Hydration of Nonpolar Molecules

The established correspondence of the ratios A%H°/NS, AgCp/Vs, and AgS°/N, for such different molecules as those of noble gases and multi-atomic hydrocarbons shows that the observed heat effect of the transfer of a nonpolar molecule to water is caused mainly by the changes in the water contacting the nonpolar molecule, i.e., by hydration of these molecules. 

It appears that there are two temperatures of a universal nature that describe the thermodynamic properties for the dissolution of liquid hydrocarbons into water. The first of these, 7h is the temperature at which the heat of solution is zero and has a value of approximately 20°C for a variety of liquids. The second universal temperature is Ts, where the standard-state entropy change is zero and, as noted, Ts is about 140°C. The standard-state free energy change can be expressed in terms of these two temperatures, requiring knowledge only of the heat capacity change for an individual substance 

The temperature dependence of the solubility may also be expressed in terms of the parameters given generally by Eq. (17) or for the simple case of constant heat capacity change from Eq. (18). In the latter situation, we rearrange Eq. (18) to give A G°/T, which is -Rln(solubility), to obtain the result 

The first-order expansion of this equation gives the result 

As this equation shows, there is a maximum value for the function A G0/T, or a minimum in the solubility at the temperature TH where the heat of solution is zero. 

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