Microstructure of sulfosuccinates based microemulsions

Abstract

The system made by water, isooctane and a blend of Sodium dioctyl sulfosuccinate (AOT) with its more hydrophilic homologue sodium dihexyl sulfosuccinate (SDHS) can give raise to Winsor I, III and II equilibria at room temperature by slight changes in salinity. The phase transitions have been tracked using diffusion NMR, and solubilized volume fractions while small-angle X-ray scattering (SAXS) provided insight into the characteristic lengths (ξ) within the microemulsions' structures. Our findings reveal a peculiar linear correlation between the reciprocal of these characteristic lengths (ξ⁻¹) and the HLD values. For HLD <0, ξ⁻¹ decreases linearly with HLD with a slope = −0.0056 Å⁻¹ but just above HLD = 0 the slope reverses (+0.0056 Å⁻¹) and ξ⁻¹ increases linearly with HLD demonstrating that the HLD reflects the direction of interfacial curvature. However, at the optimal composition where the curvature is null and HLD = 0, the linear trends observed for ξ⁻¹ below and above HLD = 0 converge at a ξ⁻¹ value that is significantly greater than 0. This indicates that HLD is a measure of the interface/volume ratio and not of the interface curvature. This work aims to deepen the understanding of the relationship between the semi-empirical HLD equation and the spontaneous curvature of microemulsions.