Functionalized Silica Nanoparticles as Additives for Polymorphic Control in Emulsion-Based Crystallization of Glycine

Emulsion-based crystallization to produce spherical crystalline agglomerates is an attractive route to control the size and morphology of active pharmaceutical ingredient (API) crystals, which in turn improves downstream processability. Here, we demonstrate the use of silica nanoparticles modified with different surface functional groups (hydroxyl, amino, carboxylic, imidazolim chloride, and chloride) as additives in water-in-oil emulsion-based crystallization of glycine, a model API molecule. Spherical agglomerates of glycine obtained under different experimental conditions are characterized by powder X-ray diffraction (XRD) and scanning electron microscopy. Our observations reveal the strong influence of particle functionalization on polymorphic outcome at near-neutral (pH ∼6) conditions, where we are able to selectively crystallize the least stable β-polymorph of glycine or tune the relative ratio of α- and β-polymorphs by selecting appropriate experimental conditions. Mixtures of α- and γ-glycine are typically obtained under acidic solutions (pH ∼3), irrespective of the functional groups used. We examine the influence of charge and immobilization density of surface functional groups and nanoparticle concentration on the polymorphic outcome and rationalize our results by analyzing molecular and functional group speciation.