Persistence of birefringence in sheared solutions of wormlike micelles.

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.

Wormlike micelles of a C22-tailed zwitterionic betaine surfactant: from viscoelastic solutions to elastic gels.

The 22-carbon-tailed zwitterionic surfactant erucyl dimethyl amidopropyl betaine (EDAB) forms highly viscoelastic fluids in water at low concentrations and without the need for salt or other additives. Here, semidilute aqueous solutions of EDAB are studied by using a combination of rheological techniques, small-angle neutron scattering (SANS) and cryo-transmission electron microscopy (cryo-TEM). EDAB samples show interesting rheology as a function of temperature. At low temperatures (approximately 25 degrees C), a 50 mM EDAB sample behaves like an elastic gel with an infinite relaxation time and viscosity. Upon heating to approximately 60 degrees C, however, the sample begins to respond like a viscoelastic solution; that is, the relaxation time and zero-shear viscosity become finite, and the rheology approaches that of a Maxwell fluid. The same pattern of behavior is repeated at higher EDAB concentrations. Cryo-TEM and SANS reveal the presence of giant wormlike micelles in all EDAB samples at room temperature. The results imply that, depending on temperature, EDAB wormlike micelles can exhibit either a gel-like response or the classical viscoelastic ("Maxwellian") response. The unusual gel-like behavior of EDAB micelles at low temperatures is postulated to be the result of very long micellar breaking times, which, in turn, may be due to the long hydrophobic tails of the surfactant.

Viscosity increase with temperature in cationic surfactant solutions due to the growth of wormlike micelles.

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.

Anionic wormlike micellar fluids that display cloud points: rheology and phase behavior.

We report our investigations into the self-assembly of sodium oleate (NaOA) in the presence of a binding salt (triethylammonium chloride, Et(3)NHCl) simple salt (potassium chloride, KCl). Both salts promote the growth of long, wormlike micelles in NaOA solutions, thereby increasing the fluid viscosity. The significant difference with the Et(3)NHCl salt is that it also modifies the phase behavior of NaOA solutions. Specifically, NaOA/Et(3)NHCl solutions display cloud points upon heating, followed by phase separation into two liquid phases. Such cloud point behavior is rarely observed in ionic surfactant systems, although it is common in nonionic surfactant solutions. Interestingly, while cloud points are not observed with KCl, the addition of KCl to NaOA/Et(3)NHCl solutions further lowers the cloud point temperature. Also, in the case of tetraethylammonium halide salt, neither a cloud point nor an increase in viscosity is observed. The clouding in the case of Et(3)NHCl is attributed to the temperature-induced aggregation of anionic micelles whose surface is covered by bound counterions.