Electrophoresis Electrostatic

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Electrophoresis (qv), ie, the migration of small particles suspended in a polar Hquid in an electric field toward an electrode, is the best known effect. If a sample of the suspension is placed in a suitably designed ceU, with a d-c potential appHed across the ceU, and the particles are observed through a microscope, they can all be seen to move in one direction, toward one of the two electrodes. AH of the particles, regardless of their size, appear to move at the same velocity, as both the electrostatic force and resistance to particle motion depend on particle surface this velocity can be easily measured. 

Response to Electric and Acoustic Fields. If the stabilization of a suspension is primarily due to electrostatic repulsion, measurement of the zeta potential, can detect whether there is adequate electrostatic repulsion to overcome polarizabiUty attraction. A common guideline is that the dispersion should be stable if > 30 mV. In electrophoresis the appHed electric field is held constant and particle velocity is monitored using a microscope and video camera. In the electrosonic ampHtude technique the electric field is pulsed, and the sudden motion of the charged particles relative to their counterion atmospheres generates an acoustic pulse which can be related to the charge on the particles and the concentration of ions in solution (18). 

General References Gascoyne and Vykoukal, Electrophoresis, 23, 1973-1983 (2002). Pohl, in Moore (ed.), Electrostatics and Its Applications,... 

An example of a practical dielectrofilter which uses both of the features described, namely, sharp electrodes and dielectric field-warping filler materials, is that described in Fig. 20-33 [H. J. Hall and R. F. Brown, Lubric. Ens., 22, 488 (1966)]. It is intended for use with hydraulic fluids, fuel oils, lubricating oils, transformer oils, lubricants, and various refinery streams. Performance data are cited in Fig. 20-34. It must be remarked that in the opinion of Hall and Brown the action of the dielectrofilter was "electrostatic and due to free charge on the particles dispersed in the liquids. It is the present authors opinion, however, that both electrophoresis and dielectrophoresis are operative here but that the dominant mechanism is that of DEP, in which neutral particles are polarized and attracted to the regions of highest field intensity. 

A review of preparative methods for metal sols (colloidal metal particles) suspended in solution is given. The problems involved with the preparation and stabilization of non-aqueous metal colloidal particles are noted. A new method is described for preparing non-aqueous metal sols based on the clustering of solvated metal atoms (from metal vaporization) in cold organic solvents. Gold-acetone colloidal solutions are discussed in detail, especially their preparation, control of particle size (2-9 nm), electrophoresis measurements, electron microscopy, GC-MS, resistivity, and related studies. Particle stabilization involves both electrostatic and steric mechanisms and these are discussed in comparison with aqueous systems. 

Figure 2.1 Mass spectrometric approach. Dl, direct inlet GC, gas chromatography HPLC, high performance liquid chromatography CZE, capillary zone electrophoresis El, electron ionization Cl, chemical ionization ESI, electrospray ionization DESI, desorption electrospray ionization APCI, atmospheric pressure chemical ionization MALDI, matrix assisted laser desorption ionization B, magnetic analyzer E, electrostatic analyzer...

Figure 6.2 Electrostatic adsorption mechanism of Brunelle [1] (a) surface polarization as a function of pH (b) measurement of PZC of some oxides (equivalent to isoelectric point) by electrophoresis.

Anions and uncharged analytes tend to spend more time in the buffered solution and as a result their movement relates to this. While these are useful generalizations, various factors contribute to the migration order of the analytes. These include the anionic or cationic nature of the surfactant, the influence of electroendosmosis, the properties of the buffer, the contributions of electrostatic versus hydrophobic interactions and the electrophoretic mobility of the native analyte. In addition, organic modifiers, e.g. methanol, acetonitrile and tetrahydrofuran are used to enhance separations and these increase the affinity of the more hydrophobic analytes for the liquid rather than the micellar phase. The effect of chirality of the analyte on its interaction with the micelles is utilized to separate enantiomers that either are already present in a sample or have been chemically produced. Such pre-capillary derivatization has been used to produce chiral amino acids for capillary electrophoresis. An alternative approach to chiral separations is the incorporation of additives such as cyclodextrins in the buffer solution. 

The order of elution of peptides (charged compounds) is governed by a combination of electrophoresis and partitioning, with hydrophobic as well as electrostatic contributions. In this study it was demonstrated that sulfonic acid functionalities in the methacrylate monolith provide high stability and maintain a constant EOF over a wide range of pH (2—12). It was also demonstrated that a better separation of a mixture of therapeutic peptides was obtained at high pH values (Figure 16) due to the suppression of electrostatic attraction. 

Efficiency, of commercial columns, 40 Eicosyltrichlorosilane, 133 Electrophoresis, comparison to RPC, 224 Electrostatic interactions, 120, 208, 213 calculation of 208-211 dipoles, 208, 209 at high ionic strength, 209-210 in ion-pair chromatography, 238 at low ionic strength, 209 monopoles, 209-211... 

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. 

Interactions between the solutes and the capillary wall also have a negative effect on the efficiency in capillary zone electrophoresis. Both hydrophobic interactions and electrostatic interactions of cations with the negatively charged capillary wall can be the cause of solute adsorption. Significant adsorption has been found for high-molecular-weight species, e.g., peptides and proteins. Because of the increased surface-area-to-volume ratio of narrow-bore capillaries, this effect is even more pronounced. 

Several different analytical and ultra-micropreparative CEC approaches have been described for such peptide separations. For example, open tubular (OT-CEC) methods have been used 290-294 with etched fused silicas to increase the surface area with diols or octadecyl chains then bonded to the surface.1 With such OT-CEC systems, the peptide-ligand interactions of, for example, angiotensin I-III increased with increasing hydrophobicity of the bonded phase on the capillary wall. Porous layer open tubular (PLOT) capillaries coated with anionic polymers 295 or poly(aspartic acid) 296 have also been employed 297 to separate basic peptides on the inner wall of fused silica capillaries of 20 pm i.d. When the same eluent conditions were employed, superior performance was observed for these PLOT capillaries compared to the corresponding capillary zone electrophoresis (HP-CZE) separation. Peptide mixtures can be analyzed 298-300 with OT-CEC systems based on octyl-bonded fused silica capillaries that have been coated with (3-aminopropyl)trimethoxysilane (APS), as well as with pressurized CEC (pCEC) packed with particles of similar surface chemistry, to decrease the electrostatic interactions between the solute and the surface, coupled to a mass spectrometer (MS). In the pressurized flow version of electrochromatography, a pLC pump is also employed (Figure 26) to facilitate liquid flow, reduce bubble formation, and to fine-tune the selectivity of the separation of the peptide mixture. 

The composition of the surface-bound species must be considered they contribute to the stability of the dispersions of metal nanoparticles. In the case of electrostatically stabilized dispersions, the techniques to measure the interfacial electronic phenomena, including electrophoresis, electroosmosis, etc., are useful (54). In order to understand the composition (as well as structures) of the chemical species bound in the surface of metal particles, spectroscopic measurements used for common organic substances are used as well as the elemental analysis. 

Hattori, T., Hallberg, R., Dubin, P.L. (2000). Roles of electrostatic interaction and polymer structure in the binding of p-lactoglobulin to anionic polyelectroytes measurement of binding constants by frontal analysis continuous capillary electrophoresis. Langmuir, 16, 9738-9743. 

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