Study on the mechanism of the effect of coumaric acid with different hydroxyl substituent positions on constructing light-regulated wormlike micelles

Light-regulated wormlike micelles have unique physicochemical properties and application prospects, therefore it has been widely studied in recent years. The interesting phenomenon of trans coumaric acid isomerides exhibits different solution properties deserves in-depth research when compounded with different types of surfactant. In this work, a pyrrolidinium-based surface active ionic liquids and trans coumaric acid isomerides, including trans ortho-coumaric acid (TOCA), trans meta-coumaric acid (TMCA) and trans para-coumaric acid (TPCA), are employed to construct a series of light-regulated wormlike micelles. The rheology, cryogen transmission electron microscopy (cryo-TEM), UV–vis spectra, 1H NMR and the DFT theory demonstrated that the structure of micelles in solution changed before and after UV light irradiation. Besides, the mechanism of trans coumaric acid isomerides (TOCA, TMCA, and TPCA) exhibits different solution properties that can be explained as follow. When the type of surfactant is cationic, the order of the spacer distance between the carboxyl group and the hydroxyl group is TPCA > TMCA > TOCA. Therefore, TOCA has the strongest ability to reduce the effective headgroup area of cationic surfactant, thereby greatly promoting the growth of wormlike micelles. When the type of surfactant is zwitterionic, under the combined action of hydrogen bonding and electrostatic attraction, trans coumaric acid isomerides are more prone to exhibit a vertical structure inside the lecithin head group. The order of capable of hydrogen bonding is TPCA > TMCA > TOCA, which explains TPCA can promote the micelles growth but TMCA and TOCA do not have this effect. The above theoretical discussion explains why trans coumaric acid isomerides exhibit different solution properties when compounded with different surfactants.