The polymorphism of glycine - Thermochemical and structural aspects

X-ray, DSC and solution calorimetric investigations were carried out for alpha-, beta- and gamma-modifications of glycine. Particular attention was paid to kinetic and thermochemical aspects of gamma- --> alpha-phase transition. The temperature of this phase transition turned out to be sensitive to a) conditions under which the crystals of the gamma-modification were grown, b) tempering of crystals c) form (geometry) of crystals. Kinetics of this phase transition of single crystals of gamma-phase in rhomboedric form can be described by the equation for two-dimension nuclei growth, whereas for crystals of triangle geometry the equation for three dimension growth is valid. On the basis of energy parameters describing growth of alpha-form in gamma- --> alpha-phase transition, the kind of structure defects, which are responsible for this phase transition, was estimated. Taking into account the Delta(sol)H(m), the absolute values of the lattice energies of the investigated polymorphs in descending order are follows: gamma->alpha-->beta-modification. The obtained results are discussed with respect to the peculiarity of the crystal lattice structures, particularly the network of hydrogen bonds. The beta-modification of glycine is monotropically related to the other forms, whereas gamma- and alpha-polymorphs are enantiotropically-related phases.