An X-ray and NMR investigation of the lamellar liquid crystalline phase in the octylamine/octylammonium chloride/water system

Abstract

The lamellar liquid crystalline phase in the system octylamine/octylammonium chloride/water has been investigated by X-ray low-angle diffraction and by 2H NMR determinations of order parameter profiles from the quadrupolar splittings of deuterated surfactant. At a constant mole ratio water/surfactant 80/20, the lateral expansion of the surfactant bilayer starts when the mole fraction of charged surfactant exceeds 0.5. Accompanying conformational changes in the alkyl chains are reflected in the surfactant order parameter profile, and for most methylene segments there is a linear relationship between the area per headgroup and the measured order parameter. However, the order parameters, in particular for the α-methylene segment, show a deviation from this relationship in the amine-rich region. For high surface charge densities, the maximum in the order parameter profile occurs for the α-methylene segment, while for low surface charge densities the maximum occurs further down the chain. This is interpreted as an effect of different molecular orientations due to the interaction between the polar headgroup and the water molecules.