Crystallization and Polymorphism under Nanoconfinement

Crystallization outcomes, polymorph stability rankings, and phase transformation pathways can be significantly altered when crystallization is restricted to dimensions below 1 μm. In this review, we discuss the state of the art of crystallization and polymorphism in nanoconfinement. A primer is presented regarding the prevalent theories for nucleation, their implications for nucleation in confinement, and the dependence of solubility on crystal size. The various methods for the discovery and control of polymorphs are reviewed. Polymorphism under nanoconfinement is particularly attractive because it can be advantageous with respect to stabilizing desirable amorphous or metastable crystalline forms. Various experimental methods employing nanoconfinement are discussed along with related methods of characterization. Several critical features of crystallization under nanoconfinement─differences in the properties of confined liquid phases and their bulk counterparts, the dependence of crystal thermotropic properties on size, crystal orientation, polymorphism, and crystallization kinetics─are presented.