Influence Of Molecular Attractions On Miscibility Of Liquids And On Heat And Volume Changes During Admixture

In studying the behaviour of two liquids, A and b, when mixed together, one should consider

1. The attraction of the like molecules - of those of a for each other and of those of B for each other.

2. The mutual attraction of the molecules If the attraction of the unlike molecules is relatively so slight as to be negligible, one may expect the liquids to be non-miscible or very nearly so. With a somewhat greater relative attraction between the unlike molecules there would be miscibility within small, and it is reasonable to assume that in the process of mixing there might be slight absorption of heat and_slight expansion.

Generally, in the comparison of various pairs of liquids, as the mutual attraction of the unlike increases relatively to that of the like molecules, one would expect increasing and finally infinite misci-bility; absorption of heat at first, diminishing to zero and changing to increasing heat evolution; and diminishing expansion followed by increasing contraction. These various changes do not, in many cases, run strictly pari passu, and, among liquids which are miscible in all proportions, it is not unusual to find a small amount of contraction attended by slight heat absorption, as, for example, when a little water is added to normal propyl alcohol; but in the case of certain closely related chemical compounds, such as chlorobenzene and bromobenzene, there is neither any appreciable change of volume nor any measurable evolution or absorption of heat when the liquids are mixed together. For such substances it is probable that the different molecular attractions, a for A, B for B, and A for b, are very nearly equal, and that the relation suggested by D. Berthelot1 and by Galitzine,2 namely, that holds good. [a1.2 represents the attraction of the unlike molecules and a1 and a2 the respective attractions of the like molecules.]

There would appear, then, to be two simple cases: -1. That in which the attraction represented by a1.2 is relatively so slight that the liquids are practically non-miscible ;

1 D. Berthelot, " On Mixtures of Gases," Compt. rend., 1898, 126, 1703. 2 Galitzine, " On Dalton's Law," Wied. Ann., 1890, 41, 770.

2. That of two closely-related and infinitely miscible liquids which show no heat or volume change when mixed together.

The vapour pressures of some mixed liquids and of non-miscible pairs of liquids have been determined by Regnault, Magnus, Gernez, Konowaloff, and other observers.

Non-miscible Liquids

It was shown by Regnault in 1853 that when two non-miscible liquids are placed together over the mercury in a barometer tube, the observed vapour pressure is equal to the sum of those of the two liquids when heated separately to the same temperature. Each liquid, in fact, behaves quite independently of the other, and so long as both are present in fair quantity and one is not covered by too deep a layer of the other, it does not matter what are their relative amounts or what are the relative volumes of liquid and vapour. If, however, the upper layer is deep, the maximum pressure may not be reached for a considerable time unless the heavier liquid by shaking or stirring is brought to the surface to facilitate its evaporation.

Partially Miscible Liquids

In the case of two partially miscible liquids the vapour pressure was found to be less than the sum of those of the components, but greater than that of either one singly at the same temperature.

Infinitely Miscible Liquids

The vapour pressures of many pairs of infinitely miscible liquids have been determined by several experimenters, and, as with the changes of volume and of temperature on mixing the liquids, so with the vapour pressures of the mixtures, very different results are obtained in different cases. There can be no doubt that the behaviour of mixtures, as regards vapour pressure, depends on the relative attraction of the like and the unlike molecules. When the mutual attraction of the unlike molecules is not much more than sufficient to cause infinite miscibility - for example, with normal propyl alcohol and water - the vapour pressure, like that of a partially miscible pair of liquids, may be greater than that of either component at the same temperature. On the other hand, when that attraction is relatively very great (formic acid and water) the vapour pressure of the mixture may be less than that of either component. It seems reasonable to suppose that, when the attractions of the like and unlike molecules are equal or nearly so, the relation between vapour pressure and composition should be a simple one, and the question what is the normal behaviour of mixtures has been discussed by several investigators.

Normal Behaviour of Mixtures. Guthrie. - Guthrie1 in 1884 concluded that if we could find two liquids showing no contraction, expansion, or heat change on mixing, the vapour pressures should be expressed by a formula which may be written where P, PA and PB are the vapour pressures of the mixture, and of the two components A and b, respectively, at the same temperature, and m is the percentage by weight of the liquid a. In other words, the relation between the vapour pressure and the percentage composition by weight should be represented by a straight line.

1 Guthrie, " On some Thermal and Volume Changes attending Admixture," Phil. Mag., 1884, [V.], 18, 495.