Droplet Microfluidics XRD Identifies Effective Nucleating Agents for Calcium Carbonate

The ability to control crystallization reactions is required in a vast range of processes including the production of functional inorganic materials and pharmaceuticals and the prevention of scale. However, it is currently limited by a lack of understanding of the mechanisms underlying crystal nucleation and growth. To address this challenge, it is necessary to carry out crystallization reactions in well‐defined environments, and ideally to perform in situ measurements. Here, a versatile microfluidic synchrotron‐based technique is presented to meet these demands. Droplet microfluidic‐coupled X‐ray diffraction (DMC‐XRD) enables the collection of time‐resolved, serial diffraction patterns from a stream of flowing droplets containing growing crystals. The droplets offer reproducible reaction environments, and radiation damage is effectively eliminated by the short residence time of each droplet in the beam. DMC‐XRD is then used to identify effective particulate nucleating agents for calcium carbonate and to study their influence on the crystallization pathway. Bioactive glasses and a model material for mineral dust are shown to significantly lower the induction time, highlighting the importance of both surface chemistry and topography on the nucleating efficiency of a surface. This technology is also extremely versatile, and could be used to study dynamic reactions with a wide range of synchrotron‐based techniques

Confinement generates single-crystal aragonite rods at room temperature

The topic of calcite and aragonite polymorphism attracts enormous interest from fields including biomineralization and paleogeochemistry. While aragonite is only slightly less thermodynamically stable than calcite under ambient conditions, it typically only forms as a minor product in additive-free solutions at room temperature. However, aragonite is an abundant biomineral, and certain organisms can selectively generate calcite and aragonite. This fascinating behavior has been the focus of decades of research, where this has been driven by a search for specific organic macromolecules that can generate these polymorphs. However, despite these efforts, we still have a poor understanding of how organisms achieve such selectivity. In this work, we consider an alternative possibility and explore whether the confined volumes in which all biomineralization occurs could also influence polymorph. Calcium carbonate was precipitated within the cylindrical pores of track-etched membranes, where these enabled us to systematically investigate the relationship between the membrane pore diameter and polymorph formation. Aragonite was obtained in increasing quantities as the pore size was reduced, such that oriented single crystals of aragonite were the sole product from additive-free solutions in 25-nm pores and significant quantities of aragonite formed in pores as large as 200 nm in the presence of low concentrations of magnesium and sulfate ions. This effect can be attributed to the effect of the pore size on the ion distribution, which becomes of increasing importance in small pores. These intriguing results suggest that organisms may exploit confinement effects to gain control over crystal polymorph

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