Abstract
Recent theoretical and experimental studies have proposed a two-step mechanism for crystal formation in which crystal nucleation is preceded by formation of disordered molecular assemblies. Here, we investigated whether similar intermediates might also form as crystals dissolve, effectively the reverse process. A model system of glycine in water was studied, and the resultant solutions were characterized using small-angle X-ray scattering, dynamic light scattering, and nanoparticle tracking analysis. Invariably, dissolution of glycine crystals into water was observed to produce scattering nanospecies with liquid-like properties and a mean diameter of about 250 nm, at near saturation concentration. The nanospecies persisted indefinitely in solution in the presence of excess glycine crystals and were identified as glycine-rich nanodroplets with an equilibrium population of about 109 per mL. The time to appearance of glycine crystals from quiescent supersaturated solution (S = 1.1) containing either a low po...
Highlights
Understanding of the mechanism of nucleation of crystals from solution is crucial for many natural and industrial processes, from biomineralization of bones and teeth to production of pharmaceuticals and nanomaterials
On analyses using small-angle X-ray scattering (SAXS), nanoparticle tracking analysis (NTA), and dynamic light scattering (DLS), it was observed that a small population of nanoscale scattering species with a median diameter of around 250 nm was present
The smaller molecular clusters were unaffected by nanofiltration through either PTFE or Anotop filters and exhibited Small-Angle X-ray Scattering (SAXS) behavior identical to that previously reported for glycine solutions (Figure 1b)
Summary
Understanding of the mechanism of nucleation of crystals from solution is crucial for many natural and industrial processes, from biomineralization of bones and teeth to production of pharmaceuticals and nanomaterials. With liquid-like properties, have been reported in concentrated protein solutions[7−10] and glycine aqueous solutions[5] and implicated as possible metastable intermediates on the crystallization pathway. These results, combined with theoretical studies, led to the proposal of an alternate two-step process of crystallization[11−13] in which disordered metastable molecular assemblies, containing relatively large numbers of solute (and solvent) molecules, are produced prior to the nucleation of more ordered nascent crystals. A crystallization pathway involving nanoscale intermediates has been reported for mesostructured crystals in which nanoparticles
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