Electro-osmosis

Electroosmosis (also electroendosmosis, rarely electroendoosmosis ) is to create a standing parallel to a surface electric field. The effect occurs, inter alia, in capillary and the movement of a liquid is then called electroosmotic flow.

Basics

The reason for the effect of electro-osmosis is that a liquid, although in the volume (ie, in the interior ) is electrically neutral, but forming an electrochemical double layer at a surface; This is about one nanometer thick, the layer thickness depends on the ions dissolved in the liquid. The liquid is thus not electrically neutral at the surface. If one applies an electric field parallel to the surface, so a force acts on the liquid, and it is to flow. This is possible only in case of insulating surfaces; Metals and other electrical conductors would short-circuit a field parallel to the surface.

Electroosmotic flow

The electroosmotic flow (EOF) is closely connected to the electrophoresis. It occurs as a result of an interfacial phenomenon between the capillary and the electrolyte solution when an electric field. For certain materials, such as glass, amorphous quartz, Teflon, paper, agarose or silica gel, consisting of the electrophoretic carrier plates and capillaries occur surface charges, leading to a printed above the electrolyte solution to form a double layer. While the charges of the solid side remain stationary bound to follow the moving charges in the electrolyte solution to the field and thus put the liquid along the interface in motion.

Thus, the driving force produced thereby directly to the capillary wall so that the resulting movement of liquid over the entire cross section of the capillary is uniform. However, a liquid pumped through the capillary was braked on the wall to near zero speed, and have its maximum velocity in the center of the lumen. This is a decisive advantage for the selectivity of the electrophoresis.

The capillaries usually used for the capillary electrophoresis of amorphous quartz show a dissociation of silanol groups (SiOH → SiO ), leading to a negative charge to the inner capillary walls. The resulting negative surface charge, together with positive charged ions of the electrolyte solution from a star bilayer. The positive charges in the electrolyte are attracted to the cathode and move the Kapillarinhalt therefore there.

However, for the separation of anions, it is necessary to generate an EOF toward the anode. Use of cationic surfactants (known EOF modifiers ) in the electrolyte solution, it is possible to build up an intermediate layer on the capillary wall, the lumen through which carries positive charges. Then induces a negative boundary layer in the electrolyte, which makes this flow towards the anode. Has proven particularly useful as an EOF modifier CTAB ( cetyltrimethylammonium bromide ) and better yet tetradecyltrimethylammonium.

The electroosmotic flow is described by the following equation:

Veo = μeo · e

Veo here describes the electro-osmotic flow (EOF), μeo the electroosmotic mobility and E is the electric field strength.

Since the surface charge is strongly pH dependent on the capillary wall, the electroosmotic flow varies with the pH of the electrolyte. At low pH it becomes smaller at a high correspondingly larger. The strength of the EOF is also dependent on the ambient temperature and the electrolyte concentration. Increases the electrolyte concentration decreases the EOF, and vice versa. By the addition of organic solvents such as methanol is decreased to the EOF.

Impact and applications

Since the force acts only in a very thin layer of liquid, electroosmosis can be observed in thin capillaries ( a few nanometers up to a few microns). For thicker liquid layers or columns, the effects of the volume outweigh ( ionic conduction, electrolysis, electrophoresis) by far. Electroosmosis can therefore in microchannels that are used for capillary electrophoresis small quantities of liquid, strong kick ( electroosmotic flow ) in appearance.

With the help of electro-osmosis, a "nano pump " can be implemented, will be issued a carefully dosed with small amounts of liquid. Typical voltages for such applications are a few hundred to over a thousand volts per centimeter. Thus, a pressure of about 10 bar is reached, the liquid velocities are in the range of microns to a few millimeters per minute. The low liquid velocities depend to the fact that the flow resistance in so thin capillaries ( mostly water with dissolved substances) is very high due to the viscosity of the liquid, and that the force so effective only in close proximity to the surface, where the flow particularly strongly by viscosity is impeded. Therefore, the efficiency of such pumps is also theoretically in the ideal case, well below 10%.

Which method electroosmosis is suitable for drying of masonry, in the article Electrophysical wall drainage is treated.