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 Cd0.83PS3(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.

The single perovskite slab alkylammonium lead iodides (C n H2 n +1NH3)2PbI4, n = 12, 16, 18, display two phase transitions, just above room temperature, associated with changes in the alkylammonium chains. We have followed these two phase transitions using scanning calorimetry, X-ray powder diffraction, and IR and Raman spectroscopies. We find the first phase transition to be associated with symmetry changes arising from a dynamic rotational disordering of the ammonium headgroup of the chain whereas the second transition, the melting of the chains in two dimensions, is characterized by an increased conformational disorder of the methylene units of the alkyl chains. We examine these phase transitions in light of the interesting optical properties of these materials, as well as the relevance of these systems as models for phase transitions in lipid bilayers.

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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 0.83 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 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 13 C NMR spectroscopies have been used to establish the conformation of the methylene chains in the intercalated normal bilayer and interdigitated bilayer phases of Cd 0.83 PS 3 (CTA) 0.34 and the dispersion of the delocalized methylene wagging (ν 3 ) and rocking-twisting (ν 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 conformation, whereas, in the normal bilayer phase of Cd 0.83 PS 3 (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.

A structural analysis of alkyl chain conformation of an intercalated cationic lipid bilayer is described. Dialkyl dimethylammonium ions (di-C n DA) were ion exchanged into the galleries of layered cadmium thiophosphate to give Cd 0.83 PS 3 (di-C n DA) 0.34 . The grafting density and interlayer expansions were identical to those for the intercalated single chain alkyl trimethylammonium (C n TA) bilayers. The increased methylene chain density in the galleries, however, forces the intercalated lipid to adopt a more trans ordered structure. Progression bands arising from the coupling of vibrational modes of trans methylene units are used to establish the extent of trans registry. Two types of ordered structures of the intercalated cationic lipid may be distinguished. One in which both alkyl chains adopt an all-trans geometry, and one in which the methylene bond adjacent to the headgroup on one of the alkyl chains is gauche. The latter structure is typically found in the crystalline state of these cationic lipids. The concentrations of the two structures were determined from the ratio of the intensities of the progression bands and were found to remain unchanged with temperature.