Self-aggregation properties of new ester-based gemini surfactants and their rheological behavior in the presence of cosurfactant — monolaurin

A series of new quaternary ammonium gemini surfactants having different ethylene oxide (EO) units as spacer linked with ester functionality have been synthesized and characterized. The self-aggregation and thermodynamic properties of these new gemini surfactants have been investigated in respect to the effect of increasing ethylene oxide spacer units. The critical micelle concentration increases with increase in EO spacer units and other surface properties have been found to be dependent on the length of EO spacer units. Micellar solutions of these surfactants demonstrated greater ability to solubilize a large amount of nonionic amphiphile monolaurin (ML) at 25 °C. An addition of ML to the aqueous solutions of these surfactants in a dilute region reduces the micellar curvature to less positive and favors sphere to rod to transient networks (a transparent solution of viscoelastic wormlike micelles). Viscosity increases by three orders of magnitude of that of pure solvent. The rheology study shows that the length of the polar EO spacer units of these surfactants affects the rheology; the maximum viscosity is achieved with gemini surfactant/H2O/ML having single EO unit as spacer and regular decrease in viscosity is observed with increase in EO spacer units for gemini surfactant/H2O/ML system. These systems exhibit viscoelastic behavior described by the Maxwell mechanical model with a single stress relaxation mode.