increasing molar ratio of DAS/AOT and DAS concentration to an extent that the conductivity maximum overlapped with the percolation peak. The observed effect and a complementary viscosity and DSC analysis suggested clustering of mixed droplets in the low-water region. It has been confirmed that DAS enables formation of different aggregation states by influencing the rigidity of the oil/water interface in water/AOT-DAS/n-heptane mixed reverse micelles, w/o microemulsions. Our results have shown that by controlling DAS and water content it is possible to extensively modify the microstructure properties of the water/AOT-DAS/n-heptane system.