Structure and transport in concentrated micellar solutions with a lower consolute boundary

Abstract

Self-diffusion coefficients and viscosities of aqueous solutions of dodecyltributylammonium bromide (C₁₂NBu₃Br) have been measured as a function of concentration and temperature by pulsed-field-gradient ¹HNMR and couette viscometry. These solutions form no liquid-crystalline phases, remaining isotropic at all compositions, but demix above a lower critical temperature of 58 °C between 8 and 80 wt.% solution. Diffusion results are first interpreted using the conventional approach in which the partition of both surfactant and water between bulk and micelle bound states is examined. An alternative approach to self-diffusion in concentrated surfactant solutions incorporating the viscosity results has proven more fruitful, revealing a change in solution structure near the high-concentration side of the two-phase body. Based on this a mechanism for the phase separation involving hydration of the surfactant is proposed and compared with previous results on this and related systems.