Conformation of methylene chains in an intercalated surfactant bilayer

Infrared and Raman spectroscopy have been used to establish the conformatio n and orientation of the methylene "tail" of cetyl trimethylammonium ions i ntercalated in an inorganic layered host, CdPS3. Intercalation, effected by a two-step ion-exchange process leading to the formation of Cd0.83PS3(CTA) (0.34), occurs With a lattice dilation of 26.5 Angstrom with the surfactant ions within the galleries adopting a bilayer structure. The frequencies of the conformationally sensitive methylene stretching modes in the infrared and Raman spectra indicate that a majority of the bonds in the methylene ch ain are in a trans conformation. The methylene chains are tilted with respe ct to the inorganic layer. The rift angle as determined from orientation-de pendent infrared spectra is similar to 35 degrees. Although the population of gauche conformers is low, it has been possible to identify specific conf ormational sequences containing a gauche bond, in the interior and termini of the intercalated methylene chains, by their characteristic frequencies i n the infrared spectrum. These high-energy conformers, except for the ones at the chain end, disappear on cooling, leaving on average all 15 methylene units of the intercalated cetyl trimethylammonium ion in trans conformatio nal registry at 40 K. The terminal gauche defects are significant in preven ting interdigitation of the low-density intercalated surfactant bilayer.