n-Caprylammonium Chloride: A Solid Membrane Model

Article abstract of DOI:10.1021/j100192a064

n-Caprylammonium chloride (C8Cl) undergoes three thermotropic phase transitions between 304 and 311 K.Its high-temperature phases (β, γ, and α) are very disordered and resemble smectic mesophases found in biomembranes.The δ-phase (stable between 278 and 304 K) of C8Cl is studied by means of single-crystal X-ray diffraction.Chlorine and nitrogen atoms form a 3-periodic arrangement, but the carbon atoms oscillate about the long axes of the n-caprylammonium chains.These exist as several conformers which can be approximated as a single, entirely extended chain.The cell of the δ-phase allowed us to index powder X-ray photographs of the other phases of C8Cl and to deduce a structural description for them.Percentages of conformers, G forms, and GTG'/GTG sequences in the various phases are estimated from the analysis of characteristic IR and Raman bands, especially of the ω-bideuterated derivative.By a combination of our conclusions with previous NQR and NMR results, a rather complete picture of the phase transitions is deduced: they are brought about by the disordering of the aliphatic chains on one hand and of the hydrogen bonding scheme at the Cl-N interface on the other hand.The dynamics of these two subsystems behave quite differently indeed: changes in the chains occur very gradually, whereas variations in the interface happen quite abruptly.C8Cl successfully mimics the temperature dependence of the bilayer thickness, chain cross section, and order as well as the chain separation/interdigitation transition in biological membranes.