STRUCTURAL CHARACTERIZATION OF SURFACTANT AND CLAY SURFACTANT FILMS OF MICROMETER THICKNESS BY FT-IR SPECTROSCOPY

Reflection absorption IR (RAIR) spectroscopy was used for structural s tudies of films cast from tetraalkylammonium bromides and their compos ites with colloidal clay. The dependence of p-polarized spectra on the angle of incidence of source radiation was used to find reference tra nsition dipoles. Orientations of the symmetric SiO4 stretch of clay in composite films and the symmetric C-N stretch in hexadecyltrimethylam monium bromide films were found to be nearly normal to their film plan es. These bands were used as references to estimate orientations of ot her transition dipoles. Hydrocarbon chains were found to tilt 20-40-de grees to the normal. Frequencies for CH stretching and bending were se nsitive to surfactant hydrocarbon chain conformations, which control t he thermotropic phase of these films. Band positions for surfactant fi lms in solidlike and liquid crystal phases were similar to reported va lues for these respective phases for a lamellar water-dialkyldimethyla mmonium system. Phase-sensitive band frequencies for surfactant in com posite films gave poor correspondence with the latter system. Water in teracts mainly with head groups in the pure surfactant films, but did not influence RAIR bands in composite films. Results are consistent wi th a previously proposed multibilayer structure of the films, but clea rly reveal the tilt of the surfactant hydrocarbon tails.