Interdigitation of an intercalated surfactant bilayer

Abstract

The cationic surfactant cetyltrimethylammonium (CTA) has been ion-exchange intercalated from an aqueous solution of the surfactant by a two-step process into the galleries of layered cadmium thiophosphate to give Cd_0.83PS₃(CTA)_0.34. Two phases of the intercalated product are observed. One is a phase characterized by a lattice expansion of 26.5 Å, in which the methylene chains of the CTA ion adopt a tilted bilayer arrangement. This phase, when left in the reaction media, transforms to a phase characterized by a lattice expansion of 12.5 Å with no change in chemical stoichiometry. Using X-ray diffraction and orientation-dependent infrared spectroscopy, it has been possible to establish that the observed collapse of the interlayer spacing is a consequence of the interdigitation of the methylene "tails" of the intercalated CTA ion. Infrared, Raman, and ¹³C NMR spectroscopies have been used to establish the conformation of the methylene chains in the intercalated normal bilayer and interdigitated bilayer phases of Cd_0.83PS₃(CTA)_0.34 and the dispersion of the delocalized methylene wagging (ν₃) and rocking-twisting (ν₈) modes as a probe of the planarity of the chains. The results indicate that a majority of the methylene chains in the intercalated interdigitated bilayer adopt an all-trans planar conformation, whereas, in the normal bilayer phase of Cd_0.83PS₃(CTA)_0.34, although a majority of the methylene units are in a trans configuration, the presence of a few gauche defects is sufficient to destroy the planarity of the methylene chain. Planarity of the methylene chains is the key factor for the absence or occurrence of interdigitation of the intercalated surfactant bilayer.