Electrokinetic Second

Electrokinetic processes only develop in dilute electrolyte solutions. The second phase can be conducting or nonconducting. Processes involving insulators are of great importance, since they provide the only way of studying the structure and electrical properties of the surface layer of these materials when they are in contact with the solution. Hence, electrokinetic processes can also be discussed as one of the aspects of insulator electrochemistry. 

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 washing of capillaries with dilute alkaline solution is advisable before analysis. The alkaline solution can be followed by deionized water and buffer. Capillaries can be washed between runs too. Samples can be introduced into the capillary by hydrodynamic and electro-kinetic methods. The hydrodynamic method applies a pressure difference (5-10 sec) between the two ends of the capillary. The pressure difference can be achieved by overpressure, vacuum or by creating a height difference between the levels of the buffer and sample reservoirs. In the case of electrokinetic injection, the injection end of the capillary is dipped into the sample for a few seconds and a voltage of some thousand volts is applied. 

Electrokinetic Measurements. Electrophoretic mobilities were measured with a flat-cell apparatus manufactured by Rank Brothers, Cambridge, England. In addition, several mobility values were checked for accuracy with a Zeta Meter, New York. Mobilities were determined with a small volume of the suspension (approximately 25 cc) that had been prepared for the adsorption experiments. The pH of the solution was measured prior to determining the electrophoretic mobilities, which involved measuring the velocities of five to ten particles in each direction. An average value of the mobilities was recorded. Samples containing the flocculated particles were dipped into an ultrasonic bath for approximately one second prior to making the pH and mobility measurements. 

Two conditions must be met to justify comparisons between f values determined by different electrokinetic measurements (a) the effects of relaxation and surface conductivity must be either negligible or taken into account and (b) the surface of shear must divide comparable double layers in all cases being compared. This second limitation is really no problem when electroosmosis and streaming potential are compared since, in principle, the same capillary can be used for both experiments. However, obtaining a capillary and a migrating particle wiih identical surfaces may not be as readily accomplished. One means by which particles and capillaries may be compared is to coat both with a layer of adsorbed protein. It is an experimental fact that this procedure levels off differences between substrates The surface characteristics of each are totally determined by the adsorbed protein. This technique also permits the use of microelectrophoresis for proteins since adsorbed and dissolved proteins have been shown to have nearly identical mobilities. 

Chapters 11-13 of the second edition, which discussed van der Waals forces (old Chapter 11), electrical double layers (old Chapter 12), electrokinetic phenomena (old Chapter 13), and colloid stability (old Chapters 11 and 12), have been restructured and new materials on colloid stability and polymer/colloid interactions have been added. For example ... 

For electrokinetic injection, the capillary is dipped in the sample and a voltage is applied between the ends of the capillary. The moles of each ion taken into the capillary in t seconds are... 

Kim, Kim and Kim (2005) evaluated the treatment of arsenic in two fine-grained soils with an ex situ electrokinetic technology, which consisted placing the samples in a three-compartment chamber with a platinum anode and titanium cathode on opposite ends. One soil consisting of a Korean kaolinite was spiked with 1500 mg kg-1 of As(V). The second soil sample, which contained 3210 mg kg-1 of arsenic, was collected from the abandoned Myungbong gold mine in southern South Korea. The soils were treated with NaOH or KH2P04 electrolyte solutions. Deionized water was used with control samples to establish a baseline. 

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. 

Figure 7.7 Electrochromatograms of noradrenaline and dopamine, respectively. Electrokinetic injection at 20kV/cm for 2 seconds followed by separation at 30 kV/cm, L = 54 cm (total length of capillary), 1 = 40 cm (length of capillary up to detector) [22].

Pumera et al. [34] used gold nanoparticle enhanced open channel NCEC for the separation of p-aminophenol, o-aminophenol, and m-aminophenol in a bare glass channel and polydiallyl dimethyl ammonium chloride (PDADMAC) gold coated channel. The mobile phase used was acetate buffer (20 mM, pH 5.0) with electrokinetic sample injection at 1.5kV/cm for 3 seconds and 2.0kV/cm as separation voltage (Fig. 7.15). Broyles... 

The use of centrifugation to separate the liquid from solid phases in traditional batch or tube techniques has several disadvantages. Centrifugation could create electrokinetic effects close to soil constituent surfaces that would alter the ion distribution (van Olphen, 1977). Additionally, unless filtration is used, centrifugation may require up to 5 min to separate the solid from the liquid phases. Many reactions on soil constituents are complete by this time or less (Harter and Lehmann, 1983 Jardine and Sparks, 1984 Sparks, 1985). For example, many ion exchange reactions on organic matter and clay minerals are complete after a few minutes, or even seconds (Sparks, 1986). Moreover, some reactions involving metal adsorption on oxides are too rapid to be observed with any batch or, for that matter, flow technique. For these reactions, one must employ one of the rapid kinetic techniques discussed in Chapter 4. 

Mixer 4 [M 4] Electrokinetic Instability Electroosmotic Flow Micro Mixer, Second-generation Device... 

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).

Figure 1.13 Ensemble-averaged temporal evolution of voxel-averaged spatial intensity PDFs for the electrokinetic instability micro mixer, second-generation device. Each ensemble consists of nine realizations.

Figure 1.14 Two-dimensional power spectra of various mixing chamber images for the electrokinetic instability micro mixer, second-generation device, (a) Large frequency components along the vertical direction owing to the initial layered distribution of the dye. (b) Larger spatial frequencies are introduced by the EKI stirring within the chamber, (c) The attenuation of large spatial frequencies corresponds to a nearly homogeneous intensity profile [25] (by courtesy of ACS).

Another IEF/CGE separation was carried out on a PC chip for protein analysis. First dimension in one horizontal channel was used to perform denaturing IEF and second dimension in an array of 10 channels was used for SDS-gel electrophoresis, as shown in Figure 6.46. Instead of sequentially sampling proteins eluted from IEF, they are electrokinetically transferred in a parallel fashion... 

Abstract Investigations of alternate adsorption regularities of cationic polyelectrolytes a) copolymer of styrene and dimethylaminopropyl-maleimide (CSDAPM) and b) poly(diallyldimethylammonium chloride) (PDADMAC) and anionic surfactant - sodium dodecyl sulfate (SDS) on fused quartz surface were carried out by capillary electrokinetic method. The adsorption/desorption kinetics, structure and properties of adsorbed layers for both polyelectrolytes and also for the second adsorbed layer were studied in dependence on different conditions molecular weight of polyelectrolyte, surfactant and polyelectrolyte concentration, the solution flow rate through the capillary during the adsorption, adsorbed layer formation... 

It can be shown that the CVI tends to a nonzero limiting value at very high electrolyte concentrations, as in the case of other electrokinetics of soft particles (Chapters 21 and 24). This is a characteristic of the electrokinetic behavior of soft particles, which comes from the second term of the right-hand side of Eq. (25.45). 

---