Investigation of Surface-Potential Controlled Nucleation Using an Acoustic Levitation Apparatus

The focus of this thesis was to investigate the effect that droplet surface potential has upon nucleation of a solute from levitated solution droplets; there being virtually no such investigations thereof in the current literature. An increase in droplet surface potential resulted in significant promotion to solute nucleation, as determined by the number of crystals observed, for sodium chloride, ammonium nitrate, α-cyano-4-hydroxycinnamic acid, and 2,4,6-trihydroxyacetophenone monohydrate solution droplets that were individually levitated using an electrodynamic levitation trap. A change in NaCl crystal habit was also observed for a population of levitated droplets once a surface charge density threshold of ~ 9 × 10-4 e•nm-2 was surpassed; but was not observed for individually levitated droplets. Since the effects of surface potential and the presence of an external electric field on solute nucleation cannot be differentiated using an electrodynamic levitation trap as a means of levitation, an acoustic levitator was designed and constructed for this purpose. For acoustically levitated NaCl solution drops, both the presence of drop surface potential (SCDmax ~ ± 3.44 x 10-4 e•nm-2) and an external electric field (Eext = 6.0 x 105 Vm-1) were each found to promote solute nucleation as determined by the increase in the number of crystals observed in each drop relative to the number of crystals observed in the absence of drop surface potential and Eext. The effect of both drop surface potential and the presence of Eext on the crystallization of the concomitant dimorphic m-nitrophenol and trimorphic anthranilic acid systems with respect to polymorphic fraction were investigated for acoustically levitated solution drops. For anthranilic acid trials conducted in n-butanol under conditions of negative drop surface potential, the polymorphic ratio changed from approximately 0 : 100 : 0 to 0 : 91 : 9 (form I : form II : form III). A more dramatic shift to approximately 0 : 68 : 32 was observed for the same solution drops in the presence of Eext. The enhancement of form III appeared correlated to the decreases observed in drop supersaturation. No change in polymorphic ratio was observed for m-nitrophenol drops under similar conditions.