Solvation, decreases free energies

Modem understanding of the hydrophobic effect attributes it primarily to a decrease in the number of hydrogen bonds that can be achieved by the water molecules when they are near a nonpolar surface. This view is confirmed by computer simulations of nonpolar solutes in water [15]. To a first approximation, the magnimde of the free energy associated with the nonpolar contribution can thus be considered to be proportional to the number of solvent molecules in the first solvation shell. This idea leads to a convenient and attractive approximation that is used extensively in biophysical applications [9,16-18]. It consists in assuming that the nonpolar free energy contribution is directly related to the SASA [9],... 

This equation has the expected behavior that AG< becomes more positive with decreasing solubility of the solute. However, free energies of solvation for different solutes cannot be related to their relative solubilities unless the vapor pressures of the different solutes are similar or one takes account of this via Equation 76. Furthermore, if the solubility is high enough that Henry s law does not hold, then one must consider finite-concentration activity coefficients, not just the infinite-dilution limit. 

Entropy effects. The replacement of the coordination shell of the cation by a multidentate ligand has also the very important effect of decreasing the free energy of the system by the increase in translational entropy of the displaced water molecules. If there were no variation in solvation and internal entropies of the free ligand and of the complexes, the increase in translational entropy would amount to about 8 e.u., where x is the number of displaced solvent molecules minus one (38). This estimate is, however, very inaccurate large deviations are expected, especially in the case of complicated multidentate ligands for which complex formation may produce appreciable internal and solvation entropy changes. 

Fig. 6 compares the nuclearity effect on the redox potentials [19,31,63] of hydrated Ag+ clusters E°(Ag /Ag )aq together with the effect on ionization potentials IPg (Ag ) of bare silver clusters in the gas phase [67,68]. The asymptotic value of the redox potential is reached at the nuclearity around n = 500 (diameter == 2 nm), which thus represents, for the system, the transition between the mesoscopic and the macroscopic phase of the bulk metal. The density of values available so far is not sufficient to prove the existence of odd-even oscillations as for IPg. However, it is obvious from this figure that the variation of E° and IPg do exhibit opposite trends vs. n, for the solution (Table 5) and the gas phase, respectively. The difference between ionization potentials of bare and solvated clusters decreases with increasing n as which corresponds fairly well to the solvation free energy of the cation deduced from the Born solvation model [45] (for the single atom, the difference of 5 eV represents the solvation energy of the silver cation) [31]. 

The stronger the solvating ability of a solvent is, the more it decreases the thermodynamic activity of the reactants and their reactivity, that is, their availability for the reaction. A linear correlation has been found between the activation Gibbs free energy of a series of Sn2 reactions and the acceptor number (AN) of the solvents (Marcus, 1998). 

A very common and important effect of the solvent on enzymatic reactions is that of affecting the solvation of the substrates and products of the reaction catalyzed. The solvation of the substrate influences its free energy and thereby its reactivity. Solvents which are able to dissolve a substrate very efficiently lower the free energy, and the rate of the catalyzed reaction is thereby decreased. The solvent also influences the equilibrium position of reactions, and here the solvation of both substrates and products must be considered. 

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