Estudo de espalhamento Raman nos ácidos palmítico e esteárico: forma C

Abstract

In this work vibrational properties of crystals of palmitic and stearic acids have been studied by Raman spectroscopy. Initially we were able to crystallize the fatty acids using the method of slow evaporation of the solvent. Two solvents with di®erent levels of polarity (ethanol and chloroform) were used at two di®erent controlled temperatures at » 16 ±C and another at » 0 ±C. Thus, we obtained single crystals with two polymorphic phases known in the literature as Bm and C forms, both with monoclinic symmetry of space group P21/a (C5 2h) with Z=4, which were identi¯ed by technique of X-ray di®raction. In order to evaluate the thermodynamic stability of the crystals obtained in this study, we measured Raman spectra at room conditions, low temperatures and high pressures. Firstly, in order to determine the vibration normal modes of the crystals of stearic and palmitic acids both in the C form, experiments were performed using polarized Raman spectroscopy at room conditions in two di®erent scattering geometries, in this case, Z(YY)Z and Z(XX)Z at the spectral range between 30 and 3000 cm¡1. The mode assignment of the observed bands associated to the Raman active modes was done based on Raman data correlations already published for compounds such as stearic, palmitic and oleic acids among others. Secondly, the behavior of the polarized Raman spectra of the stearic acid crystal in C form was studied as function of the temperature from room temperature down to 8 K in the 30-3000 cm¡1 spectral region. From the changes observed in several spectral regions on cooling, it was noticed at least two phase transitions undergone by the crystal: a ¯rst transition is related to change of the point group C2h to C2 (210-180 K); and the second transition (30-8 K) from the latter group to another corresponding to a higher symmetry. Additionally, we analyzed the evolution of the Raman bands of the crystal of stearic acid in the C form by varying the pressure from 0.0 GPa up to about 9.5 GPa in three di®erent spectral regions: 25-200 cm¡1, 800-1200 cm¡1 and 2800-3100 cm¡1. With increase of the pressure, various modi¯cations have been observed in both the external and the internal modes and they were understood as follows: from 0.0 up to » 2.5 GPa, the crystal presents conformational changes; for values above 3.3 GPa the crystal undergoes one ¯rst order phase transition; and between 3.3-3.8 GPa, it undergoes another phase transition of conformational nature. Additionally, Raman spectroscopy studies on palmitic acid crystal in the C form at high pressures have been performed in the spectral region between 25 and 3120 cm¡1, by varying the pressure from 0.0 up to 21.0 GPa. Pronounced modi¯cations were observed in all regions of the measured spectra, especially in the region of the lattice modes. Such modi¯cations, are possibly associated with a series of phase transitions undergone by the crystal that can be understood as ¯rst and second order transitions that occurs near (or within) the following pressure intervals [0;1.0] GPa,[3.0;5.5] GPa and [5.5;9.7] GPa. Furthermore, anomalies are observed in internal modes at about 14 GPa; they were associated with conformational changes and possibly are related with rotations of the molecule. Some modi¯cations above of 18.0 GPa were considered as beginning of a process of loss of crystallinity, although the process allows the crystal recovers its original structure at atmospheric pressure.