Resumen de On the polymorphism and structural characterization in the family of even saturated carboxylic acids
The polymorphism, structural and morphological characterization of the family of even saturated normal carboxylic acids between C10H19O2H and C2sH3s02H Is discussed within this work. The family of carboxylic acids is a multipolymorphic system; each member can adopt up to seven different crystalline forma at room temperatura: A,upar(Pl, Z=6), Aa (Pl, 2=2), E0 (Poca, 2=8), Ba (Pbca, 2=8), Em (P21/c, Z=4), Bm (P21/c, 2=4) and C (P21/c, 2=4). The "low-temperature" phases undergo a solid-solid transition before the melting point to render the "high-temperature" form called the C form. The thermodynamically stable phase at room temperature is the Asuper; at high temperatura the stable one is the C form. To rationalise the crystal structures of each polymorph the general methodology for crystal structure determination from powder X-ray diffraction data has been improved by taking into account the energy of the system. In addition, the advantages and disadvantages of using third generation radiation sources are evaluated to conclude that synchrotron sources do not necessarily improve the results obtained in terms of accuracy of the atomic positions. The structural analyses on carboxylic acids reveáis that, in general, the different solid phases are constituted by dimers in a r22 (8) arrangement and these dimers are packed into bilayers giving rise to pure methyl and pure carboxyl group interfaces, except for the Asuper form in which the packing is rather different. The morphology of these crystalline materials is rationalised on the basis of their crystal structures and different approaches to predict crystal morphology are investigated.
