The Entropy of Mixing of Binary Liquid Mixtures

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Abstract
Three factors which may contribute to the excess entropy of mixing of binary solutions have been explored. These are the relative volumes of the molecules, the spatial distribution of the molecules about a given reference molecule, and the lack of randomness of the orientational distribution of the molecules about a reference molecule. Of these, the spatial distribution, which depends upon the energy of mixing, is the smallest. It yields a negative value for the excess entropy and becomes zero at infinite dilution. The effect of differences in the volumes of the molecules of the components is positive and may be appreciable. The greatest effect is obtained from the orientational distribution. An estimation of the lack of randomness in this distribution for the pure components is obtained from the differences of the entropies of evaporation of the components and that of argon compared under corresponding conditions. This difference is positive and is comparable, at least in order of magnitude, to the observed partial molal excess entropy of mixing at constant pressure and at infinite dilution for the six binary mixtures composed of benzene, carbon tetrachloride, cyclohexane, and methanol. The contribution of the change in the orientational distribution of the components to the partial molal excess entropy at infinite dilution is then considered to be caused by two effects, one in which the orientational distribution of the solute becomes completely random when dissolved in a solvent at infinite dilution and the other in which the orientational distribution of the solvent is changed by the substitution of a solute molecule for a solvent molecule. The individual behavior of each solvent is very evident and is consistent with its molecular structure.

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