Molecular Conformation and Liquid Structure of Oleic Acid

采用差示扫描量热法，通过测定浓度、粘度、自扩散系数、红外光谱、溶解于酸中的DPH的荧光极化、四种自旋探针在酸中的ESR阶参数、^1H和^<13>C核磁共振弛豫时间T_1和T_2以及x射线衍射，研究了顺式-9-十八烯酸在纯态下分子构象和液体结构的温度依赖关系。在实验结果的基础上，得出酸在不同温度下具有三种不同的液体结构：(1)第一种结构，在熔点至303K温度范围内，由簇状的准晶液晶组成；这个。星团是随机定向的，它们的大小小于可见光的波长。(2)在303 - 323K的温度范围内，可能由结构不太有序的团簇组成。(3)第三种似乎是在323K以上的各向同性液体。液体油酸的近红外光谱的温度依赖性表明，这种由温度冻结引起的相变是由于酸分子的环二聚体的解离引起的，这些酸分子由羧基的氢键连接在一起。

Temperature dependence of molecular conformation and liquid structure for cis-9-octadecenoic acid in the neat state has been studied by means of differential scanning calorimetry and by measurement of the density, viscosity, self-diffusion coefficient, infrared spectrum, fluorescence polarization of DPH dissolved in the acid, ESR order parameter for four kinds of spin probe incorporated in the acid, ^1H and ^<13>C NMR relaxation times T_1 and T_2, and X-ray diffraction.On the basis of the experimental results, it is concluded that the acid has three kinds of liquid structure depending on temperature : (1)The first structure, in temperatures from the melting point to 303K, consists of clusters of a quasi-smectic liquid crystal ; the. clusters are randomly orientated and their sizes are smaller than the wavelength of visible light. (2)The second one, in the temperature range between 303 and ca. 323K, presumably consists of clusters having a less ordered structure. (3)The third one appears to be an isotropic liquid above ca. 323K.Temperature dependence of near infrared spectrum for liquid oleic acid suggests that such the phase transitions resulted from the temperature qrizing are due to the dissociation of the cyclic dimer of the acid molecules linked together by a hydrogen bond of their carboxyl groups.