Towards an understanding of induced-charge electrokinetics at large applied voltages in concentrated solutions.
By Martin Z. Bazant, M. Kilic, Brian D. Storey, A. Ajdari. Advances in Colloid and Interface Science, 2009
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The venerable theory of electrokinetic phenomena rests on the hypothesis of a dilute solution of point-like ions in quasi-equilibrium with a weakly charged surface, whose potential relative to the bulk is of order the thermal voltage (kT/e approximately 25 mV at room temperature). In nonlinear electrokinetic phenomena, such as AC or induced-charge electro-osmosis (ACEO, ICEO) and induced-charge electrophoresis (ICEP), several V approximately 100 kT/e are applied to polarizable surfaces in microscopic geometries, and the resulting electric fields and induced surface charges are large enough to violate the assumptions of the classical theory. In this article, we review the experimental and theoretical literatures, highlight discrepancies between theory and experiment, introduce possible modifications of the theory, and analyze their consequences. We argue that, in response to a large applied voltage, the "compact layer" and "shear plane" effectively advance into the liquid, due to the crowding of counterions. Using simple continuum models, we predict two general trends at large voltages: (i) ionic crowding against a blocking surface expands the diffuse double layer and thus decreases its differential capacitance, and (ii) a charge-induced viscosity increase near the surface reduces the electro-osmotic mobility; each trend is enhanced by dielectric saturation. The first effect is able to predict high-frequency flow reversal in ACEO pumps, while the second may explain the decay of ICEO flow with increasing salt concentration. Through several colloidal examples, such as ICEP of an uncharged metal sphere in an asymmetric electrolyte, we show that nonlinear electrokinetic phenomena are generally ion-specific. Similar theoretical issues arise in nanofluidics (due to confinement) and ionic liquids (due to the lack of solvent), so the paper concludes with a general framework of modified electrokinetic equations for finite-sized ions.
Subfields: Biomedical Engineering, Physical and Theoretical Chemistry
View Paper: https://www.semanticscholar.org/paper/868803a1140dc526bf03830e55fedb88b5abac26
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Citations in the corpus
- Ion-specific hydration effects: Extending the Poisson-Boltzmann theory (2011, background): “[16] Bazant MZ, Kilic MS, Storey BD, Ajdari A, Towards an understanding of induced-charge electrokinetics at large applied voltages in concentrated solutions, Adv Colloid Interfac 2009; 152:48-88.”
- Reviewing the fundamentals of supercapacitors and the difficulties involving the analysis of the electrochemical findings obtained for porous electrode materials (2020, background): “It is worth mentioning that the stationary capacitance ( C Sts ) was assumed to be connected in series with the diffuse layer as recommended in the literature [200].”