LIQUID-LIQUID PHASE-SEPARATION IN CATIONIC MICELLAR SOLUTIONS

We have investigated the phase equilibria and liquid isotropic phase of solutions of the cationic surfactant dodecyltributylammonium bromide in water and D2O. The extremely hygroscopic solid surfactant forms an isotropic liquid phase up to at least 92 wt % in water and undergoes a phase separation on warming onto two liquid isotropic phases, with a critical point at 46 wt % surfactant and 48°C in D2O. Small-angle neutron scattering spectra were measured on solutions with compositions between 1 and 80 wt % surfactant and at temperatures from 20 to 60°C in order to determine micellar aggregation numbers and charges and interfacial areas per molecule of the surfactant. Intermicellar monomer concentrations are also determined from scattering spectra by a novel method, and these are used to explain the range and nature of the concentration-dependent intermicellar interactions which give rise to phase separation. © 1990 American Chemical Society.