Perfluorocarbons polarizability

Highly fluorinated molecules have a nonpolar character and an extremely low polarizability, inducing only weak intra- and intermolecular interactions. As a consequence, perfluorocarbons behave almost like ideal liquids they are very compressible and have very high vapor pressure. For example, the physical properties of perfluoro-hexane, heptafluorohexane, and hexane are reported in Table 1.2 The effect of the polar character of the hemifluorinated compound (heptafluorohexane) on the dielectric constant value is remarkable. 

The lipophilicity of perfluorocarbons is remarkably enhanced by introducing a more polarizable atom into the compound such as bromine or hydrogen (C8F18 CST +37 CC, C8F17Br CST —24.5 C, C8F17H CST +21.3 C). Thus, Rt —RH diblock compounds such as C6F13C3H7 or C8Fl7C8Hl7 are mostly totally miscible with hydrocarbons at room temperature.21... 

Another consequence of the low polarizability of perfluorocarbons is the occurrence of large miscibility gaps in solvent systems composed of perfluorocarbons and hydrocarbons. The occurrence of a third, fluorous , liquid phase in addition to the organic and aqueous phases has been extensively exploited in the convenient and supposedly ecologically benign fluorous chemistry, which will be discussed in detail in Chapter 3. 

Solid perfluorocarbon surfaces also have extremely low surface energies Thus, poly(tetrafluoroethylene) (PTFE, Teflon) has a y value of 18.5 dyn cm which is the reason for the anti-stick and low-friction properties used for frying pans and other applications. That this effect is directly related to the fluorine content becomes obvious on comparison of the surface energies of poly(difluoro-ethylene) (25 dyn cm ), poly(fluoroethylene) (28 dyn cm ), and polyethylene (31 dyn cm Y If only one fluorine atom in PTFE is replaced by more polarizable chlorine, the surface energy of the resulting poly(chlorotrifluoroethylene) jumps to 31 dyn cm , the same value as for polyethylene [8]. 

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