Can Simple Salts Influence Self-Assembly in Oil? Multivalent Cations as Efficient Gelators of Lecithin Organosols

Abstract

It is known that lecithin, a zwitterionic phospholipid, self-assembles into spherical reverse micelles in organic solvents. We have explored the effects of adding inorganic salts to lecithin organosols. Salts are insoluble in organic solvents, and hence their effects on reverse self-assembly have rarely been studied. Our studies show, however, that salts can indeed be dissolved in organic liquids in the presence of lecithin. More interestingly, salts of multivalent cations like calcium (Ca(2+)), magnesium (Mg(2+)), lanthanum (La(3+)), and cerium (Ce(3+)) greatly increase the viscosity of lecithin organosols and transform the samples into optically transparent organogels. In comparison, monovalent cations or transition-metal cations have negligible effect on reverse self-assembly. On the basis of data from small-angle neutron scattering (SANS), we show that gelation is accompanied by a nanostructural transition from spherical micelles to cylindrical micelles/filaments. The varying abilities of different cations to induce gelation is shown to correlate with their binding tendencies to the phosphocholine headgroups of lecithin. A two-component gelator such as lecithin/Ca(2+) could be attractive for applications due to its negligible cost and nontoxic nature. We demonstrate how such a gelator combination can convert a liquid fuel such as kerosene into a gel without the use of heat or shear. The same gel can also further be ungelled by addition of a few drops of alcohol.