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The theophylline-oxalic acid co-crystal system: solid phases, thermodynamics and crystallisation
KTH, School of Chemical Science and Engineering (CHE), Chemical Engineering and Technology, Transport Phenomena.
KTH, School of Chemical Science and Engineering (CHE), Chemical Engineering and Technology, Transport Phenomena.
2012 (English)In: CrystEngComm, ISSN 1466-8033, E-ISSN 1466-8033, Vol. 14, no 14, 4644-4655 p.Article in journal (Refereed) Published
Abstract [en]

The solid phases in the theophylline-oxalic acid co-crystal system have been investigated by thermal analysis techniques, X-ray diffraction analysis (XRD) and solubility measurements. The work includes the 2 : 1 co-crystal of theophylline and oxalic acid, polymorphs of theophylline, the theophylline monohydrate and solid oxalic acid. The DSC curve of the co-crystal presents two endothermic peaks, one at about 230 degrees C where the oxalic acid in the co-crystal decomposes and carbon dioxide is liberated, and another one at 279 degrees C where the remaining theophylline melts. At equilibrium with the solid co-crystal, the theophylline concentration is only 60% of the corresponding value for the pure solid theophylline. Using the solubility data, the standard Gibbs free energy of formation of the co-crystal from the pure solid phases is estimated to be approximately -6.0 kJ mole(-1). In a chloroform/methanol mixture (4 : 1 v : v) the commercial form of pure theophylline (Form II) has been observed to transform into a more stable non-solvated form (Form I). The data suggest that the polymorphs are enantiotropically related with an approximate transition temperature of 70 degrees C. The 2 : 1 theophylline-oxalic acid co-crystal can be successfully produced by cooling crystallisation in chloroform/methanol mixture (4 : 1 v : v) and by slurry conversion crystallisation also in more benign solvents.

Place, publisher, year, edition, pages
2012. Vol. 14, no 14, 4644-4655 p.
National Category
Chemical Engineering
Identifiers
URN: urn:nbn:se:kth:diva-99096DOI: 10.1039/c2ce25299fISI: 000305530500016Scopus ID: 2-s2.0-84862876852OAI: oai:DiVA.org:kth-99096DiVA: diva2:541464
Note
QC 20120718Available from: 2012-07-18 Created: 2012-07-13 Last updated: 2017-12-07Bibliographically approved

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