Dynamic fluorescence probing of the microenvironment of sodium dodecyl sulfate micelle solutions: Surfactant concentration dependence and solvent isotope effect

We have studied the surfactant concentration dependence and solvent isotope effect on the slow solvation dynamics and orientational dynamics of solvatochromic coumarin dyes in the aqueous sodium dodecyl sulfate (SDS) micelle solutions using picosecond fluorescence spectroscopy. The solvation time constants for SDS micelle H2O,solutions are about 130 ps and 2.5 ns. Both the faster and slower solvation time constants do not depend on the SDS concentration in the range 16.2-810 mM, though the observed solvation component becomes larger with increasing of SDS concentration. The solvation dynamics in SDS micelle D2O solutions is about 1.2 times slower than that in SDS micelle H2O solutions. The retardation of the solvation dynamics in micelle solutions could be due to the slower process of the hydrogen bond dynamics in the hydration layer around the micelle, as well as simple hydrogen -bonding liquids. The surfactant concentration dependence and solvent isotope effect on the orientational dynamics of the fluorescence probe is correlated with those on the solvation dynamics in SDS micelle solutions.