Scientia Sinica

No. 1 HUANG, YANG: ACTIVITY COEFFICIENTS OF NONELECTROLYTES 63

The chief drawback of Debye’s theory is that he only considered the electrostatic force and ignored the role of forces of other types that may play in the salt effect. According to Debye’s theory, any salt should produce a salting-out effect, if the nonelectrolyte lowers the dielectric constant of water. Actually for the same nonelectrolyte, some ‘salts cause its salting-out and some, especially those with large anions, bring about salting-in.

The effect of the nature of salt was considered by some authors. Larsson!)

recognized the additivity of the effects of the positive and negative ions in the salt. Gross! went one step further by assuming that the cation salts-out and the anion salts-in the nonelectrolyte and that the resulting effect 1s due to the balance of the antagonistic action of the two ions. Actually some large cations salt-in the nonelectrolyte, although such data are extremely rare.

At present, there are two types of theories dealing with the role of ions playing in the salt effect besides mere electrostatic interaction. In the first type, it is assumed that ions break the structure of water and cause a change of its volume!) or a change of its entropy"!. In the second type, it is assumed that the existence of van der Waals-London’s dispersion force between ton and molecule has also influence on the course of salt effect. We shall deal only with the second type here.

In 1930, London" worked out a general theory of intermolecular force, of which the dispersion force is an important part. According to London, the dispersion energy U between two different molecules, 1 and 2, is

32 jQy AvohV02 (5)

U=2r6 Avo + hvo2

In Eq. (5), r is the distance between molecules 1 and 2, 4 the Planck constant, a, and vq, are the polarizability and characteristic frequency in the unperturbed

state for molecule 1, & and ) have similar meanings for molecule 2. U ~

has the following properties: (1) it is the energy of attraction, (2) it increases with the increase of polarizability and therefore with the increase of the size ‘of the molecule, and (3) it has additive property").

Kortum" pointed out that the optical properties of salt solutions cannot be explained by electrostatic effect, and hence it is necessary to consider the part played by vander Waals’ force. He suggested the use of dispersion force to explain the salting-in of nonelectrolyte by large ions. From an experimental study of salt effect on the solubilities of m-nitrobenzoic acid in water, Huang, Chu and Han"? pointed out that salting-out decreases and salting-in increases with the increase of the size of the ion. This apparently is the effect of London’s dispersion force. Bockris, Bowler-Reed and Kitchener'*! used London's equation and deduced a salting-in formula. They said, “The effect could be treated by regarding the water in the salt solution as having a ‘pseudo-tem-