Self-diffusion, dynamical molecular conformation, and liquid structures ofn-saturated and unsaturated fatty acids

The translational movement, the dynamical molecular conformation, and the a ggregate structure in the liquid state for three kinds of cis-type unsatura ted fatty acids (cis-6-, cis-9-, and cis-11-octadecenoic acids), trans-type unsaturated acid (trans-9-octadecenoic acid), and n-saturated acid (octade canoic acid) have been studied through the measurements of density, self-di ffusion coefficient, C-13 NMR spin-lattice relaxation time, and X-ray diffr action. The magnitude of the self-diffusion coefficient, D, fell in sequenc e of cis-type unsaturated acids > trans-type acid > saturated acid below 35 0 K. Above 350 K the value of D for the trans-type acid rose to those for t he cis-type acids. On the other hand, the density rho for the acids also fe ll in the order of cis-type acids > cans-type acid > saturated acid; the or der of the density is just opposite to that expected from the results of th e self-diffusion coefficient and other physical properties such as melting point and heat of fusion. This discrepancy between D and rho values for the acids is well explained by the models of the clusters in their liquids.