Iskanje po katalogu digitalne knjižnice

Opis: Recent advances in experimental studies of nanoparticle-driven stabilization of chiral liquid-crystalline phases are highlighted. The stabilization is achieved via the nanoparticles’ assembly in the defect lattices of the soft liquid-crystalline hosts. This is of significant importance for understanding the interactions of nanoparticles with topological defects and for envisioned technological applications. We demonstrate that blue phases are stabilized and twist-grain boundary phases are induced by dispersing surface-functionalized CdSSe quantum dots, spherical Au nanoparticles, as well as MoS2 nanoplatelets and reduced-graphene oxide nanosheets in chiral liquid crystals. Phase diagrams are shown based on calorimetric and optical measurements. Our findings related to the role of the nanoparticle core composition, size, shape, and surface coating on the stabilization effect are presented, followed by an overview of and comparison with other related studies in the literature. Moreover, the key points of the underlying mechanisms are summarized and prospects in the field are briefly discussed.
Ključne besede: liquid crystals, nanoparticles, quantum dots, reduced-graphene oxide, calorimetry, microscopy, blue phases, twist-grain boundary phases, disclination lines, screw dislocations
Objavljeno v DKUM: 09.08.2024; Ogledov: 97; Prenosov: 9
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Opis: The first calorimetric study of coupling between organocuprate, derived from Grignard reagent (vinyl magnesium chloride), and primary alkyl halide (e.g. (S)-ethyl 3-(tert-butyldimethylsilyloxy)-4-iodobutanoate) has been conducted. This transformation is paramountly important for efficient preparation of (R)-ethyl 3-(tert-butyldimethylsilyloxy)hex-5-enoate – a key lactonized statin side chain precursor. The results obtained give thorough calorimetric insight into this complex low-temperature synthesis as well as a new understanding of the suggested reductive elimination of the final intermediates in the coupling reaction. Namely, the surprising unexpected spontaneous three-step exothermal event has been observed during controlled progressive heating of the mixture of the final intermediates to the room temperature. This phenomenon confirms that coupling between functionalized primary alkyl halide and vinylic organocuprate reagent is not a simple $S_N2$ substitution reaction. The presented study provides among others the first reported values of reaction enthalpies and corresponding adiabatic temperature rises of reaction mixture for all exothermic events that occurred in the (R)-ethyl 3-(tert-butyldimethylsilyloxy)hex-5-enoate synthesis. The obtained results ensure consequential thermal process safety knowledge which can be incorporated into safe process scale-up as well as design of reactor system with sufficient cooling capacity for industrial production of (R)-ethyl 3-(tert-butyldimethylsilyloxy)hex-5-enoate. Moreover, the results provide a basic guidance for other organocuprate coupling reaction systems.
Ključne besede: Grignard reagents, organocuprates, coupling reaction, reaction calorimetry, statins
Objavljeno v DKUM: 17.08.2017; Ogledov: 1134; Prenosov: 107
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Opis: Al-based alloyed powders, appropriate for the sintering procedure (powder metallurgy, P/M) contain the alloying elements with a high solid solubility in Al, enabling reaction and liquid-phase sintering. They are surface oxidised because of a high affinity of Al to oxygen. Besides, this type of powders contains a polymeric lubricant (wax), which reduces the friction on die walls during automatic die compaction into the final compact shape of a product. This lubricant has to be removed slowly during the first stage of sintering in order to prevent deformations and cracking of the product. Consequently, its sintering is very complex. Generally, these powders are sintered in pure nitrogen with a low dew point. The optimum sintering conditions are generally determined on the basis of light and scanning electron microscopy. The investigation can also be completed very successively with differential scanning calorimetry and thermo gravimetry. The first one allows an insight into the endo- and exothermic reactions, taking place during the heating and cooling of a compacted metal powder, and the second one allows an insight into the processes, connected with the mass loss (a reduction, a lubricant removal, etc.) or mass increase (an oxidation). The DSC/TG of three commercial Al-based alloyed powders was performed in the frame of our investigations. The results were compared with the theoretical thermodynamic-based calculations and the optimum sintering conditions were proposed.
Ključne besede: aluminium powders, sintering, differential scanning calorimetry and thermo gravimetry
Objavljeno v DKUM: 15.03.2017; Ogledov: 1384; Prenosov: 138
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Opis: The study of two different methods of obtaining chiral alcohols is proposed herein. The requirement of the relatively new paradigm of green chemistry associated with clean technologies such as biocatalysis or non-conventional solvents, dense gases, was focused at. Indeed, the two methods of production of chiral alcohols were: - the conversion of acetophenone into (R)-1-phenylethanol in dense gases catalysed by Lactobacillus brevis alcohol dehydrogenase and its coenzyme, NADP/H; - the resolution of (±)-trans-1,2-cyclohexanediol by cocrystal formation with tartaric acid followed by supercritical extraction. In both cases high enantiopurities were achieved (ee>99%).
Ključne besede: High-pressure technologies, enantiomers, green chemistry, R-1-phenylethanol, Lactobacillus brevis, alcohol dehydrogenase, NADP, liquid propane, enzyme deactivation, resolution, trans-1, 2-cyclohexanediol, tartaric acid, cocrystal, supercritical carbon dioxide, extraction, X-ray diffraction, differential scanning calorimetry
Objavljeno v DKUM: 02.02.2011; Ogledov: 3053; Prenosov: 146
Celotno besedilo (6,25 MB)