Search Results

You are looking at 1 - 9 of 9 items for

  • Author or Editor: G. Perlovich x
  • All content x
Clear All Modify Search

Abstract  

Crystallisation is generally regarded as a nucleation — growth mechanism of a solid phase and often studied using thermo chemical methods. The present work postulates an analogy to melting processes, looking at melting as nucleation — growth of a liquid phase. The melting process of acetylsalicylic acid single crystals was investigated by DSC measurements under isothermal conditions. The fraction of material molten after a certain time period, α(t), was calculated by integrating the DSC curves. The resulting kinetic curves were fitted using the Avrami-Erofeev equation: –ln(1–α)=kt n, where parameter n was analysed. According to established methods, functions I('2')=[t('2')]/[t('2')+t('3')]100% and I('3')=[t('3')]/[t('3')+t('2')]100% were introduced, where t('2') and t('3') is the absolute time of consumption two- and three-dimension nuclei growth, respectively. Applying correlation analysis, relationships between two- or three-dimensional growth and the independent variables describing the single crystals (for strictly definite trajectories into the space of sizes) were found. Particular correlations were:a) Two-dimensional growth is a function of the total surface area of the crystal, S, and of the surface area of the (ac)-face, S ac; b) Three-dimensional growth is a function of S/M (where M is the mass of the single crystal). It is also a function of S ac/M and of S. The obtained experimental data are explained by the ‘layer’ structure of crystals of acetylsalicylic acid.

Restricted access

Polymorphism of paracetamol

Relative stability of the monoclinic and orthorhombic phase revisited by sublimation and solution calorimetry

Journal of Thermal Analysis and Calorimetry
Authors: G. Perlovich, Tatyana Volkova, and Annette Bauer-Brandl

Abstract  

The thermodynamic relationship between crystal modifications of paracetamol was studied by alternative methods. Temperature dependence of saturated vapor pressure for polymorphic modifications of the drug paracetamol (acetaminophen) was mea sured and thermodynamic functions of the sublimation process calculated. Solution calorimetry was carried out for the two modifications in the same solvent. Thermodynamic parameters for sublimation for form I (monoclinic) were found: ΔGsub298=60.0 kJ mol−1; ΔHsub298=117.9�0.7 kJ mol−1; ΔSsub298=190�2 J mol−1 K−1. For the orthorhombic modification (form II), the saturated vapor pressure could only be studied at 391 K. Phase transition enthalpy at 298 K, ΔHtr298(I→II)=2.0�0.4 kJ mol−1, was derived as the difference between the solution enthalpies of the noted polymorphs in the same solution (methanol). Based on ΔHtr298 (I→II), differences between temperature dependencies of heat capacities of both modifications and the vapor pressure value of form II at 391 K, the temperature dependence of saturated vapor pressure and thermodynamic sublimation parameters for modification II were also estimated (ΔGsub298=56.1 kJ mol−1; ΔHsub298=115.9�0.9 kJ mol−1; ΔSsub298=200�3 J mol−1 K−1). The results indicate that the modifications are monotropically related, which is in contrast to findings recently reported found by classical thermochemical methods.

Restricted access

Abstract  

Single crystals of the N,N-dimethylformamide (DMF) solvate (1:1) of flurbiprofen (FBP) were grown for the first time and characterised by X-ray diffraction, IR spectrophotometry, DSC and solution calorimetric methods. The structure may be characterised as a layer-structure, where DMF double-sheets are arranged between FBP double-sheets. The FBP and DMF molecules are linked to each other by a hydrogen bond, which is formed between the hydroxyl group of FBP and the carbonyl group of DMF. The conformation of FBP molecules in the DMF solvate differs from analogous enantiomers in the unsolvated form. The differences are discussed from the point of view of the influence of the nature of the solvent on selective crystallisation of the enantiomers. A peculiarity of the solvate is its low melting point, 37.30.2C, with respect to the unsolvated phase, 113.50.2C. Based on solution enthalpies of the solvated and unsolvated phases dissolved in DMF, the difference in crystal lattice energies, 9.8 kJ mol-1, was calculated and the difference in entropies, 33 J mol-1 K-1 estimated. A possible mechanism explaining the low melting point of the solvate is discussed.

Restricted access

Abstract  

A new device, based on the inert gas flow method, for measuring the vapour pressure and the determination of the enthalpy of sublimation in a wide range of temperatures (up to 573 K) is described in this paper. The limits of the flow rate as important experimental parameter were determined for the given instrument. The results of calibration showed a good precision and reproducibility of the measurements of the enthalpy of sublimation. The results of the determination of some derivatives of pyrimidine were presented.

Restricted access

Abstract  

Temperature dependences of solubility, saturated vapour pressure and crystal heat capacity of [4-(Benzyloxy)phenyl]acetic acid were determined. The solubility of this compound was investigated in n-hexane, buffered water solutions with pH 2.0 and 7.4 and n-octanol. The enthalpy of sublimation and vaporization as well as the fusion temperature were determined. Solvation and solubility processes have been analyzed. The thermodynamics of transfer processes from one buffer to another (protonation process), from buffers to 1-octanol (partitioning process), and from n-hexane to the applied solvents (specific interaction) have been calculated and compared to those of other NSAIDs. The relevant shares of specific and non-specific interactions in the process of solvation have been investigated and discussed.

Restricted access

Abstract

Solution and dilution enthalpies of aqueous solutions of potassium diclofenac salt (K_DC) were measured by an isoperibolic calorimeter at 298.15 and 318.5 K. Heat capacities of the solutions with concentrations 0.002–0.09 mol kg−1 were obtained at the temperature interval of 288.15–318.15 K using a scanning adiabatic microcalorimeter. The virial coefficients were derived from Pitzer's model, and the excess thermodynamic functions of both the solvent and the solute of the solution were calculated. The concentration and temperature dependencies of thermodynamic characteristics of the solution were analyzed and discussed.

Restricted access

Abstract  

Enthalpies of solution and dilution of aqueous solutions of sodium diclofenac salt were measured by isoperibolic calorimeter at 293.15, 298.15, 303.15, 308.15 and 318.15 K. The concentration of the electrolyte was restricted to solubility salt at various temperatures and did not exceed 0.035–0.057 mol kg−1 values depending on the studied temperature. The virial coefficients were derived from Pitzer’s model and the excess thermodynamic functions of both the solution and the components of the solution were calculated. The analysis of thermodynamic characteristics of the solution from concentration and temperatures was carried out and discussed.

Restricted access