Is there a region of highly structured water around a nonpolar solute molecule?

Published measurements of water proton chemical shifts for dilute solutions of alcohols and other hydrocarbon derivatives surprisingly seem to imply that hydrophobic groups enhance water structure near 0°C but disrupt it at elevated temperatures. A model is presented which allows these observations to be rationalized and is consistent with experimental values of enthalpies and heat capacities of solution of hydrocarbon gases. It requires the assumption that hydration-shell H-bonds have higher bond-breaking enthalpies and entropies than those in bulk water. These quantities are evaluated from available thermochemical data. Using the corresponding free energies of bond breaking, it is then calculated that the fraction of broken H-bonds is larger in the hydration shell than in the bulk liquid even at temperatures near the freezing point. The Model does not invoke formation of extended ordered regions that could be described as icebergs and that “melt” when the solutions are heated.

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