Solvation of water

The values of 7r are derived from the transfer of a side chain from water to a hydrophobic solvent and so measure the relative energies of solvation of water relative to the organic solvent. R. Wolfenden and coworkers have measured the incremental free energies of transfer of amino acid side chains from water to near vacuum (low gas pressures) by measuring the vapor pressure of a series of substituted compounds (Table 11.5). These numbers are useful in understanding, foj example, how mutations in proteins affect the energies of the denatured state (Chapter 17). 

Considerable progress has been made in going beyond the simple Debye continuum model. Non-Debye relaxation solvents have been considered. Solvents with nonuniform dielectric properties, and translational diffusion have been analyzed. This is discussed in Section II. Furthermore, models which mimic microscopic solute/solvent structure (such as the linearized mean spherical approximation), but still allow for analytical evaluation have been extensively explored [38, 41-43], Finally, detailed molecular dynamics calculations have been made on the solvation of water [57, 58, 71]. 

For example, the lithium halides have solubilities roughly in the reverse order LiBr > LiCl > Lil > LiF. The solubilities show a strong hard-hard interaction in LiF that overcomes the solvation of water, but the weaker hard-soft interactions of the other halides are not strong enough to prevent solvation and these halides are more soluble than LiF. Lil is out of order, probably because of the poor solvation of the very large iodide ion, but it is still about 100 times as soluble as LiF on a molecular basis. 

The efficient solvation of water [96,110,112, 113] outlined above can be exploited in condensation reactions, in which water formed during the course of the reaction is deactivated by absorption , which can improve conversion and/or selectivity. For example, in the catalytic oxidation of an alcohol to the aldehyde [140, 141], water is formed as by-product which can undergo further reaction with the aldehyde to yield unwanted carboxylic acid. In ionic liquids, neither a reduced rate of reaction nor the formation of carboxylic acid is observed, even in the presence of up to 1 equiv. of water. However, at higher concentrations of water (absorption from air or accumulated by-product) the selectivity of aldehyde decreases, and the carboxylic acid is formed instead. Upon vigorous drying of the catalyst-ionic liquid mixture, the selectivity of the system is restored [141],... 

Specifically for liquid water, the solvation of water in pure water paved the way to answer questions such as What is the structure of water and How much is this structure changed when a solute is added The details and the scope of application of the new measure were described in the monograph by Ben-Naim (1987). 

Rick SW, Berne BJ. The aqueous solvation of water a comparison of continuum methods with molecular dynamics. J Am Chem Soc 1994 116 3949-3954. 

We shall discuss at length solvation quantities in Chapter 3. Here, we present some values of the thermodynamics of solvation of water in pure water. It should be noted that in the traditional approach to solvation, only solvation of one component in very dilute solution in a solvent can be defined and measured. In the definition used here, the concept of solvation can be applied to any molecule in any liquid at any concentration. We define the solvation process as the transfer of a molecule from a fixed position in an ideal gas phase to a fixed position... 

Again, we start with the Gibbs energy of solvation of water in liquid water... 

Propylene carbonate. This system has a miscibility gap at 298 K, so only the organic solvent-rich region was dealt with (Marcus 1990). The preferential solvation of water around propylene carbonate, 8xws 0 for the homogeneous region Xg > 0.68, but the water-water preference, Sx w has a maximum of 0.36 near Xg = 0.76 and this preference extends even to the third solvation shell, where 5xww(m >x) = 0.040 still. This preference is expected, in view of the phase separation that takes place. 

FIGURE 6.15. Preferential solvation of water-methanol and water-ethanol as a function of the mole fraction of water (component A). 

Solvation of water in pure water (at 25°C). For pure water the molar volume is = 18 cm moF hence. 

S. W. Rick and B. J. Berne,/. Am. Chem. Soc., 116, 3949 (1994). The Aqueous Solvation of Water A Comparison of Continuum Methods with Molecular Dynamics. 

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