Electrophoretic mobilities

The electrophoretic mobility, pep, is a specific parameter for a given compound. It determines the velocity of a compound in an applied electric field. Compounds with different pep can be separated from each other. 

When an ion of charge q is placed into an electric field E, it experiences an electric force Fef. 

This electric force accelerates positively charged ions towards the negative electrode (cathode). Negatively charged ions are accelerated towards the positive electrode (anode). 

The movement of the ions is opposed by the frictional force, Ffr of the medium molecules. This force is directly proportional to the radius of the ion, r, and its electrophoretic migration velocity, Vgp, as well as the viscosity of the medium, i]. 

In a constant electric field, equilibrium is reached between the frictional and the electric force (equation 3.3). Hence, ionic particles move with a constant velocity, Vep (equation 3.4). 

The movement (migration) of a charged species under the influence of an applied field is characterized by its electrophoretic mobility, fie, which has units of cm2 sec 1 V 1. Mobility is dependent not only on the charge density of the solute (the overall valence and size of the solute molecule), but also on the dielectric constant and viscosity of the electrolyte. Mobility is also strongly dependent on temperature, increasing by approximately 2% for each Kelvin rise in temperature.2 In the presence of electroosmotic flow (Section 4.3.3), the apparent mobility is the sum of the electrophoretic mobility of the analyte, /ze, and the mobility of the electroosmotic flow, /XGO. 

The apparent mobility, /jl, may be determined, experimentally, using the equation3 

Electrophoresis is the movement of a charged molecule relative to a stationary liquid by an applied electric field. For weak enough electric fields, tiie average velocity u of the molecule is proportional to the electric field E, 

Diffusion Coefficients of Poiyeiectroiyte Chains in Diiute and Semidiiute Soiutions 

Source Muthukumar, M., Dynamics of polyelectrolyte solutions, J. Chem. 

Note Cp and are, respectively, the concentrations of the polymer and monovalent added salt. 

Let us consider a rigid spherical particle of radius R, bearing a net charge Q. Let E be the externally applied uniform electric field and jo be the shear viscosity of the solution. The charged particle experiences an electric force Fg/ directed toward the oppositely charged electrode. 

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There are a number of complications in the experimental measurement of the electrophoretic mobility of colloidal particles and its interpretation see Section V-6F. TTie experiment itself may involve a moving boundary type of apparatus, direct microscopic observation of the velocity of a particle in an applied field (the zeta-meter), or measurement of the conductivity of a colloidal suspension. 

Rowell and co-workers [62-64] have developed an electrophoretic fingerprint to uniquely characterize the properties of charged colloidal particles. They present contour diagrams of the electrophoretic mobility as a function of the suspension pH and specific conductance, pX. These fingerprints illustrate anomalies and specific characteristics of the charged colloidal surface. A more sophisticated electroacoustic measurement provides the particle size distribution and potential in a polydisperse suspension. Not limited to dilute suspensions, in this experiment, one characterizes the sonic waves generated by the motion of particles in an alternating electric field. O Brien and co-workers have an excellent review of this technique [65]. 

The presence of surface conductance behind the slip plane alters the relationships between the various electrokinetic phenomena [83, 84] further complications arise in solvent mixtures [85]. Surface conductance can have a profound effect on the streaming current and electrophoretic mobility of polymer latices [86, 87]. In order to obtain an accurate interpretation of the electrostatic properties of a suspension, one must perform more than one type of electrokinetic experiment. One novel approach is to measure electrophoretic mobility and dielectric spectroscopy in a single instrument [88]. 

These effects can be illustrated more quantitatively. The drop in the magnitude of the potential of mica with increasing salt is illustrated in Fig. V-7 here yp is reduced in the immobile layer by ion adsorption and specific ion effects are evident. In Fig. V-8, the pH is potential determining and alters the electrophoretic mobility. Carbon blacks are industrially important materials having various acid-base surface impurities depending on their source and heat treatment. 

Fig. V-8. Electrophoretic mobility of carbon black dispersions in 10 KNO3 as a function of pH. (From Ref. 93.)...

Surface active electrolytes produce charged micelles whose effective charge can be measured by electrophoretic mobility [117,156]. The net charge is lower than the degree of aggregation, however, since some of the counterions remain associated with the micelle, presumably as part of a Stem layer (see Section V-3) [157]. Combination of self-diffusion with electrophoretic mobility measurements indicates that a typical micelle of a univalent surfactant contains about 1(X) monomer units and carries a net charge of 50-70. Additional colloidal characterization techniques are applicable to micelles such as ultrafiltration [158]. 

Fig. XIV-4. Electrophoretic mobility of n-hexadecane drops versus the pH of the emulsion. (From Ref. 12.)...

The velocity with which a solute moves through the conductive medium due to its electrophoretic mobility (Vep). 

Electroosmotic Mobility When an electric field is applied to a capillary filled with an aqueous buffer, we expect the buffer s ions to migrate in response to their electrophoretic mobility. Because the solvent, H2O, is neutral, we might reasonably expect it to remain stationary. What is observed under normal conditions, however, is that the buffer solution moves toward the cathode. This phenomenon is called the electroosmotic flow. 

First, solutes with larger electrophoretic mobilities (in the same direction as the electroosmotic flow) have greater efficiencies thus, smaller, more highly charged solutes are not only the first solutes to elute, but do so with greater efficiency. Second, efficiency in capillary electrophoresis is independent of the capillary s length. Typical theoretical plate counts are approximately 100,000-200,000 for capillary electrophoresis. 

A form of capillary electrophoresis in which separations are based on differences in the solutes electrophoretic mobilities. 

Electroultrafiltration (EUF) combines forced-flow electrophoresis (see Electroseparations,electrophoresis) with ultrafiltration to control or eliminate the gel-polarization layer (45—47). Suspended colloidal particles have electrophoretic mobilities measured by a zeta potential (see Colloids Elotation). Most naturally occurring suspensoids (eg, clay, PVC latex, and biological systems), emulsions, and protein solutes are negatively charged. Placing an electric field across an ultrafiltration membrane faciUtates transport of retained species away from the membrane surface. Thus, the retention of partially rejected solutes can be dramatically improved (see Electrodialysis). 

Electroosmotic flow in a capillary also makes it possible to analyze both cations and anions in the same sample. The only requirement is that the electroosmotic flow downstream is of a greater magnitude than electrophoresis of the oppositely charged ions upstream. Electro osmosis is the preferred method of generating flow in the capillary, because the variation in the flow profile occurs within a fraction of Kr from the wall (49). When electro osmosis is used for sample injection, differing amounts of analyte can be found between the sample in the capillary and the uninjected sample, because of different electrophoretic mobilities of analytes (50). Two other methods of generating flow are with gravity or with a pump. 

The reaction center is built up from four polypeptide chains, three of which are called L, M, and H because they were thought to have light, medium, and heavy molecular masses as deduced from their electrophoretic mobility on SDS-PAGE. Subsequent amino acid sequence determinations showed, however, that the H chain is in fact the smallest with 258 amino acids, followed by the L chain with 273 amino acids. The M chain is the largest polypeptide with 323 amino acids. This discrepancy between apparent relative masses and real molecular weights illustrates the uncertainty in deducing molecular masses of membrane-bound proteins from their mobility in electrophoretic gels. 

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