Alan Turing is rightly famous for his contributions to computer science. But one of his core concepts – an autonomous system that can generate complex behavior with a few simple rules – also has applications in unexpected places, such as animal behavior. One area where Turing applies the concept himself is chemistry, and he published a paper describing how a single chemical reaction can create complex patterns like streaks if certain conditions are met.
It took us decades to figure out how to actually implement Turing’s ideas about chemistry, but we managed to create some reactions that exhibit the behavior he described. And now a team of Chinese researchers has figured out how to make something practical: a highly efficient desalination membrane.
From hypothesis to chemistry
Many chemical reactions eventually complete, with all possible reactants doing their thing and producing a product that is evenly distributed in the reaction chamber. But under the right conditions, some chemical reactions do not reach equilibrium. These reactions interested Turing, because 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 just mix the two in a reaction, the result just depends on the balance between these two chemicals. But as Turing showed, interesting things can happen when you spread them in a response from different locations. And if the two chemicals diffuse at different rates, you can get complex patterns or reaction products, such as spots or tiger stripes.
Turing’s paper describing these reactions was published in 1952; it wasn’t until the 1990s that anyone figured out how to make this possible. Now researchers may have discovered a way to put Turing’s ideas into practice.
The use they focused on was the production of membranes used in desalination. We already know how to control chemical reactions to make very thin membranes with many pores by putting reactants in separate solvents that do not mix. In this way, the membrane only forms at the interface between the water-based solution and the organic-based solution. While these membranes are very effective, they usually have to do with a trade-off: if you make a membrane so that water passes through it more easily, you also tend to let more salt through.
Stripes or dots?
To provide better control over a membrane reaction, the researchers used a system in which the chemical that makes up the membrane polymer was in an organic solvent, and a separate chemical that caused this reaction was dissolved in water. Separately, a molecule that inhibits the reaction was placed in the organic solvent so that the reaction was confined to the interface with water.
To make this a truly Turing-like system, the researchers dissolved a large molecule in water. This resulted in the water becoming more viscous, which slowed down the diffusion of the activator. In addition, the molecule was chosen so that the activator would stick to it, slowing it down even further. The end result was a system similar to the one defined more than half a century ago.
And it behaved just like Turing’s description. Depending on the precise details of the two solutions involved, the researchers could modify the system so that it would form a dense array of dots or a thick stripe pattern. Outside these areas, the membrane formed as usual, creating an impenetrable barrier.
Imaging of the features shows that instead of simply thickening the membrane, the membrane maintained the same width in these areas; instead, it bulged to form the structures. This is critical, because the amount of surface area exposed to a saline solution should affect the amount of water that passes through the membrane. The researchers even confirmed that more water was purified when the new membranes were used (the version with the stripes outperformed the dotted one). Unfortunately, the researchers are not comparing this system with commercially available membranes.
The researchers wanted to confirm that the visible (using microscopy) physical features were responsible for the improved performance and not unusual polymer-level chemistry. So they put some nanoparticles in the water they pushed through the membrane, which eventually settled around the bulges in the membrane, confirming that more water flowed through the membrane at these points.
It took 40 years for someone to follow up on Turing’s hypothesis and another 25 to produce something potentially useful. Science doesn’t necessarily give us a quick return on investment, but improvements in desalination and water treatment can be a big help. A lack of clean water has already led many countries to try desalination and water recycling, and the energy savings from a more efficient membrane can be significant.
Science2017. DOI: 10.1126/science.aar6308 (About DOIs).