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III.4.4. VLE and LLE of Alkane and Perfluoroalkane Mixtures
Modeling
A significant amount of experimental data is available for perfluoroalkane + alkane
systems but, even so, the thermodynamic behaviour of these systems is not completely
understood. Althought both alkanes and perfluoroalkanes are nonpolar species and thus it
would be expected that their mixtures behave almost ideally, in fact they display large
positive deviations from ideality (Hildebrand and Scott, 1950).
The first attempts to account for this anomalous behaviour appeared in the early 50s
just after the publication of the experimental data for symmetric perfluoralkane + alkane
mixtures (Hildebrand et al., 1950; Simons and Dunlap, 1950 and Simons and Mausteller,
1952) showing substantial regions of liquid-liquid immiscibility. To that date it was
believed that such mixtures would be completely miscible in all proportions and that the
regular solution theory would also describe these mixtures, at least in a qualitatively way.
In 1958 Scott (Scott, 1958) reviewed the various explanations proposed and
concluded that none of the suggestions accounted fully for the anomalous behaviour of all
systems investigated. Among the various explanations were the interpenetration between
neighboring C-H groups giving an abnormally strong hydrocarbon-hydrocarbon interaction
energy leading to the failure of the geometric combining rule proposed by Simons and
Dunlap (1950), an empirical shift in the solubility parameter, δ, for hydrocarbons in order
to fit the data proposed by Hildebrand (1950), corrections to the regular solution theory to
include the effect of volume changes, that occur on mixing proposed by Simons and
Dunlap (1950) and modification of regular solution theory to take into account size and
ionization potential differences between the two components proposed by Reed (1955).
The author concluded that none of these explanations could account for the behaviour of
all of the anomalous systems.
In the early 70s Dantzler and Knobler (1971) measured the second virial
coefficients of n-alkanes, perfluoro-n-alkanes, and their mixtures and concluded that the
anomalous behavior of alkane + perfluoroalkane systems was due to the failure of the
geometric mean rule to describe the unusually weak hydrocarbon-fluorocarbon attractive
interaction. They discussed possible reasons for such a weak unlike interaction between the
two components: noncentral forces, large ionization potential differences, and large size
differences, concluding that the observed deviation from the geometric-mean prediction
was most likely due to the latter. This explanation had previously been suggested by Dyke
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