Nonlinear electroosmosis in hierarchical monolithic structures.

Abstract

We studied the dependence of electroosmotic flow (EOF) velocity and separation efficiency for neutral analytes in 100 microm ID capillary monoliths on a variation of the mobile phase ionic strength and applied electrical field strength, i.e., we covered a range for the concentration of Tris buffer from 10(-5) to 10(-2) M and applied electrical field strengths up to 10(5) V/m. The silica-based monoliths are hierarchically structured having intraskeleton mesopores and interskeleton macropores. While a linear dependence of the average EOF velocity on applied field strength could be observed with 5 x 10(-3) M Tris (turning slightly nonlinear at a higher concentration due to thermal effects), this dependence becomes systematically nonlinear as the Tris concentration is reduced towards 10(-4) M. Increased velocities by more than 50% compared to those expected from linear behavior are realized at 10(5) V/m. Concomitantly, as the Tris concentration is reduced from 10(-3) to 10(-4) M, we notice an improvement in plate heights by a factor of more than 2 (they approach 2 microm for ethylbenzoate). We complementary analyzed the onset of the nonlinear EOF dynamics in a hierarchical monolith and the significantly reduced axial dispersion in view of nonequilibrium electrokinetic effects which may develop in porous media due to the presence of ion-permselective regions, e.g., the mesoporous monolith skeleton. In this respect, a decreasing mobile phase ionic strength favors the formation of nonequilibrium concentration polarization in strong electrical fields, and a coupling of the electrostatics and hydrodynamics then may explain nonlinear EOF velocities and increasing separation efficiencies depending on the Tris concentration and applied field strength.