Understanding the early stages of crystallization by X-ray scattering and laser irradiation

Early stages of crystallization (nucleation) plays a decisive role in determining the final solid properties, hence it is necessary to understand the fundamentals of nucleation to obtain products with desired properties. This research aims to develop a comprehensive picture of nucleation by understanding the structure of supersaturated solutions. In an attempt to achieve this goal two distinct routes were taken. In the first approach, small angle X-ray scattering was utilized to directly examine the formation of prenucleation clusters in aqueous solutions of glycine and urea. Prior to crystallization, a transformation from mass fractal to surface fractal arrangement was observed, which was interpreted as rearrangement of fractal clusters into more compact structures to form crystal nuclei. The findings suggested a two-step nucleation model, rather than the mechanism proposed by the classical nucleation theory. Glycine molecules were found to exist as dimers in aqueous solutions, producing the α-form composed of cyclic dimers, and monomers in acetic acid-water mixtures, producing the γ-form built of monomeric growth units, which suggested a direct link between the molecular assembly process in solutions and the polymorphic outcome. In the second approach, non-isothermal non-photochemical laser induced nucleation (NPLIN) was employed as a probe for understanding the fundamentals of nucleation. The achievement of NPLIN in supersaturated aqueous urea, glycine, L-alanine and DL-methionine solutions supported the two-step model. Laser pulses were capable of inducing nucleation only in certain solvents, which suggested that the outcome of NPLIN might provide information on the impact of solvent composition on the nucleation mechanism. The findings indicated that the differences in the nucleation temperatures obtained by linearly and circularly polarized light could be linked to the solution structure. Urea and L-alanine solutions were suggested to contain chain-like structures, while DL-methionine was suggested to exist as cyclic dimers, which were consistent with the molecular arrangements in crystal structures. These results provided an additional support for the link between the molecular association in solution and the structure of the crystallized solid, and demonstrated that the non-isothermal NPLIN can be a potential technique to understand the nature of molecular clustering in supersaturated solutions.