Density, structural lifetime, and entropy of H-bond cages promoted by monohydric alcohols in normal and supercooled water

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Abstract
Density data for aqueous solutions of monohydric alcohols down to supercooling are presented and combined with data concerning viscosity of the same systems, and with available data on pure water. Two conceptually different families of longer‐lived, high‐connectivity H‐bond structures can be sorted out in the irregular, frequently restructured network of H bonds: spontaneous structures, as in pure water, and alcohol‐induced structures. Molar volumes for both are obtained and compared, allowing microscopic conclusions which agree quantitatively with available thermodynamic data. For the three alcohols studied, the well‐known negative excess entropy of mixing shows a strict proportionality to the fraction of water molecules sorted out in our study as taking statistically part in alcohol‐promoted cages. The remarkable proportionality extends to all alcohols, all concentrations, and all temperatures studied. Apparent (and expected) geometric distortions of alcohol‐promoted cages do not affect this proportionality. However, they can be related to disturbing effects on the singular behavior of several properties of cold and supercooled water. These results are further combined with the only available quantitative data on the modulation by alcohols of the hydrophobic contribution to the functional conformational switching of a biomolecule. This allows for the first time deriving, from experiments on a proteinsolution, the statistical number of water molecules and associate entropy change directly involved in a specific protein function (oxygen uptake/release by human hemoglobin HbA in the specific case). Compared to the bare protein, this functional unit is largely stabilized as a consequence of the remarkably higher dimensionality in its phase space.