Solvent Isotope Effects On the Rheology and Micostructure of Cationic Wormlike Micelles near the Isotropic to Nematic Transition

We study the solvent isotope substitution (D₂O-H₂O) at the isotropic to nematic (I-N) transition and the associated rheology and shear banding behavior of aqueous CTAB solutions. Linear and nonlinear rheology as well as flow birefringence and particle image velocimetry measurements are used to characterize the CTAB/H₂O system at compositions and temperatures near the I-N transition. These measurements are compared to those reported previously [Helgeson et al., J. Rheol., 2009, 53, 441; Helgeson et al., Soft Matter, 2009, 5, 3858] for CTAB in D₂O. Substantial isotope effects were observed when H₂O is substituted by D₂O at comparable molar concentrations. These are: (1) a shift of the equilibrium I-N phase transition to lower mole fractions and (2) a significant broadening of the shear banding region for both the range of shear rate and the composition. These effects are consistent with the fact that D₂O is a more polar and more structured solvent than H₂O, which results in a reduction of the flexibility in the WLM, as confirmed by the values of the persistence length calculated using flow-birefringence measurements.