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MrPlow writes:

Submitted via IRC for SoyCow3941

A chemical reaction [Alan Turing] suggested can now be done, and it makes a great membrane.

Many chemical reactions end up going to completion, with all the possible reactants doing their thing and producing a product that's distributed uniformly within the reaction chamber. But under the right conditions, some chemical reactions don't reach equilibrium. These reactions are what interested Turing, since they could generate complex patterns.

Turing's paper on the topic focused on a reaction that could be controlled by the addition of two chemicals: an activator that promotes it and an inhibitor that slows it down. If you simply mix the two into a reaction, the outcome will simply depend on the balance between these two chemicals. But as Turing showed, interesting things can happen if you diffuse them into a reaction from different locations. And if the two chemicals diffuse at different rates, you can get complex patterns or reaction products like spots or tiger stripes.

Turing's paper describing these reactions came out in 1952; it wasn't until the 1990s that someone actually figured out how to make this happen. Now, researchers may have discovered a way to put Turing's ideas to practical use.

The use they focused on was the production of membranes used in desalination. We already know how to arrange chemical reactions to make very thin membranes with lots of pores by putting reactants in separate solvents that don't mix. That way, the membrane only forms at the interface between the water-based solution and the organic-based solution. While these membranes are highly effective, they typically face a trade-off: if you make a membrane so that water passes through more easily, you tend to allow more salt to pass through as well.

To provide finer control over a membrane reaction, the researchers used a system in which the chemical that forms the membrane polymer was in an organic solvent, and a separate chemical that triggered this reaction was dissolved in water. Separately, a molecule that inhibits the reaction was placed in the organic solvent, ensuring that the reaction was limited to the interface with water.

To make this a true Turing-style system, the researchers dissolved a large molecule in water. This had the effect of making the water more viscous, which slowed the diffusion of the activator. In addition, the molecule was chosen so that the activator would stick to it, slowing things down even further. The end result was a system similar to the ones defined over a half-century ago.

Source: https://arstechnica.com/science/2018/05/alan-turings-contribution-to-chemistry-used-to-filter-salt-water/

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