Specific cohesion

In (8.1), is the specific cohesive energy, v = 1/p is the specific volume and the reciprocal of the density p. Vq is the specific volume at zero pressure as shown in Fig. 8.1. The final parameter a is constrained by the relation for the bulk modulus... 

Physical Properties.—Arsenic trichloride is a colourless, transparent oily liquid at ordinary temperatures. The following values for the density,13 specific cohesion, surface tension14 and molecular surface energy were determined by Jager 15... 

Temperature, °0. Specific Cohesion, a2. Vapour Pressure, mm. X1 (Dutoit and Mojoiu). (Ramsay and Shields). (Ramsay and Rose Innes). 

A quantity that is derived from the cohesion energy is known as the specific cohesion or the cohesion density, EJV, where V is the molar volume. The solubility parameter, S, is the square root of the specific cohesion, and it is expressed by the equation... 

The molecular interaction is characterized by the specific cohesion or cohesion density UjV, where V is the molecular volume (Table 33). Another closely related quantity is the so-called internal pressure of a liquid. 

The specific cohesion, or the internal pressure, plays a part especially in mixtures. For the case of an exclusive London interaction we have (see p. 347) ... 

When a gas molecule dissolves in a liquid a hole must first be made in which it can be placed. The energy necessary for this will be proportional to the specific cohesion UjV (or to the internal pressure). If the molecules of the solvent are not polar, this is proportional to a2/ 3. It will, therefore, be expected that for gases of small polarizability the solubility will decrease with increasing polarizability of the solvent, for gases of large polarizability it is exactly the opposite. 

The solubility will be but small in solvents in which the cohesion is based for a great part on dipole-dipole interaction (in particular through hydrogen bonds). The large specific cohesion energy of the solvent is only compensated to a small extent by the Debye interaction with the solute molecule. 

Bearing in mind that for a liquid which obeys the Van der Waals equation of state the specific cohesion U/V is ajV2, this formula can be written more generally as ... 

It is again the specific cohesion energy UjV which determines the behaviour of the substances. The square root of the specific cohesion energy is also called the solubility parameter (Table 33). 

Van Arkel s formula has the important advantage over that of Scatchard in that it can be seen from it that the heat of mixing does not in fact become zero for liquids with the same specific cohesion if this cohesion has a different origin. 

Incomplete miscibility at room temperature or above is rare with non-polar liquids. Only Snl4, molten phosphorus and molten sulphur, all with a very high specific cohesion, are exceptions to this statement. With increasing cohesion of the second substance, for example of sulphur with benzene, naphthalene and anthracene, respectively, the critical solubility temperature falls below the melting line but the partial miscibility is still present in the metastable state (Kruyt, Kete-laar and Jibben). ... 

Although aniline does possess a dipole moment the cohesion energy is predominantly of the London type. The critical solubility point—technically actually the i i demixing point—with hydrocarbon mixtures (aniline point) is an important characteristic quantity. The aniline point rises with the molecular weight in a homologous series. For aromatic hydrocarbons the critical solubility point is the lowest, for paraffins the highest, olefins and cyclic hydrocarbons are in between, in agreement with the variation of the specific cohesion (Table 33). 

With V = 100 cm3,VUAjVA — VUBjVB must, therefore, be at least 3.4 so that Tc 300° K which indicates a very appreciable difference in specific cohesion energy, so that partial miscibility is rare in non-polar substances. In most systems of two liquids one of the components is associated. 

The above formula for the solubility, in which also the corrections for the temperature dependence of the latent heat of fusion AH were taken into account, has been very extensively tested by Hildebrand on the solubility of iodine in a very large number of solvents (violet solution, p. 342). Since the specific cohesion (p. 359) of iodine is very high (y/UjV — 14.1), the solubility line only approaches the ideal line for solvents with similarly large specific cohesion, such as Snl4 (11.7) and Sg (12.7), molten sulphur (Table 33). 

Also for the solubility (degree of swelling) of polymers in solvents it holds in general that this is only appreciable if the specific cohesion of polymer and solvent do not differ too much13. Thus rubber (VU/V 7.9) is soluble in most solvents... 

Instead of the surface tension, a, the so-called specific cohesion is sometimes used. This was defined by Poisson as ... 

Walden 0 found that the formula for the specific cohesion ( LVIIIG) ... 

Several attempts have been made to connect the surface tension o, or the specific cohesion ( l.Vni G) a =2alQ=2av, where =density and z =sp. vol., with the latent heat of evaporation, 4 g.cal. per g. Waterston gave an equation equivalent to ... 

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