Electroosmosis
Definition and meaning of Electroosmosis in chemistry.
Electroosmosis is the bulk movement of a liquid through a stationary porous medium, membrane, or narrow capillary that occurs when an electric field is applied across it. The flow arises because the field acts on mobile ions in the electric double layer at the solid-liquid interface, dragging the surrounding fluid along with them.
In more detail
Most solid surfaces develop a surface charge when in contact with an electrolyte solution (for example, deprotonated silanol groups give fused-silica capillary walls a negative charge above pH 3). This charge attracts a diffuse layer of mobile counterions from the solution, forming an electric double layer. When an electric field is applied along the capillary, these counterions migrate toward the oppositely charged electrode and, through viscous coupling, carry the bulk liquid with them, producing a nearly flat flow profile. The resulting electroosmotic flow (EOF) velocity is described by the Smoluchowski equation, v = εζE/η, where ζ is the zeta potential, ε the permittivity, E the field strength, and η the viscosity.
Key facts
| Field | Physical Chemistry |
|---|---|
| Governing relation | Smoluchowski equation: v = εζE/η |
| Requires | Electric double layer and nonzero zeta potential (ζ) |
| Key application | Capillary electrophoresis and microfluidic pumping |
In capillary electrophoresis, the negatively charged wall of a fused-silica capillary generates electroosmotic flow toward the cathode strong enough to carry even anionic analytes past the detector, alongside neutral and cationic species.
Frequently asked questions
How does electroosmosis differ from electrophoresis?
Electroosmosis moves the bulk liquid past a fixed charged surface, whereas electrophoresis moves charged particles or molecules through a stationary liquid; both are electrokinetic effects driven by an applied field acting on interfacial or particle charge.
What controls the direction and speed of electroosmotic flow?
The sign of the zeta potential sets the flow direction, and the speed scales with the applied field strength and zeta potential and inversely with the liquid's viscosity, as given by the Smoluchowski equation.