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 phase. 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 H-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 position of the hydrogen atoms and thus of the intramolecular conformation and intermolecular hydrogen bonding. Our molecular dynamics simulations suggest that such disorder is dynamic in character, and allow analyzing the dynamic H-bonding motif of the metastable phase in terms of composition, length and energy of the H-bonds, and molecular conformations involved. Charge conduction is mainly ionic in both crystalline phases and exhibits the typical temperature dependence of disordered solid electrolytes, rather than crystalline ones. Dielectric characterization indicates that, upon crystallization of liquid EAM, a small fraction of the sample (approximately 5%) remains amorphous, a situation similar to that reported for other H-bonded systems such as glycerol and nbutanol.

Crystal Growth Kinetics of an Industrial Active Pharmaceutical Ingredient: Implications of Different Representations of Supersaturation and Simultaneous Growth Mechanisms

“…17,44,45 For instance, birth-and-spread relies on a highenough supersaturation to overcome the free energy penalty of creating a new step edge, 57 which is better supplied at higher antisolvent composition. Another possibility is that the dissolved solute molecule must overcome an energetic barrier whose magnitude varies with solution composition, as in, e.g., conformational isomerism, 58 causing steric obstacles for surface attachment. Alternatively, it is possible that at lower acetonitrile content desolvation is the rate limiting step or that a new API molecule would have to overcome an impurity or solvent component layer adsorbed on certain (fast growing) crystal surfaces to be incorporated in the crystal lattice.…”

Solid-solid Phase Transitions between Crystalline Polymorphs of Organic Materials

“…Because the temperature is constant, the enthalpy that is registered can be directly related to the entropy change of the equilibrium under study (i.e., ∆S = ∆H/T at equilibrium). Dielectric and conductivity measurements have been carried out on the two solid phases of ethanolamine 93 . During the irreversible transition of the metastable form II into form I, both the conductivity and the dielectric data changed.…”

“…During the irreversible transition of the metastable form II into form I, both the conductivity and the dielectric data changed. The metastable form contains static disorder, which could be observed by the dielectric loss parameters 93 .…”