Infrared active surface modes found in thin films of perfluoroalkanes reveal the dipole-dipole interaction and surface morphology.

Infrared (IR) spectra of an organic thin film are mostly understood by considering the normal modes of a single molecule, if the dipole-dipole (D-D) interaction is ignorable in the film. When the molecules have a chemical group having a large permanent dipole moment such as the C=O and C-F groups, the D-D interaction induces vibrational couplings across the molecules, which produces an extra band as a surface phonon or polariton band because of the small thickness. Since the dipole moment of an organic compound is much less than that of an inorganic ionic crystal, we have a problem that the extra band looks like a normal-mode band, which are difficult to be discriminated from each other. In fact, this visual similarity sometimes leads us to a wrong direction in chemical discussion because the direction of the transition moment of the extra band is totally different from those of the normal modes. Here, we show useful selection rules for discussing IR spectra of a thin film without performing the permittivity analysis. The apparent change in the spectral shape on decrease in the thickness of the sample can be correlated with the morphological change in the film surface, which can also be discussed with changes in the molecular packing. This analytical technique has effectively been applied for studying the chemical properties of perfluoroalkanes as a chemical demonstration, which readily supports the stratified dipole-array theory for perfluoroalkyl compounds.

Perfluoroalkanes remain on water surface even after volatilization: Affinity analysis of fluorinated solvent with water surface.

Perfluoroalkyl (Rf) compounds are known to have a poor solubility for most solvents except fluorinated solvents, which is known as a fluorous property. In Langmuir (L) film studies of Rf compounds, fluorinated solvents such as perfluoro-n-alkanes are generally used as a good solvent for depositing a sample monolayer on the water surface. On the other hand, a single Rf chain with a short length such as C6F13- is known to exhibit a totally different character from a condensed matter to have a strong affinity to a water molecule on the water surface via the dipole-dipole interaction, which is known as the dipole interactive (DI) property. On considering the DI property, the solvents of perfluoro-n-alkanes would remain on water for a long time, which may disturb the formation of L film on water. In the present study, details of a liquid layer of perfluoro-n-alkanes on water are investigated by using infrared external reflection (IR ER) spectrometry. Although the perfluoro-n-alkanes are highly volatile, the relevant vibration bands did not disappear even after two hours, which means that they remain on the water surface. Fortunately, however, the remained solvent, C6F14, has been found no disturbing factor for preparation of L films.