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 4000 similar to 400 cm(-1) and 30 similar to 100 degrees C. The C=O 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 C=O bond and O-H bond at temperature rise. Before the phase transition temperature, the absorption peak strength of C=O 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 C=O 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 C=O, which leads to the increase of the C=O 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.