Capillary wall, zeta potential

In the following, the results under two different ap- pared. Figures 3a and b show the electrokinetic wall effects proaches, the complete numerical solution of Eq. (12) and for a given particle size dp = 5 p,m under different charge the analytical solution based on Eq. (27), will be com- conditions of the capillary wall. The wall zeta potentials... 

Fig. 17.3. Potential difference (i//) at the internal wall of a silica capillary, because of the distribution of charges, x is the length in cm from the center of charge of the negative wall to a defined distance, the zeta potential, 1 = the capillary wall, 2 = the Stern layer or the inner Helmholtz plane, 3 = the outer Helmholtz plane, 4 = the diffuse layer and 5 = the bulk charge distribution within the capillary.

As is derived from Equation (8), can be adjusted by changing the dielectric constant and/or the viscosity of the medium, but also C- As mentioned before, the zeta potential is mainly influenced by the distribution of charges at the capillary wall. All alterations resulting in a change of the charge distribution at the capillary wall like changes in the pH, ionic strength, valence of ions in the buffer electrolyte, etc., can be applied to adjust the velocity of the EOF. 

The second parameter influencing the movement of all solutes in free-zone electrophoresis is the electroosmotic flow. It can be described as a bulk hydraulic flow of liquid in the capillary driven by the applied electric field. It is a consequence of the surface charge of the inner capillary wall. In buffer-filled capillaries, an electrical double layer is established on the inner wall due to electrostatic forces. The double layer can be quantitatively described by the zeta-potential f, and it consists of a rigid Stern layer and a movable diffuse layer. The EOF results from the movement of the diffuse layer of electrolyte ions in the vicinity of the capillary wall under the force of the electric field applied. Because of the solvated state of the layer forming ions, their movement drags the whole bulk of solution. 

While the external electrical field approach is a method directly modifying the zeta-potential of the capillary wall, it is not applicable with commercial apparatuses. The back-pressure technique, on the other hand, has the disadvantage that the flat electroosmotic flow profile is disrupted by superposition of a pressure-driven laminar flow profile hence, the efficiency of separation deteriorates. 

In the presence of EOF, the observed velocity is due to the contribution of electrophoretic and electroosmotic migration, which can be represented by vectors directed either in the same or in opposite direction, depending on the sign of the charge of the analytes and on the direction of EOF, which depends on the sign of the zeta potential at the plane of share between the immobilized and the diffuse region of the electric double layer at the interface between the capillary wall and the electrolyte solution. Consequently, is expressed as... 

A. Effect of pH Acidic silanol groups at the surface of the capillary wall will dissociate when in contact with an electrolyte solution, as illustrated by Eq. (4.5). At high pH, the silanol groups are fully ionized, generating a dense compact layer and a high zeta potential. As a result, the magnitude of the EOF in untreated fused silica capillaries increases with increasing pH. 

D. Effect of Buffer Cation and Buffer Anion The electroosmotic flow is proportional to the potential drop across the diffuse layer of counterions associated with the capillary wall. Because the potential drop is formed by counterions in the buffer attracted to the charged silica surface, the nature of the counterions will affect the zeta potential and therefore the EOF. Figure 4.5 shows the effect of the buffer cation on the mobility of both the EOF (using mesityl oxide as the marker) and a solute (dansylalanine).16 The highest mobility is obtained with the smallest cations however, high mobility may decrease solute resolution, so care must be taken in choosing the cation. The buffer anion also affects the mobility of the EOF, although trends are less apparent. Therefore, the effect of the EOF on a separation can be altered by careful selection of both the buffer anion and cation. 

There are several ways to reduce or suppress the electroosmotic flow in capillaries. These methods involve either eliminating the zeta potential across the solution-solid interface or increasing the viscosity at this interface. One approach is to coat the capillary wall, physically, with a polymer such as methylcellulose or linear polyacrylamide. Because of the difficulty in deactivating the capillary surface reproducibly, however, alternative methods employing dynamic reduction of solute-capillary interactions have been developed. Dynamic reduction of these interactions include the addition of chemical reagents such as methylhydroxyethylcellulose, S-benzylthiouro-nium chloride, and Triton X-100. 

Direct control of the EOF in capillary zone electrophoresis can be obtained by using an external electric field. The EOF may be increased, decreased, or even reversed in the fused silica capillaries by the application of a separate potential field across the wall of the capillary. Further, the zeta potential can be changed at any time during the analysis to achieve innovative separation results. 

Zeta potential at the surface of the particle or the capillary wall... 

Figure 4-34. Wall effects in capillary electrophoresis. The capillary walls, which are usually made of fused silica contain a small proportion of dissociated silanyl groups which bind positively charged counterions. In the region close to the wall, defining the Stern layer, these ions are relatively immobile mobility increases beyond this layer in a region which defines the Guoy-Chapman layer. The distribution of positively charged ions is termed the Zeta-potential it is fairly constant in the Stern layer, but falls off sharply with distance from the wall in the Guoy-Chapman layer.

The Helmholtz-von Smoluchowski equation indicates that under constant composition of the electrolyte solution, the electro-osmotic flow depends on the magnitude of the zeta potential which is determined by many different factors, the most important being the dissociation of the silanol groups on the capillary wall, the charge density in the Stern layer, and the thickness of the diffuse layer. Each of these factors depends on several variables, such as pH, specific adsorption of ionic species in the compact region of the electric double layer, ionic strength, viscosity, and temperature. 

There are two types of nonuniformities of electro-osmotic flow (EOF) that can contribute signihcantly to the solute peak broadening and are important for capillary electrophoresis (CE). The first is the transversal nonuniformity of the usual EOF in the capillary with the zeta potential of the walls and longitudinal electric field strength constant and independent of coordinates. The second one is the nonuniformity of EOF caused by the dependence of the zeta potential of the walls or electric field strength on coordinates. 

---