In today’s post I’d like to share with you some physical chemistry regarding the self-assembly of lignin. And here we get to the exciting world of Waterman polyhedra. Have a look at this video (not made by me):
It depicts the way you can make a large sphere with smaller spheres. At the same time, it’s quite complicated to explain mathematically, but pretty intuitive to grasp visually. As the above video illustrates, you can make a sphere by adding hexagonal and square layers of spheres onto a shape called a truncated octahedron. The simplest such structure has an edge length of two spheres. This I call a Waterman cluster, like the one below:
Just to state the obvious, why cannot a truncated octahedron have edge length of one sphere? It sort of can, but in that case the shape is a single sphere and no longer a cluster of spheres.
So what happens when colloidal lignin particles self-assemble, like in our paper with over 500 citations? Lignin is first dissolved in a solvent. In our case, the solvent is called tetrahydrofuran, or THF. THF is such a good solvent that it can dissolve individual lignin polymers. That is the supramolecular shell of THF molecules can encapsulate a single lignin molecule. While a lignin molecule is a relatively large polymer, a single lignin molecule does not show a clear shape. Only when you intentionally reduce THF’s ability to dissolve lignin are multiple lignin molecules able to form their own supramolecular structure. This is done by adding a THF solution of lignin to water. Exactly what happens here is quite complicated, so I’ll try to keep things simple.
What I think takes place is that a when there is water present in just the right amount, there isn’t just THF present anymore, but the THF forms a supramolecular shell around a solid core of water. And if the amount of water is just right, the shell isn’t too small, so that lignin can self-assemble to its natural shape of a hollow nanotubule (sort of a cylinder, except the shape is a bit more complicated).
These cylinders trapped in a THF shell have a propensity to fuse together (or crystallize) into long hollow tubules. If the conditions are just right, this crystallization takes place in a very ordered manner, forming hexagonal and square crystallization patterns.
Below is the shape depicted with the lignin tubules (comprised of several lignin molecules) already in the final alignment, but without them being fused.
However, most often, all of this takes place in a blink of an eye in a beaker. Thus, when one tries to see this shape with an electron microscope, what you usually see is this:
A bunch of more or less spherical particles. These are absolutely gold, as far as their properties go. No complaining about that. However, you can’t really see what’s taking place in their self-assembly.
However, if you know what to expect, you can do a couple of tricks. You can intentionally stop the process of self-assembly and attempt to see individual hexagonal sheets of nanotubules. And this is exactly what I did, already some (two or three) years ago:
Except when I had my results, the people I showed this image weren’t too excited. I did already have a theory of lignin nanotubules, but I didn’t have a theory of supramolecular shells of solvent. My theory just seemed too far-reaching and hypothetical.
So now I’ve come full circle and I need to show the electron microscope image of lignin self-assembly to show to the editor of Nature, that my theory of supramolecular shells isn’t pure speculation. It remains to be seen whether the editor is convinced. They don’t need to accept that what I say is correct. That’s left for the reviewers. But it’s the role of the editor to weed out manuscripts that aren’t important enough to even to send for peer review. I surely hope I’ll pass this hurdle.
Anyhow, if Steve Waterman hadn’t come up with his idea back in 1990, I’m not sure whether I’d figured all of this out by myself. So, thank you, mr. Waterman!
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