Vibrational energy relaxation of aqueous azide ion confined in reverse micelles

Abstract

Vibrational energy relaxation (VER) times have been measured by ultrafast infrared spectroscopy near 2000 cm−1 for the antisymmetric stretching ν3 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, ω=[H2O]/[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.

Vibrational population relaxation and dephasing dynamics of Fe(CN)64− in water: deuterium isotope effect of solvents

“…On the other hand, for polyatomic cases, Hochstrasser and coworkers reported that the vibrational relaxation of azide ion takes place in 1.2 ps in H 2 O and 2.4 ps in D 2 O [1,2]. Very recently, Zhong et al [18] reported that the vibrational relaxation time is 0.81 ps in H 2 O. Morita and Kato [19] performed theoretical calculation of the vibrational relaxation process for N À 3 in water. They considered the intramolecular charge fluctuation induced by the solute vibrational motion and by the external potential by the solvent.…”

Dynamics of Nanoscopic Water: Vibrational Echo and Infrared Pump−Probe Studies of Reverse Micelles

“…31,39,40 Infrared (IR) pumpprobe experiments on the azide ion confined in reverse micelles have measured the azide ion vibrational lifetime. 41 A significant increase of the lifetime of the azide ion is observed for the smallest reverse micelles. Polarization-resolved pump-probe studies of the same system showed that the orientational relaxation of azide was at least an order of magnitude slower than the reported lifetimes, which was interpreted as suggesting restricted mobility of water in reverse micelles.…”

Structure and dynamics of water in nonionic reverse micelles: A combined time-resolved infrared and small angle x-ray scattering study

“…Other studies, in particular NMR experiments and vibrational energy relaxation of probe molecules, support this observation. 23,51 From our study, we conclude that there is no water between the PEO chains, for these small reverse micelles as represented in Figure 12. However, we observe that for w • = 15, S deviates a lot from the fit.…”

“…The conformation of the hydrophilic polyethylene oxide (PEO) chain in nonionic reverse micelles is still a matter of controversy. 23,31,51,52 Numerous papers depict the structure of a nonionic reverse micelle. Mostly, they show that the PEO chain penetrates the water volume and dangles freely in the water body.…”

“…22 Probe molecules are also used in optical techniques. 23,24 Optical techniques have the advantage of having long lived excited-states, thus creating a large time window for dynamic measurements. However, the difficulty with using molecular probes lies in determining where the molecule is located inside the reverse micelle, thus making it challenging to determine which part of the water in the reverse micelle is being probed.…”