Self-Diffusion, Dynamical Molecular Conformation, and Liquid Structures of <i>n</i>-Saturated and Unsaturated Fatty Acids

Abstract

The translational movement, the dynamical molecular conformation, and the aggregate structure in the liquid state for three kinds of cis-type unsaturated fatty acids ( cis -6-, cis -9-, and cis -11-octadecenoic acids), trans-type unsaturated acid ( trans -9-octadecenoic acid), and n -saturated acid (octadecanoic acid) have been studied through the measurements of density, self-diffusion coefficient, 13 C NMR spin−lattice relaxation time, and X-ray diffraction. The magnitude of the self-diffusion coefficient, D, fell in sequence of cis-type unsaturated acids > trans-type acid > saturated acid below 350 K. Above 350 K the value of D for the trans-type acid rose to those for the cis-type acids. On the other hand, the density ρ for the acids also fell in the order of cis-type acids > trans-type acid > saturated acid; the order of the density is just opposite to that expected from the results of the self-diffusion coefficient and other physical properties such as melting point and heat of fusion. This discrepancy between D and ρ values for the acids is well explained by the models of the clusters in their liquids.