Pressure‐induced phase transitions in palmitic acid: C form

Abstract

In this article, we report a high-pressure Raman spectroscopy study of palmitic acid (PA, C form) from ambient pressure up to 21 GPa. The effects of hydrostatic pressure on the vibrational spectrum of PA are reported, and the data show that PA experiences a rich sequence of phase transformations. These changes in the crystal structure occur gradually as the pressure increases and they are related to the highly flexible crystalline structure.

Stability of lauric acid at high pressure studied by Raman spectroscopy and picosecond acoustics

“…1475 cm −1 band are observed. The global behaviour is very similar to that reported for palmitic and stearic acids [37,21]. As a conclusion, three phase transitions are observed in Raman spectra, namely at 1, 5 and 12 GPa (see Table 3.2).…”

“…The assignments of the Raman modes can be done with the help of literature data on other fatty acids such as palmitic acid [21]. Since lauric acid samples used in this study are polycrystalline, all the vibrational modes should be observed.…”

“…C18 octadecanoic acid with (CH 2 )16 chain) have already been studied by Raman spectroscopy in a DAC at room temperature [37,23]. Four phase transitions at 1, 6, 12 and 16 GPa have been identified by Raman spectoscopy in palmitic acid [37] and detailed analysis of stearic acid crystals pointed out that 3 phases can be identified below 4 GPa [21]. The raw data spectra, and the associated bands, are shown in Figs.…”

“…Phase transitions were, nevertheless, detected in other solid fatty acids : in palmitic acid [21], in oleic acid [22], or in linoleic acid [23]. However, these phase transitions are detected only in Raman scattering.…”

Low-temperature phase transformation studies in the stearic acid: C form

“…In fact, it is clearly noticeable that an increase in the intensity of the 79 cm À1 band during the cooling accompanied with a large Raman shift (Dx $ 51 cm À1 ), must be a consequence of temperature-induced anharmonic effects. Such a non-linear effect is probably due to the presence of weak hydrogen bonds [34,[38][39][40][41][42][43][44][45]. Fig.…”