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Caffeine has been found to display a low-temperatureβ- and a high-temperatureα-modification. By quantitative DTA the following data were determined: transformation temperature 141±2°; enthalpy of transition 4.03±0.1 kJ·mole−1; enthalpy of fusion 21.6±0.5 kJ·mole−1; molar heat capacity
\documentclass{aastex} \usepackage{amsbsy} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{bm} \usepackage{mathrsfs} \usepackage{pifont} \usepackage{stmaryrd} \usepackage{textcomp} \usepackage{upgreek} \usepackage{portland,xspace} \usepackage{amsmath,amsxtra} \pagestyle{empty} \DeclareMathSizes{10}{9}{7}{6} \begin{document} $$\begin{array}{*{20}c} {{\vartheta \mathord{\left/ {\vphantom {\vartheta {^\circ C}}} \right. \kern-\nulldelimiterspace} {^\circ C}}} & {100(\beta )} & {100(\alpha )} & {150(\alpha )} & {100(\alpha )} \\ {{{C^\circ _\mathfrak{p} } \mathord{\left/ {\vphantom {{C^\circ _\mathfrak{p} } {J \cdot K^{ - 1} \cdot mole^{ - 1} }}} \right. \kern-\nulldelimiterspace} {J \cdot K^{ - 1} \cdot mole^{ - 1} }}} & {271 \pm 9} & {287 \pm 10} & {309 \pm 11} & {338 \pm 10} \\ \end{array}$$ \end{document}
in good accord with drop-calorimetric data. For the constants of the equation log (p/Pa)=−A/T+B, static vapour pressure measurements on liquid and solidα-caffeine, and effusion measurements on solidβ-caffeine yielded:
\documentclass{aastex} \usepackage{amsbsy} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{bm} \usepackage{mathrsfs} \usepackage{pifont} \usepackage{stmaryrd} \usepackage{textcomp} \usepackage{upgreek} \usepackage{portland,xspace} \usepackage{amsmath,amsxtra} \pagestyle{empty} \DeclareMathSizes{10}{9}{7}{6} \begin{document} $$\begin{array}{*{20}c} {A = 3918 \pm 37; 5223 \pm 28; 5781 \pm 35K^{ - 1} } \\ {B = 11.143 \pm 0.072; 13.697 \pm 0.057; 15.031 \pm 0.113} \\ \end{array}$$ \end{document}
. The evaporation coefficient ofβ-caffeine is 0.17±0.03.
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Temperature-resolved X-ray diffractometry as a thermoanalytical method

A powerful tool for determining solid state reaction kinetics

Journal of Thermal Analysis and Calorimetry
Authors:
M. Epple
and
H. K. Cammenga

Development and experimental setup of the time-, and temperature -resolved X-ray powder diffractometry are described. This method allows far deeper insight into solid state reactions than conventional thermoanalytical methods like differential scanning calorimetry (DSC) or thermogravimetry. As an example, the dehydration of caffeine hydrate was investigated. We found that in earlier stages the reaction is nucleation controlled, whereas for higher extent of reaction diffusion limitation becomes rate-controlling.

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The glass transition of isomalt and its components, the enthalpy of solution (crystalline state, glassy state) and the enthalpy of melting are reported. From the measured data (solution enthalpy, enthalpy of fusion and heat capacities) a cycle like the BORN-HABER cycle can be constructed. It is possible to calculate the amounts of amorphous isomalt from measured solution enthalpies; however, the values obtained do not agree with those provided by X-ray powder diffraction studies.

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