ORIENTATION OF LIQUID PERFLUOROHYDROCARBONS AT THE INTERFACE WITH AIR AND WITH SOME LIQUIDS

The relationships of the surface free energy and density of the chain length were used to calculate the contribution of CF3 and CF2 groups of perfluorohydrocarbons to the surface free energy and the values of gamma-CF3 = 9.52 and gamma-CF2 = 22.56 mJ m-2 were obtained at 20-degrees-C. Simultaneously, the orientation of molecules at the liquid-air interface was evaluated as the relative occupation of the surface by CF3 groups. This value decreases from 0.955 for perfluoropentane to 0.589 lfor perfluorononane which is much steeper than for the random arrangement. Similar calculations were carried out for temperatures up to 60-degrees-C for perfluorohydrocarbons with chain lengths up to 11 carbon atoms. The degree of orientation increases with increasing temperature. The cross-sectional area of molecules perpendicular to the surface substituted for (M/p)2/3 fits the Eotvos equation. The orientation changes at the interface with water. The force equivalent to the surface free energy depends on the orientation and on the density of the molecules. The respective values were determined: for perfluorohexane, e.g., gamma-F = 11.92 at 20-degrees-C while at the interface with water gamma-Fi = 9.88 and for cohesion in the bulk gamma-Fb = 17.92 mJ m-2. The differences between these values decrease with increasing chain length. Aliphatic perfluorohydrocarbons show an acid-base interaction with water which is caused by their acid element, gamma-F(a). Hexafluorobenzene interacts through the induced base element, gamma-F(ib), and perfluorodecalin contains an acid-base component. The respective values were determined using a new hypothesis of acid-base interaction at interfaces.