Water hydration, free energy change

Values of AG and AGh can be estimated from calculations based on a model set forth elsewhere (92, 93). The free energy of hydration, AGh, which describes the interaction of the cation with the monodentate ligand water, is taken to be the sum of the free energy changes associated with the following steps ... 

The thermodynamic properties of the hydradon shell (Table VIII) show it to be slightly, but not strongly, different from the bulk water. The free-energy difference is only 0.5 kcal/mol of water, slighdy less than the ambient thermal energy. The heat capacity, enthalpy, and volume changes associated with hydration are 10—15% of the bulk water values. 

In other cases, an unfavorable enthalpy term was found to be compensated by a favorable entropy term, thus rendering negative the free energy change associated with a particular phase transformation. Lamivudine can be obtained in two forms, one of which is a 0.2-hydrate obtained from water or from methanol that contains water and the other of which is non-solvated... 

Because the concentration (activity) of water is essentially constant in dilute aqueous solutions (a mole fraction of unity), the hydration of the proton can be ignored in defining acid-base equilibria. Because the equilibrium activity of the proton and of H30 are not known separately, the thermodynamic convention sets the standard free energy change AG for reaction 10 equal to zero that is, Kt, = 1. In dealing with dilute solutions we can, because of this convention, represent the aquo hydrogen ion by H (aq), or more conveniently by H" that is. 

The water of hydration irih and of solution are analogous and almost equal to the primary rrii and secondary moisture contents, respectively (Figure 30). Equations for the free energy changes associated with the water of hydration and of solution, in analogy with Equation 32 are given by... 

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