Obtaining curvature-based HLD parameters for single ionic surfactants via Small Angle X-ray Scattering (SAXS)

The Hydrophilic-Lipophilic Difference (HLD) for ionic surfactant microemulsions (μEs) can be interpreted as the curvature normalized by the tail length parameter (L), if it is corrected by a factor “bi”, bi·HLD= HLDcurv= HLDbi= bi·ln(S) - kbi·EACN + Ccbi, with bi and kbi representing the curvature sensitivity to changes in salinity (S, g NaCl/100 mL) and in the equivalent alkane carbon number (EACN) of the oil, respectively; and Ccbi representing the normalized curvature when S = 1 %NaCl and EACN=0. This study determines the core radius (ξH) of drop-type μEs from a fractal core-shell analysis of Small Angle X-ray Scattering (SAXS) profiles of μEs obtained with different salinities (S) and oils (EACN), from where HLDcurv= -2L/ξH, or +2L/ξH, for oil-swollen micelles, or water-swollen reverse micelles, respectively. A linear regression analysis between HLDcurv, ln(S), and EACN yields bi, kbi, and Ccbi. This SAXS-based HLDcurv was tested using sodium dioctyl sulfosuccinate (AOT), sodium laureth sulfate (SLES), and cationic surfactant, dodecyltrimethyl ammonium chloride (DTAC). The bi, kbi and Ccbi parameters for these surfactants are consistent with those obtained using phase scans and solubilization studies. Various recommendations are offered to help reduce the uncertainty of the fitted bi, kbi and Ccbi values.