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.