V. A. Drebushchak

Polymorphism of glycine, Part II

The contribution summarizes the results of a systematic study of the three glycine polymorphs (a, b, g-forms), including: i) the controlled crystallization of a desirable form, ii) a comparative calorimetric study of the three forms in the temperature range between 5 K and the sublimation temperatures (»500 K).

The investigation of ancient pottery

SummaryAncient ceramic samples (single fragments and different parts of pots, unbroken and repaired; total about 180 samples) dated from the transitional period of late Bronze to early Iron Age (VIII-VI centuries BC) and early Iron Age (VII-IV centuries BC) were investigated by thermal analysis, X-ray powder diffraction, petrography, and scanning electron microscopy equipped with the energy-dispersive X-ray analyzer. In addition to that, to identify the clay sources for the ceramic manufacturing, about 15 samples of clays and soils found near archeological digs and taken from the mineralogical museum were investigated. We found out that the calcite content of ceramics is a very informative parameter for the identification of the clay source for the pottery manufactured at low technological level (low-temperature firing).

Low-temperature heat capacity of α and γ polymorphs of glycine

Low-temperature heat capacity of two polymorphs of glycine (α and γ) was measured from 5.5 to 304 K and thermodynamic functions were calculated. Difference in heat capacity between polymorphs ranges from +26% at 10 K to -3% at 300 K. The difference indicates the contribution into the heat capacity of piezoelectric γ polymorph, probably connected with phase transition and ferroelectricity. Thermodynamic evaluations show that at ambient conditions γ polymorph is stable and α polymorph is metastable.

Decreasing particle size helps to preserve metastable polymorphs. A case study of DL-cysteine

The contribution describes the effect of particle size on the interconversion between the high-temperature (I) and the low-temperature (II) polymorphs of crystalline DL-cysteine (+NH3–CH(CH2SH)–COO−) studied by X-ray powder diffraction, Raman spectroscopy, differential scanning calorimetry, scanning electron microscopy. Crystalline DL-cysteine undergoes a phase transition on cooling related to the rotation of its side-chain and a rearrangement of the H-bond network. For the crystals larger than ∼1 μm, the transition could be observed in the range 250–200 K and was characterized by a large (up to 110 K) hysteresis. If (I) was obtained as crystalline particles with characteristic size of ∼1 μm by grinding, no transformation into (II) was observed on cooling down even to 10 K. (I) was preserved down to 200 K, after which another low-temperature phase, (I)′, appeared, which was structurally related to (I), but with strongly disordered (possibly modulated) sulfhydryl side-chains. Nevertheless, (II) could be prepared as small (∼0.1 to 1 μm) particles directly at low temperatures by freeze-drying of DL-cysteine aqueous solutions. The sample could be then preserved as a metastable low-temperature polymorph on heating up to ∼333 K, and transformed completely to (I) at 373 K only. The small particles of (I) obtained by heating from small particles of (II) then transformed completely back to (II) on cooling down to 183 K. The effects are interpreted in terms of kinetic control of the polymorphic transitions and the relation between particle size, nucleation of a new phase, and relaxation of mechanical stresses.

Thermoanalytical investigation of drug-excipient interaction. Part I. Piroxicam, cellulose and chitosan as starting materials

SummaryCellulose, chitosan and piroxicam were investigated by TG and DSC at heating up to 215°C, and by X-ray powder diffraction before and after the heating. Dehydration of cellulose and chitosan comes to the end near 160°C. Thermal decomposition of chitosan starts at the final stage of its dehydration, and the mass losses after these two reactions overlap with one another. Enthalpy of dehydration is 47.1±2.4 kJ mol-1of water for cellulose and 46.2±2.0 kJ mol-1for chitosan. Thermal decomposition of chitosan is an exothermic process. Crystal structure of cellulose after heating remains unchanged, but that of chitosan contracts. Piroxicam melts at 200.7°C with the enthalpy of melting 35 kJ mol-1. Heat capacity of the liquid phase is greater than that of the solid phase by approximately 100 J mol-1 K-1. Cooled back to ambient temperature, piroxicam remains glassy for a long time, crystallizing slowly back into the starting polymorph.

Mechanochemical Synthesis of Nanocomposites of Drugs with Inorganic Oxides

The nanocomposites of piroxicam and indomethacin with fine porous inorganic oxides, alumina, silica, and magnesia, were obtained by high energy ball milling. Except for the piroxicam–alumina system, the composites revealed higher dissolution rate and solubility of the drugs as compared to the initial ones. The changes in the IR spectra suggested the interaction of the components during ball milling. It was assumed that the formation of new bonds at the contacts of particles in the composite resulted in stabilization of drugs in a metastable state inhibiting their transition into the initial crystalline form.

New interpretation of heat effects in polymorphic transitions

Overlapping endothermic and exothermic effects in DSC measurements of polymorphic transitions is often detected in molecular crystals and drugs. It is explained by the sequence of melting and crystallization. In this paper, we argue that this explanation is incorrect. In revealing the kinetic nature of the endo/exo thermal effect, we suggest another explanation, based on the nucleation. New interpretation does allow us to measure the energetic barrier in the nucleation of bulk sample, thus providing a tool for testing the nucleation models.

Phase transition at 204–250 K in the crystals of β-alanine: kinetically irreproduceable, or an artefact?

The crystals and the powder samples of β-alanine were studied in the wide temperature range by adiabatic calorimetry, differential scanning calorimetry, IR- and Raman spectroscopy and X-ray diffraction. No phase transitions could be observed. A small anomaly observed at about 256 K in the Cp(T) dependences measured for the samples re-crystallized from water was shown to be due to the presence of a small (about 0.14% of the total sample mass) amount of solvent inclusions. This anomaly was not observed in the solvent-free samples, either powders or single crystals.

Decay of (Fe1−xNix)0.96S DSC investigation

AbstractThe decay of a monosulphide solid solution (mss) with the composition (Fe1−1Nix)0.96S was investigated by means of differential scanning calorimetry in the temperature range, from 20 to 305‡C. Thermal effects of various natures were detected:i)Ordering-disordering in the Fe-Ni sublattice near 100‡C.ii)Pentlandite exsolution (exothermal peak); the peak temperature varies from 180 to 240‡C and depends on the initial composition; the higher the Ni content, the lower the exsolution temperature.iii)Magnetic-paramagnetic transition. The transition temperature decreases down to 220‡C as the Fe∶Ni ratio is decreased from 10∶0 to 4∶6. Ni atoms are the defects in the magnetic ordering of themss generated by the Fe atoms in the metal sublattice. Thus, the driving force for pentlandite exsolution is the removal of Ni atoms from the magneticmss into the nonmagnetic pentlandite. This is the reason why the Fe∶Ni ratio in the generated pentlandite is much higher than that in the initialmss.

Thermophysical theory of DSC melting peak

Melting peak for metals is described with expressions derived from thermophysical consideration of DSC operation. Three parameters govern the shape of the peak: thermophysical coefficient derived from the DSC design, enthalpy of fusion of a sample, and heating rate. Rigorous evaluation yields rather complex expressions, but simplified expressions can be used in common practice. The peak shape is described by two different expressions for two separate stages in the process of metal melting (1) the melting itself and (2) heat relaxation after the melting completion. The validity of the expressions was demonstrated after the experiments on gallium melting. The thermophysical coefficient is shown be affected to small variations by the changes in sample preparation or experimental conditions (melting Ga, In, Zn).