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Abstract: X-ray photoelectron spectroscopy (XPS) has emerged as a technique that allows for characterization and classification of hydrogen bonding and proton transfer interactions in organic crystal structures, in a way that is complementary to crystallography by X-ray or neutron diffraction. Here, we analyze the nitrogen 1s core-level binding energies (BEs) of isonicotinamide (IN) systems with proton transfer between donor and acceptor groups at short distances. We show how a careful calibration of the BE scale places these salt systems correctly on the edge of the so-called salt–cocrystal continuum. We show how performing a fitting analysis of the data that is consistent with elemental analysis, expected stoichiometry, and quantification of adventitious carbon contamination facilitates the determination of absolute BEs with accuracy and reproducibility within ±0.1 eV. The determined N 1s core-level BEs of the protonated IN acceptors suggest that the local geometric arrangements of the donor, acceptor, and proton can influence the N 1s core-level BE significantly.

Open Access

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Abstract: Mechanochemical synthesis has recently emerged as a scalable “green” approach for the preparation of MOFs, but current understanding of the underlying reaction mechanisms is limited. In this work, an investigation of the reaction pathway of the mechanochemical synthesis of MOF-74 from ZnO and 2,5-dihydroxyterephthalic acid (H4HDTA), using DMF as a liquid additive, is presented. The complex reaction pathway involves the formation of four short-lived intermediate phases, prior to the crystallization of MOF-74. The crystal structures of three of these intermediates have been determined using a combination of single-crystal and powder X-ray diffraction methods and are described here. The initial stages of the reaction are very fast, with a DMF solvate of H4HDTA forming after only 2 min of milling. This is followed by crystallization, after only 4 min of milling, of a triclinic one-dimensional coordination polymer, Zn(H2DHTA)(DMF)2(H2O)2, which converts into a monoclinic polymorph on additional milling. Highly crystalline MOF-74 appears after prolonged milling, for at least 70 min.

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Abstract: A study on the hydration behavior of fluconazole demonstrates that both sample age and polymorphic form impact the drug’s kinetic stability to hydration. For two of the polymorphs, aged samples were found to hydrate more readily than fresh samples. The aging effect was attributed to the formation of monohydrate (MH) crystals over time. The MH, despite being initially undetectable, had a seeding effect, thus impacting the hydration kinetics.

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Abstract: Edible air-in-oil systems, also referred to as oleofoams, constitute a novel promising material for healthier, low-calorie fat replacers in confectionary products. Oleofoams can be formed by whipping oleogels, which are dispersions of fat crystals in an oil phase. Understanding how the properties of the fat crystals (i.e., size, shape, and polymorphism) contained in oleogels affect the microstructure and stability of oleofoams is essential for both the efficient design and manufacture of novel food products. In this work, cocoa butter, one of the main fat phases present in confectionary productions, which is responsible for pleasant texture and mouthfeel properties, was mixed with high oleic sunflower oil and crystallized to obtain an oleogel. This was subsequently whipped to yield a stable, highly aerated oleofoam. The effect of the crystallization conditions (oleogel composition and cooling rate) on the properties of the oleogels and related oleofoams was investigated with a multitechnique characterization approach, featuring polarized light microscopy, cryogenic scanning electron microscopy, X-ray diffraction, differential scanning calorimetry, and oscillatory rheology. Oleogel crystallization was performed in a lab-scale vessel and was monitored using light turbidimetry as an in situ technique. Results showed that the concentration of cocoa butter in sunflower oil was the parameter that affected most strongly the foamability and rheology of oleofoam samples. The size and shape of cocoa butter crystals within the oleogel was found to have a less significant effect since crystals were broken or partially melted during the aeration process. Oleofoams whipped from oleogels containing 15 and 22% w/w cocoa butter displayed an overrun of 200%, corresponding to a calorific density reduction to one-third, and an increase in both the elastic and viscous moduli compared to their oleogel precursor. This was explained by a structuring effect caused by the aeration process, where cocoa butter β(V) crystal nanoplatelets (CNPs) in the oleogel rearranged to stabilize the air bubbles via a Pickering mechanism. Oleofoams prepared from 30% w/w cocoa butter oleogels, on the other hand, incorporated less air (overrun between 150 and 180%) and displayed a similar viscoelastic profile to their unwhipped precursors potentially due to air incorporation being limited by the relatively high elastic modulus of the parent oleogels. Nevertheless, the calorific density of these samples was reduced by a factor of 1.6–2.5 compared to their full-fat analogues.

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Abstract: The effect of pressure on the crystal structures of the two ambient-pressure polymorphs of the amino acid l-histidine has been investigated. Single-crystal diffraction measurements, up to 6.60 GPa for the orthorhombic form I (P212121) and 6.85 GPa for the monoclinic form II (P21), show their crystal structures undergo isosymmetric single-crystal-to-single-crystal first-order phase transitions at 4.5 and 3.1 GPa to forms I′ and II′, respectively. Although the similarity in crystal packing and intermolecular interaction energies between the polymorphs is remarkable at ambient conditions, the manner in which each polymorph responds to pressure is different. Form II is found to be more compressible than form I, with bulk moduli of 11.6(6) GPa and 14.0(5) GPa, respectively. The order of compressibility follows the densities of the polymorphs at ambient conditions (1.450 and 1.439 g cm–3 for phases I and II, respectively). The difference is also related to the space-group symmetry, the softer monoclinic form having more degrees of freedom available to accommodate the change in pressure. In the orthorhombic form, the imidazole-based hydrogen atom involved in the H-bond along the c-direction swaps the acceptor oxygen atom at the transition to phase I′; the same swap occurs just after the phase transition in the monoclinic form and is also preceded by a bifurcation. Concurrently, the H-bond and the long-range electrostatic interaction along the b-direction form a three-centered H-bond at the I to I′ transition, while they swap their character during the II to II′ transition. The structural data were interpreted using periodic-density-functional theory, symmetry-adapted perturbation theory, and semiempirical Pixel calculations, which indicate that the transition is driven by minimization of volume, the intermolecular interactions generally being destabilized by the phase transitions. Nevertheless, volume calculations are used to show that networks of intermolecular contacts in both phases are very much less compressible than the interstitial void spaces, having bulk moduli similar to moderately hard metals. The volumes of the networks actually expand over the course of both phase transitions, with the overall unit-cell-volume decrease occurring through larger compression of interstitial void space.