On the length of lecithin reverse wormlike micelles induced by inorganic salts: Binding site matters

Abstract

Previous studies have shown that the addition of inorganic salts to lecithin organosols induces long wormlike micelles that lead to organogels. The viscosity of the micellar solution greatly increases with the increasing concentration of the inorganic salt until gelation, but interestingly, it dramatically drops after the gel regime. Although the growth mechanism has been rationally proposed, the reason for the decrease in viscosity remains unclear. In this study, we investigated the rheological behaviors and the self-assembled structures of the lecithin wormlike micelles induced by LiCl, CaCl2, and LaCl3 in cyclohexane. We found that the decrease in viscosity accompanies a shortening in the length of the wormlike micelles. Meanwhile, the study on the interactions between lecithin and the inorganic salts by FTIR shows that the inorganic salts move from the saturated phosphate and choline region to the ester linkage region on lecithin as the viscosity drops. We suggest the movement of the excess inorganic salts alters the effective molecular geometry, which in turn breaks the long wormlike micelle into short ones, thus causing the viscosity to decrease. In other words, the binding sites of the inorganic salts in lecithin headgroups is a key factor that determines the self-assembled structures of lecithin reverse micelles.