二维红外光谱法对肉豆蔻酸相变过程的研究

脂肪酸由于成本低、相变潜热大、热稳定性好的特点，在有机固-液相变材料中应用较多。对脂肪酸进行热分析一般采用热重法（TG）或差示扫描量热法（DSC）得到材料宏观上的热力学性质，但难以对其微观结构变化进行深入探讨。二维红外光谱（2D-IR）在温度扰动的作用下，样品的光谱信号将随之发生动态变化。通过数学处理能够发现样品在相变过程中微观结构的变化。以肉豆蔻酸为例，采用傅里叶红外光谱仪，在4 000～400 cm-1和30～100 ℃温度范围内对肉豆蔻酸进行一系列红外光谱实验。采用二维移动窗口（MW2D）红外光谱技术，对肉豆蔻酸中的CO和O—H键进行分析，发现MW2D测出的肉豆蔻酸熔点与传统的DSC测出的基本一致，且两种化学键显示的热数据具有良好的一致性和稳定性。对光谱数据进行2D-IR分析，结果表明，由于分辨率的提高，一维光谱中单一的重叠吸收峰对应着二维光谱中的多个吸收峰，根据2D-IR的理论知识，推测可能存在二聚体肉豆蔻酸构型向单聚体肉豆蔻酸构型转变的情况。从峰强度和温度的变化关系中发现，升温时CO键和O—H键存在三个变化过程，达到相变温度之前，CO吸收峰强度基本不变，O—H吸收峰强度逐渐下降，说明O—H键偶极矩的变化比CO键更易受温度影响；相变过程中，两者吸收峰强度都显著减弱且O—H吸收峰强度下降幅度更大；达到相变温度之后，可能因O—H形成的分子间氢键，受热导致由强变弱，O—H上的电子云移向CO，导致CO吸收峰强度增大、O—H吸收峰强度减小。同时，结合密度泛函理论，对二维红外光谱的推论进行理论验证，可知存在二聚体肉豆蔻酸向单聚体肉豆蔻酸的转化过程。

Study on Phase Transition of Myristic Acid by Two-Dimensional Infrared Spectroscopy

Fatty acids have been used in organic solid-liquid transformation materials due to their low cost, high latent heat of phase transition and good thermal stability. Although thermogravimetry (TG) and differential scanning calorimetry (DSC) for fatty acids can obtain thermodynamic data, it is difficult to analyze structural changes. Under the temperature perturbation, the two-dimensional infrared spectrum (2D-IR) signal will change instantaneously, and the structure change can be found by mathematical processing. In this paper, the infrared spectra of myristic acid were tested at 4 000～400 cm-1 and 30～100 ℃. The CO bond and O—H bond were analyzed by a two-dimensional moving window (MW2D) infrared spectrum, and it was found that the melting point measured by MW2D was the same as that measured by DSC. The results of the 2D-IR analysis showed that the overlapping absorption peaks were separated due to the improvement of resolution, and it was speculated that there was a transition from dimer to monomer configuration of myristic acid. There are three changes in the CO bond and O—H bond at temperature rise. Before the phase transition temperature, the absorption peak strength of CO remained unchanged, while the absorption peak strength of O—H gradually decreased, indicating that the dipole moment change of the O—H bond was more susceptible to temperature than the CO bond. During the phase transition, both of the absorption peak intensity decreased significantly, and the absorption peak intensity of the O—H decreased greatly. After the phase transition temperature, it may be that the intermolecular hydrogen bond is heated from strong to weak, and the electron cloud on O—H moves to CO, which leads to the increase of the CO absorption peak intensity and the decrease of the O—H absorption peak intensity. At the same time, combined with density functional theory, the inferences of 2D-IR have been verified, and there exists a transformation process from dimer to monomer configuration of myristic acid.