Molecular Behaviors of n-Fatty Acids in Liquid State

Through near infrared spectroscopy and 13C-NMR spin-lattice relaxation time measurements it was revealed that the molecules of the normal fatty acids in the liquid state are strongly dimerized even at 363K by hydrogen bonding between their carboxylic groups. Thus, dimer molecules for the acids are the units in their intra- and intermolecular movements. The dynamic molecular and aggregate structures of the fatty acids (C8-C18) in the liquid state were estimated through the analyses of their self-diffusion coefficient D, viscosity η, molar volume and X-ray diffraction. The apparent hydrodynamic radius evaluated from D and η for the fatty acid was almost constant, irrespective of the hydrocarbon chain length. This suggests that only a longitudinal translation (translational movement along molecular axes) would be allowed for the dimer molecules of fatty acids. In addition, the distribution function curves obtained from the X-ray diffraction data suggest that, in the pure liquid state, rod-like fatty acid dimers highly aggregate in parallel and probably make clusters that would be randomly aligned.