ELECTROPHORESIS IN FUSED-SILICA CAPILLARIES - THE INFLUENCE OF ORGANIC-SOLVENTS ON THE ELECTROOSMOTIC VELOCITY AND THE ZETA-POTENTIAL

The influence of pH and solvent composition on the electroosmotic flow in fused-silica capillaries was studied. Binary mixtures of water and a protic (methanol, ethanol, 2-propanol) or an aprotic dipolar solvent (acetonitrile, acetone, dimethyl sulfoxide) were used for the buffer electrolyte. The dependence of the electroosmotic flow on the pH was determined in the pH range from 3 to 11 for pure aqueous solution and for solutions containing 50% (v/v) organic solvent. Adding organic solvents to the electrolyte, the infection points of the resulting curves, corresponding to the pK values of the surface silanol groups, are shifted to higher values. At high pH, increasing the fraction of organic cosolvent generally decreases the electroosmotic velocity. Zeta potentials were calculated from von Smoluchowski's equation using electroosmotic velocity data and values for the ratio of the viscosity coefficient and dielectric constant for binary mixtures, taken from own measurements and from the literature. With the exception of acetone-water mixtures, the zeta potentials obtained show a similar trend as the electroosmotic velocity: they decrease with increasing content of organic solvent. This trend is explained by changes of the dielectric properties of the electric double layer and of the charge generation on the fused-silica surface.