Mixtures containing an Aromatic Fluorocarbon

Liquid Mixtures.—Aromatic fluorocarbons first became available commercially just over 10 years ago and, in a paper published in 1960, Patrick and Prosser noted that hexafluorobenzene formed an equimolar molecular compound with benzene that melted some 18 K higher than the melting temperatures of the two pure components. In the same year Brooke et observed that a similar solid compound was formed between hexafluorobenzene and aniline. The formation of solid compounds of this type implied the existence of unusually strong fluorocarbon-hydrocarbon interactions in these mixtures, in complete contrast to similar mixtures containing an aliphatic or an alicyclic fluorocarbon. The large volume of research effort that has been undertaken in this area since 

1960 has been stimulated primarily by a desire to understand the reasons for this difference in behaviour between these two classes of fluorocarbon-containing mixture. 

The first systematic studies of fluid mixtures containing an aromatic fluorocarbon were those undertaken by Scott at U.C.L.A. and by Swinton at Strathclyde. - Scott and his co-workers measured for binary mixtures of partially fluorinated aromatics with the general formula CeH F6-n + (0 m, n 6, n m) whereas the Strathclyde group studied mixtures of CeFg with methylated aromatic and alicyclic hydrocarbons and measured G, F , and Cf. A representative selection of experimental data 

A comparison of the data in Table 1 with those in Table 4 shows that the values of the three principal excess functions G , and F for aromatic fluorocarbon + aromatic hydrocarbon mixtures are everywhere much less positive than those for mixtures containing a non-aromatic fluorocarbon. The experimental excess functions for aromatic fluorocarbons + alicyclic hydrocarbon are seen to occupy an intermediate position. Another notable trend that is obvious from the data in Table 4 is that there is a negative contribution to the excess functions associated with an increase in the degree of substitution of the hydrocarbon. This trend applies to mixtures of aromatic fluorocarbons with both aromatic and alicyclic hydrocarbons. 

As more experimental evidence accumulated the need to invoke the existence of charge-transfer forces r ded. Powell and Swinton compared the excess volumes of CeFe + methylated benzenes with the values in the series C F + methylated cyclohexanes. It was noted that, in both series, F decreased with increase in methyl substitution to virtually the same extent. A similar trend was noted for the excess enthalpies. The polarizabilities of both aromatic and 

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The properties of fluid mixtures containing an aromatic fluorocarbon differ completely in many respects from those of mixtures containing an aliphatic or alicyclic fluorocarbon. It is therefore convenient to treat these two major classes of mixture separately. The desire to interpret these extreme differences in thermodynamic behaviour, at a molecular level, is one of the main reasons for the continuing interest of many physical scientists in such systems. 

Table 4 Thermodynamic properties of equimolar mixtures containing an aromatic fluorocarbon...

The 1971 paper of Simon and Knobler described the only direct measurement of an excess function for a mixture of this class to be made since 1970. The exhaustive general reviews of Stokes and Marsh and of Kehiaian contain no mention of such measurements. The virtual cessation of experimental work in this area is due in part to the industry of Scott and his co-workers during the fifties and sixties and in part to the fact that most binary mixtures of this type exhibit great similarity in their thermodynamic behaviour. Equimolar mixtures of alicyclic or aliphatic fluorocarbons with branched or unbranched aliphatic hydrocarbons, with alicyclic or with aromatic hydrocarbons, or with other simple... 

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