Solvent-structuring at the interface between crystals and mixed organic-aqueous solvents

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Non-technical Abstract:Solution crystallization underlies a broad range of industrial, laboratory, and physiological processes. In recent years, organic and mixed organic-aqueous liquids have garnered increased interest as an alternative solvent for the preparation of crystalline materials and separation (or purification) by crystallization, in particular for high-value materials such as pharmaceuticals and fine chemicals. In contrast to crystallization from aqueous solvents, the level of understanding of the fundamental processes of crystal growth from organic solvents is severely limited. The lack of insight into the relevant fundamental mechanisms and pathways has emerged as the main obstacle to a rational approach to the optimization and control of crystallization in organic and mixed organic-aqueous media. The objective of this project is to provide understanding of the molecular processes comprising crystallization from mixed solvents focusing on the thermodynamic and kinetic consequences of potential solvent structuring.Technical Abstract:The roles that organic and mixed solvents play in crystallization are the open question addressed in this project. The solvent structures at the crystal-solution interface are characterized by advanced atomic force microscopy imaging and correlated with their effects on the thermodynamics of crystallization. How the chemical properties of the solvent affect the crystallization mechanism is elucidated by determining the steps that a solute molecule takes en route to a crystal growth site. The nature of the activation barrier for incorporation of molecules into the crystal is elucidated to establish whether solvent structuring and interactions with solvent molecules are the primary mechanisms underlying the existence of this barrier. Determining the steps in the molecular mechanism and their respective governing parameters provides tools to optimize and control crystallization from organic and mixed solvents and suppress defect-inducing instabilities. Comparing data for several porphyrin-solvent pairs elucidates the roles of the chemical moieties in the solvents and their abilities to form hydrogen and van der Waals bonds in the formation of solvent structures. Untangling the interlaced chemical and physical processes comprising crystallization and identifying the relevant governing parameters represents a new paradigm in the rational design of organic crystals.