Alkyl chain length-dependent viscoelastic properties in aqueous wormlike micellar solutions of anionic gemini surfactants with an azobenzene spacer

A homologue of carboxylate gemini surfactants with an azobenzene spacer and different lengths of the alkyl tails, referred to as C_m(azo)C_m, has been synthesized. All the surfactants formed wormlike micelles at relatively low concentrations without addition of salt. The reason was attributed to the long and rigid characteristic of the azobenzene spacer, which yielded the pseudo volume between the two tails and thus a columnar-like molecular geometry favorable for the formation of wormlike micelles. The results of rheology and FF-TEM measurements showed that the length of the alkyl tails strongly affected the viscoelastic properties of the wormlike micelle solution. With the increase of the alkyl tail length, the solutions evolved from a typically viscous fluid to a strong viscoelastic solution and then to a gel-like state. This was attributed to the different microstates as revealed by scaling law. These solutions behave as polyelectrolyte systems because of their additive-free nature.