Toward Polymorph Control in an Inorganic Crystal System by Templated Nucleation at a Microdroplet Liquid Interface: Potassium Hexacyanoferrate(II) Trihydrate

Abstract

Potassium hexacyanoferrate(II) trihydrate (KFCT) is known to manifest multiple structure types under practical conditions of crystal growth, including a monoclinic modification which is stable at room temperature and a metastable tetragonal form. Its rich polytypic nature renders it an ideal model inorganic crystal for studies of the influence of ordered structures upon the nucleation of differing crystal forms. We have utilized a microdroplet-based crystallization method to investigate the influence of self-assembled surfactant monolayers (octadecanol, stearic acid, and octadecylamine (ODA)) at a water-decanol interface upon the crystal nucleation behavior of KFCT at the isolated microdroplet level. Our results indicate that markedly different habits can be selectively nucleated, depending upon the choice of surfactant and temperature. The crystals of differing habits were analyzed by polarized light microscopy and micro-Raman spectroscopy. We have demonstrated that a droplet crystallization platform promises control of the nucleation outcome, even in a crystal system where the polymorphic crystal forms have only the most delicate energetic differences, since confinement to isolated droplets ensures single nucleation events and avoids interconversion of polymorphic forms.