The Chemical History of a Candle and Structure of an Ember - Webinar

A webinar presented on the 6th of May 2020 for the Churchill College MCR, University of Cambridge, by Jacob W. Martin.

Abstract: Grab a hot chocolate and get cosy for a fireside chat about a candle's flame and the embers left behind. Following the lead of Michael Faraday in his 1848 Royal Institution Christmas lectures, I will provide an updated chemical history of a candle - including some experiments for you to try at home. The fun doesn't end there - after a flame is extinguished, there's a wealth of discoveries to be made in the embers. Understanding the structure of these carbon materials begins with Rosalind Franklin, best-known for her work on DNA, and continues with simulations on a supercomputer. While there are still many unanswered questions around flames and carbon materials, these recent insights are enlightening and important for cleaning up our planet.

Fingerprinting soot: finding curved aromatics in soot

Fingerprints of molecules in soot particles imaged in an electron microscope showing curved species in the early flame. 

Fingerprinting is a unique way of identifying people by reading off the ridges found on a person's fingers. In a new paper, I and some colleagues used an electron microscope to image the fingerprint of aromatic molecules in early soot particles, finding evidence for a large number of curved molecules, which could be important for reducing soot pollution. Here is a link to the paper or to the open access preprint, "Flexoelectricity and the Formation of Carbon Nanoparticles in Flames". Here is a short video explaining the findings.

Soot emissions cause many deaths around the world while also contributing to global warming, but scientists are still at a loss to explain how soot is formed. We have previously suggested a new mechanism where aromatic molecules curve via pentagon integration and become electrically polarised, interacting strongly with charged species produced in the flame. However, as of yet, no one had measured just how curved (and therefore polar) the molecules are in the early soot particles.

To answer this question we imaged the molecules in soot on the nanoscale. To collect some soot we injected a small copper grid covered in amorphous carbon into a flame very similar to a candle. Soot stuck to the amorphous carbon and using an electron microscope we were able to image the molecules in the soot particles sticking to the grid. As you can see in the first image of this post we could convert the dark regions of the image, corresponding to aromatic molecules on their side, into lines. We could determine how many molecules that we imaged indicate pentagonal ring integration and we found this value to be greater than 62.5% of the fringes. This high amount suggests that they are important for soot formation.

We found even in the earliest soot particles sampled at the lowest height, which give us the most insight into soot formation, long fringes 0.9-1.0 nm in length are present (around 15 aromatic rings). We then simulated three curved aromatic molecules of the same length with one, two and three pentagonal rings, providing different amounts of curvature, and computed the tortuosity/curvature. From this analysis, we could conclude that early soot particles (at 10 mm above the flame) have a tortuosity/curvature indicating that two pentagons rings are integrated. Below is a figure of the molecule which corresponds to the average species around 1 nm in width with two pentagonal carbon rings. 

 

Computing the electric polarisation due to the curved structure, we found a large value of 5.32 debye - around three times that of water, which is substantial. Below is a plot of the electric potential around the molecules introduced earlier.

We found this molecule bound very strongly to charged chemi-ions which are produced in abundance in the flame, using computer simulations. 

The strong interactions of these polar aromatic with chemi-ions need to be explored in more detail as it might help explain many of the electrical effects that have been seen, such as the ability of an electric field to stop soot formation in certain circumstances.