Fatty acids such as oleic and stearic acids having a long hydrocarbon chain are known to exist as dimers in their melt and even in a non-polar solvent. In their melt the dimers arrange longitudinally and alternately to form clusters which resemble a smectic liquid crystal. The clusters determine the liquid properties of the fatty acids such as density, viscosity and fluidity. Then, do the dimers of fatty acid having a moderate-length hydrocarbon chain construct such the clusters? In the present study the dynamic molecular behavior and assembly structure of octanoic acid in its melt and also in CCl4 solution have been investigated by the X-ray diffraction, near infrared spectroscopy, 1H-NMR chemical shift, self-diffusion coefficient and 13C-NMR spin-lattice relaxation time measurements. From these results it has been revealed that the clusters of octanoic acid exist in its melt and also in CCl4 and that the clusters in the melt disintegrate with an increase in temperature. The dissociation profile of dimers of octanoic acid into monomers in CCl4 also has been clarified.
Fatty acids are used in many fields such as cosmetic, detergent, food and lubricant industries; they are characteristic and significant components of most lipids and play an important role in functions such as flexibility, fluidity and material transfer in biomembranes. The functions seem to be responsible to the aggregated structures of the fatty acid molecules. Thus, for various needs in the industries and also from the fundamental aspects, it is important to reveal the relationship between the functions and the aggregated structures of fatty acids.
In the previous study on the liquid structure of various fatty acids it has been clarified that these fatty acids exist mostly as dimers in their melt and even in a non-polar solvent. Namely, through the measurements of near-infrared spectroscopy (NIR) and vapor pressure osmosis it has been revealed that cis-9-octadecenoic acid (Iwahashi, Suzuki, Czarnecki and Ozaki, 1995 [
Furthermore, the dynamic molecular aspects and the assembly structures of several fatty acids having 18 carbon atoms such as cis-6-octadecenoic, cis-9-octadecenoic, cis-11-octadecenoic, trans-9-octadecenoic, and octadecanoic acids in their pure liquids were also studied at various constant temperatures (Iwahashi et al., 2000 [
The existence of the clusters most likely determines the liquid properties of fatty acids such as density and fluidity. For example, a discrepancy between self-diffusion coefficient and density among the above acids has been clearly resolved using the above cluster model (Iwahashi et al., 2000 [
Then, do the dimers of fatty acid having a moderatelength hydrocarbon chain construct such the clusters in the melt or in a non-polar solvent? If so, are the clusters stable at high temperature or in a dilute solution? To solve these questions, we measured the X-ray diffraction, self-diffusion coefficient, 13C-NMR spin-lattice relaxation time, near-infrared (NIR) spectroscopy and 1H-NMR chemical shift for the samples of octanoic acid in its melt and in its CCl4 solution.
Sample of octanoic acid (>99.9%) was kindly supplied from the Research Institute Biological Materials (Kyoto, Japan). Octanoyl chloride (98%) was purchased from Tokyo Kasei Co. They were used without further purification. Carbon tetrachloride (CCl4: 99.5% pure) purchased from Nacalai Tesque INC (Kyoto, Japan) was dried over 5 Å molecular sieves and distilled under an atmosphere of dried nitrogen. Samples for the 13C-NMR spin-lattice relaxation time T1 measurements were prepared after a 30-minite-argon gas passing, using a glove box under an atmosphere of nitrogen gas to prevent the absorption of oxygen, which would make the T1 shorter.
X-ray diffraction measurement for the sample of octanoic acid was carried out on a X-ray diffraction instrument (Rigaku model RU-300) using MoKα (wavelength λ = 0.7107 Å) radiation (40 kV × 240 mA) in the temperature range 303 - 473 K ± 0.2 K. Samples were set in glass capillary cells with 2-mm diameter and 1/100-mm thickness. Scanning intensities in the range from 0.06 to 4.603 Å−1 in s value (s = (4π/λ) sinθ, 2θ = scattering angle) were measured (Iwahashi et al., 2000 [
NIR spectra of the samples of octanoic acid and octanoyl chloride in CCl4 were measured at resolution of 1.0 nm on Hitachi-3500 spectrophotometer in a temperature range (293 - 313) ± 0.01 K at interval of 5 K (Iwahashi, Kasahara, Minami, Matsuzawa, Suzuki and Ozaki, 2002 [
The chemical shifts, δ, of the OH protons of octanoic acid samples in CCl4 were measured on a NMR spectrometer (Japan Electron Optics Laboratory (JEOL) Model EX-400), using 1%-tetramethylsilane (TMS) in DMSO-d6 contained in 1-mm inner tube as a chemical shift standard in a temperature range (303 - 323) ± 0.5 K at interval of 10 K.
The self-diffusion coefficient, D, was determined by means of the pulsed-field gradient NMR method (Farrar and Becher, 1971 [
The 13C-NMR spin-lattice relaxation time, T1, for octanoic acid samples was obtained by the inversion recovery method (Hertz, 1967 [