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

By means of microsecond flash photolysis the rate constants, hi,,, of the t hermal cis --> trans isomerization of two azo dyes were determined in depen dence on temperature in aqueous solutions of poly(ethylene glycols) (PEGs) and of a poly(ethylene oxide)-poly(propylene oxide)-poly(ethylene oxide) bl ock copolymer (F88, EO96PO39EO96) PEG/water mixtures are considered as a mo del for the PEG/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),(H2O)/k(iso) ,D2O) is larger than the viscosity solvent isotope effect of the mixtures ( eta (D2O)/eta (H2O)) Therefore we assume that beside changes of microviscos ity a dynamic hydrogen bonding effect of water contributes to the sensitivi ty of hi,, on nanostructure changes in aqueous surfactant solutions. Compar ing temperature-dependent k(iso),z data from PEG/water mixtures with hi,, v alues determined in aqueous solutions of F88, hydration numbers Z in the mi croenvironment of isomerizing dyes were approzimately estimated. From S-sha ped Arrhenius plots of k(iso) critical micellization temperatures were dete rmined, and the dependence of hi,, data on copolymer concentration is discu ssed.