Fourier transform infrared spectroscopy of azide ion in reverse micelles

The antisymmetric stretching band of the nu(3) vibration of azide ion was studied in nanosize water droplets confined in reverse micelles by Fourier transform infrared spectroscopy. In reverse micelles of nonionic nonylphenyl polyoxyethylene (NP) and cationic cetyltrimethylammonium bromide (CTAB), the nu(3) stretching frequency of the azide ion is red-shifted compared to in bulk water, whereas in anionic reverse micelles of sodium bis(2-ethylhexyl) sulfosuccinate (AOT), it is blue-shifted. The shifts at omega = 3 (omega = [H2O]/[surfactant]) for NP, CTAB, and AOT reverse micelles are -14, -31, and +4 cm(-1), respectively. In all cases, the shift decreases as omega is increased, indicating that the water interior approaches the properties of bulk water. This agrees with the changes observed in the IR spectra of water and the surfactant headgroup in NP reverse micelles as a function of omega. Comparison between reverse micelles formed with different surfactants reveals that the nu(3) azide band is sensitive to its local environment, which could be affected by interactions with the surfactant headgroups, the solvating water properties, and the presence of counterions. In addition, the aggregation number of NP reverse micelles in cyclohexane was obtained via time-resolved fluorescence qenching. The radius of the water droplet in the interior of micelle was found to increase from 13 to 34 Angstrom for omega = 3 to 10.