Polymorphism at hexadecanoic-acid crystals investigated through structural and vibrational studies

Hexadecanoic acid (HA) is a saturated fatty acid with wide industrial application due to its important chemical and physical properties. However, understanding of its polymorphic characteristics under a specific growth condition is still an open question. This paper shows HA crystals grown by the slow evaporation method with five different solvents. The polymorphism was evaluated by X-ray diffraction, Raman and Fourier-transform infrared (FT-IR) spectroscopy. Most of the crystals presented two main polymorphic phases, and the C polymorphic form common among samples appeared as a progressive increase with increasing solvent polarity. Nevertheless, solvents with slower evaporation allowed to obtain the Bm form as a minor phase. Furthermore, polar solvents with rapid evaporation allowed the growth of the Em form as a secondary phase. Vibrational analysis on the Raman- and IR-active bands indicated that almost all inter-molecular modes are more likely to gauche conformation than all-trans conformation. This behavior is due to changes in the hydrogen bonds (O−H.O) and, mainly, in the C-C bonds around the carboxylic group (COOH) that form the dimers between the pairs of molecules within the crystalline lattice.