THEORETICAL-MODEL OF ELECTROOSMOTIC FLOW FOR CAPILLARY ZONE ELECTROPHORESIS

A mathematical model of electroosmotic now in capillary zone electroph oresis has been developed by taking into consideration of the ion-sele ctive properties of silica surfaces. The electroosmotic velocity was e xperimentally determined, under both constant voltage and constant cur rent conditions, by using the resistance-monitoring method. A detailed study of electroosmotic now characteristics in solutions of singly ch arged, strong electrolytes (NaCl, LiCl, KCl, NaBr, NaI, NaNO3, and NaC lO4), as well as the phosphate buffer system, revealed a linear correl ation between the 5 potential and the logarithm of the cation activity . These results suggest that the capillary surface behaves as an ion-s elective electrode. Consequently, the zeta potential can be calculated as a function of the composition and pH of the solution with the corr esponding modified Nernst equation for ion-selective electrodes. If th e viscosity and dielectric constant of the solution are known, the ele ctroosmotic velocity can then be accurately predicted by means of the Helmholtz-Smoluchowski equation. The proposed model has been successfu lly 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.