Electroosmosis

Electro-osmosis (also known as electro-endosmosis , rarely also electro-endo-osmosis ) is the application of an electrical field parallel to a surface. The effect occurs, among other things, in capillary electrophoresis and the movement of a liquid is then called electroosmotic flow . Electro-osmotic effects were examined for the first time by Ferdinand Friedrich von Reuss and published in 1809.

Basics

The reason for the effect of electroosmosis is that a liquid is electrically neutral in volume (i.e. inside), but an electrochemical double layer forms on a surface ; this is about ten nanometers thick, the thickness of the layer depending on the ions dissolved in the liquid. The liquid is therefore not electrically neutral on the surface. If you now apply an electric field parallel to the surface, a force acts on the liquid and a flow occurs. This is only possible with insulating surfaces; Metals and other electrical conductors would short-circuit a field parallel to the surface.

Electro-osmotic flow

The electroosmotic flow (EOF) is closely related to electrophoresis . It occurs as a result of an interface phenomenon between the capillary wall and the electrolyte solution when an electric field is applied. With certain materials such as As glass , amorphous quartz , Teflon , paper , agarose or silica gel, from which electrophoretic carrier plates and capillaries are made, surface charges occur that lead to the formation of a double layer with an overlying electrolyte solution . While the charges on the solid side remain fixed, the mobile charges in the electrolyte solution follow the field and set the liquid in motion along the interface.

The driving force is generated directly on the capillary wall, so that the resulting liquid movement is uniform over the entire cross section of the capillary. In contrast, a liquid pumped through the capillary would be decelerated at the wall to almost zero speed and would have its maximum speed in the middle of the lumen. This is a decisive advantage for the selectivity of electrophoresis.

The capillaries made of amorphous quartz mostly used for capillary electrophoresis show a dissociation of silanol groups (SiOH → SiO-), which lead to a negative charge on the inner capillary walls. The resulting negative surface charge, together with positively charged ions in the electrolyte solution, forms a star double layer . The positive charges in the electrolyte are attracted to the cathode and therefore move the capillary contents there.

To separate anions, however, it is necessary to generate an EOF towards the anode. With the help of cationic surfactants (so-called EOF modifiers) in the electrolyte solution, it is possible to build up an intermediate layer over the capillary wall, which carries positive charges towards the lumen. It then induces a negative boundary layer in the electrolyte, which allows it to flow towards the anode. CTAB ( cetyltrimethylammonium bromide ) and, better still, tetradecyltrimethylammonium bromide, have proven particularly useful as EOF modifiers .

The electroosmotic flow is described by the following equation:

{\ displaystyle v_ {eo} = \ mu _ {eo} \ cdot E}

${\ displaystyle v_ {eo}}$ describes the electroosmotic flow (EOF), the electroosmotic mobility and the electric field strength. ${\ displaystyle \ mu _ {eo}}$ ${\ displaystyle E}$

Since the surface charge on the inner capillary wall is strongly pH-dependent, the electroosmotic flow changes with the pH value of the electrolyte. If the pH value is low, it becomes smaller, and if it is high, it becomes correspondingly larger. The strength of the EOF also depends on the ambient temperature and the electrolyte concentration. If the electrolyte concentration increases, the EOF decreases and vice versa. The EOF also decreases with the addition of organic solvents such as methanol.

Impact and Applications

Since the force only acts on a very thin layer of liquid, electroosmosis can only be observed in thin capillaries (a few nanometers to a maximum of a few micrometers). In the case of thicker layers or columns of liquid, the effects of volume (ionic conduction, electrolysis , electrophoresis) outweigh by far. Electroosmosis can therefore appear strongly in microchannels that are used for capillary electrophoresis of the smallest amounts of liquid (electroosmotic flow).

With the help of electroosmosis, a “nanopump” can also be implemented with which small amounts of liquid can be dispensed in a well-dosed manner. Typical field strengths for such applications are a few hundred to over a thousand volts per centimeter. This means that a pressure of over 10 bar can be achieved, and the liquid velocities are in the range from micrometers to a few millimeters per minute. The low liquid velocities are related to the fact that the flow resistance in such thin capillaries is very high due to the viscosity of the liquid (mostly water with dissolved substances), and that the force only acts in close proximity to the surface, where the flow is particularly strong due to viscosity is hindered. The efficiency of such pumps therefore remains well below 10% even in the theoretically optimal case.

The article Electrophysical Wall Drying deals with which methods electroosmosis is suitable for drying masonry .

Web links

Determining the zeta potential - why? In: company brochure. Malvern Instruments Ltd, archived from the original on September 29, 2007 ; Retrieved July 6, 2016 (section History - electrokinetic phenomena ).
Description of the electro-osmotic "pump" ( Memento from June 17, 2007 in the Internet Archive ) (PDF file; 1.7 MB)

literature

Hens Hijnen: A Theoretical Analysis of the Influence of Electroosmosis on the Effective Ionic Mobility in Capillary Zone Electrophoresis . In: Journal of Chemical Education . tape 86 , no. 7 , 2009, p. 869 , doi : 10.1021 / ed086p869 .
Stefan Bauer, Peter Fischer: A technical monument in the forest: The former "electro-osmosis" system on Lengemannschacht near Großalmerode, part 1 . In: Ceramic magazine . tape 58 , no. 3 , 2006, p. 201–204 ( Keramik-zeitschrift.info ).
Stefan Bauer, Peter Fischer: A technical monument in the forest: the former "electro-osmosis" system at Lengemannschacht near Großalmerode, part 2 . In: Ceramic magazine . tape 58 , no. 4 , 2006, p. 280-282 ( keramische-zeitschrift.info ).

Individual evidence

↑ Рейсс Ф. Ф .: О новом действии гальванического электричества . In: Memoires de la Société Imperiale des Naturalistes de Moscou . tape II , 1809, p. 327-337 .
^ ^A ^b ^c Daniel C. Harris: Textbook of Quantitative Analysis . Ed .: Gerhard Werner, Tobias Werner. 8th edition. Springer, Berlin / Heidelberg 2014, ISBN 978-3-642-37787-7 , Chapter 25: Chromatographic methods and capillary electrophoresis , p. 737 ff ., Doi : 10.1007 / 978-3-642-37788-4_26 .

[1] Рейсс Ф. Ф .: О новом действии гальванического электричества . In: Memoires de la Société Imperiale des Naturalistes de Moscou . tape II , 1809, p. 327-337 .

[harris-2] A ^b ^c Daniel C. Harris: Textbook of Quantitative Analysis . Ed .: Gerhard Werner, Tobias Werner. 8th edition. Springer, Berlin / Heidelberg 2014, ISBN 978-3-642-37787-7 , Chapter 25: Chromatographic methods and capillary electrophoresis , p. 737 ff ., Doi : 10.1007 / 978-3-642-37788-4_26 .