Interdigitation of an intercalated surfactant bilayer

The cationic surfactant cetyltrimethylammonium (CTA) has been ion-exchange intercalated from an aqueous solution of the surfactant by a two-step proce ss into the galleries of layered cadmium thiophosphate to give cd(0.83)PS(3 )(CTA)(0.34). Two phases of the intercalated product are observed. One is a phase characterized by a lattice expansion of 26.5 Angstrom, in which the methylene chains of the CTA ion adopt a tilted bilayer arrangement. This ph ase, when left in the reaction media, transforms to a phase characterized b y a lattice expansion of: 12.5 Angstrom with no change in chemical stoichio metry. Using X-ray diffraction and orientation-dependent infrared spectrosc opy, it has been possible to establish that the observed collapse of the In terlayer spacing is a consequence of the interdigitation of the methylene " tails" of the intercalated CTA ion. Infrared, Raman, and C-13 NMR spectrosc opies have been used to establish the conformation of the methylene chains in the intercalated normal bilayer and interdigitated bilayer phases of Cd0 .83PS3(CTA)(0.34) and the dispersion of the delocalized methylene wagging ( nu (3)) and rocking-twisting (nu (8)) 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 conforma tion, whereas, in the normal bilayer phase of Cd0.83PS3(CTA)(0.34), althoug h a majority of the methylene units are in a trans configuration, the prese nce of a few gauche defects is sufficient to destroy the planarity of the m ethylene chain. Planarity of the methylene chains is the key factor for the absence or occurrence of interdigitation of the intercalated surfactant bi layer.