As a consequence of the COVID-19 pandemic caused by the SARS-CoV-2 virus, the widespread daily use of face masks is promoted worldwide. Particle-size dependent filtration efficiencies (FE; d p ¼ 30 nm-10 mm), applying a particle counting approach, and additionally pressure drops (Dp) were determined for 44 samples of household materials and several medical masks. Huge FE differences were found between sample materials and for different particle sizes, spanning from <10% up to almost 100%. Minimum FE were determined for d p ¼ 50-500 nm particles with significantly larger values for d p ¼ 30 nm particles and especially for those with d p > 2.5 mm. Measurements at different numbers of layers showed that stacks of textiles can be treated as separate filters and total FE and Dp can readily be estimated from the features of the individual layers, leaving laborious measurements of individual combinations obsolete. For many materials, electrostatic attraction contributes strongly to overall FE for particles up to 100 nm diameter. Measurements with defined leaks showed that already a small fractional leak area of 1-2% can strongly deteriorate total FE. This is especially the case for particles smaller than 5 mm diameter, where FE dropped by 50% or even two thirds. Our measurements show that by stacking an adequate number of layers of many fabrics, decent filtration efficiencies can be reached for homemade face masks over large particle size ranges with acceptable pressure drop across the material. Very important, however, is good fit of the masks to minimize leak flows and selection of non-hazardous mask material.
Peroxodicarbonates are of substantial interest as potentially powerful and sustainable oxidizers but have so far been accessible only in low concentrations with unsatisfactory energy efficiency. Concentrated (> 0.9 mol L À 1 ) peroxodicarbonate solutions have now been made accessible by the electrolysis of aqueous K 2 CO 3 / Na 2 CO 3 /KHCO 3 solutions at high current density of 3.33 A cm À 2 in an efficiently cooled circular flow reactor equipped with a boron-doped diamond anode and a stainless-steel cathode. Their synthetic potential as platform oxidizers was clearly demonstrated in transformations including sulfoxidation, N-oxidation, and epoxidation.
A novel and scalable protocol for the synthesis of para-benzoquinones from inexpensive phenols is reported. In advancement of previous methods the oxidation is performed free of catalyst, mediator or terminal...
Identification of turn motifs that are stabilized by intramolecular hydrogen bonds can be useful in describing the conformation of peptide systems. However, this approach is somewhat insufficient for cyclic peptides...
A novel electrosynthetic protocol for the direct conversion of easily accessible 4-hydroxybenzaldehydes into valuable benzoquinones is reported. The transformation is enabled through anodic oxidation and is performed in a reagent-free manner devoid of terminal oxidants and redox mediators to effectively minimize waste. Environmentally benign and inexpensive graphite serves as a metal-free anode material. A broad range of substituents, including halogens, are tolerated in this electrochemical protocol, and the quinones are selectively accessible in yields up to 99%. Furthermore, the compatibility with electrochemical flow synthesis is demonstrated. Moreover, the flow protocol is successfully applied to the biopolymer lignosulfonate and provides quinones in yields up to 7.6 wt%. A mechanistic proposal for this Dakin-type reaction is offered. The sustainability and synthetic utility are evaluated quantitatively and compared to established protocols by application of different green chemistry metrics.
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