Electroosmosis

Electroosmotic flow arises from the formation of an electrical double layer at a solid-liquid interface. This is due to the presence of charged species on the solid surface; either in the form of surface ionized groups (e.g. SiO" in the case of silica) or because of the preferential adsorption of ions from the solution. In most cases it is a combination of both. The surface charges are counterbalanced by ions in solution, which form an immobile, strongly bound layer near the surface and a mobile, solvated layer extending into the liquid. Under the influence of an applied potential, the solvated layer of counterions moves, causing bulk solvent flow (Figure 1).

This means of inducing solvent flow has been successfully applied to capillary column chromatogra-phy, producing capillary electrochromatography (CEC). Using electroosmotic flow (EOF) to pump solvent through a column generates a 'plug' flow profile, which is distinct from the parabolic profile generated by hydraulic pumping. This results in reduced band broadening in CEC. It also allows the use of very fine chromatographic supports, which would be impossible to use in pressure-driven systems owing to back-pressure constraints. These factors combine to give high linear flow rates, of the order of 1 mm s"1, allowing very fast, efficient separations.

As with electrophoretic migration, the EOF velocity increases linearly with applied potential. This makes it generally desirable to apply higher potentials in order to achieve faster migration rates.

Surface silanol groups

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