How do the molecules align in carbon materials and form these coloured regions under polarised light? Image Credit: link |
tl:dr The self-assembly of disc-shaped molecules into aligned regions is important for making synthetic graphite for batteries and understanding how soot pollution forms. We showed in this paper how curving the molecules disrupts this molecular alignment a process that had been long hypothesised. Check out the preprint or the paper recently published in the journal Carbon.
Self-assembly and liquid crystal displays
Getting molecules to line up is more important than you might think. Liquid crystal displays (LCD) work by aligning and misaligning rod-shaped molecules using an electric field to let through or block polarised light.
Image Credit:link |
Image Credit: link |
The molecules in liquid crystal displays are rod-like (Calamitic) and they form ordered configurations. These are not truly crystalline with solid and liquid phases but are disordered phases and are therefore called mesophases.
Image Credit: link |
These aligned regions can be nicely visualised through cross-polarisers and provide for some stunning images.
Image Credit: link |
The most important liquid crystal structure for carbon materials science was discovered in India in 1977. Chandrasekhar, a world leader in liquid crystal structure based at the Raman Research Institute discovered that disc-like molecules can form liquid crystals. These discotic structures form similar configurations to the rod-shaped molecules used in LCD displays but have some unique electronic properties. But this type of alignment is critical for making carbon fibres, synthetic graphite for electrodes in electric motors and it is also important in making graphite for batteries. But to understand this a little bit of a historical digression is helpful.
Discovery of the mesophase
Two Australian scientists Geoffrey Taylor and J.D. Brooks were exploring the geology of the Wongawilli coal seam in New South Wales in Australia in the 1960s (see below the picture of some of this coal coming out of the ground at the beach in Sydney).
Image Credit: link |
In parts of this rock formation, ancient magma had pushed its way between the coal seam and led to some heat-treated regions of the coal (these are often called cokes). This provided a nice thermal gradient in the coal from the molten magma at thousands of degrees to low temperature as you went further away from the magma. This provided a fossilised record of the impact of heat on coals structure.
Image Credit: link |
Looking under the microscope with a polarising lens Taylor and Brooks observed spheres where the molecules were all aligned.
Image Credit: Harry Marsh |
These were called mesophase spheres and are regions where all of the graphitic molecules are aligned in stacks. This happens when the heat from the magma melts the molecules and they can start to align in a mesophase.
Image Credit: link |
In their 1965 Nature paper, Brooks and Taylor showed that by heating up specific disc-shaped (discotic) molecules extracted from coal (pitch) they could reproduce this effect in the lab. They also observed that the spheres would fuse together and form a continuously ordered phase that was only explained fully by Chandrasekhar in 1977.
Image Credit: link |
One of my favourite pictures of this is an electron microscope image showing one half where the spheres have merged and the other where they are still separated.
Image Credit: link |
Since the 1960s, this technique has allowed for synthetic graphite to be made in large quantities for electrodes, batteries and carbon fibres. However, only very special pitches from fossil fuels will form a mesophase (so-called mesophase pitches). They are hard to make meaning synthetic graphites are still expensive.
In particular, it is not clear why almost all carbon-rich materials, such as cellulose in wood, do not form mesophases and instead form disordered forms of carbon. So recently a new push has been made to understand this molecular alignment. In what follows some very recent work other groups have recently published on the molecules present in pitch and then some of our work using computer simulations to look at mesophase development.
Observing the molecules
So what do these mesophase pitch molecules look like? Only very recently have researchers been able to image the molecules using a technique called non-contact atomic force microscopy. This technique attaches a carbon monoxide atom to the end of a sharp needle. This is wobbled electronically using a tuning fork and the interaction of the carbon monoxide tip and the molecule allows for a picture of the bonding network in aromatic molecules to be imaged.
Image credit: IBM Research Zurich |
The pitch molecules can be seen in the figure below. The molecules all have a basic aromatic domain where the carbon atoms are arranged in a hexagonal "chicken wire". There are also small chains or hydrocarbons on the edge of these molecules. The raw pictures and the drawings derived from these images are shown below.
Image Credit: Used with permission from Elsevier. Scale bar in the AFM images is |
Image Credit: Jacob Martin CC-ND |
In order to look at how these molecules align in the mesophase, we made use of computer simulations.
Aligning mixtures of disc-like molecules
Image Credit: Kimberly Bowal |
Impact of curvature on the mesophase
Curvature is found in the carbon materials formed from materials that do not form a mesophase and has long been suggested to disrupt the formation of a mesophase (I have a previous blog post on how curvature is integrated into aromatic molecules through pentagonal rings). I have also recently shown that in order to simulate these curved species correctly the flexoelectric dipole must be correctly described which Kimberly Bowal and I developed in a series of papers.
We made use of the replica exchange molecular dynamics approach described before and were able to find the most stable configurations of clusters of mixed sized curved PAH (see below).
Image Credit: link |
There may be some hope for transforming more materials into graphite as curved PAH are known to orient themselves in electric fields as they are polar. Adding aliphatic chains on the edge of cPAH has also allowed for aligned columnar stacking of curved PAH.
No comments:
Post a Comment