【作者】 范雅；

【导师】 许大鹏；

【作者基本信息】 吉林大学 ， 凝聚态物理， 2011， 博士

【摘要】 高压和低温研究可以发现物质在常压和室温下不能表现出来的一些现象,进而揭示新规律、新性质,乃至发现新物质,为研究高压和低温下的物质性质和解释常压室温现象提供新的途径。随着静高压原位测量技术的迅猛发展,金刚石对顶砧(DAC)已被广泛应用于各种物质的高压物性研究。激光拉曼光谱和高压技术相结合使人们可以方便的研究在压力作用下的物质内部结构的变化,因此高压原位拉曼光谱是目前最常用的高压实验技术之一,为研究物质在高压下的相变和化学反应提供了一个非常灵敏和有效的探测手段。不饱和脂肪酸作为一类重要的膳食脂肪酸,具有特殊的生理功能。而食品超高压处理技术在食品加工和储藏业具有非常广阔的应用前景,不饱和脂肪酸作为人类食物的主要成分之一,其高压和低温相变研究对于食品储藏技术的改进具有重要的实际意义。另一方面,由于不饱和脂肪酸分子中存在碳碳双键,使其在低温和高压条件下极易发生相变,所以其相变研究也具有重要的理论价值。已有几种实验方法的研究证实油酸和亚油酸在高压下均发生了相变,但并没有任何文献给出关于油酸和亚油酸压致相变点的准确数据和高压相的准确指认,同时这两种不饱和脂肪酸的高压拉曼光谱或其他高压原位实验研究也未见报道。因此为解决以上问题,本论文以油酸和亚油酸为研究对象,利用金刚石对顶砧技术结合原位拉曼光谱,研究它们在高压下的相变。同时,为探究高压下分子间相互作用、溶剂效应等,测量了油酸溶液的高压原位拉曼光谱。另外,由于液体的高压结晶相和低温结晶相的比较是高压研究的一个重要方向,我们又利用Linkam变温实验系统与原位拉曼光谱相结合,研究了这两种不饱和脂肪酸在低温下的相变行为,和其高压相变做比较。主要研究内容及获得结论如下:一、纯油酸的高压相变通过测量并分析纯油酸在室温下加压至0.84GPa过程中的原位拉曼谱发现:在0.29-0.36GPa压力区间,纯油酸拉曼谱出现极为显著的变化,包括部分拉曼峰的消失、新拉曼峰的出现以及某些拉曼峰的频率-压力曲线斜率的突变,说明在该压力范围内油酸发生了压致相变。随着压力进一步增加至0.84GPa,并没有证据显示发生另一次相变,故在0.84 GPa以下仅存在一个高压相。光谱分析显示在油酸的高压相中,油酸分子的聚亚甲基链呈现有序全反构象,甲基链端呈现有序tt构象,烯烃基团呈现斜-顺-斜′构象。其构象特征表明油酸的高压相与其低温下的γ结晶相一致,故油酸的压致相变是由液相转变为γ结晶相,首次给出了油酸高压相的准确指认。该相变是可逆的,且为一级相变。二、溶液中油酸的高压相变通过测量并分析溶液(25%(V/V),溶剂为四氯化碳)中油酸在室温下加压至0.84GPa过程中的原位拉曼谱发现:在0.15-0.20GPa和0.28-0.40GPa两个压力区间,25%(V/V)油酸溶液的拉曼光谱发生了极为明显的变化,包括部分拉曼峰的消失、新拉曼峰的出现等,证明溶液中的油酸在上述压力范围内发生了两次压致相变。当0.15-0.20GPa的第一次相变发生时,光谱分析显示油酸分子的甲基链端由无序构象变为有序的tt构象,C=C键两侧的亚甲基链均出现了有序反式构象成分,但在甲基侧链中还保留了一定的无序邻位交叉式构象成分。根据其拉曼光谱和分子的构象特征,第一个高压相被认定为结晶相。因此溶液中油酸的第一次压致相变是由液相固化为分子构象有序度较高的结晶相。当0.28-0.40GPa的第二次相变发生时,光谱分析显示油酸分子的烯烃基团由结晶相的斜-顺-反构象转变为斜-顺-斜′构象,而甲基侧链转变成有序的全反式构象。根据其拉曼光谱和分子的构象特征,第二个高压相被认定为γ结晶相。因此溶液中油酸的第二次压致相变是由结晶相转变为更为有序的γ结晶相。溶液中油酸的固化压强比纯油酸低,相变过程中多出一中间相——结晶相,相变途径与纯油酸相比存在较大的差异,可能与四氯化碳小分子对油酸分子二聚体结构的衬垫作用有关。三、纯油酸的低温相变通过测量并分析纯油酸在–14oC ~ 18oC温度范围内的原位拉曼光谱发现:在–4oC ~ 0oC和12oC ~ 16oC两个温度区间内,油酸的拉曼光谱出现了显著的变化,说明在这两个温度范围内油酸各发生了一次温致相变。当–4oC ~ 0oC的第一次相变发生时,油酸分子的烯烃基团由斜-顺-斜′构象转变为斜-顺-反构象,甲基侧链中反式构象成分减少,邻位交叉式构象成分增加,构象有序度下降。分析表明该相变是由γ结晶相转变为结晶相,为有序-无序相变。当12oC ~ 16oC的第二次相变发生时,光谱分析显示分子的整个聚亚甲基链都变成了无序构象,甲基侧链的链端则由有序tt构象变成无序构象。分析表明该相变是由结晶相熔化为液相。油酸在低温和高压下的相变途径是不同的,说明对油酸来讲,压缩效应不完全等同于致冷效应。四、亚油酸的高压相变通过测量并分析亚油酸在室温下加压至1.29GPa过程中的原位拉曼光谱发现:在0.07-0.12GPa和0.31-0.53GPa两个压力区间,亚油酸的拉曼谱出现了显著的变化,证明在这两个压力区间内亚油酸各发生了一次压致相变。当0.07-0.12GPa的第一次相变发生时,光谱分析显示亚油酸分子的亚甲基链由无序的邻位交叉式构象变为有序的反式构象。但第一个高压相还无法准确指认,暂命名为高压I相。当0.31GPa-0.53GPa的第二次相变发生时,光谱分析显示亚甲基链保持有序的反式构象,而烯烃基团的构象有明显变化,构象有序度上升。第二个高压相同样无法指认,命名为高压II相。亚油酸的压致相变是可逆的。与油酸相比,由于亚油酸分子中多了一个顺式C=C键,分子构象灵活度更高,故其固化压强降低,在高压下的相变途径存在较大的差别。五、亚油酸的低温相变通过测量并分析亚油酸在常压下20oC ~–70oC温度范围内的原位拉曼光谱发现:在0oC ~–10oC和–30oC ~–40oC两个温度区间,亚油酸的拉曼光谱出现显著的变化,证明了两次温致相变的发生。在0oC ~–10oC的相变发生时,光谱分析显示亚油酸分子的甲基侧链保留一定无序构象成分,羧基侧链变为有序反式构象,分子中心的大烯烃基团构象也有所改变。分析表明该相变是由液相转变为HT相,HT相的构象特征与油酸的结晶相比较接近。在–30oC ~–40oC的相变发生时,光谱分析显示分子的甲基侧链转变为有序反式构象,甲基链端则转变为有序的tt构象,而分子中心的大烯烃基团构象有较大的变化。分析表明该相变是由HT相直接转变为LT相,LT相的构象特征与油酸的γ结晶相比较相似。相变过程中并未出现文献中报道的中间相——MT相,可能是由于实验中的变温速率过大造成的。由于亚油酸的高压相和低温相的拉曼谱存在着较大的差异,无法将其对应起来。说明对于亚油酸来说,压缩效应和致冷效应存在更大的差别,这可能是由于亚油酸分子中C=C键数目的增多造成在极端条件下其分子构象的变化方式更加多样化。上述研究结果为深入了解油酸和亚油酸分子在不同压力、温度下的相变过程及分子相互作用等提供了有利的实验依据,为不饱和脂肪酸进一步的性质研究和未来应用研究奠定了实验基础,同时也对提高食品高压储藏工艺有一定的指导意义。依据以上实验结果,通过选择不同液体作为溶剂及改变溶液浓度,可以系统的研究溶剂效应对高压拉曼光谱频移速度的作用,进而了解分子特征(如分子结构差异、密度、压缩系数)和浓度变化对不同分子间相互作用的影响。更多还原

【Abstract】 High-pressure and low-temperature research can discover some new phenomenon which substances cannot exhibit at normal pressure and room temperature, disclose new rules and properties, and even find new substances. It provides new approaches for investigating substances’properties at high pressure and low temperature and explaining physical phenomenon at normal pressure and room temperature.With rapid developing of static high pressure measuring technology, diamond anvil cells (DAC) has been widely applied to the high-pressure study of many materials. Raman spectroscopy combined with the high-pressure technology can make us convenient to investigate the structural changes of materials under pressure, so the high-pressure in-situ Raman spectroscopy is one of the most useful high-pressure experimental technologies, and supplies a sensitive and effective probing method for investigating phase transition and chemical reaction of the substance in DAC.As an important kind of diet fatty acids, unsaturated fatty acids have special physiological functions. The application of ultra high pressure processing technology in food industry has wide perspective of development. As one of the most commonly appearing components of human diets, unsaturated fatty acids’phase transition study at high pressure and low temperature is very helpful to improve food preservation technology. On the other hand, the presence of unsaturated double bonds C=C in their molecules makes their phase transition very easy to occur at high pressure and low temperature. Therefore, study on their phase transition also has important theoretical values.It has been proved that oleic acid and linoleic acid undergo phase transition under pressure, but so far no high-pressure Raman spectroscopy or any other in-situ experimental research on their pressure-induced phase transition has been reported, nor has accurate transition point or the structural characterization of their high- pressure phase. Thus, in this thesis, to solve above questions, with two kinds of unsaturated fatty acids (oleic acid and linoleic acid) as study objects, we have investigated their phase transition at high pressure by DAC high-pressure technology combined with in-situ Raman spectroscopy. On the other hand, to explore molecular interaction and solvent effect under high pressure, the high-pressure Raman spectra of oleic acid solution were measured. Besides, the comparison of liquids’crystalline phases obtained by high pressure and by low-temperature crystallization is an important direction of high pressure research. We also studied the phase transitions of these two unsaturated fatty acids at low temperature by in-situ Raman spectroscopy associated with Linkam variable-temperature experimental system, in comparison with their pressure-induced phase transition.The main contents and conclusions as follows:(1) Pressure-induced phase transition in pure oleic acid.By measuring and analyzing the in-situ high-pressure Raman spectra of pure oleic acid up to 0.84 GPa at room temperature, it is found that in 0.29-0.36 GPa pressure range, significant changes in Raman spectra of oleic acid have been observed, including the disappearance of some modes, appearance of new modes, and sudden changes in the slope of the frequency–pressure curves of other modes. These changes prove that oleic acid undergoes a pressure-induced phase transition in this pressure range. With the further increase in pressure, no evidence of another phase transition can be concluded, so only one high-pressure phase is present below 0.84 GPa.In the high-pressure phase, the polymethylene chains exhibit the ordered all-trans conformation with the methyl end of the chains exhibiting the ordered tt chain-end conformation and the olefin group taking the skew-cis-skew′conformation. The conformational characters indicate that the high-pressure phase of oleic acid is the same as the low-temperatureγcrystalline phase. Hence the pressure-induced phase transition in oleic acid is from liquid phase toγcrystalline phase. For the first time the high-pressure of oleic acid was accurately recognized.This phase transition is reversible and of first order.(2) Pressure-induced phase transition in oleic acid in solution.By measuring and analyzing the in-situ high-pressure Raman spectra of oleic acid in 25% (V/V) solution up to 0.84 GPa at room temperature, it is found that in 0.15-0.20 GPa and 0.28-0.40 GPa pressure ranges, significant changes in Raman spectra have been observed, including the disappearance of some modes, appearance of new modes, and sudden changes in the slope of the frequency-pressure curves of other modes. These changes indicate that oleic acid in solution undergoes two phase transitions in the two pressure ranges mentioned above.During the first phase transition in 0.15-0.20 GPa pressure range, spectral analysis indicates that in oleic acid molecule the methyl end of the chains transforms to the ordered tt conformation. The ordered trans conformers appear in the polymethylene chains on the both sides of C=C bond, but a certain number of gauche segments remain in the methyl-sided chain. According to its spectral features and conformational characters, the first high-pressure phase was recognized as crystalline phase. Therefore, for oleic acid in solution the first pressure-induced phase transition is from liquid phase to crystalline phase, in which the degree of molecular conformational order is higher.During the second phase transition in 0.28-0.40 GPa pressure range, spectral analysis indicates that in oleic acid molecule the olefin group transforms from skew-cis-trans to skew-cis-skew′conformation and methyl-sided chain takes to ordered all-trans conformation. According to its spectral features and molecular conformational characters, the second high-pressure phase was recognized asγcrystalline phase. Therefore, for oleic acid in solution the second pressure-induced phase transition is from crystalline phase toγcrystalline phase, in which the degree of molecular conformational order is higher.Oleic acid in solution has lower solidifying pressure than pure oleic acid. There is one more intermediate phase ( crystalline phase) appearing in the transition of oleic acid in solution and its transition path is different from pure oleic acid, which probably has some connection with the washer function of CCl4 molecule on the molecular dimer of oleic acid.(3) Phase transition at low temperature in pure oleic acid.By measuring and analyzing the in-situ Raman spectra of oleic acid in–14 oC ~ 18 oC temperature range at normal pressure, it is found that in–4 oC ~ 0oC and 12 oC ~ 16 oC temperature ranges, significant changes in Raman spectra have been observed, which proves the occurrence of two temperature-induced phase transitions in these two temperature ranges.During the first phase transition in–4o C ~ 0 oC temperature range, in oleic acid molecule the olefin group transforms from skew-cis-skew′to skew-cis-trans conformation. In the methyl-sided chain the number of the ordered trans conformers decreases but the number of gauche segments increases, suggesting a decrease in the degree of conformational order. This phase transition is fromγcrystalline phase to crystalline phase and an order-disorder type.Obviously the transition path of oleic acid at high pressure is different from that at low temperature, indicating that for oleic acid compression effect is not completely the same as cooling effect.(4) Pressure-induced phase transition in linoleic acidBy measuring and analyzing the in-situ high-pressure Raman spectra of linoleic acid up to 1.29 GPa at room temperature, it is found that in 0.07-012 GPa and 0.31-0.53 GPa pressure ranges, significant changes in Raman spectra of oleic acid have been observed, which proves that linoleic acid undergoes two pressure-induced phase transitions in these two pressure ranges.During the first phase transition in 0.07-0.12 GPa pressure range, spectral analysis indicates that in linoleic acid molecule the polymethylene chains transform from disordered gauche conformation to the ordered trans conformation and the degree of molecular conformational order increases. However, the first high-pressure phase of linoleic acid has not been recognized and called after high-pressure I phase. During the first phase transition in 0.31-0.53 GPa pressure range, in linoleic acid molecule the polymethylene chains remain the ordered trans conformation whereas the conformation of the olefin group significantly changes and the degree of conformational order increases. Similarly the second high-pressure phase of linoleic acid has not been recognized and called after high-pressure II phase.The pressure-induced phase transitions in linoleic acid are reversible. In Comparison with oleic acid, because of the presence of one more C=C bond, the molecular conformation of linoleic acid is more flexible, which results in the lower solidifying pressure and different transition process.(5) Phase transition at low temperature in linoleic acid.By measuring and analyzing the in-situ Raman spectra of oleic acid in 20 oC ~–70 oC temperature range at normal pressure, it is found that in 0 oC ~–10 oC and–30 oC ~–40 oC temperature ranges, significant changes in Raman spectra have been observed, which proves the occurrence of two temperature-induced phase transitions in these two temperature ranges.During the first phase transition in 0 oC ~–10 oC temperature range, in linoleic acid molecule the carboxyl-sided chain transforms to ordered trans conformation but a certain number of disordered conformational segments remain in the methyl-sided chain. The conformation of the large olefin group in the centre of molecule changes a little. This phase transition is from liquid phase to HT phase, which’s conformational characters are similar to that of the crystalline phase of oleic acid.During the first phase transition in 30 oC ~–40 oC temperature range, in linoleic acid molecule the methyl-sided chain transforms to ordered trans conformation, the methyl end of chain transforms to ordered tt conformation, and the conformation of the large olefin group in the centre of molecule also significantly changes. This phase transition is directly from HT phase to LT phase, which’s conformational characters are similar to that of theγcrystalline phase of oleic acid. During this transition there is not appearing the intermediate phase (MT phase) reported in literature, which is caused by the too large poikilothermal speed in experiment.Due to the large difference between the Raman spectra of high-pressure and low-temperature phase of linoleic acid, we cannot find their corresponding relationship. This suggests that compression effect is more different from cooling effect for linoleic acid, which maybe relate with that the presence of one more C=C bond in linoleic acid molecule leads to the various changing ways of molecular conformation under extreme conditions.In summary, the above study results provide helpful experimental evidences for deeply learning the transition progress and the molecular interactions of oleic acid and linoleic acid at different pressures or temperatures. It lays an experimental foundation for further study on the property and prospective application of unsaturated fatty acids and also has directive significance for the improvement of food preservation technology under pressure.According to the experimental results, we can systematically study the solvent effect on the frequency-shift speed of high-pressure Raman spectra by choosing different liquids as solvent and changing the concentration of solution. Furthermore, we can explore the influence of molecular characters (such as structural differences, density and compressibility coefficient) and the change of concentration on the interactions between different molecules.更多还原