SAXS Study of the Nucleation of Glycine Crystals from a Supersaturated Solution

Nucleation of crystalline solids, the first stage of crystallization from solution, is not yet fully understood. This is true for both small molecules of low molecular weight and more complicated large molecules. To obtain direct structural information about the process of nucleation and crystallization of small molecules, small-angle X-ray scattering (SAXS) has been used to study the crystallization of the amino acid glycine from its supersaturated aqueous solution. The scattering data was analyzed using the unified fit model, which is well-suited for studying complex systems that may contain multiple levels of related structural features. The results suggest that glycine molecules exist as dimers in supersaturated solution. The system obeys power-law behavior that indicates the presence of fractals in the solution. A transformation from mass fractal structure to surface fractal structure is observed during the crystallization process, which could be the signature of a two-step nucleation process.