Principle of Electroendosmosis

The static support, the stabilizing medium (e.g. the gel) and/or the surface of the separation equipment such as glass plates, tubes or capillaries can carry charged groups: e.g. carboxylic groups in starch and agarose, sulfonic groups in agarose, silicium oxide on glass surfaces. These groups become ionized in basic and neutral buffers: in the electric field they will be attracted by the anode. As they are fixed in the matrix, they can not migrate. This results in a compensation by the counterflow of H₃O⁺ ions towards the cathode: electroendosmosis.

In gels, this effect is observed as a water flow towards the cathode, which carries the solubilized substances along. The electrophoretic and electroosmotic migrations are then additive. The results are: blurred zones, and drying of the gel in the anodal area of flatbed gels.

When fixed groups are positively charged, the electroosmotic flow is directed towards the anode.

Electroendosmosis is normally seen as a negative effect, yet a few methods take advantage of this effect to achieve separation or detection results (MEKC in capillary electrophoresis and counter immunoelectrophoresis).