Effect of hydrocarbon chain length of aliphatic solvents on the reverse self-assembly of lecithin and monovalent ion mixtures

Abstract

Surfactants are used as organogelators, which form surfactant-based gels in nonpolar solvents. One of the well-known surfactants that form organogels is lecithin, which self-assembles into reverse spherical micelles in nonpolar solvents. When inorganic salts are added to lecithin solutions, the reverse spherical micelles transform into long reverse cylindrical micelles, giving rise to organogels. Although previous studies have shown that the addition of inorganic salts to lecithin solutions can induce the gel formation, the solvent effects on the gel formation have not been mapped out in detail. In this study, we systematically investigated the effects of hydrocarbon chain length of alkanes on the self-assembly of lecithin and monovalent ion mixtures. Our findings showed that LiCl, LiBr, LiI, NaBr, NaI, and KI salts formed organogels in the presence of lecithin, whereas NaCl and KBr formed viscoelastic solutions with lecithin. More importantly, with an increase in the hydrocarbon chain length of alkanes, the lecithin/salt mixtures formed gels more efficiently. The gel formation is closely related to the length of the reverse cylindrical micelles, as confirmed by small-angle X-ray scattering analysis. In addition, the length of the cylinders is affected by the interactions between the lecithin head groups and ions, as evidenced by Fourier transform infrared spectroscopy.