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.