Heat Capacities of α-, β-, and γ- Polymorphs of Glycine.

As a part of our effort to establish reliable thermodynamic data for amino acids, the heat capacity and phase behavior are reported for two stable polymorphs (α and γ) of glycine (aminoacetic acid, CAS RN: 56-40-6). Prior to heat capacity measurement, thermogravimetric analysis and X-ray powder diffraction were performed to determine decomposition temperatures and initial crystal structures, respectively. The literature heat capacities obtained by adiabatic calorimetry are available in the temperature interval (7-304). The literature data were used for validating performance of our relaxation (heat-pulse) calorimeter, which was used for measurement of the heat capacity of α-glycine in the temperature interval (2-267) K. The crystal heat capacities of the α- and γ-glycine were extended towards higher temperatures using Tian-Calvet calorimetry in the temperature interval (262-358) and power compensation DSC in the temperature interval (310-449) K. As a result, reference heat capacities and thermodynamic functions for the crystalline phase from 0 K up to 450/435 K for α/γ-glycine were developed. The literature heat capacities for β-glycine over the range 0 K to 295 K were treated in the same manner in order to provide thermodynamic data for all three polymorphs existing at the atmospheric pressure.