Nestor Veglio

Polymorphism of progesterone: Relative stabilities of the orthorhombic phases I and II inferred from topological and experimental pressure‐temperature phase diagrams

Temperatures and melting enthalpies of orthorhombic Phases I and II of natural progesterone, together with the temperature dependence of their lattice parameters and the specific volume of the melt at ordinary pressure, have been determined. With these results, a topological pressure-temperature (P-T) phase diagram accounting for the thermodynamic relationships between these phases has been constructed by way of the Clapeyron equation. The dependence of the melting temperature on the pressure has also been determined for each phase by high-pressure differential thermal analysis. It was found that, upon increasing the pressure, the melting curves converge to the I-II-liquid triple point (T(I-II-liquid) = 459.4 K, P(I-II-liquid) = 149.0 MPa), in close agreement with its topological location. This entails that Phase II should exhibit a stable phase region at higher pressure.

Overall Monotropic Behavior of a Metastable Phase of Biclotymol, 2,2′-Methylenebis(4-Chloro-3-Methyl-Isopropylphenol), Inferred From Experimental and Topological Construction of the Related P-T State Diagram

The melt from the usual monoclinic phase (Phase I) of biclotymol (T(fusI) = 400.5 +/- 1.0 K, Delta(fus)H(I) = 36.6 +/- 0.9 kJ mol(-1)) recrystallizes into another phase, Phase II, that melts at T(fusII) = 373.8 +/- 0.2 K (Delta(fus)H(II) = 28.8 +/- 1.0 kJ mol(-1)). The transformation of Phase II into Phase I is found to be exothermic upon heating either as a direct process at 363 K or through a melting-recrystallization process (II --> liquid --> I). The melting curves, obtained from differential thermal analyses at various pressures ranging from 0 to 85 MPa, diverge as the pressure increases ((dP/dT)(fusI) = 2.54 +/- 0.07 MPa K(-1), (dP/dT)(fusII) = 5.14 +/- 0.85 MPa K(-1)). A topological P-T diagram with no stable phase region for Phase II, and similar to the 4th case of the P-T state diagrams formerly published by Bakhuis Roozeboom, is drawn, thus illustrating the overall monotropic behavior of Phase II.

Polymorphism of CBr2Cl2

The polymorphism of dibromodichloromehtane (CBr2Cl2) has been studied by means of a wide set of experimental techniques as a function of temperature and pressure. From the p–V–T diagram and the derived p–T diagram the volume variations at the transition points have been calculated and compared with those obtained by means of X-ray and neutron powder diffraction. By combining the experimental techniques, it has been demonstrated the existence of a high-pressure orientationally disordered rhombohedral phase (aR ≈ 14.6 ± 0.3 A and α ≈ 89.2 ± 0.2°). The existence of a glass transition within the monoclinic (C2/c, Z = 32) low-temperature ordered phase associated with the freezing of exchange positions between Cl and Br atoms is analyzed considering the asymmetry of the intermolecular interactions by means of the study of the thermal expansion tensor. The change of the derivative of the aspherism index, previously reported as a possible “fingerprint” for this kind of glass transitions, is found.

From the two-component system CBrCl3+CBr4 to the high-pressure properties of CBr4.

The experimental phase diagram of the CBrCl3+CBr4 system has been determined by means of X-ray powder diffraction and thermal analysis techniques from 200 K to the liquid state. Before melting, the two components have the same orientationally disordered (OD) face-centered cubic phase, and solid-liquid equilibrium is explained by simple isomorphism. The application of multiple crossed isopolymorphism formalism to the low-temperature solid-solid equilibria has enabled the inference of an OD rhombohedral metastable (at normal pressure) phase for CBr4. Experimental determination of the pressure-volume-temperature and construction of the pressure-temperature phase diagrams for CBr4 reveal the existence of a high-pressure phase, the rhombohedral symmetry of which is inferred by means of the thermodynamic assessment of the experimental phase diagram and demonstrated by means of high-pressure neutron diffraction measurements. The procedure used in this work confirms the connection between the appearance of metastable phases at normal pressure and their existence at high-pressure.

Stability hierarchy between Piracetam forms I, II, and III from experimental pressure–temperature diagrams and topological inferences

The trimorphism of the active pharmaceutical ingredient piracetam is a famous case of polymorphism that has been frequently revisited by many researchers. The phase relationships between forms I, II, and III were ambiguous because they seemed to depend on the heating rate of the DSC and on the history of the samples or they have not been observed at all (equilibrium II-III). In the present paper, piezo-thermal analysis and high-pressure differential thermal analysis have been used to elucidate the positions of the different solid-solid and solid-liquid equilibria. The phase diagram, involving the three solid phases, the liquid phase and the vapor phase, has been constructed. It has been shown that form III is the high-pressure, low-temperature form and the stable form at room temperature. Form II is stable under intermediary conditions and form I is the low pressure, high temperature form, which possesses a stable melting point. The present paper demonstrates the strength of the topological approach based on the Clapeyron equation and the alternation rule when combined with high-pressure measurements.

Liquid–liquid miscibility gaps and hydrate formation in drug–water binary systems: Pressure–temperature phase diagram of lidocaine and pressure–temperature–composition phase diagram of the lidocaine–water system

The pressure-temperature (P-T) melting curve of lidocaine was determined (dP/dT = 3.56 MPa K(-1)), and the lidocaine-water system was investigated as a function of temperature and pressure. The lidocaine-water system exhibits a monotectic equilibrium at 321 K (ordinary pressure) whose temperature increases as the pressure increases until the two liquids become miscible. A hydrate, unstable at ordinary pressure, was shown to form, on increasing the pressure, from about 70 MPa at low temperatures (200-300 K). The thermodynamic conditions of its stability were inferred from the location of the three-phase equilibria involving the hydrate in the lidocaine-water pressure-temperature-mole fraction (P-T-x) diagram.

New evidence for a liquid?liquid transition in a one-component liquid

Based on specific heat, x-ray diffraction and nonlinear dielectric effect (NDE) measurements the evidence for a liquid–liquid (L–L) transition in trans-1,2-dichloroethylene at TL−L≈Tm+23.6 K≈247 K is shown. The strong pre-transitional anomaly of NDE, similar to the one observed in the isotropic phase of nematic liquid crystals, makes it possible to estimate the weakly discontinuous character of the transition. Based on NDE measurements, the pressure evolution of the L–L transition up to almost 500 MPa was also obtained. Parameterization by the modified Simon–Glatzel-type equation made possible an insight into the negative pressure domain.

Comparison of short-range-order in liquid- and rotator-phase states of a simple molecular liquid: A reverse Monte Carlo and molecular dynamics analysis of neutron diffraction data

The short-range order (SRO) correlations in liquid- and rotator-phase states of carbon tetrachloride are revisited here. The correlation of some angular magnitudes is used to evaluate the positional and orientational correlations in the liquid as well as in the rotator phase. The results show significant similitudes in the relative position of the molecules surrounding a central one but striking differences in their relative orientations, which could explain the changes in SRO between the two phases and the puzzling behavior of the local density in the liquid and rotator phases.

A Procedure to Quantify the Short Range Order of Disordered Phases

Determination of the short- and intermediate-range structure of disordered materials is a necessary step to fully understand their properties. Despite of this, no generally accepted procedure exists to date to extract structural information from diffraction data. In this paper we describe a method which enables determination of the short-range structure of disordered molecular phases. This general method is applied to one of the first studied molecular liquids, carbon tetrachloride, and to its plastic phase being able to unravel the so called local density paradox: although molecules are closer in the liquid than in the plastic phase, the density of the former is lower than that of the later. The analysis of the short range order in both phases shows that although the minimal energy configuration allows a closer approach of molecules, it hinders the formation of the face centered cubic long range ordered lattice due to the difficulty of molecules to form stacked structures.