MORPHOTROPISM, POLYMORPHISM AND BEHAVIOR ON HEATING OF ODD MONOCARBOXYLIC ACIDS CnH2nO2 IN THE HOMOLOGOUS SERIES n = 11–21
Y. M. Trushkina, E. N. Kotel'nikova
St. Petersburg State University, St. Petersburg, Russia
Keywords: monocarboxylic acids, odd fatty acids, polymorphism, morphotropism, thermal phase transitions, express test, powder X-ray diffraction, variable temperature X-ray diffraction
Abstract
X-ray diffraction characteristics are obtained for odd monocarboxylic acids (fatty acids) CnH2nO2 with n = 11, 13, 15, 17, 19, and 21. It is found that all original samples (reagents) of these acids are two-phase. There are found the following modifications: for the acid with n = 11 triclinic Tc (space group P–1) and monoclinic M3 (space group P21/c); for n = 13 triclinic Tc (space group P–1) and bilayered monoclinic 2 M (space group A2/ a); for n = 15, 17, 19, 21 two monoclinic M1 (space group P21/a) and M3 (space group P21/с). It is demonstrated that the homological series and individual homologues of fatty acids manifest noticeable crystal chemical features such as morphotropism and polymorphism, respectively. Thermal phase transformations of odd fatty acids are studied using pentadecanoic acid C15H30O2 as a representative example. It is found that near the melting point (52 °C) within the temperature range of 45–50 °C the monoclinic phase M1 of this acid undergoes a polymorphic transformation in the high-temperature bilayered monoclinic phase 2 М. The data obtained are compared to those reported in the literature. An express standardless test is developed for the determination of homologues (the n value) and polymorphic modifications of monocarboxylic acids.
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