Effect of Nanoscale Confinement on the Crystallization of Potassium Ferrocyanide

Many crystallization processes of great significance in nature and technology occur in small volumes rather than in bulk solution. This article describes an investigation into the effects of nanoscale confinement on the crystallization of the inorganic compound potassium ferrocyanide, K4Fe(CN)6 (KFC). Selected for study due to its high solubility, rich polymorphism, and interesting physical properties, K4Fe(CN)6 was precipitated within controlled pore glasses (CPG) with pore diameters of 8, 48, and 362 nm. Remarkable effects were seen, such that although anhydrous potassium ferrocyanide was never observed on precipitation in bulk aqueous solution, it was the first phase to crystallize within the CPGs and was present for at least 1 day in all three pore sizes. Slow transformation to the metastable tetragonal polymorph of the trihydrate K4Fe(CN)6·3H2O (KFCT) then occurred, where this polymorph was stable for a month in 8 nm pores. Finally, conversion to the thermodynamically stable monoclinic polymorph of KFCT was observed, where this phase was always found after a few minutes in bulk solution. As far as we are aware these retardation effects—by up to 5 orders of magnitude in the 8 nm pores—are far greater than any seen previously in inorganic systems and provide strong evidence for the universal effects of confinement on crystallization.