Oil-solubilization-induced viscosity transition in aqueous amidoamine surfactant solutions

Worm-like micelles, long, flexible aggregates of surfactant molecules, have attracted significant attention owing to their unique macroscopic rheological properties. Typically, the worm-like micellar solution, which is composed of common surfactants, decreases with increasing temperature. In previous studies, we reported that rigid rod-like micelles formed by a long-chain amidoamine derivative, N-(2-amino-ethyl)-3-{[2-(2-amino-ethylcarbamoyl)-ethyl]-octadecyl-amino}-propioamide (C18AA), underwent a phase transition into highly viscous, flexible worm-like micelles upon heating to approximately 50°C. This phase transition results from thermally induced breakage of intermolecular hydrogen bonds within the amide groups of C18AA. This study demonstrated that the solubilization of toluene into rod-like micelles of C18AA induced a phase transition into worm-like micelles, resulting in a significant increase in the viscosity of micellar solutions even at room temperature. However, when hexane was solubilized in rod-like micelles, the viscosity of the micellar solutions remained constant, suggesting that solubilization of hexane did not induce this phase transition. The contrasting effects of toluene and hexane solubilization on the rod-like micellar solutions of C18AA were attributed to the different solubilization sites. These findings underscore the importance of solvent-specific interactions in micellar morphology and offer a straightforward approach to tuning the rheology of surfactant systems without thermal stimulation.