Infrared reflection absorption spectroscopy study of smectite clay monolayers

Infrared reflection absorption spectroscopy (IRRAS), also called infrared external reflection spectroscopy, is used for studying ultrathin hybrid clay films prepared by the Langmuir-Blodgett (LB) method. Infrared spectra of the hybrid films have been obtained at the air-water and air-solid (gold) interface. The presence of a smectite monolayer is observed when cationic surfactants are spread over the surface of a dilute aqueous smectite clay suspension. The in-plane ν(Si-O) band of saponite is observed in IRRAS spectra of saponite clay monolayers at the air-water interface. In contrast, the out-of-plane ν(Si-O) band at 1063 cm^-1 is the dominant band in the p-polarized IRRAS spectra of a saponite monolayer deposited on a gold surface. These data correspond to the first-reported infrared spectra of clay minerals showing the enhancement of the out-of-plane vibrational modes and the nearly complete suppression of the in-plane vibrational modes. The results provide direct spectroscopic evidence confirming the orientation of the smectite particles parallel to the substrate.