Showing papers by "Bart Kahr published in 2021"

Imidacloprid Crystal Polymorphs for Disease Vector Control and Pollinator Protection.

Abstract: Imidacloprid, the world's leading insecticide, has been approved recently for controlling infectious disease vectors; yet, in agricultural settings, it has been implicated in the frightening decline of pollinators. This argues for strategies that sharply reduce the environmental impact of imidacloprid. When used as a contact insecticide, the effectiveness of imidacloprid relies on physical contact between its crystal surfaces and insect tarsi. Herein, seven new imidacloprid crystal polymorphs are reported, adding to two known forms. Anticipating that insect uptake of imidacloprid molecules would depend on the respective free energies of crystal polymorph surfaces, measurements of insect knockdown times for the metastable crystal forms were as much as nine times faster acting than the commercial form against Aedes, Anopheles, and Culex mosquitoes as well as Drosophila (fruit flies). These results suggest that replacement of commercially available imidacloprid crystals (a.k.a. Form I) in space-spraying with any one of three new polymorphs, Forms IV, VI, IX, would suppress vector-borne disease transmission while reducing environmental exposure and harm to nontarget organisms.

Dichroic Calcite Reveals the Pathway from Additive Binding to Occlusion

Abstract: Organic additives play key roles in controlling the precipitation of calcium carbonate in the environment, industry, and biology, where they can direct polymorphism, alter crystal morphologies, and sometimes even become occluded, modifying bulk properties. However, significant questions remain regarding the pathway from adsorption on crystal surfaces to occlusion. Here, the optical properties of calcite crystals grown in the presence of the dye Congo red are used to characterize the organization of the additives within the crystal. Complemented by the analysis of surface adsorption through in situ atomic force microscopy (AFM), molecular simulations, and changes in crystal morphologies, we show that the occluded dye molecules are randomly oriented under fast growth conditions, but that slow growth engenders ordering of the dye within islands, whose orientation is determined by the dye/terrace interaction free energy. The islands are subsequently overgrown such that their internal structure is preserved. These results reveal that the occlusion of organic macromolecules into calcite can be understood by thermodynamics operating at the adsorption stage. This new insight will ultimately enable the design of additives to give specific material properties.

Punin Ripening and the Classification of Solution-Mediated Recrystallization Mechanisms

Abstract: Ripening (also called recrystallization) is a process that occurs commonly in nature and industry that shifts the size distribution of an ensemble of crystals toward a smaller number of larger crys...

Ambient L-lactic acid crystal polymorphism

Abstract: Only one crystalline phase of the metabolite, L-lactic acid (LLA), has been described since its isolation from sour milk as long ago as 1780. Herein, we report the structures of two new crystalline polymorphs of LLA obtained from supercooled melts and characterized by X-ray diffraction and micro-Raman spectroscopy, as well as their transformations from one phase to another. None of the three crystal structures are consistent with the aggregate geometries of LLA in cold matrices, in solution, or those predicted by quantum chemical computations. The latter aggregates feature typical cyclic carboxylic acid dimers not represented in any of the crystal structures. These differences underscore the difficulty of reasoning solid state structure from associations in other media.