Understanding the effect of solvent polarity on the polymorphism of
octadecanoic acid through spectroscopic techniques and DFT calculations
CRYSTENGCOMM, 21 (2):
297-309 (2019)DOI: 10.1039/c8ce01402g
Abstract
In this work, the polymorphism of octadecanoic acid (C18H36O2, stearic
acid) was studied as a function of organic solvent polarity.
Polycrystalline samples of stearic acid were grown via a slow
evaporation method using five different solvents (hexane, heptane,
toluene, dichloromethane and methanol). The polymorphic forms were
investigated by X-ray diffraction (XRD) and two polymorphs were
identified: C (monoclinic) and B-m (monoclinic). The cell parameters of
the crystalline phases were obtained from refinement using the Rietveld
method, and their quantitative composition was estimated as a function
of the solvent polarity. The C form is the dominant structure for
crystals grown in hexane, heptane and toluene, while the B-m form is
dominant for crystals grown in dichloromethane and methanol. In
addition, Raman and Fourier-transform infrared (FT-IR) spectroscopy
combined with differential scanning calorimetry (DSC) were also used to
analyze the octadecanoic acid crystalline phases under the effect of the
solvent polarity and corroborated well the XRD studies. Besides, Raman
and infrared spectra were calculated via density functional theory to
elucidate the polymorphic behavior of octadecanoic acid which presented
a good agreement with experimental results. The DSC measurements
provided information on the thermodynamic behavior of the polymorphism
phenomenon and demonstrated that the C form of octadecanoic acid is the
most stable form at high temperatures.