We can reverse the direction of electroosmotic flow in capillary electrophoresis by adding an alkylammonium salt to the buffer solution. As shown in (b), the positively charged end of the alkyl ammonium ions bind to the negatively charged silanate ions on the capillary’s walls. The tail of the alkyl ammonium ion is hydrophobic and associates with the tail of another alkyl ammonium ion. The result is a layer of positive charges that attract anions in the buffer solution. The migration of these solvated anions toward the anode reverses the electroosmotic flow’s direction. The order of elution is exactly opposite that observed under the normal conditions shown in (a).
