Dependence of the electroosmotic flow in bare fused-silica capillaries from pH, ionic strength and composition of electrolyte solutions tailored for protein capillary electrophoresis

This paper discusses the dependence of the electroosmotic flow (EOF) generated in bare fused-silica capillaries from composition, pH, and ionic strength of electrolyte solutions tailored for controlling the protonic equilibrium in a wide pH range and masking the silanol adsorption sites for proteins. These electrolyte solutions consist of the aliphatic oligoamine triethylentetramine (TETA) or diethylentriamine (DIEN) in combination with phosphoric acid. The paper evaluates the specific adsorption of the positively charged aliphatic oligoamines at the interface between the capillary wall and the electrolyte solution, resulting in a drastic variation of the positive charge density in the compact region of the electric double layer, which reduces the zeta potential and, hence, the EOF. When the positive charge density of the adsorbed cationic species exceeds the negative charge density on the capillary wall, resulting from the ionization of the silanol groups, the zeta potential becomes positive and the direction of the EOF is reversed from cathodic to anodic. The influence of pH, ionic strength and composition of the electrolyte solution on the specific adsorption of the selected cationic species on the inner surface of the capillary is evaluated in terms of the measured variation of zeta potential with varying the investigated parameters.