FNSFood and Nutrition Sciences2157-944XScientific Research Publishing10.4236/fns.2013.49A1005FNS-36104ArticlesBiomedical&Life Sciences Dynamic Molecular Behavior and Cluster Structure of Octanoic Acid in Its Liquid and CCl<sub>4</sub> Solution ideyoMatsuzawa1MasayaTsuda1HideyukiMinami1MakioIwahashi1*Department of Chemistry, School of Science, Kitasato University, Sagamihara, Japan.* E-mail:makio-iwahashi@jcom.home.ne.jp(MI);2008201304092532May 15th, 2013June 15th, 2013 June 22nd, 2013© Copyright 2014 by authors and Scientific Research Publishing Inc. 2014This work is licensed under the Creative Commons Attribution International License (CC BY). http://creativecommons.org/licenses/by/4.0/

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.

Octanoic Acid; Oleic Acid; Stearic Acid; X-Ray Diffraction; Near-Infrared Spectroscopy; Self-Diffusion Coefficient; <sup>13</sup>C-NMR Spin-Lattice Relaxation Time; <sup>1</sup>H-NMR Chemical Shift1. Introduction

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 [1]) and several nfatty acids (C8-C11) (Iwahashi, Kasahara, Minami, Matsuzawa, Suzuki and Ozaki, 2002 [2]) in their liquid states exist as dimers even at 80˚C: the dimers are the units in intraor intermolecular movements.

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 [3]). The dimers of these fatty acids, which are stable even at high temperature, aggregate also to form clusters possessing the structure of a quasi-smectic liquid crystal: The long-chained fatty acid dimers arrange longitudinally and alternately to make an interdigitated structure in the clusters. The alignment in the longitudinal direction for the acid molecules resembles that for the dodecanoic acid molecules in A-form crystal (Lomer, 1963 [4], Goto and Ashida, 1978 [5]) and the cis-9-octadecenoic acid molecules in β1-form crystal. (Kaneko et al., 1997 [6]).

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 [3]). Furthermore, effect of additives on the cluster structure of cis-9-octadecenoic acid also was studied (Iwahashi et al., 2007 [7]); it was found that cholesterol strengthens the interaction among the acid dimers in the cluster and makes the cluster structure stable, while ethanol and benzene weaken the cluster structure.

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.

2. Experimental2.1. Materials

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.

2.2. Measurements2.2.1. X-Ray Diffraction

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 [3]). The intensities were corrected by the subtraction of the back ground intensity. Deconvolution of the diffraction signals was carried out by assuming a Lorentzian curve for each signal and determined the peak positions of the signals.

2.2.2. NIR Spectrum Measurements

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 [2]). A quartz cell having a 0.5-cm path length was used. A Hitachi temperature-regulated cell holder (No. 131- 0030) was used to maintain the temperature of the sample.

2.2.3. NMR-Chemical Shift

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.

2.2.4. Self-Diffusion Coefficient

The self-diffusion coefficient, D, was determined by means of the pulsed-field gradient NMR method (Farrar and Becher, 1971 [8]). All the measurements were made on protons at 399.65 MHz in a temperature range (303 - 323) ± 0.5 K at interval of 5 K on the same NMR spectrometer with a probe for the pulsed-field gradient NMR measurements.

2.2.5. <sup>13</sup>C-NMR Spin-Lattice Relaxation Time

The 13C-NMR spin-lattice relaxation time, T1, for octanoic acid samples was obtained by the inversion recovery method (Hertz, 1967 [9]) employing a 180-τ-90˚ pulse sequence, using also the same NMR spectrometer at 313 ± 0.5 K.

3. Results and Discussion3.1. Aggregation Structure of Octanic Acid in Its MeltX-Ray Diffraction Measurements

Figure 1 shows the X-ray diffraction spectra for the liquid sample of octanoic acid in its melt at various temperatures. There are three individual signals at a constant temperature: A broad signal exists around 0.4 Å−1 in s value, a large and sharp signal, around 1.4 Å−1 and a broad one, around 2.7 Å−1. The signal around 2.7 Å−1 would be attributable to the second-order reflection to the signal at 1.4 Å−1. The peak position of the 0.4 Å−1 signal slightly decreases and that of the 1.4 Å−1 signal apparently decrease with an increase in temperature. The X-ray diffraction spectra resembles those of the liquid samples of fatty acids having a long hydrocarbon chain such as cis-octadecenoic (oleic), trans-octadecenoic (elaidic) and octadecanoic (stearic) acids (Iwahashi et al., 2000 [3]). As mentioned in introduction, these long-hydrocarbon chained fatty acids are known to exist as dimers in the melt; their dimers arrange longitudinally

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