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Electric double layer

The electric double layer or EDL occurs at the interface between electrolytes such as salt water and electrodes such as gold. How does this happen? Does it happen just between metals and electrolytes? Understanding this is crucial to addressing the swtich from a non-polarized electrode to a polarized electrode. I should have been documenting resources as I found them but I suppose it isn't too late to start. A good start, with pictures, can be found here. Ahh, and Daniel Laser at Stanford might know something....

It seems that quite a few resources, like the ones above, talk about the EDL forming because of some sort of oxidation or reduction reaction (even without current). However, I'm interested in seeing if an EDL forms without any such reaction, like in platinum. What happens if there is no reaction, is there EDL? What if the electrode is coated with an insulating material? What if the material is non-polarizable (is that really possible)? Help!

EDL and Microfluidics - Electroosmosis
I looked at bit closer at Daniel Laser's page and realized that it was about microfluidics. This also sort of answers my question about insulators and the double layer. In this case silicon oxide makes contact with water. A reaction occurs at the surface, which makes H+ and adds electrons to the surface of the silicon oxide. A double layer forms as a result. The focus here is not the EDL but the H+ ions that are formed. When a potential is applied across the channel, the H+ ions are pulled towards the cathode (or is it the anode - ugh!) which in turns drags everything else. I found more details here. This still doesn't answer the question I had about platinum, which does not react, but it does suggest that having an insulating layer is no good if it isn't something that won't react as well.

Ahh, something interesting. In this article about electroosmosis, it states that "most surfaces spontaneously develop an electric double layer when brought into contact with either weak or strong electrolyte solutions. This charge generation is caused by electrochemical reactions at the liquid/solid interface..." Ok, so hat if there is no reaction, like platinum, or stainless steel? Is there still an EDL without applied voltage? The reference for that statement is Hunter's "Zeta Potential in Colliod Science".

EDL and AFM
EDL has recently been used to improve atomic force microscopy (AFM). For those of you that are unfamiliar with the technique, AFM basically involves a micromachined cantilever with a very sharp tip jutting out at the end. As the cantilever is dragged across the surface it basically traces out the atoms on the surface allowing to you to see the topography of the surface. How does this relate to the EDL? I found this article by Sokolov et al. One of the problems in AFM is the attractive van der Waals force between the tip and the sample. Sokolov et al. propose using an EDL to shield some of the van der Waals forces.

The answer?
I looked up "electric double layer" and "platinum" on Google, and voila, I found what I was looking for... I think. I found an article from korea about an EDL-based battery which I had skipped over ealier. A polarizable electrode made of carbon is used. In figure 2, it shows the battery under discharged and charged conditions. Clearly, when the battery is discharged there is no double layer, but when the battery is charged there is. Nicely, this gives a good review of all the different double layer models AND uses a polarizable electrode. Still it isn't exactly what I'm looking for but the references will be of great value.

The problem.
This is what it says in Webster's book.

When a polarizable electrode is in contact with an electrolyte, space charge forms in two layers at the interface to produce the half-cell potential and a capacitive reactance for current flow. If the electrode is moved so that the space charge distribution is altered, then there is momentary change of the half-cell potential until equilibrium is reestablished. If
an electrode pair is in an electrolyte and one moves with respect to the other, a potential difference appears between the two electrodes during this movement. This potential is a motion artifact associated with a polarizable electrode and can be a serious interference source in recording biopotentials. From the equivalent circuit, there are two potential sources of motion artifact: Ehe and Ese. To minimize these effects, the polarizability of the electrode-skin interface must be reduced. The best way to do this is to insure good contact between the electrode and the inner layers of the epidermis, meaning that something must e done to the skin surface (i.e., abrasion) to minimize polarization [1].

What is this half-cell potential Neuman is talking about? If there is no oxidation or reaction, then where does this come from? Oh man, I'm getting tired.

Reference
[1] M. R. Neuman, "Biopotential Electrodes," in Medical instrumentation : application and design, J. G. Webster and J. W. Clark, Eds., 3rd ed. New York: Wiley, 1998, pp. 183-232.

Posted by torque at August 11, 2003 11:56 PM | TrackBack