Aggregate evolution in aqueous solutions of a Gemini surfactant derived from dehydroabietic acid

Abstract

Innovations in surfactant structure are a feasible way to probe molecular self-assembly principles. Herein, the solution behaviour of a newly synthesized Gemini surfactant derived from dehydroabietic acid, abbreviated R-(EO)-E-R, was investigated using surface tension, fluorescence, isothermal titration calorimetry (ITC), rheology, freeze-fracture transmission electron microscopy (FF-TEM) and cryogenic transmission electron microscopy (cryo-TEM) methods. R-(EO)-E-R has two large, rigid hydrophobic groups. At low concentrations, R-(EO)-E-R forms micelles with an aggregation number of approximately 10, which is smaller than those of Gemini surfactants containing flexible alkyl tails. In addition, the micellization process is less exothermic because of the rigidity of the hydrophobic portions. As the concentration increases, R-(EO)-E-R without any additives forms wormlike micelles, endowing the solution with an obvious viscoelasticity. Further increases in the concentration lead to the coexistence of single-walled vesicles, double-walled vesicles and rarely observed long, tubular vesicles. This behaviour is attributed to the two large, rigid hydrophobic groups of R-(EO)-E-R, which increase the density of the hydrophobic portion around the ionic head groups and facilitate the formation of aggregates with lower curvatures and asymmetric morphology. Surfactants containing rigid hydrophobic portions are expected to result in more delicate, self-assembled morphologies with broad applications.