Vibrational Relaxation Dynamics of Azide in Ionic and Nonionic Reverse Micelles

Abstract

Ultrafast infrared spectroscopy has been used to measure vibrational energy relaxation (VER) and reorientation times (Tr) for the antisymmetric stretching band of azide ion (N3-) in several reverse micelle (RM) systems using cationic, anionic, and nonionic surfactants. RMs were formed using H2O for all surfactants and D2O for the anionic and cationic surfactants. The vibrational dynamics depended on the RM charge. The charge dependence is attributed to differences in ion location in the RM because of Coulombic interactions. The VER times in anionic (AOT, sodium bis(2-ethylhexyl) sulfosuccinate) RMs are indistinguishable from those in bulk solution and Tr times are longer only for the smallest RMs studied. In cationic (CTAB, cetyltrimethylammonium bromide) RMs, VER and Tr times are longer and weakly depend on the RM water content and water pool size. The results are attributed to the azide anion being attracted toward the RM wall when it is cationic and repelled into the bulklike center when it is anionic. VER times are also longer in small nonionic RMs (NP and Brij-30) but, unlike cationic RMs, approach bulk behavior as the RM size is increased. Comparative studies are performed using mixtures of water and tri(ethylene glycol) monomethyl ether (TGE) in which the latter resembles the hydrophilic portion of the nonionic surfactants. The similar results for nonionic RMs and TGE/water mixtures provide evidence that water penetrates into and hydrates the poly-oxo chains in the nonionic RMs before a water pool is formed. The interfacial region in nonionic RMs include the poly-oxo chains, which appear to be hydrated, so that the boundary between the interface and the water core is less clearly defined than for the ionic RMs.