(121c) Solvent Isotope Effects On the Equilibrium and Shear Induced Isotropic to Nematic Transition in Wormlike Micellar Solutions

Isotropic to nematic (I-N) transitions of wormlike micellar (WLM) cetyltrimethyl ammonium bromide solutions in H2O and D2O was studied as a function of temperature and concentration. Linear and non-linear rheology of the solutions was performed in the vicinity of the I-N transition Theoretical predictions of the single-mode Giesekus model in the nonlinear-viscoelastic regime were compared to the non-linear rheological data with fairly good agreement. Discrimination between banding and non-banding behavior of the WLM solutions was possible through the value of the fitted drag anisotropy parameter. Substantial isotope effects were observed when H2O is substituted by D2O at comparable molar concentrations of surfactant, namely: (1) an increase of about 15 °C in the equilibrium I-N transition temperature; (2) the critical value of the order parameter describing the relative distance to the equilibrium I-N transition [Helgeson et al., Soft Matter 2009, 5, 3858] increased in about 0.8 wt% or about 4 °C; (3) the shear banding region in the non-equilibrium state diagram expanded to larger Wissenberg number and larger compositional order parameter. The isotope effect in the phase transition temperature can be explained by the fact that D2O is a more polar solvent than H2O, which reduces flexibility in the WLM.