Electroendosmotic flow

Electroendosmosis or electro-osmotic flow describes the bulk flow of liquid through a fine capillary due to the effect of an applied electric field on the solution charged double layer which in this context is at the capillary wall of a fused silica column [85]. The double layer charge has its origins in 

When a voltage is applied across the capillary, cations in the diffuse area of the double layer migrate towards the cathode and because they are solvated their movement drags the bulk solution in the direction of the negative electrode. This electro-osmotic flow (EOF) and is the fundamental process which drives capillary electrophoresis. EOF is defined by 

Where e is the dielectric constant, rj is viscosity, is the zeta potential and E the field strength. The corresponding electroendosmotic mobility of the 

The speed of electro-osmotic flow is typically at least an order of magnitude greater than that due to the electrophoretic mobility of the ions—and thus all species cations, neutral molecules and anions will be carried towards the cathode. The cations migrate fastest, the anions slowest with the neutral molecules being carried at the speed of the EOF. Herein lies the power and potential of CE as an analytical technique. 

While EOF is beneficial and is exploited in CE it must be controlled since if it is too high the analyte species will have migrated to the cathode before separation has occurred. The EOF can be fine tuned by adjusting the following parameters. 

---

This is a more recently developed technique which is a hybrid between HPLC and capillary electrophoresis. The capillary is packed with HPLC media and the mobile phases are aqueous buffers. A voltage is applied to generate an electroendosmotic flow and the analytes separate by interaction with the stationary phase and electrophoretic forces no pump being required as for HPLC. Improved separation efficiencies have been reported. 

Figure 9.2 Formation of an electrical double layer responsible for electroendosmotic flow in an uncoated fused-silica capillary. The negative charges on the surface of the capillary are neutralized by positive charges of cations present in the buffer, which form an electrical layer near the surface of the capillary. When the electric held is apphed, the positive charges migrate toward the negative electrode, generating a bulk flow of the solution contained within the column. Electroosmosis exhibits a flat prohle, in contrast to hydraulic flow, which is parabolic.

The basic principle is shared by several methods In chromatographic or electrophoretic systems where the liposomes (vesicles) are immobilized, pseudostationary, or carried by an electroendosmotic flow, migrating amphiphilic drug molecules partition between the water outside the liposomes, the lipid bilayer of the liposome, and the aqueous compartment within the liposome (Fig. 3). In all cases the migration rate basically reflects the par-... 

Alkaline proteins with similar pi values (e.g., ribosomal and nuclear proteins with pis in the range of 10.5-11.8) can be differentiated by narrow range (pH 10-12 or 9-12) IPG gels. Substitution of DMAA (N,N-dimethylacrylamide) and addition of propan-2-ol to the rehydration solution provide high reproducibility and suppresses the reverse electroendosmotic flow. To obtain better resolution for this type of... 

De Nobili, M., Bragato, G., and Mori, A. (1998). Combined effects of molecular size and electroendosmotic flow on the capillary electrophoretic behaviour of humic substances. Acta Hydrochim. Hydrobiol. 26,186-190. 

The resulting experimental speed is shown in Fig. 3. Special attention to the electroendosmotic flow will be given later. 

The interaction between the paper and the buffer when the field is applied causes the electroendosmotic flow. 

It is clear that evaporation is superimposed on the electroendosmotic flow and that partial measurements on the open strip account for both phenomena at once (Fig. 12). 

Fig. 12. Influence of evaporation and electroendosmosis. Glucose applied at Sj, C, and S2 Ev, evaporation flow E, electroendosmotic flow. Upper run stopped after 2 hours lower run stopped after 4 hours. Strips developed by heat.

Capillary electrochromatography uses electroendosmotic flow (EOF) to perform highly efficient separations in small-diameter fused-silica capillaries, packed with HPLC-type stationary phases. It can be considered as a combination of capillary electrophoresis (CE) and HPLC. The separation of solutes is based on electrophoretic mobility (for charged species) and interaction with the stationary pha.se, allowing the separation of both neutral and charged compounds. 

A preparative SDS-PAGE system, exploiting the electroendosmotic flow, generated during electrophoresis, for the elution of the proteins from the gel, can be used for purification of nearly all the protein components of wine (Vincenzi et ah, 2003). 

The true distances of migration of a substance in borate buffer are obtained by correcting for movement due to electroendosmotic flow toward the cathode i. e. the true distances of migration are usually greater than the apparent distances. 

A flow of electrolyte, known as electroendosmotic flow, EOF, is generated during the lectrophoretic run. (Do not bother about EOF now we will explain this later.). This EOF results in a flow of the solution along the capillary usually towards the detector. As such it makes the sample ions move faster. EOF can therefore significantly reduce einalysis times. 

At low pH the silanols are unionised and therefore the flow rate is much reduced or can become zero at very low pH values. Let us give some mathematical consideration to the electroendosmotic flow. 

The extent of the electroendosmotic flow (EOF) is a function of the charge on the capillary, the buffer viscosity and dielectric constant of the buffer. The magnitude of the EOF is described by the following relationship ... 

The electroviscous effects are observed as variations of viscosity upon appHcation of outer electric fields, and as build-up of potential gradients upon flow of such fluids. See also electroconvection, electrorheological fluid, -> sedimentation potential, electrokinetic effect, -> electroendosmotic flow. 

---