Vibrational energy relaxation of aqueous azide ion confined in reverse micelles

Vibrational energy relaxation (VER) times have been measured by ultrafast infrared spectroscopy near 2000 cm⁻¹ for the antisymmetric stretching ${\nu }_3$ band of azide ion in water pools of nonionic reverse micelles (RM). The water pool radii were varied in the 1–3 nm range by adjusting the water-to-surfactant molar ratio, $\mathrm{\omega =[}{\mathrm{H}}_2\mathrm{O]/[surfactant]}.$ Compared to the value measured in this work for bulk water (0.81±0.06 ps), the VER decay times are about three times longer (2.5±0.2 ps) for the smallest RM studied (ω=1) and become shorter with increasing ω and RM size but do not reach the bulk value at the largest ω studied. Solvent shifts of the azide vibrational band in RMs have been previously reported [Langmuir 18, 7401 (2002)], and in a manner similar to the VER rates, tend toward the bulk water value with increasing ω. Studies of the VER dynamics of azide ion in RMs are used to investigate confinement effects on solvation and to explore the effects of continuously modifying the solute–solvent interaction by varying ω in the RMs.