Studies in solid state chemistry on a solid-state mechanism for the polymorphic transformation of orthorhombic to tetragonal PbO.

Abstract

In this work, different solvents and mixtures were used to prepare the polymorphs (α and β form) of azelaic acid by cooling crystallization at 308.15–263.15 K, including ethanol, propionic acid, n-butanol, methanol, and ethyl acetate. As a result, the α form was successfully prepared in propionic, while the β form was obtained in ethanol. Then the structures and morphologies of both polymorphs were characterized by various means. Moreover, the dissolving properties of the two forms in water, ethanol, and propionic acid from 263.15 K to 308.15 K were investigated by both experiments and molecular dynamic (MD) simulation for the first time. Results show that solubility of the two forms increase with temperature, which are well fitted by the Modified Apelblat model and NRTL model. What's more, the solubility of each form in ethanol is the highest compared to that in propionic acid and water. Particularly, the solubility of α polymorph is always higher than that of β polymorph from 280.15 K to 308.15 K indicating that the latter should be the stable form. The same results were also obtained through the calculation of molecular interaction in the solid, which are −5.02 kJ/mol of α form and −11.46 kJ/mol of β form. Finally, the diffusion coefficients and solvation free energies were calculated for the α and β conformations in solution, and the results reveal that high diffusion rate and solvation free energy seem stay the same with high solubility in different solvents.