Hydrocarbon chain conformation in an intercalated surfactant monolayer andbilayer

Cetyl trimethyl ammonium (CTA) ions have been confined within galleries of layered CdPS3 at two different grafting densities. Low grafting densities a re obtained on direct intercalation of CTA ions into CdPS3 to give Cd0.93PS 3(CTA)(0.14). Intercalation occurs with a lattice expansion of 4.8 Angstrom with the interlamellar surfactant ion lying flat forming a monolayer. Inte rcalation at higher grafting densities was effected by a two-step ion-excha nge process to give Cd0.93PS3(CTA)(0.14), with a lattice expansion of 26.5 Angstrom. At higher grafting densities the interlamellar surfactant ions ad opt a tilted bilayer structure. C-13 NMR and orientation-dependent IR vibra tional spectroscopy on single crystals have been used to probe the conforma tion and orientation of the methylene 'tail' of the intercalated surfactant in the two phases. In the monolayer phase, the confined methylene chain ad opts an essentially all-trans conformation with most of the trans chain ali gned parallel to the gallery walls. On lowering the temperature, molecular plane aligns parallel, so that the methylene chain lies flat, rigid and ali gned to the confining surface. In the bilayer phase, most bonds in the meth ylene chain are in trans conformation. It is possible to identify specific conformational sequences containing a gauche bond, in the interior and term ini of the intercalated methylene. These high energy conformers disappear o n cooling leaving all fifteen methylene units of the intercalated cetyl tri methyl ammonium ion in trans conformational registry at 40 K.