Rate Constants of the Thermal Cis → Trans Isomerization of Azo Dyes as Kinetic Probes in Aqueous Solutions of Poly(ethylene glycols) and Poly(ethylene oxide)−Poly(propylene oxide)−Poly(ethylene oxide) Block Copolymer F88

Abstract

By means of microsecond flash photolysis the rate constants, kiso, 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 kiso 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 (kiso,H2O/kiso,D2O) is larger than the viscosity solvent isotope effect of the mixtures (ηD2O/ηH2O). Therefore we assume that beside changes of microviscosity a dynamic hydrogen bonding effect of water contributes to the sensitivity of kiso on nanostructure changes in aqueous surfactant solutions. Comparing temperature-dependent kiso,Z data from PEG/water mixtures with kiso 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 kiso critical micellization temperatures were determined, and the dependence of kiso data on copolymer concentration is discussed.