THEORETICAL-MODEL OF ELECTROOSMOTIC FLOW FOR CAPILLARY ZONE ELECTROPHORESIS

A mathematical model of electroosmotic now in capillary zone electrophoresis has been developed by taking into consideration of the ion-selective properties of silica surfaces. The electroosmotic velocity was experimentally determined, under both constant voltage and constant current conditions, by using the resistance-monitoring method. A detailed study of electroosmotic now characteristics in solutions of singly charged, strong electrolytes (NaCl, LiCl, KCl, NaBr, NaI, NaNO3, and NaClO4), as well as the phosphate buffer system, revealed a linear correlation between the 5 potential and the logarithm of the cation activity. These results suggest that the capillary surface behaves as an ion-selective electrode. Consequently, the zeta potential can be calculated as a function of the composition and pH of the solution with the corresponding modified Nernst equation for ion-selective electrodes. If the viscosity and dielectric constant of the solution are known, the electroosmotic velocity can then be accurately predicted by means of the Helmholtz-Smoluchowski equation. The proposed model has been successfully applied to phosphate buffer solutions in the range of pH from 4 to 10, containing sodium chloride from 5 to 15 mM, resulting in similar to 3% error in the estimation of the electroosmotic velocity.