Wormlike Micelles of a C22-Tailed Zwitterionic Betaine Surfactant:  From Viscoelastic Solutions to Elastic Gels

Abstract

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.

Understanding viscosity reduction of a long-tail sulfobetaine viscoelastic surfactant by organic compounds

“…This is evidence that microemulsions were present in the solution. 5,7,8,30 Cryo-TEM revealed the existence of microemulsions along with short and thin cylindrical micelles in the surfactant solution with 3 wt% n-decane equilibrated at 60 C (Fig. 11).…”

“…Long-tail surfactants have been of interest of research due to their various applications as viscosifying agents. 5 Moreover, long-tail sulfobetaine surfactants have been reported to be a good candidate for especially high-temperature applications such as well stimulation. 33 Despite this interest, micellar changes of long-tail sulfobetaine surfactants induced by organic compounds have not been published.…”

“…These organic compounds reduce the viscosity of the surfactant solutions by solubilizing in the core of cylindrical micelles. 5,6,8,[28][29][30] Solubilization disrupts the cylindrical micelle structure, changing longer cylindrical micelles into shorter cylindrical micelles, Kralchvesky and coworkers 31,32 determined the solubilization kinetics of non-polar oils in mixed cylindrical micelles of a nonionic surfactant and a triblock copolymer. The micelles adsorb on the oil-aqueous solution interface before solubilization takes place.…”

“…This mechanism was also proposed for the changes induced by sodium salicylate (NaSal) and sodium hydroxynaphthalene carboxylate (NaHNC) on EDAB. 5 Because the surfactant was a sulfobetaine, it resisted a change in charge with reducing pH. Therefore, the micelles were insignicantly affected.…”

“…Erucyl-tailed amphoteric surfactants such as erucyl dimethyl amidopropyl betaine (EDAB), 5 EDAS 34 and erucyldimethyl amidopropyl amine oxide (EMAO) 38 have such behavior within attributed to gels. 5 But the existence of the Newtonian region at low shear rates (Fig. 3) and G 0 (Fig.…”