Crystallinity of pharmaceuticals

X-ray powder diffractometry is widely used to determine the degree of crystallinity of pharmaceuticals. X-ray diffractometric methods were originally developed for determining the degree of crystallinity of polymers. Many polymers exhibit properties associated with both crystalline (e.g., evolution of latent heat on cooling from the melt) and noncrystalline (e.g., diffuse x-ray pattern) materials. This behavior can be explained by the two-state model, according to which polymeric materials consist of small but perfect crystalline regions (crystallites) that are embedded within a continuous matrix [25]. The x-ray methods implicitly assume the two-state model of crystallinity. 

XRD is widely used to determine the degree of crystallinity of pharmaceuticals. The procedure developed by Hermans and Weidinger is based on three assumptions. First, it must be possible to demarcate and measure the crystalline intensity (/J and amorphous intensity (4) from the powder pattern. Usually, the integrated line intensity (area under the curve), rather than the peak intensity (peak height), is measured. Second, there is a proportionality between the experimentally measured crystalline intensity and the crystalline fraction (xc) in the sample. Finally, a proportionality exists between the experimentally measured amorphous intensity and the amorphous fraction (x ) in the sample. The degree of crystallinity (or percent crystallinity), is given by the expression. 

C. J. Strachan, T. Rades, D. A. Newnham, K. C. Gordon, M. Pepper and P. F. Taday, Using terahertz pulsed spectroscopy to study crystallinity of pharmaceutical materials, Chem. Phys. Lett., 2004, 390, 20-24. 

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