Electrokinetic First

The 2eta potential (Fig. 8) is essentially the potential that can be measured at the surface of shear that forms if the sohd was to be moved relative to the surrounding ionic medium. Techniques for the measurement of the 2eta potentials of particles of various si2es are collectively known as electrokinetic potential measurement methods and include microelectrophoresis, streaming potential, sedimentation potential, and electro osmosis (19). A numerical value for 2eta potential from microelectrophoresis can be obtained to a first approximation from equation 2, where Tf = viscosity of the liquid, e = dielectric constant of the medium within the electrical double layer, = electrophoretic velocity, and E = electric field. 

Electrokinetics. The first mathematical description of electrophoresis balanced the electrical body force on the charge in the diffuse layer with the viscous forces in the diffuse layer that work against motion (6). Using this force balance, an equation for the velocity, U, of a particle in an electric field... 

Fig. 3-3. Comparison of the values of enantiomeric resolution of different DNP-D,L-amino acids at different deconvolution stages of a cyclic hexapeptide sublibrary. Resolution values in a cyclo(Arg-Lys-X-X-X-P-Ala) sublibrary, in the first line, are compared to those obtained in sublibraries with a progressively increasing number of defined positions. All the sublibraries were 30 mM in the running buffer while the completely defined cyclo(Arg-Lys-Tyr-P-Tyr-P-Ala) peptide is used at 10 mM concentration. Conditions cyclopeptide sublibrary in 20 mM sodium phosphate buffer, pH 7.0 capillary, 50 pm i.d., 65 cm total length, 57 cm to the window V = -20 kV, I = 40 electrokinetic injection, -10 kV, 3 s detection at 340 nm. (Reprinted with permission from ref. [75]. Copyright 1998, American Chemical Society.)...

If the electric field E is applied to a system of colloidal particles in a closed cuvette where no streaming of the liquid can occur, the particles will move with velocity v. This phenomenon is termed electrophoresis. The force acting on a spherical colloidal particle with radius r in the electric field E is 4jrerE02 (for simplicity, the potential in the diffuse electric layer is identified with the electrokinetic potential). The resistance of the medium is given by the Stokes equation (2.6.2) and equals 6jtr]r. At a steady state of motion these two forces are equal and, to a first approximation, the electrophoretic mobility v/E is... 

During the formation of polycation-polyanion multilayer coatings on halloysite, we monitored the surface potential (electrokinetic zeta potential). Initially negative halloysite (—40 mV) was converted to a positive surface with polycation layer adsorption in the first step of the LbLassembly (figure 14.10). Adsorption of polyanions in the second step re-established the negative charge which was reversed... 

The word electrokinetic implies the joint effects of motion and electrical phenomena. We are interested in the electrokinetic phenomena that originate the motion of a liqnid within a capillary tube and the migration of charged species within the liquid that surrounds them. In the first case, the electrokinetic phenomenon is called electroosmosis whereas the motion of charged species within the solution where they are dissolved is called electrophoresis. This section provides a brief illns-tration of the basic principles of these electrokinetic phenomena, based on text books on physical chemistry [7-9] and specialized articles and books [10-12] to which a reader interested to stndy in deep the mentioned theoretical aspects should refer to. 

The first group, consisting of Sections 2.2-2.4, covers sedimentation. After some preliminaries, we discuss Stokes s law, a hydrodynamic equation that will appear again when we discuss electrokinetic phenomena in Chapter 12 and the kinetics of coagulation in Chapter 13. Stokes s law is a key relationship in understanding the rate of sedimentation and is used in the derivation of the sedimentation equation for spherical particles. Following this, the equation for the sedimentation coefficient, a... 

The first three electrokinetic processes are our concern in this chapter, with the emphasis on electrophoresis. 

When one thinks of electrokinetic phenomena in the context of a first-level course on colloid and surface chemistry, the first thought that probably comes to mind is the use of such phenomena to measure zeta potentials and charges of colloidal species. But, as we have already seen in Chapter 1 and as we see later in this chapter, electrokinetic phenomena play a significant role in many other applications. We take a look at one such application here and see why the topics we consider in this chapter and in others are important in that context. 

First, the very existence of a surface of shear implies some interesting behavior within the fluid phase of the system under consideration. In our discussion of all electrokinetic phenom-... 

Probstein, R. F., Physicochemical Hydrodynamics, 2d ed., Wiley-Interscience, New York, 1994. (A primarily first-year-graduate-level textbook on colloidal and hydrodynamic phenomena of interest in practice. Has good discussions on electrokinetic phenomena.)... 

Electro-osmotic oscillation (first observed by Teorell [1]—[4] in a laboratory set-up devised to mimic nerve excitation) may likely represent a common source of oscillations in various natural or synthetic electrokinetic systems such as solid microporous filters, synthetic ion-exchange membranes or their biological counterparts. The original experimental set-up, which contained all essential elements to look for when the electro-osmotic oscillations are suspected in a natural system, is schematically as follows. 

In addition to the foregoing, it is customary to include under electrochemistry (I) processes for which the net reaction is physical transfer, e g., concentration cells (2) electrokinetic phenomena, e.g.. electrophoresis. eleclroosmnsis, and streaming potential (3) properties ot electrolytic solutions, if they are determined by electrochemical or other means, e g.. activity coefficients and hydrogen ion concentration (4) processes in which electrical energy is first converted to heal, which in turn causes a chemical reaction that would not occur spontaneously at ordinary temperature. The... 

The separation of phospholipids by micellar electrokinetic capillary electrophoresis (MEKC) has been described (17-19). In this technique, solutes are separated based on their distribution between a mobile (usually aqueous) and a pseudostationary (micellar) phase. Szucs et al. found that the major soybean phospholipids were fully resolved in only 7 minutes using deox ycholic acid for micelle formation in combination with 30% n-propanol at 50°C (18). However, quantification of the separated compounds remains troublesome. This is due first of all to the fact that only UV detection can be used, thus making the response highly dependent on the degree of unsaturation of the phospholipids. Besides, the comparison of peak areas in MEKC is more complicated than in HPLC, because all compounds are moving with different velocities. 

Fig. 15 Separation of ropivacaine and propranolol enantiomers using an MIP plug composed of (S)-ropivacaine MIP and (S)-propranolol MIP. The capillary was 100 cm in total length and 91.5 cm in effective length. The electrolyte contained acetonitrile/2 mol L-1 acetic acid adjusted to pH 3 by the addition of triethanolamine (90/10, v/v). The separation voltage was 15 kV, and the capillary column was thermostated to 60 °C. The MIPs were injected hydrodynamically at 50 mbar for 6 s each, and the sample was composed of 50 pmol L 1 rac-propranolol (first eluting) and rac-ropivacaine injected electrokinetically at 16 kV for 3 s. Detection was performed at 214 (top) and 195 nm (bottom) [42]...

In 1990, Bushey and Jorgenson developed the first automated system that coupled HPLC with CZE (19). This orthogonal separation technique used differences in hydrophobicity in the first dimension and molecular charge in the second dimension for the analysis of peptide mixtures. The LC separation employed a gradient at 20 (xL/min volumetric flow rate, with a column of 1.0 mm ID. The effluent from the chromatographic column filled a 10 pU loop on a computer-controlled, six-port micro valve. At fixed intervals, the loop material was flushed over the anode end of the CZE capillary, allowing electrokinetic injections to be made into the second dimension from the first. 

Mixer 3 [M 3] Electrokinetic Instability Electroosmotic Flow Micro Mixer, First-generation Device... 

Figure 1.7 Design of an electrokinetic instability EOF micro mixer, first-generation device [25] (by courtesy of ACS).

Figure 1.8 Design of an electrokinetic instability micro mixer, second-generation device, based on the results obtained with the first design given in Figure 1.7. The electrokinetic instability is confined to the square mixing chamber shown in the center of the schematic and, to a small extent, to fluid channel regions attached to it [25] (by courtesy of ACS).

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