Diffusion, viscosity and activity of supersaturated solutions of potassium dihydrogen phosphate, ammonium dihydrogen phosphate, and triglycine sulfate and the growth of crystals in microgravity

The factors that affect the growth of crystals from water solution in microgravity have been examined. Three nonlinear optical materials have been studied, potassium dihydrogen phosphate (KDP), ammonium dihydrogen phosphate (ADP) and triglycine sulfate (TGS). TGS is a crystal formed composed of three glycine molecules and one molecule of sulfuric acid. The diffusion coefficient of the solutions in the supersaturated region was measured using Gouy interferometry. The viscosity of the solutions in the supersaturated region was measured using a capillary viscometer. At low supersaturations the diffusion coefficient is changing rather slowly, while the viscosity is rising more rapidly. Micron-sized droplets of solution were suspended in an electrodynamic levitator. From this experiment, the width of the metastable region was found to be much wider than that ordinarily obtained in bulk solutions and the activity of the solute was measured over this region. Theories of cluster formation were used to estimate values for the extent of solute clustering and the critical cluster size using these data. Column gradient experiments showed no sign of clustering at low supersaturations and there was no measurable time change of the viscosity. The crystal growth rate is strongly affected by the change of the diffusion coefficient with concentration.