Evaluation of two concepts of crystallinity using calcium gluceptate as a model compound

Abstract

According to the USP, solids are either crystalline, non-crystalline (amorphous) or a mixture of the two. The degree of crystallinity depends on the fraction of crystalline material in the mixture (two-state model). An alternative concept is that the degree of crystallinity has a value between 0% (amorphous) and 100% (perfect crystal) depending on the state of disorder in the lattice (one-state model). On grinding dehydrated calcium gluceptate (II) for increasing times, there were marked increases in apparent water solubility, decreases in the intensity of X-ray diffraction peaks, and heats of solution changed from endothermic to exothermic. The results are attributed to decreases in crystallinity, since surface area measurements showed that they could not be due to particle size reduction. Density values obtained for II using a liquid suspension method changed progressively with grinding. Lack of separation into two fractions on dispersion in the suspending liquid suggests that the decrease in crystallinity was not due to a decrease in the proportion of crystalline material in a crystalline-amorphous mixture. It is concluded that grinding decreases the crystallinity of II by increasing lattice disorder according to the one-state model, and thereby increases the apparent water solubility.