Water Dynamics at the Interface in AOT Reverse Micelles

The orientational dynamics of water molecules at the interface in large Aerosol-OT (AOT) reverse micelles are investigated using ultrafast infrared spectroscopy of the OD stretch of dilute HOD in H2O. In large reverse micelles (similar to 9 nm diameter or larger), a significant amount of the nanoscopic water is sufficiently distant from the interface that it displays bulk-like characteristics. However, some water molecules interact with the interface and have vibrational absorption spectra and dynamics distinct from bulk water. The different characteristics of these interfacial waters allow their contribution to the data to be separated from the bulk. The infrared absorption spectrum of the OD stretch is analyzed to show that the interfacial water molecules have a spectrum that peaks near 2565 cm(-1) in contrast to 2509 cm(-1) in bulk water. A two-component model is developed that simultaneously describes the population relaxation and orientational dynamics of the OD stretch in the spectral region of the interfacial water. The model provides a consistent description of both observables and demonstrates that water interacting with the interface has slower vibrational relaxation and orientational dynamics. The orientational relaxation of interfacial water molecules occurs in 18 +/- 3 ps, in contrast to the bulk water value of 2.6 ps.