Solvent

Summary

A solvent (from the Latin solvō, "loosen, untie, solve") is a substance that dissolves a solute, resulting in a solution. A solvent is usually a liquid but can also be a solid, a gas, or a supercritical fluid. Water is a solvent for polar molecules and the most common solvent used by living things; all the ions and proteins in a cell are dissolved in water within the cell. Major uses of solvents are in paints, paint removers, inks, and dry cleaning. Specific uses for organic solvents are in dry cleaning (e.g. tetrachloroethylene); as paint thinners (toluene, turpentine); as nail polish removers and solvents of glue (acetone, methyl acetate, ethyl acetate); in spot removers (hexane, petrol ether); in detergents (citrus terpenes); and in perfumes (ethanol). Solvents find various applications in chemical, pharmaceutical, oil, and gas industries, including in chemical syntheses and purification processes. Solutions and solvation When one substance is dissolved into another, a

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A solvent- and vacuum-free route to large-area perovskite films for efficient solar modules

Yuhang Wang, Fei Ye

Recent advances in the use of organic-inorganic hybrid perovskites for optoelectronics have been rapid, with reported power conversion efficiencies of up to 22 per cent for perovskite solar cells1-9. Improvements in stability have also enabled testing over a timescale of thousands of hours(10-14). However, large-scale deployment of such cells will also require the ability to produce large-area, uniformly high-quality perovskite films. A key challenge is to overcome the substantial reduction in power conversion efficiency when a small device is scaled up: a reduction from over 20 per cent to about 10 per cent is found(15-21) when a common aperture area of about 0.1 square centimetres is increased to more than 25 square centimetres. Here we report a new deposition route for methyl ammonium lead halide perovskite films that does not rely on use of a common solvent(1,2,4-15) or vacuum3: rather, it relies on the rapid conversion of amine complex precursors to perovskite films, followed by a pressure application step. The deposited perovskite films were free of pin-holes and highly uniform. Importantly, the new deposition approach can be performed in air at low temperatures, facilitating fabrication of large-area perovskite devices. We reached a certified power conversion efficiency of 12.1 per cent with an aperture area of 36.1 square centimetres for a mesoporous TiO2-based perovskite solar module architecture.

Nature Publishing Group2017

Influence of pH on dehalorespiring consortia & first steps in the development of a methodology for acid neutralization in enhanced in situ bioremediation

Nicole Munz

Enhanced in situ bioremediation of chlorinated solvents is, especially in source zone remediation, often limited through the accompanied high acidification of groundwater. This acid built-up originating from the dechlorination itself and fermentation products, such as organic acids, presents a draw-back in the efficiency of dehalorespiring bacteria. It is therefore indispensable to develop and implement appropriate buffering strategies to ensure a constant transformation of toxic chloroethenes to the final non-toxic product ethene. In this study pH tolerance of different dehalorespiring bacteria was tested. The limiting acidic pH varies significantly among the consortia. The consortium SL23b4 (PCE to ethene) grows at all the examined pH (pH 5 to pH 7.5) with a pH optimum between 6 and 7 (depending on the dechlorination step), whereas Maroc 6b (PCE to ethene) was only found to grow at incubation pH of 7, and very slowly at pH 6.5. The consortium 2SO (cis-DCE to ethene) seems to be inhibited at acidic pH (no growth at pH 5 and 5.5) having a pH optimum above neutral pH. In most cases incomplete dechlorination was observed in cultures growing at acidic pH or below the pH optima, accumulating toxic intermediate dechlorination products as a consequence. Additionally, high rates of dechlorination were always accompanied by pH drops up to 1.2 pH units, giving evidence of high acid production. pH had also influence on the community composition. In the consortium SL23b4 a shift from a Dehalococcoides- (pH 6-7.5) to a Sulfurospirillum spp.- (pH5 -5.5) dominated community was observed. Consortium Maroc 6b showed significantly different community compositions due to a pH change of only 0.5 pH units. Dehalococcoides spp. and Dehalobacter spp. were dominating at pH 7 and pH 6.5, respectively. Consortium 2SO is dominated by Dehalococcoides spp. at all pH, although at pH 7 a much higher diversity seems to prevail, with the threefold number of OTUs. Tests on the tolerance of the dehalorespiring consortia for elevated PCE concentration showed similar patterns for the different dechlorination steps, as for pH cultures, and confirmed a generally higher sensitivity towards PCE and pH changes for the last steps of dechlorination. Conclusively, Dehalococcoides spp., the only known genera able to perform complete dechlorination, seem to be highly sensitive to pH and PCE changes. Further, abiotic experiments on silicate mineral dissolution, using forsteritic olivine, were performed to simulate the buffer capacity of the minerals for the acid production during microbial reductive dechlorination. In these experiments, the buffer capacity was insufficient to counterbalance the constant acid titration rate, resulting in a final pH between 3 and 4 (initial pH of 7). The formation of precipitates, such as iron oxide, and the re-precipitation of magnesium were observed in some cases, showing a decrease in magnesium concentrations.

2010

Polymer brushes are a specific class of polymer thin films in which each chain is tethered to the underlying substrate via one chain end. In sufficiently densely grafted assemblies, the individual polymer chains are forced into an extended chain conformation and form a brush-like structure, colloquially referred to as polymer brush. Upon incubation in a good solvent, polymer brushes experience swelling which leads to a tension at the polymer brush - substrate interface. This tension is believed to mechanically facilitate cleaving reactions at the tethering point resulting in the detachment of polymer chains, a process which is known as degrafting. Degrafting, and in the case of crosslinked polymer brushes delamination, has been reported for a large variety of polymer brushes and surface materials. Even though this phenomenon is already known for more than two decades, in depth understanding of the underlying mechanism is lacking and quantitative description of the process is rare. This thesis aims to gain further understanding of the phenomenon by quantitative description of the degrafting process. Further, swelling induced mechanical activation at the tethering point of the polymer to the underlying substrate is discussed in the context of polymer mechanochemistry and presented as a tool to selectively facilitate mechanophore cleavage at the interface. Chapter 1 provides a short historical review on the development of polymer mechanochemistry and gives an overview of commonly applied techniques in the field. The concept of polymer mechanochemistry is then extended to polymer brushes and the discussion results in a number of issues and research questions which are addressed in the subsequent chapters. The preparation and characterization of polymer brushes is not trivial. In Chapter 2 limitations and challenges with focus on experimental reproducibility and thin film characterization are discussed. Chapter 3 discusses the influence of a number of factors, i.e. grafting density and molecular weight of the polymer brush, solvent quality and salt concentration in the incubation medium, on the degrafting process of hydrophilic brushes in aqueous media. Further, the degrafting behavior of polymer brushes prepared from different monomers is compared. It is attempted to correlate degrafting behavior with polymer brush swelling. Contradicting results concerning the actual cleaving point of chains in polymer brushes are found in literature. In Chapter 4 presents structures incorporating only one single potential cleaving point. However, non-specific degrafting could not completely be suppressed. Further, a disulfide bond and oxy-norbornene moiety, commonly used mechanophores, were introduced at the anchoring point. In Chapter 5 the limitation of unwanted background hydrolysis was circumvented by investigating degrafting processes in dry organic media. Specific cleavage of the two mechanophores, disulfide and oxy-norbornene, could be demonstrated.

EPFL2021

Influence of pH on dehalorespiring consortia & first steps in the development of a methodology for acid neutralization in enhanced in situ bioremediation

Nicole Munz

2010