Hydration Entropy and Energy

In their study of krypton hydration, Durell and Wallqvist also reported a calculation of the enthalpy of hydration evaluated by the direct method of Eq. [31]. Both constant volume and constant pressure enthalpies were determined by varying the volume of the krypton solution. Their results are displayed in Table 1. The enthalpy of hydration in the constant volume case (—6.3 1.3 kcal/mol) is significantly more exothermic than in the constant pressure case (—3.4 1.3 kcal/mol). The latter number agrees very well with the experimental value of —3.3 kcal/mol, also obtained at constant pressure. The calculated enthalpies of solvation were decomposed into solute-water and water-water (solvent reorganization) terms. The solute-water contribution is comparable and favorable (—5.4 kcal/mol) in both the constant volume and constant pressure calculations. The solvent reorganization term, in contrast, shows a large ensemble dependence. In the constant-pressure case, the solvent reorganization term has a value of 2.0 1.3 kcal/mol. The overall favorable enthalpy of hydration of krypton at constant pressure therefore results from the solute-water attractions rather than from a 

Stabilization of the hydrogen bond network. This is in apparent conflict with solvent ordering predicted by the iceberg hypothesis. 

Tomas-Oliveira and Wodak used a test particle approach and Eq. [32] to investigate the free energy, energy, and enthalpy of solvation of small cavities as a function of size in both water and hexane. The test particle approach allows for determination of thermodynamic quantities as a function of cavity size from a single simulation. Their results illustrate the importance of solvation entropy when determining solvation free energies near room temperature. 

Several studies of the temperature dependence of hydration free energies 

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