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.     

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.     

Stirring of a viscous fluid

Zusammenfassung Untersucht wird die stationäre strömende Geschwindigkeit, die durch eine kreisförmige rührende Bewegung eines starren Zylinders induziert wird. Die Amplitude dieser kreisförmigen Bewegung wird im Vergleich zum Durchmesser des Zylinders als klein angenommen. Es wird gezeigt, dass bei genügend hohen Frequenzen der nicht-stationäre Wirbeleffekt auf eine dünne Stokes-Scherwellenschicht beschränkt ist, dass aber die induzierte Strömung ausserhalb dieser Region bestehen bleibt. Für den besonderen Fall eines runden Zylinders ergibt sich, dass diese stationäre Strömung von der Reynolds-Zahl unabhängig ist.     

A study of the perovskite solid solution series LaxSr1−xRuO3 and LaxCa1−xRuO3 by ruthenium-99 Mössbauer spectroscopy

The magnetic, electronic, and structural properties of the solid solutions La_xSr_1−xRuO₃ and La_xCa_1−xRuO₃ have been studied by ⁹⁹Ru Mössbauer spectroscopy and other techniques. The La_xCa_1−xRuO₃ phases are reported for the first time and have been shown by powder X-ray diffraction measurements to be orthorhombically distorted perovskites. Electrical resistivity measurements on compacted powders show that all the phases are metallic with p 10⁻³, ohm-cm. Progressive substitution of Sr²⁺ by La³⁺ in ferromagnetic SrRuO₃ leads to a rapid collapse of the magnetic hyper-fine splitting at 4.2°K. For x = 0.25 some ruthenium ions still experience a magnetic field but for 0.4 x 0.75 only single, narrow resonance lines are observed, consistent both with the complete removal of the ferromagnetism and with the presence of an averaged ruthenium oxidation state in each phase, i.e., La_x³⁺Sr_1−x²⁺Ru^(4−x)+O₃ rather than La_x³⁺Sr_1−x²⁺Ru_x³⁺Ru_1−x⁴⁺O₃. LaRuO₃ and CaRuO₃ both give essentially single-line spectra at 4.2°K, indicating that the ruthenium ions in these oxides are not involved in long-range antiferromagnetic order but are paramagnetic. The solid solutions La_xCa_1−xRuO₃ (0 < x 0.6) give sharp symmetrical singlets with chemical isomer shifts (relative to the Ru metal) which move progressively from the value characteristic of Ru⁴⁺ (−0.303 mm sec⁻¹) toward the value for Ru³⁺ (−0.557 mm sec⁻¹), consistent with the presence of intermediate ruthenium oxidation states in these phases also.     

New derivatives of poly-hexafluoropropylene oxide from the corresponding alcohol

Poly-hexafluoropropylene oxide (poly-HFPO) alcohol was prepared by the reduction of the corresponding methyl ester using NaBH_4, NaBD₄, or LiAlH₄ (LAH). This alcohol is a versatile starting material for the preparation of additional interesting compounds. Because of its expected poor nucleophilicity, before attempting additional reactions, we have found it advantageous to prepare a corresponding salt. We have found that the resulting salt behaved like a normal hydrocarbon alkoxide. The reaction of this salt with either acetyl chloride or allyl chloride (or bromide) gave the expected products which proved to be normal nucleophilic substitutions. In addition, the bromination of the allyl ether was studied using NMR and GC/MS techniques.