Polymorphism with Conformational Isomerism and Incomplete Crystallization in Solid Ethanolamine

Abstract

We employ calorimetry, Rietveld refinement of X-ray powder diffraction, broadband dielectric spectroscopy, and molecular dynamics simulations to investigate the phase behavior, crystal structure, dc conductivity, and dielectric response of the condensed phases of ethanolamine (EAM), an ethane derivative with two distinct hydrogen-bonding groups. EAM is found to exhibit, besides its stable crystal phase, a metastable crystalline phase obtained from the supercooled liquid. The metastable phase can only be maintained at low temperature; upon heating, an irreversible exothermic transformation to the stable crystal structure takes place at 210 K, with a low transition enthalpy of ca. 2.4 J/g. The observed polymorphism is accompanied by differences in the molecular conformation and degree of structural order. Each EAM molecule participates in six hydrogen bonds in both crystalline phases. While the stable phase is fully ordered, the metastable phase appears to be characterized by a statistical disorder in the p...