Rate constants k iso of the thermal cis-trans isomerization of four 4,4'-nitro-aminoazobenzenes with different amino groups have been determined in homogeneous aprotic solvents and polyglykol oligomers, primarily by means of conventional flash photolysis. The rate constants have been correlated with polarity (according to max from UV/Vis absorption spectra of the trans isomers) and bulk viscosity of the solvents. Qualitative conclusions about the influence of varying concentrations of water with respect to polarity and hydrogen bonding on k iso -and max -values in acetone/water mixtures were derived. Based on these results the data from microheterogeneous solutions have been interpreted.In microheterogeneous water/surfactant solutions k iso -values of selected azo dyes were strongly dependent on the concentrations of SDS, Triton ᭨ X-100, C 12 EO 8 in water, and varied with the composition of bicontinuous microemulsions of Igepal ᭨ CA-520/ heptane/water. The large spread of isomerization rate constants is in part due to varying microviscosity.Replacement of H 2 O by D 2 O in aqueous surfactant solutions produced surprisingly large kinetic solvent isotope effects.

In aqueous solutions of poly(ethylene oxide) (PEO)−poly(propylene oxide) (PPO)−PEO triblock copolymers (Pluronics P85, F88, and L64), structure formation (micellization) on increasing temperature was followed by determination of rate constants k iso of the thermal cis−trans isomerization of 4,4‘-nitroanilinoazobenzene by means of flash photolysis in H2O and D2O. The kinetic solvent isotope effect k iso,H 2 O/k iso,D 2 O indicates that the azobenzene dye molecules are solubilized in a water-rich environment. From the nearly constant solvatochromic UV/vis absorption band maxima λ max of the dye, it is concluded that the S shape of the ln k iso vs 1/T curves is mainly due to microviscosity changes on micellization. Critical micelle temperature values derived are in satisfactory agreement with those from self-diffusion coefficients of the copolymer molecules dependent on temperature determined by means of pulsed field gradient nuclear magnetic resonance measurements. The self-diffusion experiments allow conclusions on the size of the diffusing particles in H2O and D2O and the influence of dye molecules on aggregation. The hydrodynamic radii of the diffusing species are larger in H2O than in D2O. The reason is seen in the stronger hydrogen bonds between EO units and D2O compared to those between EO units and H2O. On gelation of 25% (w/v) F88 in water at 31° C, the bulk viscosity increases sharply but the microviscosity around the dye molecules does not.

We report new observation of cooperative paramagnetic fluctuations of Ru 4+ spins that coexist with the non-Fermi liquid state in CaRuO3 perovskite below T 22 K. Detailed electrical, magnetic and neutron scattering measurements reveal that the Ru 4+ ions reside in magnetic field independent random domains with dynamic properties that are reminiscent of the cooperative paramagnetic fluctuations. The linear (E/T ) scaling of the dynamic susceptibilities and divergence of the mean relaxation time as T → 0 K suggest quasi-local critical nature of the spin fluctuations. We argue that the non-Fermi liquid behavior arises due to the quantum critical nature of the cooperative paramagnetic fluctuations in CaRuO3.

By means of microsecond flash photolysis the rate constants, k iso, of the thermal cis → trans isomerization of two azo dyes were determined in dependence on temperature in aqueous solutions of poly(ethylene glycols) (PEGs) and of a poly(ethylene oxide)−poly(propylene oxide)−poly(ethylene oxide) block copolymer (F88, EO96PO39EO96). PEG/water mixtures are considered as a model for the PEO/water interface layer of micelles of F88. The dependence of k iso on bulk viscosity of PEG/water mixtures varying with the number of water molecules, Z, per ethylene oxide unit is stronger than that in water-free solutions, and the kinetic solvent isotope effect (k iso,H 2 O/k iso,D 2 O) is larger than the viscosity solvent isotope effect of the mixtures (η D 2 O/η H 2 O). Therefore we assume that beside changes of microviscosity a dynamic hydrogen bonding effect of water contributes to the sensitivity of k iso on nanostructure changes in aqueous surfactant solutions. Comparing temperature-dependent k iso, Z data from PEG/water mixtures with k iso values determined in aqueous solutions of F88, hydration numbers Z in the microenvironment of isomerizing dyes were approximately estimated. From S-shaped Arrhenius plots of k iso critical micellization temperatures were determined, and the dependence of k iso data on copolymer concentration is discussed.

In aqueous solutions of the triblock copolymer F88 (EO96PO39EO96) micelle formation has been studied by means of dual fluorescence of the probe molecule 1,3-bis(1-pyrene)propane monitoring viscosity changes in its microenvironment. Mobility changes of the polymer molecules on micellization were followed by means of pulsed field gradient (PFG) NMR. These results complement a recent detailed study of cis−trans isomerization of azo dyes as “kinetic probes” for structure formation in aqueous solutions of F88 (Langmuir 2001, 17, 2907). Differences of the critical micellization temperatures determined by various methods, and in H2O and D2O as solvents, are discussed. Our results reflect the particular large content of polye(ethylene oxide) in F88 (80%) compared to other triblock copolymers of this type.