EAGER: Exploring the Application of Transition Zone Theory to Crystallization from Solutions

North Carolina State University, Raleigh NC

Investigators

Abstract

Part I: Non-Technical Summary Crystallization was one of the earliest chemical processes studied by humans, as indicated by evidence of the practice of salt crystallization from seawater in pre-historic times. Today, the ability to control crystal growth is essential to technological devices such as solar cells and lasers as well as in preparation of pharmaceutical agents. However, the fundamental mechanisms underlying crystal growth are not fully understood. Classical nucleation theory, which is the widely accepted model for how crystals grow, does not fully account for experimental measurements. In this project, supported by the Solid State and Materials Chemistry program within the Division of Materials Research, the PI will continue to develop a new model of crystal growth initiated in his laboratory for formation of crystals from molten materials. The new project extends this theory to crystal growth from solutions by studying the arrangement of starting materials in the solutions and the speeds at which solid crystals form under varying conditions. Taken together, these experiments will increase understanding of crystal growth, eventually allowing scientists to develop better procedures for growing high-quality crystals for applications. The project also encompasses training of students and postdoctoral researchers, including their participation in advanced experiments at national synchrotron facilities. Part II: Technical Summary The recently developed Transition Zone Theory of crystallization, based on the cooperativity of crystallizing units rather than a classical particle attachment/detachment conception, accurately describes the crystal growth rates from melts of diverse materials. This project, supported by the Solid State and Materials Chemistry program within the Division of Materials Research, tests the hypothesis that similar entropic and cooperative influences are responsible for controlling crystallization from solution. This work extends the model to encompass crystallization from solution by studying the kinetics of crystal growth across binary phase diagram systems to determine the respective activation energy requirements to form long-range order in crystals and to exclude solvent from the crystallizing solute. Using representative systems of hydrated metal salts and aromatic organic molecules, the project uses spectroscopic and diffraction methods to investigate 1) the structure of saturated solutions, 2) comparative crystallization rates as a function of solvent/solute concentration from the pure phase to eutectic compositions, and 3) comparative rates of crystallization out of diverse solvents for which strong solvent-solute interactions are likely to inhibit crystallization. The long-term goal is that an extension of the Transition Zone Theory model can provide the basis for a comprehensive mechanistic understanding of crystal growth across a continuum of systems from pure metals to saturated solutions. 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.