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Abstract  

The systematics of isotopic exchange in a heterogeneous batch system between a dilute aqueous solution and an adsorbate or the solid adsorbent are discussed. In particular, the influence of the experimental precision on the detection of an eventual non-reactive fraction of the adsorbate is considered, down to ~ 5% of which is detectable. The exchange with the solid adsorbent is illustrated by the case of45Ca2+ and calcite. The minimum layer thickness observable in surface exchange is ~ 0.05 m. The same formalism can be applied to uptake due to recrystallization of the solid. It enables distinction between this phenomenon and diffusion into the solid.

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Journal of Thermal Analysis and Calorimetry
Authors: M. Matthews, I. Atkinson, Lubaina Presswala, O. Najjar, Nadine Gerhardstein, R. Wei, Elizabeth Rye, and A. Riga

Abstract  

Dielectric analysis (DEA), supported by thermogravimetric analysis (TG), differential scanning calorimetry (DSC), powder X-ray diffraction analysis (PXRD) and photomicrography, reveal the chiral difference in the amino acids. The acids are classified as dielectric materials based on their structure, relating chirality to the vector sum of the average dipole moment, composed of the constant optical (electronic) and infra-red (atomic) polarizabilities, as well as dipole orientation. This study encompasses 14 L-and D-amino acid isomers. Physical properties recorded include AC electrical conductivity, charge transfer complexes, melting, recrystallization, amorphous and crystalline phases, and relaxation spectra, activation energies and polarization times for the electrical charging process.

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Abstract  

Isomorphic substitution, accompanied by surface adsorption, of a trace cation into a crystalline host matrix from its saturated solution can be studied by a double radiotracer experiment. Recrystallization of the matrix and uptake of the trace cation are thus measured simultaneously. This paper gives the formulation of the radiotracer experiment and the precision with which mole fractions of the trace cation in both phases, may be derived from it. The final result is thus a set of coexisting mole fractions and their uncertainties as a function of time. As an illustration, the case of calcite as the host matrix and CdII as the trace cation is considered.

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Abstract  

Differential scanning calorimetry (DSC) was used to construct phase diagrams of binary mixtures of alkylcyclohexanes and to characterize metastable phases formed in the binary mixtures. The experimentally measured liquidus curves were compared to the liquidus curves calculated using ideal solution theory. The measured phase diagrams of pentadecylcyclohexane/nonadecylcyclohexane and octadecylcyclohexane/nonadecylcyclohexane binary mixtures are consistent with theoretical phase diagrams constructed based on the assumption that these mixtures form eutectic systems. It was also observed that a metastable phase formed in some binary mixtures of pentadecylcyclohexane/nonadecylcyclohexane under fast cooling conditions. It is hypothesized that this metastable phase recrystallizes into the eutectic phase upon heating.

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A simple and sensitive method for separation and quantitative determination of antidiabetic drugs in pharmaceutical preparations has been established and validated. Commercial formulations of five antidiabetic drugs (metformin, pioglitazone, rosiglitazone, glibenclamide, and gliclazide) were chosen for the studies. The compounds were extracted, isolated, purified, recrystallized, and characterized by measurement of melting point, λ max , and IR. Quantitative determination was performed by HPLC, TLC, and column chromatography supplemented with UV spectrophotometry. Two of the combinations, metformin + pioglitazone and metformin + gliclazide, were separated by open-column chromatography. Detection was by UV spectrophotometry in HPLC and by use of iodine vapor in TLC.

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The polymorphic forms II and III of paracetamol were obtained by melting the marketed form I. Under the melting and cooling conditions used, it was possible to obtain forms I, II and III. The recrystallization conditions and the physical properties of forms II and III were investigated by means of various techniques: thermomicroscopy, DSC analysis, infrared microspectrometry and X-ray powder diffraction at room temperature and as a function of temperature. Form III was found to be very unstable. However, its formation seems to be an important intermediate step in the preparation of form II.

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Journal of Thermal Analysis and Calorimetry
Authors: P. Espeau, B. Nicolaï, R. Céolin, M. Perrin, L. Zaske, J. Giovannini, and F. Leveiller

Abstract  

Investigation into the thermal behavior of orthorhombic Forms I and II of spironolactone, by means of differential scanning calorimetry and high-resolution X-ray powder diffraction, showed that Form I melts then recrystallizes into Form II at 373–393 K, i.e. in the temperature range within which high resolution X-ray powder diffration showed that Form I transforms into Form II. Refinements of the lattice parameters of the two forms indicated that Form I is denser than Form II in the range from 298 K up to the temperature at which it melts.

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Abstract  

The multiple melting behaviour of isothermally crystallized bulk poly(trimethylene terephthalate) (PTT) observed using DSC has been correlated to the total depolarized light intensity (DLI) of thin films using hot-stage polarized light optical microscopy. The observation of partial melting, recrystallization and final melting in the DSC is correlated to the observation of the partial decrease, sudden increase and final decrease in DLI under the same heating conditions. Integration of real-time visible spectra of the transmitted light was used to separate the effects of retardation from pure birefringence of the colorful spherulitic thin-film PTT samples. The correlation of the results from these two methods has demonstrated clearly that the observed DSC multiple melting behaviour of this particular polymer is the illustrated effect of a process of continuous partial melting/recrystallization/final melting in the material during thermal analysis. The observed thermal behaviour of these metastable spherulitic materials is a complex function of their thermal history including crystallization temperature and anneal conditions, including scanning rate during thermal analysis.

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The characteristics of crystallization, melting and spherulitic growth of a random propylene copolymer (PRC) containing small amount of ethylene were studied in the presence of a selective Β-nucleating agent (calcium pimelate). It was established that the products of isothermal and non-isothermal crystallization are very rich in Β-modification but have mixed polymorphic composition. The formation of α-modification may be attributed to Βα-transition on the surface of growing Β-spherulites resulting in αΒ-twin-spherulites. During melting of PRC of Β-modification, the characteristics observed with Β-nucleated propylene homopolymers, namely, a Βα-recrystallization of recooled samples and separated melting of non-recooled samples (i.e. the melting memory effect), as well as a ΒΒ-recrystallization leading to a perfection of the structure within the Β-modification, are also demonstrated. The disturbance of regularity of the polymer chain highly reduces the tendency to Β-crystallization. In contrast to the observations with propylene homopolymers, the growth rate of α-modification (G α) is higher than that of Β-modification (G β) and no critical crossover temperature can be found (T(Βα)=413 K) below whichG α>G β. The experimental results show that a partial disturbance of chain regularity by incorporation of comonomer units considerably reduces the tendency to Β-crystallization.

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Journal of Thermal Analysis and Calorimetry
Authors: F. Giordano, A. Rossi, R. Bettini, A. Savioli, A. Gazzaniga, and Cs. Novák

Abstract  

The thermal behavior of binary mixtures of paracetamol and a polymeric excipient (microcrystalline cellulose, hydroxypropylmethylcellulose and cross-linked poly(vinylpyrrolidone)) was investigated. The physical mixtures, ranging from 50 to 90% by mass of drug, were submitted to a heating-cooling-heating program in the 35–180C temperature range. Solid-state analysis was performed by means of differential scanning calorimetry (DSC), hot stage microscopy (HSM), micro-Fourier transformed infrared spectroscopy (MFTIR), and scanning electron microscopy (SEM). The polymeric excipients were found to address in a reproducible manner the recrystallization of molten paracetamol within the binary mixture into Form II or Form III. The degree of crystallinity of paracetamol in the binary mixtures, evaluated from fusion enthalpies during the first and second heating scans, was influenced by the composition of the mixture, the nature of the excipient and the thermal history. In particular, DSC on mixtures with cross-linked poly(vinylpyrrolidone) and hydroxypropylmethylcellulose with drug contents below 65 and75%, respectively, evidenced the presence only of amorphous paracetamol after the cooling phase. Microcrystalline cellulose was very effective in directing the recrystallization of molten paracetamol as Form II.

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