Varying the rheological behavior of a micellar solution <i>via</i> modified microscopic structures in the presence of graphene oxide

We experimentally and numerically investigate the aggregation structure of cationic wormlike micelles in the presence of graphene oxide (GO), in connection with the change in the rheological properties of their aqueous dispersion. We first confirm that the macroscopic viscoelastic properties under oscillatory shear vary non-monotonically with the addition of GO flakes. We then carried out three distinct experiments-small-angle X-ray scattering (SAXS) measurements, time-domain nuclear magnetic resonance (TD-NMR) measurements, and molecular dynamics (MD) simulations-to elucidate the structural modifications likely responsible for the rheological changes. The results of the SAXS and TD-NMR measurements suggest that surfactant molecules preferentially remain in a worm-like microparticle form when bonded to the GO surface but not completely covering the GO surface. Moreover, using MD simulations, we confirmed that the attractive interaction between negatively charged functional groups on the GO surface and cationic surfactants indeed leads to adsorption. Together with the results of the rheology measurements, the SAXS, TD-NMR, and MD simulation results suggest that GO flakes tend to form three-dimensional aggregates bridged by the wormlike micelles. Our results can be utilized to control the rheological properties of micellar solutions and provide a new paradigm for designing microscopic structures of GO.