Apolar Solutes in Water

Kinetics of Reactions Involving Apolar Solutes in Water. . 256... 

The most conceptually attractive model for these solutions is to consider that the organization of water resembles that in the clathrate hydrates (p. 225), the structure being based on pentagonal dodecahedra of hydrogen bonded water molecules (Glew and Moelwyn-Hughes, 1953). This model receives some support from the observation that there is an optimum molecular radius of 4-5 x 10 8 cm for solubility of apolar solutes in water (Franks and Reid, 1973). 

For apolar solutes in water, we have seen that AG > 0, AH < 0, TAS < 0 and 7ASe > A//e (p. 248). At the other end of the scale when x2 = 1, we are concerned with the thermodynamic properties of an apolar solute in a non-aqueous solvent. Because the solubility is usually larger for such solutes in these solvents, AGf (gas -> solvent) will be less. Thus the effect of going from a solution in water to a solution in a pure co-solvent should be as predicted in (32). Also ASf (gas - co-solvent) will not be as negative as in the... 

The further conclusion which has to be drawn is that dissolution of ions in water is stmcturally totally distinct from that for dissolution of apolar solutes in water. The way in which the stmcture of pure water is modified on forming aqueous solutions of these two types of solutes is crucially different, and raises the question as to whether the changes of entropy on dissolving an apolar or non-polar solute in water can be used as a base-line for interpreting changes in entropy for dissolution of ions in water (see Section 13.16.3). 

Smithrud, D.B. Diederich. F. Strength of molecular complexation of apolar solutes in water and in organic solvents is predictable by linear free energy relationships A general model for solvation effects on apolar binding. J. Am. Chem. Soc. 1990. 772. 339-343. 

B16. Solute-Solute Potential of Mean Force for Apolar Solutes in Water. 

In the literature, sometimes the high ced of water has been taken as the ultimate cause of the hydrophobicity of apolar solutes. However, it has been argued that this is not correct and that probably the difference between the ced and the internal pressure is a better rational for the low solubility of apolar solutes in water. 

In the previous section we have seen that the formation of hydrophobic hydration shells aids the dissolution of apolar solutes in water. Upon increasing concentration and/or size of the solute it is inevitable that, at a critical concentration, the large hydrophobic hydration shells start to overlap, leading to mutually destructive breakdown of these water arrangements (Fig. 2.6). This sacrifice of H-bonding interactions results in a solvent-induced sticking... 

These trends are now more or less established, also in other types of measurements. For instance, the solubility of small apolar solutes in water has a minimum as a function of T, and enthalpy-entropy compensation has also been found by others [19-21]. 

Smithrud, D. B. and Diederich, F. (1990) Strength of Molecular Complexation of Apolar Solutes in Water and in Organic Solvents Is Predictable by Linear Free Energy Relationships A General Model for Solvation Effects on Apolar Binding , J. Am. Chem. Soc. 112, 339-343. 

As shown by Frank and Evans 41 , solutions of apolar substances in water are characterized by a large entropy of mixing, leading to a high positive free energy of dissolving. 

The low solubility of hydrocarbons and other mainly apolar substances in water has been ascribed phenomenologically to the hydrophobic interaction. The hydro-phobic free energy can be defined4 as the difference between the standard chemical potentials of an apolar solute at infinite dilution in a hydrocarbon solvent juhc and in water... 

For a more generalized approach to this solubility problem, the question of reproducibility of experimentally known properties of water seems to be in order. Previous computational studies have suggested that the insertion of compounds in aqueous solution is vastly dependent upon the ability of the water model to reproduce structural properties at the desired conditions. Even so, this comparative type of analysis would only be effective in ideal solutes or apolar solutes in the solution. If the solute was to deviate from this spherical shape, the method would need to be modified accordingly. One may even suggest that the CO2 may be treated as more of a single LJ sphere with a series of charges that would reproduce the quadrapole moment, and therefore, coordinate the water in an appropriate solvation, although further study of the aqueous C02 system would be needed to confirm this. 

Thus, for this small ion, activation is not accompanied by significant reorganization of water in the solvent co-sphere as in the case of apolar solutes (p. 256). The volume of activation for the hydrolysis of t-BuEtMeS+ is positive, 6 cm3 mol-1 at 313 K (Brower and Wu, 1970) from which it has been concluded that the reaction involes breaking of a C—S bond. However, the complexity of volume properties of solutes in water indicates that AV values should be interpreted with caution. It would not be surprising if AV were controlled to a considerable extent by changes in the solvent co-sphere. 

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