Crystallization in Nanospaces: Consequences for Nucleation and Polymorphism

Classical crystal growth models posit that crystallization outcomes are determined at a critical size where the volume free energy of nuclei begins to offset the unfavorable surface free energy arising from the interface with the growth medium. Crystallization under nanoscale confinement offers an opportunity to examine nucleation and phase transformations at length scales corresponding to the critical size, at which kinetics and thermodynamics of nucleation and growth intersect and dramatic departures in stability compared to bulk crystals can appear. Crystallization of organic compounds in nanoporous matrices provides a snapshot of the earliest stages of crystal growth, with insights into nucleation, size-dependent polymorphism, and thermotropic behavior of nanoscale crystals. These matrices can be used to screen for crystal polymorphs and assess their stability as nanocrystals, while crystallization in aligned cylindrical pores affords insight into the competitive nature of nucleation, critical sizes, and the role of stereochemical (chiral) recognition at crystal interfaces of enantiomorphs. Collectively, these investigations have increased our understanding of crystallization at length scales that are deterministic while suggesting strategies for controlling crystallization outcomes and producing new materials.