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), transtype 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 F 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 F values for the acids is well explained by the models of the clusters in their liquids.

The self-association of octan-1-01 in the pure liquid state and in decane solutions has been studied by nearinfrared spectroscopic observation over the temperature range 298.1-348.1 K and also by viscosity, self-diff usion and NMR measurements. The absorption bands assigned to the hydroxy group are superimposed on several bands attributed to the methylene groups belonging to both the alcohol and decane molecules. A superior technique using a computer was adopted for the cancellation of the absorption bands due to the methylene groups so as to obtain the net OH absorption bands. The analysis of the sharp band at 1410 nm (the first overtone band of the OH-stretching vibration attributed to the free OH-monomer and partly to the OH-polymer) leads to the conclusion that the mean degree of association n (i.e. the average size of the multimer in monomeric units) in the polymer of octan-1-01 is ca. four, both in the liquid and in decane solutions under the temperature conditions given above. From the arguments about the aggregated species, a cyclic tetramer was proposed as the associated alcohol species in the liquid and in decane solutions. The association equilibrium constant and other thermodynamic properties such as changes in enthalpy, entropy and Gibbs energy for tetramer formation were also determined.

The dissociation of dimers of cis-9-octadecenoic acid into its monomeric species in its pure liquid was studied by near-infrared spectroscopic measurements over a temperature range of 8.6-84.4 OC. An absorption band at 1445 nm, which is attributed to the free OH of the fatty acid molecules, appeared even at relatively low temperatures; its peak height increased with increasing temperature. This shows that the dissociation of the acid dimers occurs even at low temperatures and increases with increasing temperature. The molar absorption coefficient at 1445 nm for the monomeric OH stretch mode in the carboxyl group was obtained through the absorbance measurements in carbon tetrachloride solution at very low concentrations. The molar absorption coefficient and the absorbance for the neat sample at various temperatures, T, give the temperature dependence of the degree of dissociation, a, for dimeric cis-9-octadecenoic acid into monomers in the liquid state. The a-T curve has two break points at 30 and 55 OC. The break-point temperatures correspond to the transition temperatures in the liquid structures of cis-9-octadecenoic acid: 30 OC corresponds to a quasi-smectic liquid crystal to a more disordered liquid crystal, and 55 OC corresponds the disordered liquid crystal to isotropic liquid.' Namely, the transitions in the liquid state of cis-9-octadecenoic acid are attributable to the presence of the monomeric molecules produced by the dissociation of the dimeric acid: The monomeric molecules existing as an impurity would initiate the transition in the pure liquid state consisting mainly of cis-9-octadecenoic acid dimers.

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