Estimation and comparison of ζ-potentials of silica-based anion-exchange type porous particles for capillary electrochromatography from electrophoretic and electroosmotic mobility

Mobilities of different chromatographic particles obtained from two electrokinetic methods were determined and compared. The particles were all based on porous silica, between 3 and 15 mum diameter, and were either native, or derivatized. As intermediate of chemical modification 3-mercaptopropyl-modified silica particles (TP-silica) are obtained. These particles were finally transformed into weakly basic anion exchangers with O-9-(tert-butylcarbamoyl)quinine (tBuCQN) as chiral selector. The electrophoretic mobility of the particles was determined from their migration velocity in an electric field using microelectrophoresis. Electrokinetic chromatography with a capillary column packed with the same particles was used to measure the electroosmotic flow generated. All measurements were carried out in background electrolytes of equal ionic strength (10(-2) mol/L), at pH varying between 3.5 and 9.5. From these data a rough estimation of the xi-potential was made, taking Helmholtz-Smoluchowski conditions into consideration. With both methods the xi-potential of the native silica particles is negative throughout, and its value increases with pH. The weakly basic tBuCQN particles have positive xi-potentials at pH lower than about 7.5, but exhibit a negative xi-potential above this pH, indicating the dominating effect of residual silanol groups at the silica surface. The xi-potential for these anion-exchange particles ranged between +30 and -40 mV. The xi-potentials derived with electrophoresis and electroosmosis agree, showing the adequacy of the approach, although many limitations must be taken into account in the treatment of the electrokinetic phenomena in such porous systems. These restrictions in interpreting mobility and xi-potential were discussed.