New insight into the transition mechanism of pH-tunable wormlike micelles based on experiments and DPD simulation

A new gemimi-like surfactant (named C14-N6-C14) formed by a mixed system of itaconic acid tetradecyl ester (C14) and 1,6-hexamethylenediamine (N6) at a molar ratio of 2:1 was developed to investigate the transition mechanism of pH-tunable wormlike micelles. pH-tunable properties of gemini-like surfactant were characterized by macroscopic observation, rheology, and 1H NMR measurements. The experimental results indicated that the pH-tunable properties could be ascribed to different protonation degrees and noncovalent interactions of the diamine to carboxyl groups of C14 at different pH values. The gemini-like surfactant C14-N6-C14 could be changed into itaconic acid monosodium salt surfactant (C14-ONa) when the pH was high enough. The formation of wormlike micelles was a result from the synergy between "gemini" and "unimer". In addition, the pH-tunable mechanism of the C14-N6-C14 system was proposed and further confirmed by mesoscopic morphological simulations using the dissipative particle dynamics (DPD) modul. The simulation and experimental results showed a good consistency, revealing the formation and destruction processes of wormlike micelles in the long chain acids and short amines mixed system. Our studies have lighted new views on understanding of the noncovalent interactions behind the structural changes by experiments and DPD simulation.