Vibrational Spectroscopy and Dynamics of Water Confined inside Reverse Micelles

In this work, we combine atomistic molecular dynamics simulations with theoretical vibrational spectroscopy to study the properties of water confined inside bis(2-ethylhexyl)sulfosuccinate (AOT) reverse micelles. This approach is found to successfully reproduce the experimental spectra, rotational anisotropy decays, and spectral diffusion time-correlation functions as a function of micelle size. These results are interpreted in terms of water molecules in different hydrogen bonding environments. One interesting result from our simulation, not directly accessible experimentally, involves the distance from the surfactant headgroup/water interface over which the dynamical properties of water become bulk-like. We find that this distance varies with micelle size, casting doubt on the core/shell model. In particular, the distance increases with decreasing micelle size, and hence decreasing radius of curvature of the interface. We suggest that this arises from curvature-induced frustration. We also find that the dynamics in the smallest micelle studied is extremely slow-relaxation is still incomplete by 1 ns. As in other glassy systems with collective relaxation, our time-correlation functions can be fit to stretched exponentials, in this case with very small exponents.