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I.2. Perfluoroalkanes
General Introduction
Perfluoroalkanes (PFCs) are synthetic fluorinated hydrocarbons. They are
formulated by substituting fluorine to replace hydrogen atoms on common organic
compounds (Sargent and Seffl, 1970 and Drakesmith, 1997). Fluorine’s extreme
electronegativity endows C-F bonds to possess relatively high ionic bond characters,
resulting in the strongest bond with carbon as shown in Table I.1. Moreover, addition of
fluorine atoms influences adjacent bond energies. For example, CF3-CF3 bond is 10
kcal/mole stronger than CH3-CH3 bond (Banks, 1970).
Table I.1. Average Bond Dissociation Energies (kcal/mole) (Banks and Tatlow, 1986)
C-F C-Cl C-Br C-I C-H C-O C-C C-N
116 81 68 51 99 86 83 72
Fluorine’s high ionization potential and relatively low polarizability lead to very
weak inter-molecular forces (Fielding, 1979) which, together with the strong intramolecular
forces, are the main responsible for the interesting properties presented by
perfluoroalkanes when compared with the corresponding hydrocarbons, as for example:
- higher solubility for gases (the highest known among organic liquids)
- exceptional chemical and biological inertness
- excellent spreading characteristics and high fluidity
- lower surface tensions
- lower refractive indices (lower than 1.3)
- higher vapour pressures
- higher densities
- higher viscosities
- higher isothermal compressibilities
- lower internal pressures
- poorer solvency for organics
- non-polar molecules
- nearly ideal liquids
Table I.2 quantifies some of the properties given above.
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