Electrophoresis and Electroosmosis?

[dttom]

Electroosmosis is the movement of bulk fluid under the application of a voltage across. This seems to be also essential for electrophoresis, obviously, no emf, no electrophoresis. But I was told that electroosmosis could be a hindrance to movement in electrophoresis. My teacher said that, the factor of electroosmosis only comes in when a charged supporting medium (the substratum) is used, molecules to be separated converged to the medium, their covergence bring with water (to balance the osmotic potential), when the voltage is turned on, all the water blast towards to electrodes leaving those molecule stationary. In this explanation I understand to the point where molecules accumulate onto the medium and draw in water, but I can't see why when the switch turns on, water will rush towards electrodes. Could anybody help or give other explanation how electroosmosis hinders electrophoretic movement?

 

I have thought of a simple explanation, that is the charged medium holds and retard the molecules hence it is one of the resistive forces. Another explanation I could understand is that the molecules are trapped inside pores of the medium, and if the medium is charged it will have its boundary coated with a layer of ions (from the bulk liquid/electrolyte), this layer of ions will shield the emf applied , force pulling molecules force will then be weakened.

 

Comments on my own intepretation are welcome. Thanks.

[CharonY]

This sounds a bit convoluted to me. Also I do not understand why EOF is a prerequisite of electrophoresis. It is true however that under conditions where electrophoresis occurs EOF does too.

EOF is based on the fact that in a conductive medium a double layer forms between the the medium and the container/capillary if surface charges are present. If you got a NaCl solution, for instance and a negatively charged capillary, the Na+ ions will accumulate near the surface, creating a diffuse charge layer with a given length (the Debye length). These charges (Na+ in this case) will move in an electric field, dragging the bulk fluid with them. Obviously in systems where the volume is big compared to the surface EOF will be very small and confined to areas near the walls, whereas in capillaries or microfluidic systems they may be major driving forces.

In a normal system this can interfere with electrophoresis as they add another flow to the net movement of the analytes. Depending on the direction of the respective flows they can accelerate the movement (if the flow is in bot cases in the same direction) or slow it down (if both point to different directions. The larger the system the smaller the interference is, though. In cases where the EOF is stronger than electrophoresis, the net movement will reverse.

[AaliyahDykes]

General methods such as filtering, column chromatography and distillation are not easy to separate molecules, thus, electrophoresis and electro-osmosis techniques are used. Electrophoresis process separate molecules based on their sizes and it is relatively easy and inexpensive. Electro-osmosis is the technique of moving a liquid by applying an electric field to the material.

Edited September 4, 2015 by Phi for All
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