LIQUID-CRYSTAL STRUCTURES OF BILAYER-FORMING SYNTHETIC AMPHIPHILES, DIALKYLDIMETHYLAMMONIUM BROMIDES

Liquid crystal structures of simple synthetic amphiphiles, dialkyldimethylammonium bromides (2C(n)N+2C1Br-), were studied by means of X-ray diffraction and thermal analyses. Below the transition temperature, specimens of both 2C(n)N+2C1Br- powder and 2C(n)N+2C1Br--water mixture gave X-ray diffraction patterns essentially similar to those from the corresponding single crystals; in which amphiphiles are arranged with a tilting angle of about 45-degrees in the bimolecular layer. Further, in the case of the 2C(n)N+2C1Br--water mixture, it was found that water molecules are separated from the amphiphile crystal and have no influence on the bilayer structure. On the contrary, the X-ray diffraction pattern from the liquid crystal of 2C(n)N+2C1Br- was different from that of 2C(n)N+2C1Br- water mixtures. The diffraction pattern from the former had several Debye rings in the wide angle region, which suggested a regular arrangement of bromide anions and nitrogen cations in the hydrophilic layer, just like that in the crystalline state. According to the analysis of the bilayer structure carried out by using a one-dimensional rectangular electron density distribution, alkyl chains in the hydrophobic region of 2C(n)N+2C1Br- keep tilting even in the liquid crystal state. The tilting of the molten alkyl chains was attributed to the consequence of the regular arrangement of the head moieties since the alkyl chains connect directly to the cationic nitrogen atom. On the contrary, in the liquid crystal state of the 2C(n)N+2C1Br--water mixture. amphiphile molecules were arranged side by side and perpendicular to the layer surface in the same way as those usually found in lipid water mixture systems.