Influence of crowding on hydrophobic hydration-shell structure.

Abstract

The influence of molecular crowding on water structure, and the associated crossover behavior, is quantified using Raman multivariate curve resolution (Raman-MCR) hydration-shell vibrational spectroscopy of aqueous tert-butyl alcohol, 2-butyl alcohol and 2-butoxyethanol solutions of variable concentration and temperature. Changes in the hydration-shell OH stretch band shape and mean frequency are used to identify the temperature at which the hydration-shell crosses over from a more ordered to less ordered structure, relative to pure water. The influence of crowding on the crossover is found to depend on solute size and shape in a way that is correlated with the corresponding infinitely dilute hydration-shell structure (and the corresponding first hydration-shell spectra are invariably very similar to pure water). Analysis of the results using a Muller-like two-state equilibrium between more ordered and less ordered hydration-shell structures implies that crossover temperature changes are dictated primarily by enthalpic stabilization of the more ordered hydration-shell structures.