la048408n_si_003.avi (17.99 MB)
Nonequilibrium Electrokinetic Effects in Beds of Ion-Permselective Particles
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posted on 2004-12-21, 00:00 authored by Felix C. Leinweber, Ulrich TallarekElectrokinetic transport of fluorescent tracer molecules in a bed of porous glass beads was investigated
by confocal laser scanning microscopy. Refractive index matching between beads and the saturating fluid
enabled a quantitative analysis of intraparticle and extraparticle fluid-side concentration profiles. Kinetic
data were acquired for the uptake and release of electroneutral and counterionic tracer under devised
conditions with respect to constant pressure-driven flow through the device and the effect of superimposed
electrical fields. Transport of neutral tracer is controlled by intraparticle mass transfer resistance which
can be strongly reduced by electroosmotic flow, while steady-state distributions and bead-averaged
concentrations are unaffected by the externally applied fields. Electrolytes of low ionic strength caused
the transport through the charged (mesoporous) beads to become highly ion-permselective, and concentration
polarization is induced in the bulk solution due to the superimposed fields. The depleted concentration
polarization zone comprises extraparticle fluid-side mass transfer resistance. Ionic concentrations in this
diffusion boundary layer decrease at increasing field strength, and the flux densities approach an upper
limit. Meanwhile, intraparticle transport of counterions by electromigration and electroosmosis continues
to increase and finally exceeds the transport from bulk solution into the beads. A nonequilibrium electrical
double layer is induced which consists of mobile and immobile space charge regions in the extraparticle
bulk solution and inside a bead, respectively. These electrical field-induced space charges form the basis
for nonequilibrium electrokinetic phenomena. Caused by the underlying transport discrimination
(intraparticle electrokinetic vs extraparticle boundary-layer mass transfer), the dynamic adsorption capacity
for counterions can be drastically reduced. Further, the extraparticle mobile space charge region leads to
nonlinear electroosmosis. Flow patterns can become highly chaotic, and electrokinetic instability mixing
is shown to increase lateral dispersion. Under these conditions, the overall axial dispersion of counterionic
tracer can be reduced by more than 2 orders of magnitude, as demonstrated by pulse injections.
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immobile space charge regionsbeadsbulk solutionNonequilibrium Electrokinetic Effectscounterionic tracerextraparticle bulk solutionintraparticle mass transfer resistancespace charge regionflux densities approachconcentration polarization zoneconfocal laser scanning microscopynonequilibrium electrokinetic phenomenaflowdiffusion boundary layer decrease
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