Electroosmotic flow, effect

V. P. Andreev and E. E. Lisin, Investigation of the electroosmotic flow effect on the efficiency of capillary electrophoresis, Electrophoresis 13 832 (1992). 

Figure 5a also shows that, as the inlet volumetric flow rates of the two liquids increase, the electroosmotic flow effect on the pressure-driven flow becomes weaker. At the flow rate of 1.2 mL/h, it seems that the holdup of NaQ, a, remains constant though the voltage varies from —0.8 to 0.6 kV. For a typical electroosmotic flow. 

The relationship between the NaCl holdup, a, at different flow rates under the fixed electric field is shown in Fig. 5b. The NaCl holdup remains the same (0.35) for different volumetric flow rates in the absence of an externally applied electric field. This is because the volumetric flow rate ratio between the two liquids is kept unchanged at 1 1. This agrees very well with the previous theoretical and numerical study reported in the literature. From Fig. 5b, it can be seen that, as the flow rate increases, holdup a converges to a constant value, 0.35 that is, the value without the externally applied electric field. The reason is that larger pressure-driven flow velocity makes the electroosmotic flow effect almost insignificant. 

Manz, B Stilbs, P Jonsson, B Soderman, O Callaghan, PT, NMR Imaging of the Time Evolution of Electroosmotic Flow in a Capillary, Journal of Physical Chemistry 99, 11297, 1995. Matthew, JB Hanania, GIH Gurd, FRN, Electrostatic Effects in Hemoglobin Bohr Effect and Ionic Strength Dependence of Individual Groups, Biochemistry 18, 1928, 1979. 

Fig. 30. Effect of field strength and percentage of acetonitrile in the mobile phase on electroosmotic flow in a packed capillary column. (Reprinted with permission from [35]. Copyright 2000 Elsevier). Conditions capillary column 100 pm i.d., total length 38 cm, active length 8.5 cm packed with 0.5-pm C8 silica beads, mobile phase acetonitrile/25 mmol/1 TRIS-HCl buffer pH = 8, temperature 20 °C, marker thiourea...

Effects of buffer composition on electroosmotic flow in capillary electrophoresis. /. Microcol. Sep. 2, 176-180. 

If water movement in the membrane is also to be considered, then one way to do this is to again use the Nernst—Planck equation. Because water has a zero valence, eq 29 reduces to Pick s law, eq 17. However, it is also well documented that, as the protons move across the membrane, they induce a flow of water in the same direction. Technically, this electroosmotic flow is a result of the proton—water interaction and is not a dilute solution effect, since the membrane is taken to be the solvent. As shown in the next section, the electroosmotic flux is proportional to the current density and can be added to the diffusive flux to get the overall flux of water... 

The effect of the electroosmotic flow on the resolution is also evident from Eq. (16). A high electroosmotic flow in the direction of the moving ions can significantly diminish resolution. Theoretically, infinite resolution of two peaks could be reached when jlEOF is equal but opposite to the average mobility m.Av. In this case one of the solutes would migrate in the direction of the detector and the other one in the opposite direction. In other words, the separation run would be infinitely long. Thus, for a practical separation the electroosmotic flow should be controlled in a way to achieve baseline resolution (R = 1) at minimal separation time. 

Electrophoretic migrations are always superimposed on other displacements, which must either be eliminated or corrected to give accurate values for mobility. Examples of these other kinds of movement are Brownian motion, sedimentation, convection, and electroosmotic flow. Brownian motion, being random, is eliminated by averaging a series of individual observations. Sedimentation and convection, on the other hand, are systematic effects. Corrections for the former may be made by observing a particle with and without the electric field, and the latter may be minimized by effective thermostating and working at low current densities. 

Even in the absence of a colloid, an electrolyte solution will display electroosmotic flow through a chamber of small dimensions. Therefore the observed particle velocity is the sum of two superimposed effects, namely, the true electrophoretic velocity relative to the stationary liquid and the velocity of the liquid relative to the stationary chamber. Figure 12.10a shows the results of this superpositioning for particles tracked at different depths in the cell. The particles used in this study are cells of the bacterium Klebsiella aerogenes in phosphate buffer. Rather than calculated velocities or mobilities, Figure 12.10a shows the reciprocal of the time... 

Solution-. The observed effect is the sum of two contributions, one of which is the electro-osmotic flow of the medium through the cell. The latter has its maximum value at the center since the layer of fluid adjacent to the walls is stationary. The particles tracked at the center of the cell therefore possess the maximum increment in velocity due to electroosmotic flow. Since the cell is a closed compartment, the liquid displaced by electroosmosis along the walls must circulate by a backflow down the center of the tube. Since the total liquid flow in a closed cell must be zero, the appropriate value from Figure 12.10a to use for the velocity is the average of observations made at all depths. ... 

This result shows that electroosmotic flow and backflow in the capillary cancel when the factor (2r1/R — 1) equals zero. This condition corresponds to r/Rc = 0.707. Thus at 70.7% of the radial distance from the center of the capillary lies a circular surface of zero liquid flow. Any particle tracked at this position in the capillary will display its mobility uncomplicated by the effects of electroosmosis. This location may also be described as lying 14.6% of the cell diameter inside the surface of the capillary. Experimentally, then, one establishes the inside diameter of the capillary and focuses the microscope 14.6% of this distance inside the walls of the capillary. Corrections for the effect of the refractive index must also be included. Additional details of this correction can be found in the book by Shaw (1969). 

PERRIN RULE. Ions of charge opposite to that of a diaphragm have by far the greatest effect on endosmosis, The higher their valence (or opposite sign) the greater the reduction of electroosmotic flow. 

The nature of the functional monomer can sometimes be of importance. For example, if the MIP monolith is to be used as stationary phase for electrochromatography, the presence of charged groups may be essential for the generation of an electroosmotic flow. These can be simply generated by the use of MAA as the functional monomer [171], but in some cases a combination with other monomers was necessary to improve the imprinting effect [172,173]. 

L. E. Locascio, C.E. Perso and C.S. Lee, Measurement of electroosmotic flow in plastic imprinted microfluid devices and the effect of protein adsorption on flow rate, J. Chromatogr. A, 857 (1999) 275-284. 

---