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Crystallization of carboxylic acids as studied by NMR Spectrometry

The temperature dependences of the narrow 1H NMR signals, in the temperature range below phase transition point from the plastic to brittle crystal for pivalic acid (PA) confined in the controlled pore size glasses with mean diameters of 7.9 and 23.9 nm have been investigated in order to determine the depression of the crystallization temperature and the pore size distribution parameters. The obtained parameters and the temperature dependences of the narrow 1H NMR signals in the solid phases of PA and other carboxylic acids (hexadecanoic (C16), heptadecanoic (C17), octadecanoic (C18), nonadecanoic (C19)) have been used for studying the pore size distributions appeared during the natural crystallization. It was found that the pore size distribution for the even-numbered acids (C16 and C18) is spread over much wider range of the sizes comparing with those for odd-numbered acids (C17 and C19). The maximal value of the pore radius R = 40.4 nm has been found at the temperature just below the melting point of C18, which is about eight times bigger than that found for C17.