Rheology of cetyltrimethylammonium tosilate-water system. 1. Relation to phase behavior

Abstract

The partial phase behavior of CTAT/water is investigated here as a function of temperature by WAXS, DSC, polarizing microscopy, conductometry, 1H-NMR, and FTIR spectroscopy. Oscillatory strain and temperature sweeps are also reported. The Krafft temperature (TK) of CTAT/water is 23°C. Below this value, triclinic crystals of CTAT coexist with an isotropic solution. Above TK and at low concentrations, spherical micellar solutions are Newtonian and exhibit low viscosities. At higher concentrations (ct), cylindrical micelles form and viscosity increases dramatically with CTAT concentration, but no elastic effects are noticed. When micelles are long enough to entangle (0.9-27 wt % at 25°C), clear viscoelastic solutions form. At higher concentrations and up to 47 wt %, an hexagonal phase appears. This phase exhibits yield stress and viscoelasticity. At higher concentrations, a nonelastic, viscous solid paste forms. Micellar solutions and hexagonal phase depicts three regimes of viscoelasticity with temperature. These regimes are bounded by Tk and by the temperature (Tτ) at which the system exhibits its main relaxation time. Tτ moves to lower temperatures as CTAT concentration increases indicating that the main relaxation time decreases upon increasing concentration. © 1995 American Chemical Society.