Turing structures at the nanoscale
Turing structures arise when imbalances in diffusion rates make a stable steady-state system sensitive to small heterogeneous perturbations. For example, Turing patterns occur in chemical reactions when a fast-moving inhibitor controls the motion of a slower-moving activator. Tan et al. grew polyamide membranes by using interfacial polymerization, where the reactions occur at the interface between oil and water layers. The addition of polyvinyl alcohol to the aqueous phase reduced the diffusion of the monomer. This process generates membranes with more bumps, voids, and islands, which prove to be better for water desalination.
The emergence of Turing structures is of fundamental importance, and designing these structures and developing their applications have practical effects in chemistry and biology. We use a facile route based on interfacial polymerization to generate Turing-type polyamide membranes for water purification. Manipulation of shapes by control of reaction conditions enabled the creation of membranes with bubble or tube structures. These membranes exhibit excellent water-salt separation performance that surpasses the upper-bound line of traditional desalination membranes. Furthermore, we show the existence of high water permeability sites in the Turing structures, where water transport through the membranes is enhanced.
A chemical reaction he suggested can now be done, and it makes a great membrane.
Alan Turing is rightly famed for his contributions to computer science. But one of his key concepts—an autonomous system that can generate complex behavior from a few simple rules—also has applications in unexpected places, like animal behavior. One area where Turing himself applied the concept is in chemistry, and he published a paper describing how a single chemical reaction could create complex patterns like stripes if certain conditions are met.
It took us decades to figure out how to actually implement Turing’s ideas about chemistry, but we’ve managed to create a number of reactions that display the behaviors he described. And now, a team of Chinese researchers has figured out how to use them to make something practical: a highly efficient desalination membrane.
From hypothesis to chemistry
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.
Stripes or dots?
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.
And it behaved much like Turing’s description. Depending on the precise details of the two solutions involved, the researchers could tweak the system so that it would form a dense array of dots or a thick pattern of stripes. Outside of these areas, the membrane formed as usual, creating an impermeable barrier.
Imaging of the features show that rather than simply thickening the membrane, the membrane retained the same width in these areas; instead, it bulged out to form the structures. That’s critical, as the amount of surface area exposed to a salt solution should influence how much water gets through the membrane. In fact, the researchers confirmed that more water was purified when the new membranes were used (the version with the stripes outperformed the dotted one). Unfortunately, the researchers don’t compare this system to commercially available membranes.
The researchers wanted to confirm that the visible (using microscopy) physical features were responsible for the improved performance and not some unusual chemistry at the polymer level. So they put some nano-sized particles in the water that they pushed through the membrane, which ended up clustered around the bulges in the membrane, confirming that more water was passing across the membrane at these points.
It took 40 years for anyone to follow up on Turing’s hypothesis and another 25 to produce something that’s potentially useful. Science doesn’t necessarily give us a quick return on investment, but improvements in desalination and water purification could be a big help. A scarcity of clean water has already caused many countries to try desalination and water recycling, and the energy savings of a more efficient membrane could be substantial.