Dynamical structures of normal alkanes, alcohols, and fatty acids in the liquid state as determined by viscosity, self-diffusion coefficient, infrared spectra, and 13C NMR spin-lattice relaxation time measurements.

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The dynamical structures of molecules of normal alkanes (C5–C14) and normal fatty acids (C8 and C9) in the liquid state were estimated through analyses of their self-diffusion coefficients (D), viscosities (η), molar volumes, and 13CNMR spin-lattice relaxation time (T1). The apparent hydrodynamic radius evaluated from D and η for the alkanes was ca. 2.5×10−10 m and that for the fatty acids, ca. 3.1×10−10 m, irrespective of their hydrocarbon chain length: The former value is almost equal to that of the van der Waals radius (2.4×10−10 m) of the cross section of a hydrocarbon chain; the latter is in agreement with that of the normal alcohols reported in a previous paper. It is thus concluded that in the pure liquid of a rod-like compound such as normal alkane, alcohol, and fatty acid, the free rotational (end-over-end) as well as the transverse motion of the rod-like molecule is severely restricted due to the entanglements of the molecules; only a longitudinal translation is allowed. In addition, the infrared-spectrum results suggest that a unit in translational motion for a normal fatty acid is a dimer.
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