Rheological behaviors and mechanism of pH-stimulus wormlike micelles variation induced by ortho-substituent types of benzoic acid

Abstract

pH-responsive wormlike micelles (WLMs) have been widely constructed by the mixture of surfactants and organic acids. However, the effect of different ortho-substituents on the rheological behaviors of WLMs remains unclear. Here in, three pH-responsive WLMs were prepared by combining N-erucamidopropyl-N, N-dimethylamine (UC22AMPM) with o-phthalic acid (o-PA), salicylic acid (HSal) and benzoic acid (BA) at the molar ratio of 2:1, respectively (named as o-EAPA, EAHSal, EABA). The phase behaviors, viscoelasticity and thickening mechanism were researched by macroscopic observation, rheological measurements, cryo-TEM, surface tension and 1H NMR. The results indicated that, the hydroxyl group at ortho position (HSal) could form the intramolecular hydrogen bond and prevented the rotation of carboxyl group, which decreased the steric hindrance of carboxyl group. Therefore, EABA system exhibited the weaker viscoelasticity than EAHSal system. The carboxyl group in ortho position (o-PA) can construct the “pseudo” Gemini surfactant by electrostatic attraction, and o-PA2− entered into aqueous phase due to its strong hydrophilicity, which further eliminated the steric hindrance of benzene ring. As a result, o-EAPA solution had the highest viscoelasticity among the three systems. In addition, by circularly changing the pH for several times, the three systems could maintain its original viscoelasticity without being weakened in the slightest.