Conformation of an alkane chain in confined geometry: cetyl trimethyl ammonium ion intercalated in layered CdPS₃

Abstract

Cetyl trimethylammonium (CTA) ions have been confined at low grafting densities within the galleries of layered CdPS₃ by an ion-exchange intercalation reaction to give Cd_0.93PS₃(CTA)_0.14. Intercalation occurs with a lattice expansion of 4.8 Å with the methylene "tail" of the intercalated surfactant lying closer to the walls of the galleries, rather than in the center. ¹³C NMR and orientation-dependent IR vibrational spectroscopy of single crystals of the intercalated Cd_0.93PS₃(CTA)_0.14 have been used to probe the conformation and orientation of the methylene tail of the isolated, interlamellar surfactant ions. We find that the confined methylene chain adopts an essentially all-trans conformation, even in the absence of intermolecular interactions, with a large section of the molecular plane of the trans chain aligned parallel to the walls of the galleries. Upon lowering of the temperature, increasingly larger sections of the molecular plane align parallel, so that the methylene chain lies flat, rigid, and aligned to the confining surface.