Origin of the Immiscibility of Alkanes and Perfluoroalkanes

Perfluoroalkanes are considered generally to have weak inter- and intramolecular forces compared to alkanes, explaining their relatively low boiling points, low surface tensions, and poor solvent properties. However, currently accepted models do not satisfactorily explain several trends in their properties-for instance, boiling point trends as size increases. Herein, we report a comprehensive computational study of the intermolecular interactions of alkanes and perfluoroalkanes, demonstrating that perfluoroalkanes have a higher intrinsic ability for dispersive interactions than their alkane counterparts and that dispersion in perfluoroalkane dimers mainly stems from fluorine-fluorine interactions. In addition, the reasons for relatively weak intermolecular forces in perfluoroalkanes compared to alkanes are their ground-state geometries, which are increasingly unsuitable for intermolecular interactions as the carbon chain length increases, and their rigidity, which makes deformation from the ground-state geometries unfavorable. Overall, these trends are reflected in a dependence of the bulk properties of perfluoroalkanes on the carbon chain length as the fluorine content decreases and the interaction geometries become increasingly unsuitable.