Temperature dependence of hydrogen bonding and freezing behavior of water in reverse micelles.

The mid-infrared spectra of H2O and D2O confined in Aerosol OT (AOT) reverse micelles at various water/surfactant molar ratios (wo) were measured. Previous descriptions of reverse micellar (RM) water have identified three different hydrogen bonding populations in the water pool. (Onori, G.; Santucci, A. J. Phys. Chem. 1993, 97, 5430-5434.) Fitting of the O-H and O-D stretching vibrational modes to Gaussian components corresponding to these three H-bonding populations was used to determine the temperature dependence of the hydrogen bonding populations and to observe the freezing behavior of the encapsulated water pool. The H-bond network behavior of the RM water pool exhibits a strong dependence on wo and does not approximate that of bulk water until wo = 40. The freezing temperature of RM water was wo-independent. The infrared spectra of frozen RM samples has also led us to suggest a mechanism for the low-temperature phase transition behavior of AOT reverse micelles, a subject of interest for cryoenzymology and low-temperature structural biology.