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The confusion of indexing aspirin crystals.

Aubrey-Medendorp C, Parkin S, Li T

Department of Pharmaceutical Sciences, University of Kentucky, Lexington, Kentucky 40536-0082.

Much of the existing literature dealing with crystalline aspirin is vague or ambiguous with regard to indexing of the crystal faces. The inconsistency with which the indices of the dominant faces have been assigned leads to confusion in analysis of surface properties. To clarify this, we have conducted crystal growth experiments on aspirin, and indexed the crystal faces with X-ray diffraction (XRD), paying special attention to the placement of symmetry elements. The space group was confirmed as P2(1)/c, and the dominant face was (100). Contact angle measurements made on the two major faces of aspirin indicate the (100) face to be more hydrophobic than the (001) face, likely due to the acetyloxy moiety, not the carboxyl, exposed on the (100).

Published 4 March 2008 in J Pharm Sci, 97(4): 1361-7.
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