Molecular shape and intermolecular liaison: Hydrocarbons and fluorocarbons

Whereas aliphatic hydrocarbon/fluorocarbon mixtures show mutual 'phobicity' (e.g., positive deviations from Raoult's Law), their aromatic counterparts show the opposite behavior. Hexafluorobenzene forms co-crystals with many aromatic hydrocarbons, but co-crystals are unknown in the aliphatic series. This remarkable contrast between the behaviors of aliphatic and aromatic hydrocarbon/fluorocarbon mixtures calls for explanation, and we attempt to provide one in terms of the difference in overall molecular shape - disk vs. cylinder - using the new PIXEL method for calculating intermolecular energies.