Non-Classical Mechanisms of Solution Crystallization Studied Using Colloidal Experiments and Simulations

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Non-technical abstract:Crystallization is a widely-encountered phase transition that affects many areas of science and technology. From concentrated phases, crystals nucleate and grow through the addition of monomers, and are well described by classical single-step processes. From dilute solutions, however, the factors controlling crystallization remain hotly contested. This lack of understanding hinders the production of single-crystal materials for electronic and optical applications and the precipitation of chemicals and pharmaceuticals, as two examples. It also poses a significant challenge for the treatment of diseases that involve crystallization of proteins and large molecules, including kidney stones, gout, and hemoglobin CC diseases. Whereas crystals in solution often nucleate and grow using multi-step rather than single-step pathways, the factors that determine the pathway used by a given system are not well understood. This project develops experimental and computational model systems consisting of micron-sized colloidal particles suspended in liquids and identifies the pathways for crystal nucleation and growth from solution as the interactions between the particles are varied. The knowledge gained from these studies will provide the fundamental understanding needed to control crystallization and self-assembly from solution. Educational outreach efforts conveying these results and their importance for society to K – 12 students and the general public include activities at science camps at the University of Houston and at science festivals held in downtown Houston.Technical abstract:Crystals in solution are widely reported to nucleate and grow using multi-step pathways that deviate from classical mechanisms, which involve single-step processes. Although many examples of non-classical assembly have been reported, physical understanding of the factors controlling the choice of nucleation and growth pathways remains limited. The objective of this project is to deploy colloid-imaging experiments and computational models to understand how intermolecular interactions dictate the choice of classical (single-step) versus non-classical (multi-step) nucleation and growth mechanisms for crystals in solution. The driving hypothesis is that the shape of interaction potentials controls the choice of pathways by which crystals nucleate and grow. The team tests this hypothesis in analogues for molecules, suspensions of submicron colloidal particles, by integrating complementary expertise in particle synthesis and confocal microscopy, light scattering, and advanced simulation techniques to address two specific aims: (1) identify the role of nonspecific interactions in the transition from direct (classical) nucleation to multistep nucleation, by tuning both long-range repulsive and short-range attractive interactions between particles in experiment and simulation, and (2) investigate the effects of particle interactions on mechanisms of non-classical growth, by characterizing addition of clusters to crystals grown on a template or in solution. The intellectual merit of this project is fundamental insight into how non-classical nucleation and growth mechanisms affect the formation of ordered crystalline assemblies in solution, which paves the way for the development of novel strategies to rapidly and controllably assemble large-scale crystals and generate well-controlled structures with precisely placed constituents. This project also has significant broader impacts for society: crystallization underlies myriad industrial and pathological processes. The project continues the team’s ongoing efforts to broaden participation in science through mentored research opportunities for undergraduate and graduate students recruited from the diverse study body at the University of Houston, a designated Hispanic-Serving and an Asian American and Native American Pacific Islander-Serving Institution.This award reflects NSF's statutory mission and has been deemed worthy of support through evaluation using the Foundation's intellectual merit and broader impacts review criteria.