tim's journal

More insight on EDL

The EDL is more complex than I ever imagined... in fact, all the existing models are deficient in some way. There are a number of possible mechanisms and the truth is probably a combination of everything. Since superposition works for potentials, we can break down the EDL into several sub-EDLs. A book I found (citation to come later) on this topic broke it up into ionic and electronic double layers. Let's start with the electronic double layer.

Electronic double layer
Imagine am uncharged conductor, say a piece of platinum. Electrons, being free, protrude a little bit outside the boundary of the ions. The classical way of thinking about this is that the electrons are kept from flying out by the positively-charged ions, but they have some room to go a bit further. The quantum mechanical way to think about this is that with any finite barrier, the probability density leaks out, decaying exponentially. Either way, electrons exist a bit outside the boundaries of the conductor. This exterior cloud of electrons forms one layer of the the double layer. The positive ions left behind form another. Now, outside the electron cloud no charge is seen, since the net charge of the conductor is 0. Remember that the surface integral of the electric field flux is equal to the net charge enclosed. So, this clarifies an earlier question I had. If you plunk down a non-reactive conductor into a liquid you will not cause an ionic layer to form around the conductor. This can only happen if the conductor is charged.

Ionic double layer
The ionic double layer forms when the electrode has extra charge, in which case, ions outside the electrode in the electrolyte redistribute themselves (statistically). Ions with charge opposite the charge of the electrode will have a greater concentration by the electrode, while ions with the same charge will have a smaller concentration. How can the electrode have extra charge? Here's where it gets a bit complex. If the electrode is polarizable, meaning that there are no reactions, then the only way to give extra charge is to do something like charging it up with a battery. If the electrode is non-polarizable, it can become charged up because a chemical reaction at the interface contributes or consumes an electron. Reactions which are energetically favorable will occur, leaving the electrode charged, and an EDL.