Dependence of the Electroosmotic Flow in Bare Fused-Silica Capillaries from pH, Ionic Strength and Composition of Electrolyte Solutions Tailored for Protein Capillary Zone Electrophoresis D. Corradini* / L. Sprecacenere Istituto di Metodologie Chimiche del CNR – Area della Ricerca di Roma – P.O. Box 10, I00016 Monterotondo Stazione (Rome) Italy; E-Mail: danilo.corradini@mlib.cnr.it Key Words Capillary electrophoresis Electroosmotic flow Alkylamine phosphate buffers Proteins Summary 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. Introduction The electroosmotic flow (EOF) in open capillary tubes is generated by the effect of the applied electric field across the tube on the uneven distribution of ions in the electric double layer at the interface be- tween the capillary wall and the electro- lyte solution. In bare fused-silica capillaries, ionizable silanol groups are present at the surface of the capillary wall exposed to the electrolyte solution. In this case, the electric double layer is the result of the excess of cations in the solution in contact with the capillary tube to balance the negative charges on the wall arising from the ionization of the silanol groups. Part of the excess cations are firmly held in the region of the double layer closer to the capillary wall (the compact or Stern layer) and are believed to be less hydrated than those in the diffuse region of the double layer [1]. When an electric field is applied across the capillary, the remain- ing excess cations in the diffuse part of the electric double layer move toward the cathode, dragging their hydration spheres with them. Since the molecules of water associated with the cations are in direct contact with the bulk solvent, all the electrolyte solution moves toward the cathode producing a plug like flow hav- ing a flat velocity distribution across the capillary diameter [2]. Velocity and direction of the electroosmotic flow de- pend on the composition, pH and ionic strength of the electrolyte solution [3–6]. Both pH and ionic strength influence the protonic equilibrium of fixed charged groups on the capillary surface and of ionogenic substances in the electrolyte solution, which affect the charge density in the electric double layer and, conse- quently, the zeta potential. In addition, the ionic strength influences the thickness of the electric double layer (j )1 ) [7]. Increasing the ionic strength causes a decrease in j )1 , which results in decreas- ing the zeta potential and, consequently, the electroosmotic flow [8]. The charge density in the electric double layer, and hence the electroosmotic flow, is also influenced by the adsorption of potential- determining ions in the Stern region of the electric double layer [7]. Certain ions can be firmly held in the compact region of the electric double layer by forces additional to those of simple coulombic origin. The specific adsorption of coun- ter-ions at the interface between the DOI: 10.1365/s10337-003-0098-3 2003, 58, 587–596 0009-5893/03/11 $03.00/0 Ó 2003 Friedr. Vieweg & Sohn Verlagsgesellschaft mbH Original Chromatographia 2003, 58, November (No. 9/10) 587