Unravelling micellar structure and dynamics in an unusually extensive DDAB/bile salt catanionic solution by rheology and NMR-diffusometry

Abstract

The mixed didodecyldimethylammonium bromide (DDAB)–sodium taurodeoxycholate (STDC)– 2H 2O catanionic system forms a large isotropic (L 1) phase at 25 °C. The evolution of microstructure along different dilution lines has been followed by means of rheology and NMR diffusometry. In general, the L 1 phase is characterised by a weak viscoelasticity and Newtonian response. In the STDC-rich regime ( W s = [ DDAB ] / [ STDC ] = 0.2 ), 5 wt% is an overlapping concentration at which the discrete-to-rodlike micellar transition occurs as indicated from the total surfactant concentration ( C s ) dependency of both zero-shear viscosity ( η 0 ∼ C s 3.7 ) and surfactant self-diffusion ( D s ∼ C s −3.0 ). As the surfactant molar ratio ( W s ⩾ 1 ) increases, i.e., DDAB concentration increases, and at constant C s , η 0 decreases and D s increases, indicating the formation of a multiconnected micellar network.