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Nonlinear electroosmosis in hierarchical monolithic structures
Author(s) -
Nischang Ivo,
Tallarek Ulrich
Publication year - 2004
Publication title -
electrophoresis
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 0.666
H-Index - 158
eISSN - 1522-2683
pISSN - 0173-0835
DOI - 10.1002/elps.200405990
Subject(s) - electrokinetic phenomena , monolith , ionic strength , field strength , electric field , materials science , mesoporous material , nonlinear system , analytical chemistry (journal) , non equilibrium thermodynamics , tris , phase (matter) , chemical physics , chemistry , thermodynamics , nanotechnology , chromatography , physics , biochemistry , organic chemistry , quantum mechanics , aqueous solution , magnetic field , catalysis
We studied the dependence of electroosmotic flow (EOF) velocity and separation efficiency for neutral analytes in 100 μm 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 5×10 −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 μm 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.