Molecular aspects of solutions

In this section we shall outline certain points of view, actually quite extraneous to pure thermodynamics, concerning the molecular interpretation of solutions. The reader will observe that the arguments are far from precise but they will be referred to again in Chapter 14.

(a) Causes of solubility. It is often said that solubility is due to the molecular forces. That this is incorrect may be seen from the fact that two gases mix in all proportions and have infinite mutual solubility; the mixing is due, not to any interaction, but to the motion of the molecules and to the fact that a mixed state is immensely more probable than an unmixed one. The mutual solubility of gases is therefore an aspect of the statistical origin of the second law.

However, as soon as the molecules are brought into close proximity, as in condensed phases, the molecular forces have a decisive influence and may either increase or decrease the tendency towards mixing. Let it be supposed that two liquids A and B are contained in an isolated vessel so that the condition of equilibrium is a maximum of the entropy or of the number Ω of complexions. The intermingling of the molecules, if it were the only effect to be considered, would certainly cause an increase in the entropy because there are a vastly larger number of complexions for the mixed state than for the unmixed one.