Scientia Sinica

64+ SCIENTIA SINICA Vol, V

perature increase’ from its expanded density”. For the. salting-in of benzoic acid by tetraalkyl-ammonium iodides, results calculated from their formula agree rather poorly with the observed ones. Summarizing the works of our predecessors, we propose the following mechanism for the salt effect on the solubility of nonelectrolyte in water.

Let an aqueous salt solution be saturated with a nonelectrolyte. We assume that for the salt, ion of one kind of charge is much larger in size than the ion of opposite charge, and that the molecule of nonelectrolyte is much bigger than that of water. The forces giving rise to the phenomenon of salt effect are mainly two, namely, the electrostatic force and the London dispersion force. The nature of salt effect depends upon which of the two dominates the system. If the electrostatic force predominates, salting-out results; if the dispersion force predominates, salting-in results. The simplified picture how this happens seems to be as follows. Through electrostatic interaction between ion and dipole, water molecules gather around the ions (mainly around the smaller ones). This decreases the amount of “free water” serving as solvent for the nonelectrolyte and produces a salting-out effect. Between the large _ ion and large nonelectrolyte molecule, there exists, however, a strong dispersion force, which draws the latter around the former. This decreases the amount of nonelectrolyte in free water and makes room for more nonelectrolytes to dissolve, i.e., to produce a salting-in effect. The balance of the two antagonistic effects gives the observed salt effect.

We shall describe in more detail how ions of large size bring about saltingin of nonelectrolyte. The large ions can be divided into two kinds according to the symmetry and asymmetry of their structures. If the ion has a symmetrical structure, (for example, R,N*), it probably behaves like a large charged sphere. Since the distance between the centre of sphere and water molecule in closest approach is large, electrostatic interaction between them is small. On the other hand, the ion and the nonelectrolyte molecule, both being large, possess high polarizabilities, and the dispersion force between them is significant. When this force exceeds the electrostatic force between ion and water, salting-in results.

If the large ion has an asymmetrical structure, (for emample,€ » COO’),

then the electrical charge is concentrated in the small polar portion, (e.g.

COO), while the rest portion, (eg. < >) remains nonpolar. Then the

water molecules will be attracted to the small polar portion by electrostatic force, while the nonelectrolyte molecules will be attracted to the large nonpolar portion by dispersion force. If the latter force is large, salting-in effect is produced.