PROBING THE INTERFACE OF CAST SURFACTANT FILMS AND AN UNDERLYING METAL BY SURFACE-ENHANCED RAMAN-SCATTERING SPECTROSCOPY

Surface-enhanced Raman scattering (SERS) was used to evaluate orientat ions of molecules at interfaces between cast surfactant films and silv er island underlayers. Films examined included those of dialkyldimethy lammonium bromides, used previously as hosts for redox catalysts, and of cetyltrimethylammonium bromide and cetylpyridinium chloride, for wh ich SERS of adsorbates from aqueous solutions has been reported. Compa risons with normal Raman spectra of aqueous dispersions of the surfact ants showed that bands caused by head group vibrations were more stron gly enhanced by SERS than were bands from the hydrocarbon tails. Resul ts suggest that surfactant head groups are oriented down on the metal surface and hydrocarbon tails extend away. Changes in SERS intensities when films were treated with water suggested an increase in the avera ge distance of surfactant molecules from the metal surface consistent with interactions of water with head groups and perhaps the Ag surface . Results from SERS, reflection absorption IR spectroscopy, and phase transition studies taken together suggest that interactions of water w ith head groups in DDAB and DODAB films are integral to the molecular structure at the underlying solid, as well as to their fluid liquid cr ystal states.