Filtration Suppresses Laser-Induced Nucleation of Glycine in Aqueous Solutions

Abstract

We demonstrate that nanofiltration of aqueous glycine solutions has a pronounced effect on laser-induced nucleation. Two nucleation regimes were observed in nonfiltered, irradiated solutions under isothermal conditions: a rapid initial regime associated with laser-induced nucleation and a second much slower spontaneous nucleation regime. Filtration of the solutions prior to irradiation greatly suppressed the rapid regime, while the slow regime was similar regardless of filtration or irradiation, for all supersaturations studied. A clear effect of filtration on crystal polymorphism was also observed. Nonfiltered irradiated solutions at a lower supersaturation almost exclusively yielded the α-polymorph, while at higher supersaturations there was significant presence (∼40%) of the γ-polymorph. On the other hand, filtered solutions almost exclusively yielded the α-polymorph of glycine at all supersaturations studied. These surprising results challenge some established ideas about laser-induced nucleation, showing that previously reported laser-induced nucleation phenomena in glycine aqueous solutions can be effectively suppressed by filtration, so that the underlying mechanism is unlikely to be based on molecular scale interactions involving just the solute and the solvent alone. Instead, laser-induced nucleation in this system appears to be related to either colloidal scale solution clusters or foreign solid or molecular impurities that can be removed by nanofiltration.