Conformation of methylene chains in an intercalated surfactant bilayer

Abstract

Infrared and Raman spectroscopy have been used to establish the conformation and orientation of the methylene "tail" of cetyl trimethylammonium ions intercalated in an inorganic layered host, CdPS₃. Intercalation, effected by a two-step ion-exchange process leading to the formation of Cd_0.83PS₃(CTA)_0.34, occurs with a lattice dilation of 26.5 Å 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 chain are in a trans conformation. The methylene chains are tilted with respect to the inorganic layer. The tilt angle as determined from orientation-dependent infrared spectra is ~35°. Although the population of gauche conformers is low, it has been possible to identify specific conformational sequences containing a gauche bond, in the interior and termini of the intercalated methylene chains, by their characteristic frequencies in 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 conformational registry at 40 K. The terminal gauche defects are significant in preventing interdigitation of the low-density intercalated surfactant bilayer.