Control of electroosmotic flow in zirconia-coated capillaries

The inner surface of a silica capillary has been modified using an anhydrous sol-gel method: zirconium propoxide reacted with silanol groups to give, after hydrolysis, a "zirconia-like" surface. The electroosmotic properties of zirconia-modified capillaries have been studied under various conditions: working parameters were nature and concentration of the electrolyte, ionic strength, pH and solvent composition. Using different ions such as sodium, potassium, chloride, nitrate or methanoate, a positive electroosmotic flow (cathodic flow) was observed in the pH range 5-11. But below pH 5, a negative electroosmotic flow (anodic flow) occurred, corresponding to a permanent positive surface charge of the capillary walls. The value of the electroosmotic flow (including the sign) can be easily controlled by adding multivalent ions in the electrolyte. Some of these modifications of surface charge can be made irreversible. The observed electroosmotic flow has been related to surface characteristics using the triple layer model. With the large set of working parameters, the optimisation of separations can be facilitated. An example of flow control is given for the separation of four antihistaminic compounds.