electrokinetic phenomena

Stern layer The classic picture of an aqueous electrolyte near a charged surface is shown to the right.  The charged surface, and the oppositely charged counter-ions that compensating the surface, for an electrical double layer.  Motion of the ions in response to an electric field causes electrophoresis, drifting of the ions in response to the field.  Near the surface, there are more counter- than co-ions, so the drifting ions in response to a field parallel to the surface induces a net flow in the fluid, known as electroosmotic flow (EOF).

It has long been known that the simplest models (Gouy-Chapman) predict too much EOF.  This is one reason why in 1924 Stern proposed that fluid near the surface is immobile.  This supposedly stagnant layer is now called the Stern layer.  The expected fluid velocity profile near the surface is shown in the figure.  Since the effective charge now driving EOF only comes from outside the Stern layer (the diffuse layer), the Gouy-Chapman-Stern theory does a good job describing EOF.

Gouy-Chapman-Stern theory does well for EOF, i.e. motion of the fluid, but it gets into trouble with regard to motion of the ions.  It predicts too little ion current.  This has led colloid scientists to propose the dynamic Stern model, which says that ions are mobile but water in immobile in the Stern layer.

silica/water interfaceUsing our microscopic model for the amorphous silica/water interface, a prototypical double layer system,  (see figure to the left and our other page on the silica water interface), we have investigated the microscopic basis of electrokinetic phenomena near silica.  We find that a realistic theory can simultaneously account for both EOF and ion current.  The Stern model is really an effective model that correctly predicts fluid velocity far from the walls.  However, it cannot be taken literally near the walls.  The dynamic Stern model is not a microscopically realistic picture of the electrical double layer.  It is neither tenable, nor required to correctly describe both EOF and ion current.

This research is supported by the National Science Foundation.

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