Hi Fred
for what it’s worth, this has had me thinking too but I (think!) I have the measure of it (-ish) :-)
Going to separate the thing into two aspects as trying to deal with them both at the same time might get a wee bit confusing, and I think the two are not necessarily connected anyway.
Ok, firstly let’s do the colour thing first.
‘particularly black... For some daft reason I thought of tar, for example, as somehow not carbon related.. But found I was wrong’
I also thought about tar but then realised that tar is the charred remains of organic stuff like coal, oil, wool and so on. So what you have is particulate carbon suspended in high viscosity oils.
Good for roads when mixed with ground rock and solid matter.
Note, the particulate carbon.
Then I got to thinking about Bitumen (a.k.a. Asphalt in the US).
Bitumen is higher distillation fractions of oil and will consist of a grand mish-mash of phenols, napthols, long chain molecules and generally high molecular weight molecules.
Very thick, awful odour and if not black, certainly very dark brown. The colour will derive from the mix of the high molecular weight aromatics.
Note, there is no free carbon as such and so the bitumen will be expected to NOT conduct electricity.
I remember my dad constructing an underground power line to the hut which was at the bottom end of the garden-he dug up a channel starting close to an exit point from the house to lay the electric cabling in and made a junction box so that he could take power to the garage as well, the junction box was a large jam jar which he put the electrical connection block into with the appropriate wires connected to each other, then the jar was filled with molten bitumen. After it set he buried it and filled in the channel with earth and so on. The point being that the ‘junction jar’ was totally waterproof and insulated. (N.B. Not commenting on the pros/cons of the power line from the house...)
The thing is though that although bitumen is highly coloured, it’s not so dark as you might think and a thin layer will not show as dark as you think it would. I have in the past used bitumen as a darkening agent in my violin varnishes but don’t tell anyone :-)
Now Aniline Black (AB), that wonder stuff...
Massive conjugated aromatic ring system stabilised by 3 Chromium oxide ligands per AB molecule.
The dark/black colour happens because of the huge amount of conjugation with each other and aniline/benzene side chains and also, I suspect because of (the charge transfer between) the Chromium oxide ligands.
Now the crucial difference between this molecule and the high molecular weight fused aromatic mush that you’ll find in bitumen is that AB has Nitrogen atoms in it’s chain.
Immediately, that means that there is the possibility of interesting things happening upon the addition of an acid (i.e. H(+) ).
If you look at studies of Polyanilines (PANIs, of which AB is a member) you will see that electrical conductivity studies are done on them after acidification.
The long and the short of it is that upon acidification, the molecule will change form to enable conduction by a mechanism of extremely rapid bond rearrangement along the molecule to enable electron transfer along the molecule. Electron transfer equals carrying a current.
This is classical physics/chemistry Fred so don’t get over excited (I’ll take the pun) about the possibility of Quantum mechanics being involved.
Now this is where the base solvent for the paint comes in I think.
When it’s wet, it will be in an aqueous or alcoholic medium, which will be enough to protinate the AB so enabling this mechanism of electron transfers above. When dry, the AB will exist in the deprotinated state, so very much reduced to no ability for electron transfer.
Initially, I thought it was all about the Chromium ligands.
As for the black plastics, they seem to use aniline dyes, of which AB is an example.
By the way, why no?
‘Also, it seems its also probably safer not to wind a coil onto anything black! ;-) -Fred
Roy
