Erucyl Bis Hydroxyethyl Methylammonium Chloride Research Articles

Shear-induced phase separation (SIPS) with shear banding in solutions of cationic surfactant and salt

The rheological behavior and microstructure of branched, cationic wormlike micellar (WLM) solutions of 40 mM erucyl bis(hydroxyethyl)methylammonium chloride (EHAC) are studied as a function of added salt (sodium salicylate) concentration, temperature, and shear rate via Rheosmall-angle light scattering (Rheo-SALS). These WLM solutions exhibit shear-enhanced concentration fluctuations leading to shear-induced phase separation (SIPS), manifested as visual turbidity under shear and the appearance of a characteristic “butterfly” scattering pattern in Rheo-SALS experiments. Flow kinematics measurements in a Couette geometry are used to determine the relationship between SIPS and shear banding, i.e., the splitting of the flow into shear bands with different local shear rates. Modeling using the Giesekus constitutive equation aids in discrimination between banding and nonbanding solutions. The combination of Rheo-SALS, dynamic rheology, velocimetry, and constitutive equation modeling allows detailed exploration of the relationship between SIPS, shear banding, fluid microstructure, and the equilibrium phase behavior.

Rheology of Viscoelastic Solutions of Cationic Surfactant. Effect of Added Associating Polymer

Rheological studies were performed with aqueous salt solutions of viscoelastic cationic surfactant erucyl bis(hydroxyethyl)methylammonium chloride (EHAC) and its mixtures with hydrophobically modified polyacrylamide. The solutions of surfactant itself above the concentration of crossover of wormlike micelles exhibit two regions of rheological response. In the first region, they behave like polymer solutions in semidilute regime characterized by viscoelastic behavior with a spectrum of relaxation times. In the second region, unlike polymer solutions their relaxation after shear is dominated by a single relaxation time. Being composed of "living" micelles, the EHAC solutions easily lose their viscosity at the variation of the external conditions. For instance, heating from 20 to 60 degrees C reduces viscosity by up to 2 orders of magnitude, while added hydrocarbons induce a sudden drop of viscosity by 3-6 orders of magnitude. Polymer profoundly affects the rheological properties of EHAC solutions. The polymer/surfactant system demonstrates a 10,000-fold increase in viscosity as compared to pure-component solutions, the effect being more pronounced for polymer with less blocky distribution of hydrophobic units. A synergistic enhancement of viscosity was attributed to the formation of common network, in which some subchains are made up of elongated surfactant micelles, while others are composed of polymer. At cross-links the hydrophobic side groups of polymer anchor EHAC micelles. In contrast to surfactant itself, the polymer/surfactant system retains high viscosity at elevated temperature; at the same time it keeps a high responsiveness to hydrocarbon medium inherent to EHAC.

Mixed Spherical and Wormlike Micelles: A Contrast-Matching Study by Small-Angle Neutron Scattering

Small-angle neutron scattering studies were used to investigate the effect of adding an alcohol ethoxylate nonionic surfactant (d-C12E20) to aqueous solutions of a cationic surfactant, erucyl bis(hydroxyethyl) methylammonium chloride (EHAC), with and without salt (KCl). The systematic use of contrast-matching, by alternately highlighting or hiding one of the surfactants, confirms that mixed micelles are formed. In salt-free solutions, mixed spherical micelles are formed and a core-shell model combined with a Hayter-Penfold potential was used to describe the data. The core radius is dominated by the EHAC tails and the outer radius determined by the ethoxylate headgroups of the nonionic surfactant. Addition of KCl promotes micellar growth; however, results of varying the solvent contrast revealed that when the nonionic surfactant is incorporated into the wormlike structure micellar breaking is promoted. Thus, mixed wormlike micelles with shorter contour lengths compared to the pure EHAC worms are formed.

Shear-Induced Phase Separation in Solutions of Wormlike Micelles

Polymer solutions in the vicinity of the theta-point are known to undergo shear-induced turbidity or phase separation. The present study shows that a similar phenomenon also occurs for certain wormlike micellar solutions. Wormlike micelles are the self-assembled counterparts of polymers and are characterized by their ability to reversibly break and recombine. In the system of interest, the micelles are formed by the cationic surfactant erucyl bis(hydroxyethyl)methylammonium chloride (EHAC), in conjunction with a salt such as sodium chloride (NaCl) or sodium salicylate (NaSal). Micellar samples that become turbid under shear show evidence of critical concentration fluctuations and may contain predominantly branched micelles. The shear-induced turbidity in these samples correlates with the appearance of flow-dichroism in rheooptic experiments and with an increase in low-q scattering in small-angle light scattering under flow (flow-SALS) experiments. The characteristic "butterfly" pattern, with enhanced scattering in the flow direction and a dark streak perpendicular to the flow direction, is typically observed in flow-SALS. The results suggest that the turbidity is due to a shear-induced growth of concentration fluctuations, which in turn manifests as large anisotropic domains, typically oriented along the vorticity axis.

Rheology, Cryogenic Transmission Electron Spectroscopy, and Small-Angle Neutron Scattering of Highly Viscoelastic Wormlike Micellar Solutions

To understand the strong viscoelastic response showed by aqueous solutions of erucylbis(hydroxyethyl)methylammonium chloride (EHAC) in the presence of potassium chloride (KCl), steady-state rheology, small-angle neutron scattering (SANS), and cryogenic transmission electron microscopy (cryo-TEM) experiments were performed. This cationic surfactant has the ability to self-assemble into giant wormlike micelles. The effect of surfactant concentration, added salt, and temperature were investigated. The surfactant solutions have a gellike behavior at room temperature and become Maxwellian as the temperature is increased. It was found that the low-shear viscosity has a strong dependence on salt concentration and temperature. Small-angle scattering indicated the formation of wormlike micelles. The high-Q range was fitted using the Kratky−Porod wormlike chain model, and a cross-sectional radius of gyration (Rg,xs) of 21 A was obtained. Additionally, cryo-TEM images revealed changes in the structure of the entangl...