Characterization of frozen aqueous solutions by low temperature X-ray powder diffractometry

Purpose. A low temperature X-ray powder diffractometric (XRD) technique has been developed which permits in situ characterization of the solid-state of solutes in frozen aqueous solutions. Methods. A variable temperature stage, with a working temperature range of-190 to 300°C, was attached to a wide-angle XRD. The stage was calibrated with a sodium chloride-water binary system. Results. When aqueous nafcillin sodium solution (22% w/w) was frozen, eutectic crystallization of the solute was not observed. However, annealing at -4°C, caused crystallization of the solute. With increasing annealing time, there was a progressive increase in the crystallinity of the solute. Studies were carried out with sodium nafcillin solutions ranging in concentration from 20 to 50% w/w. The solid-state of the phase crystallizing from solution was independent of the solute concentration. Next, solutions of mono- and disodium hydrogen phosphate were individually frozen. Only the latter crystallized as the dodecahydrate (Na₂HPO₄·12H₂O). However when an aqueous buffer mixture of mono- and disodium hydrogen phosphate was frozen, the former inhibited the crystallization of the latter. Conclusions. Since freezing of solutions is the first step in lyophilization, the XRD technique can provide a mechanistic understanding of the alterations in solid-state that occur during freeze-drying. DSC has so far been the technique of choice to study frozen systems. The advantage of XRD is that it not only permits unambiguous identification of the crystalline solid phase(s), but it also provides information about the degree of crystallinity. While overlapping thermal events are difficult to interpret in DSC, XRD does not suffer from such a limitation.