| Abstract
| - A simple two-state structural model of solute hydration has been developed. In this model, both water in thebulk state and water of solute hydration are assumed to consist of two structural species: a high density/highenthalpy species, structurally similar to ice III, and a low density/low enthalpy species, structurally similar toice I. It is assumed that structural and thermodynamic distinctions between bulk and hydration water originatesolely from the differential fractional composition, whereas the two structural species and thermodynamicparameters associated with each species are identical for bulk and hydration water. This model has been usedin conjunction with volumetric data reported in the literature to analyze the hydration properties of charged,polar, and nonpolar groups at 25 °C. The equilibrium between the two structural species of water of hydrationof charged and polar groups is shifted toward the high density/high enthalpy species. In contrast, the equilibriumbetween the two species of water solvating nonpolar groups is shifted toward the low density/low enthalpyspecies. Solvent reorganization was found to be thermodynamically unfavorable for any atomic groupindependent of its chemical nature. However, the enthalpy, entropy, and heat capacity of solvent reorganizationare strongly dependent on the chemical nature of solvent exposed groups. In the aggregate, our results providefoundation for more reliable interpretations of thermodynamic data in terms of hydration. In addition, theseresults underscore the importance and potential usefulness of combining volumetric and calorimetric data fora more complete thermodynamic description of microscopic events, in particular, solute hydration.
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