NMR and Fluorescence Studies on Self‐assembling Behavior of Dialkyldimethylammonium Salts in Aqueous Solutions

Proton NMR and fluorescence spectra of three long chain dialkyldimethylammonium salts were studied to elucidate the characteristic self‐assembling behavior, which includes formation of bilayer structures analogous to natural lipid membranes. The line widths of the methylene and N‐methyl proton resonance signals are strongly affected by the alkyl chain length as well as by the temperature. The largest line width was observed with the ammonium salt having two octadecyl groups, which also showed a clear phase transition point detected by 1 H NMR. The fluorescence polarization of 1,6‐diphenyl‐1,3,5‐hexatriene and the excimer/monomer ratio of pyrene fluorescence shows temperature‐dependence characteristic of each compound. All of these spectroscopic data indicate that the bilayer structure obtained with dioctadecyldimethylammonium salt is the tightest but it is still softer than natural lipid membranes. As the chain length of one of the two alkyl groups is reduced, the molecular assembly gets looser: octadecyloctyldimethylammonium salt finally gives an aggregate quite close to cetyltrimethylammonium bromide micelle.