28 October 2011 - Organic chemistry
Publication: Cergol et al., Angewandte 2011 DOI
Extension of: Dendralene chemistry
Key transformation: Grieco elimination
Challenges met: only one of several methods worked
Synthetic steps: a] chloroprene, magnesium, zinc bromide,dibromoethane (Grignard reagent) b] vinylidene chloride, NiCl2(dpppe), THF c] Mg, THF, ZnBr2,formaldehyde, e] 2-nitrophenyl selenocyanate, tributylphosphine f] meta-chloroperbenzoic acid , chloroform , 0°C.
Properties: decomposes above 0°C via multiple Diels-Alder pathways
Envisioned application: Rapid synthesis of complex molecules, reacts with three equivalents of maleimide to a complex hexacycle.
Properties along a gradient
24 October 2011 - Physical chemistry
|This blog is the proud owner of a 500 ml volumetric flask filled to the rim with PET granules, a souvenir from a botched career in plastics recycling. Should the flask ever become the topic of scrutiny in a chemical laboratory, the lab technicians may be surprised to find that the melt viscosity of the granules increase going from the bottom of the flask to the top. Pretty irrelevant of course, the flask was filled with PET samples in order of viscosity for no reason at all. |
Functionally graded materials on the other hand are well known in industry and are often based on composites with varying composition along a gradient. Moving on to the micro- and nanoscale, gradient materials are an active research field. Imagine a surface that is superhydrophobic on one end and smoothly becomes superhydrophilic when going to the other end. The surface gradient material demonstrated by Yu in 2006 (DOI) is based on a gold layer covered by gold clusters by electrodeposition. This surface is very rough and therefore very hydrophobic (see Cassie's law). Hydrophobicity is further increased by covering the surface with a monolayer a long aliphatic thiol (ethanol solution dip) and because the surface coverage is related to immersion time a gradient presents itself. The unoccupied positions are then filled by a thioalcohol again by dipping making the surface increasingly hydrophilic. In similar configurations a water droplet set loose on a gradient surface will be able to propel itself to the other side.
Goldflam et al. have recently demonstrated a gradient in a so-called split-ring resonator used in negative refractive index research(DOI). The SSR is built on a sapphire layer from vanadium oxide by lithography. In the far-infrared an absorption maximum is evident regardless of the position on the layer. However when a voltage is applied each spot on the layer now has its own absorption characteristics. The cause is unclear but it has to do with local heating inducing a vanadium oxide phase transition from an insulator to a metal. The link with refraction is not directly obvious.
Ahmed et al. let gravity to all the work (DOI). They are into the gradient porous material business and mention nature's engineering skills in bone biosynthesis. Their recipe for gradient porous material starts with an oil-in-water emulsion made from cyclohexane and aqueous polyvinyl alcohol. This emulsion is centrifuged and rapidly cooled and then solvent is removed. The material can be fortified by adding silica particles (Stöber process) which travel in the opposite direction as the oil droplets.
We can even narrow our view to the molecular level to look for gradients, for example in gradient copolymers. In one demonstration Nakamo et al. polymerise racemic propylene oxide to polypropylene carbonate using a special chiral catalyst (DOI). This catalyst converts the (S) monomer twice as fast as the (R) monomer making the chain a stereogradient starting completely isotactic and then gradually becoming isotactic. Turning this copolymer into a gradient material is another challenge and then finding something useful to do with it an even bigger one. One surprising property is higher thermal decomposition temperature compared to either enantiomeric polymers of the stereoblock copolymer: something to do with defeating intramolecular stereocomplex formation.
Early carbon dioxide warning system
09 October 2011 - Nice to have
|The next invention may be of interest to you if your government plans to build a carbon dioxide storage facility right where you live. Remember that CO2 is naturally present in air in a concentration of 0.04%, a concentration of 3% is uncomfortable and 4% is lethal. A low-tech early warning device is more than welcome. In a recent publication an Australian team with first author Darwish (DOI) describes one based on a solution containing a special dye. Bubble air through it and a specific color change will signal when to start run for it, just like with that coal mine canary.|
Here is how it works. The commercially available photochromic dye 1 was converted in two steps into amidine 4. In a methanol/water solvent the merocyanine zwitterion state 5 is more stable. The solution colors purple. Adding carbon dioxide first forms carbonic acid which then protonates the phenolic group and also the amidine group to 6. A concentration of 1% CO2 is sufficient to color the solution yellow and this reaction is completely reversible. With respect to interference from other gases, sulfur dioxide is reported to spoil the system but not carbon monoxide.
|You could easily mistake the image on the left for a collection of stars in the ever expanding universe but that was yesterdays physics Nobel. It is in fact the diffraction pattern of an icosahedral Zn-Mg-Ho quasicrystal.|
Dan Shechtman was on nobody's shortlist for the 2011 edition of the Nobel Prize in Chemistry but at least Wikipedia was well prepared. The quasicrystal article was started already in 2002 with heavy involvement of mathematicians so it appears and not chemists or even physicists. The Shechtman bio page was started in 2006. Shechtman had a hard time convincing his science colleagues that crystals do not always repeat themselves when he made the discovery back in 1982. He was even expelled from one of his workgroups as a heretic.
Do read Derek (Paulings questionable involvement), do read chembark (a physicist stole the chemistry prize) or the very brief drfreddy. More background at physorg.com, CEN, nobel.org or NYT
The regular media should have no problem shipping the news to the general public. Even if the discovery seems esoteric, anyone can appreciate the similarity between a nanoscopic quasicrystal and a macroscopic Penrose tiling. However, just as last year the two main television news bulletins in the Netherlands choose to ignore it. The plight of the Japanese otter and children's eating habits (no vegetables!) were deemed more important.
And finally, anyone in for a conspiracy theory? In terms of practical applications we hear a lot of hardened cooking ware or extra sharp razor blades. However, a quick internet search leaves us empty handed. You cannot buy anything. But how about secret quasicrystal coatings for certain stealth bombers? Several UFO aware websites are full of it and a few patents make a mention. Was Shechtman discouraged from pursuing his research by the Americans, afraid their big secret got out? Investigate!
02 October 2011 - Making it move VI
|As part of our ongoing coverage of chemical making-it-move experiments, see part V, IV, III, II and I, now a brief summary of the recent attempt by Pavlick et al. at making a bead of silica move purposely in an organic solvent swimming pool (DOI). Here is the basic idea. |
Take silica microspheres (see Stöber process), form a dense monolayer on a glass slide by pulling the slide through a biphasic system of hexanes and silica in ethanol and drying. Add a layer of chromium (7 nm) and then gold (56 nm) by sputter deposition. Free the spheres from the slide by sonication. The particles have now only one hemisphere covered by gold and hence are called Janus particles. Add a linker synthesised from 3-(chloro)propyl trimethoxy silane and 1-methylimidazole to the silica covered part and functionalise further with Grubbs' catalyst. Then set a microsphere loose in a trichloroethane solution of norbornene and track movement via digital video.
The spheres were found to move around at a speed of 0.30 micrometer per second and with some sense of direction. In one occasion a particle started of in a northbound direction then steered west and then headed south. The track was wobbly at best. A similar particle but without the gold treatment also moved around a lot but ended his random walk exactly where it had started.
The Grubbs' catalyst obviously converts norbornene to its polymer but only on one side of the particle. At the silica front monomer is depleted and a solvent flow starts up towards the back as a result of osmosis creating a motion against the flow. This proof of concept is for a change not all-play-and-no-work as Pavlick notes that in biology the bacterium listeria is known to propel itself by actin polymerization.