Influence of co-solvent on the rheological behavior of aqueous viscoelastic surfactant solutions

We have studied the influence of glycerol on the rheological behavior of viscoelastic surfactant solutions. The measurements were carried out on the system CTAB and NaSal. This system shows two viscosity maxima with increasing NaSal concentration. When water is replaced by glycerol, the viscosities and the structural relaxation times at the viscosity maxima decrease while they increase at the viscosity minimum. As a result of the opposite changes the two maxima are replaced by a single maxima when 40% water is replaced by glycerol. The change of the viscosities is a consequence of a change of the structural relaxation times while the storage moduli remain constant. The opposite changes are a result of different mechanisms for the relaxation times. At the maxima, the relaxation times are reptation controlled and depend on the intermicellar interaction energy. At the minimum, relaxation times are kinetically controlled and depend on the intramolecular interaction.