Solid–liquid phase behavior of binary fatty acid mixtures: 1. Oleic acid/stearic acid and oleic acid/behenic acid mixtures

https://doi.org/10.1016/j.chemphyslip.2003.09.014Get rights and content

Abstract

Solid–liquid phase behavior of binary fatty acid mixtures was investigated by means of differential scanning calorimetry (DSC) and Fourier transform infrared spectroscopy (FT-IR) for the mixture composed of oleic acid (OA) and stearic acid (SA) and that composed of OA and behenic acid (BA). The DSC results provided a monotectic type TX phase diagram for these mixtures, from which it was suggested that the two fatty acid species are completely immiscible in a solid phase regardless of the two polymorphs of OA, i.e., α-form or γ-form. The solid phase immiscibility was confirmed by the FT-IR observation that the spectra obtained for the mixtures correspond to the superposition of the two spectra for respective components. Thermodynamic analysis of liquidus line demonstrated that OA and SA form an ideal mixture in a liquid phase, whereas the mixing of OA and BA in a liquid phase is slightly non-ideal.

Introduction

Fatty acids are distributed abundantly in nature as a main constituent of complex lipids such as oils and fats, phospholipids, glycolipids, sphingolipids, and so on (Gurr and Harwood, 1991). It is believed that the physical properties or phase behavior of the macroscopic lipid assemblies are responsible for their biological function in living bodies (Bloom et al., 1991). Most of the complex lipids contain different fatty acid species in the molecule, and the combination of the fatty acid species determines the physical properties of the lipid molecular assemblies. For example, the oil molecules in plant prefer short-chained saturated fatty acids as well as unsaturated ones, and this provides a low melting temperature to oils. On the other hand, the fat molecules in animals prefer long-chained saturated fatty acids, because of which the fats have a high melting temperature. The melting temperature of oils and fats is critically important for their role in living bodies as conservative lipids.

As for the phospholipids which are main constituents of biological membranes, fatty acids connected to the position 1 of the glycerol backbone are mostly saturated fatty acids, while those to the position 2 are unsaturated ones, in particular, oleic acid (OA). This combination of saturated and unsaturated fatty acids provides a flexible property to the molecular assemblies of phospholipids, which is essential for the function of biological membranes. Besides of the interest as a fundamental science, the phase science of lipid mixtures is also an important research subject in connection with the food industry; a number of mixtures of oils and fats are utilized as daily foods, and the quality of the food materials depends on the physical properties of the oils and fats mixtures as exemplified by chocolates and margarines. These examples demonstrate that the physico-chemical study of fatty acid mixtures is an interesting research subject being relevant to the whole understanding of the biological function of lipid molecules as well as to the practical application of lipid materials in food industry.

Physical properties of materials, including phase behavior, are governed by the molecular interaction; especially, in the case of mixture systems, the balance of the interaction between the same molecules and that between different molecules is responsible for the phase behavior. In order to understand the physical properties of complex lipids and their mixtures, the study of phase science of the constituent fatty acid mixtures should be useful, because through this kind of studies, it is expected to get insight into the acyl–acyl interaction which must play an important role to determine the physical properties and phase behavior of lipid mixtures. From these points of view, we have been studying the phase science of binary fatty acid mixtures. In this series of studies, we intend to draw a general rule for the phase behavior of fatty acid mixtures, through a systematic study on various combinations of fatty acid species including saturated, unsaturated, even-carbon, and odd-carbon fatty acids.

The solid–liquid phase behavior of binary fatty acid mixtures has so far been reported for some mixture systems containing oleic acid (Yoshimoto et al., 1991, Yoshimoto et al., 1991, Inoue et al., 1992, Inoue et al., 1993). Oleic acid (cis-9-octadecenoic acid) is a cis-monounsaturated fatty acid with 18 carbon atoms, and it must be the most important fatty acid for living bodies, because natural occurrence of oleic acid is most abundant in all fatty acid species (Padley et al., 1994). We focused our attention on this acid as a first component in the binary mixtures. In the present communication, we report the solid–liquid phase behavior of the mixtures of oleic acid and saturated fatty acid with 18 and 22 carbon atoms, i.e. stearic acid (SA; octadecanoic acid) and behenic acid (BA; docosanoic acid). The phase behavior was investigated by means of differential scanning calorimetry (DSC) and Fourier transform infrared spectroscopy (FT-IR). DSC can detect sensitively the temperature corresponding to the phase boundary, so this was used to construct the TX (temperature-composition) phase diagram for the binary fatty acid mixtures. FT-IR was used to characterize each phase and to monitor the molecular events associated with the temperature change of the mixtures.

Section snippets

Materials

Highly pure (>99.9%) samples of oleic acid (OA), stearic acid (SA), and behenic acid (BA) were obtained from the Research Institute of Structure and Function (Nara, Japan). They were prepared by the following procedure. The raw material of fats was hydrolyzed using lipase to obtain fatty acid mixture, from which fatty acid species were separated crudely by the urea adduct method and the acid soap crystallization method. Then, molecular distillation was applied to obtain highly pure fatty acid

DSC results of OA/SA mixture

Fig. 1 shows the DSC curves obtained for OA/SA mixtures with various compositions. The heat effect observed at about −3 °C for pure OA is ascribed to the polymorphic transformation from γ- to α-form (Suzuki et al., 1985). This polymorphic transformation in a solid phase is characteristic of OA and some other cis-monounsaturated fatty acids (Suzuki et al., 1988, Hiramatsu et al., 1990, Yoshimoto et al., 1991, Yoshimoto et al., 1991). The difference between γ- and α-form is seen in the

Conclusion

Present study has revealed the mixing behavior of OA/SA mixture and OA/BA mixture on the basis of the solid–liquid phase diagram of the mixtures. In both mixtures, the two fatty acid species are completely immiscible in a solid phase, while the mixing in a liquid phase is ideal or close to ideal. In view of the intermolecular interaction, the complete demixing of the two components happens when the non-ideality parameter of mixing, w (Eq. (5)), has a large positive value; i.e., the interaction

Acknowledgements

We are grateful to Prof. Makio Iwahashi of Kitasato University for his useful discussion and suggestion regarding the liquid structure of fatty acid mixtures. This work was supported in part by funds from the Central Research Institute of Fukuoka University (No. 015002) and the Research Institute of Structure and Function.

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