X-ray structural studies of bilayers composed of monoalkyl-monocation type artificial amphiphiles

The membrane structure of hydrated monoalkyl-monocation type artificial amphiphiles was investigated by an X-ray diffraction method. The amphiphiles showed a thermotropic and lyotropic crystal-liquid crystal transition similar to that of phospholipids in the biological membranes. The artificial amphiphile/water system with various amphiphile concentrations in the region from C=0.8 to C=0.1 (weight fraction) gave basically the same X-ray diffraction patterns. A one-dimensional structural analysis of the membrane was carried out for the specimen of C=0.5. According to this analysis, the repeating period of 4.20 nm consists of the water layer (0.52 nm) and the bimolecular layer (3.68 nm) in which hydrocarbon chains from both surfaces of the bimolecular layer.are mutually inserted and almost perpendicular to the surface. Considering the thickness of the water layer, water molecules seem to be packed neatly and fixed strongly by the hydrogen bonds with bromide anions which are located at the surfaces of the bimolecular layer. As the number of carbon atoms in the alkyl group changed to 10, 12, and 14, the crystal-liquid crystal phase transition temperature was raised to 303, 310, and 332 K, respectively. The thickness of the bimolecular layer increased linearly with 0.26 nm per two alkyl groups, while the thickness of the water layer remained constant (ca. 0.52 nm). The wide angle diffractions of 0.47, 0.40, and 0.38 nm were independent of the length of the alkyl chain. This indicates that the above three amphiphiles were very similar in molecular conformation and packing arrangement.