Phase behavior, micellization and rheology of erucic acid-based ionic liquid surfactant in deep eutectic solvents

Abstract

Fatty acid-based ionic liquid surfactants have attracted significant attention due to their environmental friendliness, showing significant potential in some specific applications (e.g., personal & house care products). Deep eutectic solvents (DES) are a kind of new green and promising solvent. However, the self-assembly of ultra-long-chain fatty acid-based ionic liquid surfactants in DES has rarely been reported to date. In this paper, erucic choline (ErCho), as a representative of ultra-long-chain ionic liquid surfactants, was synthesized and characterized, and then its micellization behavior in three different DES (ChGly, ChEG, and ChPh, which were obtained by mixing glycerol, ethylene glycol, phenol, and choline chloride in a 2:1 M ratio) was investigated. ErCho can self-assemble into micelles in DES, as well as in water, but a relatively larger critical micelle concentration (cmc) was obtained in DES due to a small hydrophobic association. The hydrophobic association of ErCho in ChGly was the smallest, and the standard free energy of micellization per mole of monomer unit was the largest (-37.67 kJ·mol−1). However, at high shear rates, typical shear-thinning was observed in the ChGly solution of ErCho, showing pseudoplastic fluids behavior, which was not found in ChEG, ChPh, and H2O. This reflects the presence of large aggregates, and cryo-TEM observation confirmed the formation of wormlike micelles in the ChGly solution of ErCho with a small flow activation energy (∼43 kJ·mol−1). In addition, ErCho in ChGly solution can further evolve into a hexagonal phase, and then to lamellar crystalline phases upon increasing concentration.