Zeta potential silica

Figure 4 The variation of the zeta potential of vitreous silica as a function of pH in potassium nitrate solutions. (Based on Wiese, G. K., James, R. O., and Healy, T. W., Trans. Farad. Soc., 52, 1298, 1971.)...

Capillary electrophoretic separations are performed in small diameter tubes, made of Teflon, polyethylene, and other materials. The most frequently used material is fused silica. Fused silica capillaries are relatively inexpensive and are available in different internal and external diameters. An important advantage of a fused silica capillary is that the inner surface can be modified easily by either chemical or physical means. The chemistry of the silica surface is well established due to the popularity of silica surfaces in gas chromatography (GC) and liquid chromatography (LC). In capillary electrophoresis, the silica surface is responsible for the EOF. Using surface modification techniques, the zeta potential and correspondingly the EOF can be varied or eliminated. Column fabrication has been done on microchips.13... 

Fig. 14.3 Zeta-potential of halloysite, and silica and alumina nanoparticles (for comparison).

Fig. 6. Plateau-values, I"P1 /mg m 2, of adsorption isotherms of lysozyme (LSZ), ribonuclease (RNase), a -lactalbumin (aLA), calcium-depleted (X -lactalbumin (aLA(-Ca )) and bovine serum albumin (BSA) on hydrophobic polystyrene (PS) and hydrophilic hematite (a — Fe203) and silica (Si02) surfaces. An indication of the charge density of the surface is given by the zeta-potential, C, and of the proteins by + and signs. Ionic strength 0.05 M T = 25°C. (Derived from Currie et al. 2003).

Many investigators of steric stabilization have measured colloidal stability without taking the effort to find out whether the stability actually resulted from electrostatic stabilization. In many published articles it has been concluded that steric stabilization had been attained and further study showed this was not the case. One such example is a recent paper on "steric" stabilization by an additive of the same type used in this work. (12) The published photograph shows the silica particles in oil stabilized at interparticle separations several times the distances provided by the adsorbed films no electrical measurements had been made, but it they had, this particular dispersant would have provided about -200 mV of zeta-potential and given excellent electrostatic repulsion. The reader should be wary of any claims of steric stabilization unless the electrostatic contribution has been measured. 

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.

Fig. 12.7 (a) Fabrication schematic of a graphene-encapsulated metal oxide, (b) Zeta potentials of APS-modified silica ( ) and graphene oxide ( ) in aqueous solutions with various pH values, (c) and (d) typical SEM, and (e) transmission electron microscopy (TEM) images of graphene-encapsulated silica spheres. Reprinted with permission from [87]. Copyright 2010, John Wiley Sons, Inc. 

For oxide CMP, the purpose of the solution is two fold. First, water weakens the Si—O bond in a silicon dioxide film and softens the surface as it becomes hydrated with Si—OH bonds [6,7]. Figure 10 shows the reaction mechanism. Second, the solution is to provide a basic environment (pH > 10), which accelerates the hydration rate. An environment with high pH values will allow the polishing-induced reaction to be further accelerated because the surface Si(OH) species will be partially dissolved into water. In the meantime, the zeta potential of silica increases with increasing pH values. At high zeta potentials silica particles will repel each other, whereby a better-suspended slurry is formed. 

Experiment 6.1 Zeta potential measurements at the silica/water interface... 

The magnitude of the electrostatic potential at the silica surface is, as expected from the law of mass action, pH-dependent. The variation in surface (or zeta) potential with pH must therefore be dependent on the dissociation constant of the surface silanol (Si-OH) group. 

In this experiment a zeta-meter is used to determine the variation in the zeta potential of silica at constant pH (5.7) over a range of concentrations of a cationic surfactant CTAB, which should adsorb on the ionized silanol groups on the silica particle surface. 

Surface polarity can also be independently evaluated by physical means. deMayo and coworkers have assigned surface polarity of silica gel particles by observing shifts in the absorption spectra of absorbed spiropyrans which are sensitive to solvent polarity . Darwent and coworkers have shown that kinetic salt effects follow surface charge on colloidal titanium dioxide and, with zeta potential measurements, that surface area and charge could be separately evaluated... 

Figures 4 and 5 summarize the zeta potentials that were measured on pure silica, 0% B20, glass, and 6% B,0, glass—in water and silane solutions. The zeta...

Figure 4. The zeta potential of crushed, untreated 6% B.()- E-glass fibers (O) and silica glass (Oi in 10 M KNO, solution and in a 4% solution of y-aminopropylsilane f ) and ( ) measured using a Rank Brothers Electrophoresis analyzer.

This electrokinetically driven micro mixer uses localized capacitance effects to induce zeta potential variations along the surface of silica-based micro channels [92], The zeta potential variations are given near the electrical double layer region of the electroosmotic flow utilized for species transport. Shielded ( buried ) electrodes are placed underneath the channel structures for the fluid flow in separate channels, i.e. they are not exposed to the liquid. The potential variations induce flow velocity changes in the fluid and thus promote mixing [92],... 

Figure 5.9 Illustration of the effect of electrolyte on colloid stability. The photomicrographs A through D show how 1.1 tm size silica particles are progressively coagulated by increasing additions of alum (0, 10, 30, 40 ppm, respectively). The corresponding zeta potentials are -30 mV (A), -14 mV (B), -6 mV (C), and -0 mV (D). From Zeta-Meter [544], Courtesy L.A. Ravina, Zeta-Meter, Inc., Staunton, Va.

Kirby, B.J., Wheeler, A.R., Zare, R.N., Fruetel, J.A., Shepodd, T.J., Programmable modification of cell adhesion and zeta potential in silica microchips. Labchip 2003, 3, 5-10. 

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. 

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. 

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