Solvent isotope effect on the microstructure and rheology of cationic worm-like micelles near the isotropic-nematic transition

Abstract

D2O–H2O isotope substitution is commonly used to study self-assembled surfactant mesophases by nuclear magnetic resonance and small angle neutron scattering, but the effects of this substitution are not trivially predictable. Here, 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 one of the simplest shear-banding wormlike micellar solution models: the aqueous salt-free 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 previous reports for CTAB in D2O and H2O. Substantial isotope effects are observed when H2O is substituted by D2O at comparable molar concentrations. These are: (1) an increase of about 10 °C or equivalent decrease of 0.002 mole fractions in the equilibrium I-N transition, (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 greater polarity of D2O compared to H2O, which results in a stiffening of the WLM in D2O. This is confirmed by measurements of the persistence length from flow-birefringence.