The morphology of freeze-dried rubidium chloride powder

We have formed powders of a strongly ionic compound, RbCl by freeze-drying. Stock solutions varying from very low concentrations with 4 gm RbCl reagent dissolved in 100 cc water, to saturated solutions with 80 gm reagent in 100 cc water sprayed into isopentane at /minus/160/degree/C. The droplet size in the spray was varied from relatively large 4-mm diameter droplets to extremely small droplets in an aerosol spray. It was determined that both the concentration of the stock solution and the droplet size affect the average size and the size distribution of the primary particles formed and the way in which these primary particles are bonded together. Unlike the powders of many less ionic compounds that are produced by freeze-drying, the primary particles in these RbCl powders are crystalline rather than amorphous. Analysis with an x-ray diffractometer with a cold stage indicates that crystallization occurs during the freezing cycle rather than during the sublimation period of the freeze-drying process. 11 refs., 8 figs.