In my line of work sometimes it gets difficult explaining certain aspects of why we do what we do when making alcohol. One of these involves carbon, and more specifically, just how does it work. So I thought it was about time to clear up the confusion and get the story straight. The following is more a of a scientific explanation of just how carbon works, but hopefully it will give you better idea of exactly why we carbon filter.
First thing, some substances have the ability to attract and hang on to certain molecules by electrostatic attraction. Forget all the buzz words like Van der Waals forces and London forces (if you’ve ever read into how carbon works you may have come across these terms before). They all come down to essentially the same thing as a charged comb attracting a bit of paper, the only difference being the distance the attraction works over. Carbon is particularly good at this attraction thing which might explain why diamonds attract so many bits of fluff (diamonds are packed with dense carbon particles)
The other thing about carbon is that it can be prepared so that its structure resembles a sponge, with millions of tiny passages and holes in it (usually by a process of special aeration). This preparation is loosely called “activation”. These passages and holes can control what molecules get deep inside the carbon, and which cannot…..a purely physical matter of size. It is this physical structure that primarily that primarily governs wether a particular ‘activated’ carbon can be used to ‘target’ molecules of a particular range of sizes. If you are making a respirator, then you will want to know all about that, as some respirator cartridges are best for one range of gases, but not others. In our case, we need not be as fussy as all we are dealing with are very small molecules of water and relatively huge hydrocarbon molecules….the alcohols etc. The water can penetrate all the way into average activated carbon, but the large hydrocarbons can only penetrate by various amounts, according to how big they are. Just a simple sieve thing.
The other thing about a sponge structure is that it presents a huge surface area for molecules to stick to (the use of special manufacturing techniques results in highly porous charcoals that have surface areas of 300-2,000 square metres per gram….per gram!!!!)
So ‘activating’ carbon not only greatly increases its ability to deal with the quantity by having a huge surface area, but also offers a degree of selectivity by physically controlling access to this surface area.
The question of which hydro carbons are attracted strongly to the carbon surfaces they encounter, and those which are attracted weakly, can get a bit complicated. It is not simply a matter of size. Some hydrocarbon molecules, which alone would be attracted weakly, can form a loose association with water and then be strongly attracted to carbon. Water has a boomerang shape, with the two hydrogen atoms at the tips of the boomerang and the oxygen atom in the middle. This positive charge on the oxygen side (egghead/cocktail party term #1: it’s bi-polar). This enables water molecules to stick to parts of some hydrocarbons and give them a bunch of electrostatic anchors to hang onto a carbon surface.
Bottom line is that if a big hydrocarbon molecule can get to a carbon surface, and it is ‘sticky’ enough, then the carbon will onto it (egghead/cocktail party term #2: it absorbs the hydrocarbon molecule).
Bottom line to bottom line… it won’t necessarily stay stuck! Other molecules can come charging in and knock the molecule off the carbon surface it was stuck to (egghead/cocktail party term #3 adsorption is subject to dynamic equilibrium).
Absorption is primarily an electrostatic thing, so it follows that it is desirable to have some charge imbalance on the molecule you won’t to be held strongly. Yes, water is absorbed onto carbon, but you may recall that I said that absorption is also a dynamic equilibrium thing. Water molecules are tiny compared with hydrocarbon molecules, and their mass pales to insignificance when you add up the mass of all atoms in a large hydrocarbon molecule. Think of a large passenger liner coming into to dock and berthing. If any small boats are tied up at the quayside, what do you think their chances are of staying where they are when that massive liner comes bearing down on them? In a sumo wrestling match, the biggest guy usually wins!
Note: Alcohol is made up of hydrocarbons, the fusel oils and volatiles, which make up the ‘bad stuff’ spectrum and are what we are attempting to filter out, are made up of a different type of hydrocarbon, that is more attracted to carbon and slightly larger molecule the normal hydrocarbons.
In summary what have we learnt. Activated carbon is a type of treated carbon, which creates millions of passage ways to trap large hydrocarbon molecules (alcohol and in particular ethanol, which is what we produce when we make our own spirits, is made up of hydrocarbons) and let smaller particles through.
So if the fusel oils and volatiles are made up of bigger, and more attracted particles, so in turn more of them will stick to the carbon, and eventually they will knock most other particles off the carbon, resulting in the ‘bad stuff’ sticking, and the good stuff flowing through.
I hope this has somewhat helped you in understanding just how carbon has helped..