Wormlike micellar solutions based on ionic surfactants typically show an exponential decrease in viscosity upon heating. Here, we report the unusual observation of an increasing viscosity with temperature in certain cationic wormlike micellar solutions. The solutions contain a cationic surfactant with an erucyl (C22, mono-unsaturated) tail and an organic salt, sodium hydroxynaphthalene carboxylate (SHNC). When these solutions are heated, their zero-shear viscosity increases over a range of temperatures. In some cases, the viscosity reaches a peak at a certain temperature and then decreases with further heating. The magnitude of the viscosity increase, the onset of this increase, and the peak temperature can all be tuned by varying the SHNC concentration. Small-angle neutron scattering is used to study the origin of this unusual rheological behavior. The data reveal that the contour length of the micelles increases with temperature, in tandem with the rise in viscosity. A possible explanation for the contour length increase, based on a temperature-dependent counterion binding, is discussed.

When aqueous solutions containing wormlike micelles (worms) are sheared, the micellar chains tend to align with the flow, which in turn leads to flow-birefringence. When shear is stopped, the worms rapidly revert to an isotropic state in typical samples, and the birefringence disappears. In this study, we present a system of cationic worms that shows a different behavior: not only do the samples become intensely birefringent when sheared but the birefringence also persists for hours (and even days) after shear is stopped. These results suggest that shear-aligned worms in the sample are trapped in their aligned state for long periods of time, an aspect that is confirmed by cryo-transmission electron microscopy (cryo-TEM). We seek to determine the origin of this unusual behavior. Our results show that the persistent birefringence is observed for cationic worms induced by hydroxy-naphthoate but not salicylate counterions. These observations suggest that the micellar alignment is stabilized by intermicellar attractive interactions (such as pi-pi and cation-pi) that arise when large aromatic counterions are anchored within the micelles.