The structure of water and the thermodynamic properties of aqueous solutions.

Abstract

The model proposed in the preceding paper for the theoretical derivation of the thermodynamic parameters of liquid water is extended to a treatment of the structure and the thermodynamic behavior of aqueous solutions of hydrocarbons. It is shown that the net intermolecular interaction energies of the solute with non-hydrogen-bonded water molecules differ from the interaction energies with the water molecules participating in the formation of hydrogen-bonded clusters. The differences are reflected in a change of the coordination number of the hydrogen-bonded water molecules near the solute. As a result, the amount of hydrogen bonding of the water in the immediate neighborhood of the solute is increased over its average value in pure water. The hydrogen-bonded clusters extend around part of the solute molecule, resulting in the formation of an incomplete cage. Equations are derived, expressing the mole fractions of molecular species of water having various numbers of hydrogen bonds within the first layer next to the solute. The partition function derived earlier for liquid water is modified in order to describe the water in this first layer. It is used to compute the contribution of the structural changes of water in this layer to the total free energy of solution. The contribution of the solute to the free energy is estimated in terms of changes in molecular configuration and van der Waals interactions when the solute is transferred from a nonpolar environment into water. The number of water molecules in the first layer surrounding the hydrocarbon, and the magnitude of the various intermolecular interaction energies, are introduced as variable parameters; their values used in the computations are shown to be of physically reasonable magnitude. The standard free energies, enthalpies, and entropies of solution are calculated for the normal saturated aliphatic hydrocarbons containing 1 to 8 carbon atoms, and for benzene and several of its homologs within the temperature range from 0° to 70°C. Observed values available for the same quantities in the literature are summarized and analyzed. The calculated thermodynamic parameters are shown to be in good agreement with the limited number of experimental data. The model also explains the unusually high heat capacity of the solutions discussed; the values calculated are in fair agreement with the available experimental data. The use of the term ``increased ice-likeness'' for the characterization of the changes occurring in the formation of aqueous solutions of nonpolar substances is shown to be justified on the basis of the model. The volume changes of solution are derived semiquantitatively, and an estimate of the volume change is given. Possible extensions of the method to other classes of nonpolar solutes are indicated.