Electrophoretic mobility, velocity measurement

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. 

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. 

The velocity of particle migration, v, across the field is a function of the surface charge or zeta potential and is observed visually by means of an ultramicroscope equipped with a calibrated eyepiece and a scale. The movement is measured by timing the individual particles over a certain distance, and the results of approximately 10-15 timing measurements are then averaged. From the measured particle velocity, the electrophoretic mobility (defined as v/E, where E is the potential gradient) can be calculated. 

The zeta potential can be measured by electrophoresis, which determines the velocity of particles in an electric field of known strength [144]. This particle velocity, v, can then be related to the electrical field strength, E, as the electrophoretic mobility, fi. This is shown by... 

Electrophoretic mobility measurements can be performed by laser Doppler anemometry (LDA). LDA is fast and capable of high resolution of particle velocities [144]. It measures particle velocity, which is measured in the stationary... 

A measurement of electrophoretic mobility /v, gives x = 0.02 cm s-1, where v is the velocity observed under a field, and x the mobility. Calculate the zeta potential of the colloid concerned. (Bockris)... 

EXAMPLE 12.4 Electrophoretic Mobility of Bacteria. It is proposed to evaluate the electrophoretic mobility of the bacteria cells shown in Figure 12.10a by multiplying the appropriate value of time-1 by the distance of particle displacement and then dividing by E. Criticize or defend the following proposition It is appropriate to use the maximum apparent velocity since this is measured at the center of the cell and is therefore subject to the least interference by wall effects. 

The electrophoretic mobility, tE is defined as the electrophoretic velocity divided by the electric field gradient at the location where the velocity was measured ... 

In practical applications of electrophoresis, collections of colloidal particles in bounded systems are usually encountered and the experimentally measured electrophoretic mobility is actually the average value for the entire suspension. It is therefore necessary to determine the average electrophoretic velocity for a suspension of colloidal particles. For dilute dispersions, the first order correction to the mobility of an isolated particle can be determined from the... 

The size and charge analysis was done using a Coulter DELSA 440SX (Coulter Beckman Corp., Miami, FL). This particular instrument measured the size distribution on the basis of photon correlation spectrometry (PCS) and was limited to particle diameters between 0.02 pm and 3 pm. Measurements were taken at four different angles simultaneously with 256-channel resolution each. Comparison of the spectra allowed for the detection of very small particles. The zeta potential was assessed on the basis of electrophoretic mobility (laser Doppler anemometry, LDA). This was defined as the particle velocity per unit of applied electrical field, with units usually given as pm s 1/V cm-1, while zeta potential is defined as the electrical potential between the bulk solution and the... 

To characterize a surface electrokinetically involves the measurement of one of the above electrokinetic effects. With disperse colloidal systems it is practical to measure the particle electrophoretic mobility (induced particle velocity per unit applied electric field strength). However, for a nondisperse system one must measure either an induced streaming potential or an electro-osmosis fluid flow about the surface. 

The application of laser Doppler velocimetry (LDV) to measure the electrophoretic mobility n of charged colloidal particles is known as laser Doppler electrophoresis (LDE). In a typical LDE experiment, an applied electric field drives the collective motion of charged colloidal particles. The particles pass through an interference pattern created by a dual-beam experimental setup (Section III.A.2). The collective electrophoretic velocity of the particles is then determined via intensity- or spectrum-based analysis of the scattered light, and the electrophoretic mobility n is calculated by dividing the velocity by the applied electric field strength. 

The results of measurements by the microscopic method show that the electrophoretic mobility of the particles varies with the distance from the wall of the cell particles close to the wall move in a direction opposite to that in which those in the center migrate. In any event, the results show an increase in velocity from the walls to the center of the cell. The explanation of this fact lies in the electro-osmotic movement of the liquid a double layer is set up between the liquid and the walls of the cell and under the influence of the applied field the former exhibits electro-osmotic flow. For the purpose of obtaining the true electrophoretic velocity of the suspended particles it is neceasary to observe particles at about one-fifth the distance from one wall to the other. A more accurate procedure is to make a series of measurements at different distances from the side of the cell and to apply a correction for the electro-osmotic flow. The algebraic difference of the corrected electrophoretic velocity and the speed of the particles near the walls gives the electro-osmotic mobility of the liquid in the particular cell. If the solution contains a protein which is adsorbed on the surface of the walls of the vessel and on the particles, it is possible to compare the electrophoretic and electro-osmotic mobilities in one experiment reference to the significance of such a comparison was made on page 532. 

We are Interested in measuring the electrophoretic mobility u of individual colloidal particles. This requires the preparation of a sol, appl)dng an external electric field E and determining the velocity (or velocities) v of the partlcle(s). 

The parameter normally measured in capillary electrophoresis is migration (retention) time, /. In a given CE system this parameter is inversely proportional to the electrophoretic mobility, pi. The pt (cm /V) is a normalized parameter allowing for comparison of data obtained in different CE systems. If a series of analytes are analyzed under the same conditions then the 1/r and pt are equivalent. There are only a few reports on QSRR analysis of CE data. This may suggest the unsuitability of routinely determined mobility parameters as the LEER descriptors of analyte behaviour. Probably the reproducibility of analyte migration times in CE is poor due mainly to the non-reproducible electroosmotic flow velocity 26. ... 

Determination of the Electrophoretic Mobility, To evaluate the equation for the double-layer interaction (eq 5), the zeta potential, must be known it is calculated from the experimentally measured electrophoretic mobility. For emulsions, the most common technique used is particle electrophoresis, which is shown schematically in Figure 4. In this technique the emulsion droplet is subjected to an electric field. If the droplet possesses interfacial charge, it will migrate with a velocity that is proportional to the magnitude of that charge. The velocity divided by the strength of the electric field is known as the electrophoretic mobility. Mobilities are generally determined as a function of electrolyte concentration or as a function of solution pH. 

The motion of the droplet in an applied electric field distorts the relationship between the charged center and the outer layer to create a dipole. With this simplistic view, it is possible to understand the principle behind electrophoretic mobility, whereby the relative motion of particles or emulsion droplets is measured with an applied electric field. These measurements often depend upon microscopic observation of the droplet motion in the applied electric field and a calculation of droplet velocities to determine their electrophoretic mobility. Figure 11 is a schematic of a typical experimental setup. 

Electrophoretic light-scattering is a technique which determines the electrophoretic velocities by measuring the Doppler shifts of scattered laser light. The electrophoretic mobility can be expressed by... 

Some types of electrophoretic cells are stationary layer problem free , but in the other cells the electroosmotic flow can lead to erroneous results. The observed velocity of particles is a sum of the electroosmotic flow of the fluid and the velocity of particles with respect to the fluid. The latter is a function of the potential of the particles and the former is a function of the position in the cell cross section. Hydrodynamic calculations make it possible to find the stationary levels, i.e. the positions in the cell cross section where the electroosmotic flow equals zero. Certainly the position of stationary levels in commercial electrophoretic cells can be found in the user s manual, and there is no need to perform any calculations. The fastest method to determine the electrophoretic mobility is from the velocity at one stationary level, but such a procedure can lead to substantial errors. For example, when the cell position is adjusted at room temperature and then measurements taken... 

Instrumentation. A Rank Bros, micro-electrophoresis unit was used in those studies, with a specially made quartz cell having a 6 cm. path length of rectangular inside cross-section (l mm thick, 10 mm deep) in which the Komagata equation (25) predicts zero mobility of the liquid phase in planes located at 0.612 of the distance b from the center plane of the cell to the wall. In electrophoresis experiments 300 to 1200 volts were applied to the cell and mobilities measured in planes a distance h from the center plane. The results were graphed as observed velocity versus (h/b)z as proposed by van Gils (26J, and if the straight lines characteristic of perfect parabolic flow resulted, the electrophoretic mobilities (v ) observed at h/b=0.612 were considered acceptable for calculation of zeta-potential. Zeta-potentials were calculated by the Huckel equation (27) ... 

Thus since the last term is zero for a closed cell by equation (8), the electrophoretic mobility is equal to the mean mobility uThe electrophoretic velocity may thus be obtained by making measurements at a series of levels, and obtaining a mean value from a graphical or other integration of the results with aid of equation 10. 

An indirect way to obtain information about the potential at foam lamella interfaces is by bubble electrophoresis, in which an electric field is applied to a sample causing charged bubbles to move toward an oppositely charged electrode. The electrophoretic mobility is the measured electrophoretic velocity divided by the electric field gradient at the location where the velocity was measured. These results can be interpreted in terms of the electric potential at the plane of shear, also known as the zeta potential, using well-known equations available in the literature (29—31). Because the exact location of the shear plane is generally not known, the zeta potential is usually taken to be approximately equal to the potential at the Stem plane (Figure 11) ... 

Good descriptions of practical experimental techniques in conventional electrophoresis can be found in references 29, 30, and 32. For the most part, these techniques are applied to suspensions and emulsions, rather than foams. In bubble microelectrophoresis, the dispersed bubbles are viewed under a microscope, and their electrophoretic velocity is measured taking the horizontal component of motion, because bubbles rapidly float upwards in the electrophoresis cells (33, 34). A variation on this technique is the spinning cylinder method, in which a bubble is held in a cylindrical cell that is spinning about its long axis. An electric field is applied, and the electrophoretic mobility is determined (2, 35). Other elec-trokinetic techniques, such as the measurement of sedimentation potential (36) have been used as well. 

The electrophoretic mobility, pE, is defined as the electrophoretic velocity divided by the electric field gradient at the location where the velocity was measured. It remains then to relate the electrophoretic mobility to the zeta potential (f). Two simple relations can be used to calculate zeta potentials in limiting cases ... 

Electrophoresis The motion of colloidal species caused by an imposed electric field. The term replaces the older term cataphoresis. The species move with an electrophoretic velocity that depends on their electric charge and the electric field gradient. The electrophoretic mobility is the electrophoretic velocity per unit electric field gradient and is used to characterize specific systems. An older synonym, no longer in use, is kataphoresis. The term microelectrophoresis is sometimes used to indicate electrophoretic motion of a collection of particles on a small scale. Previously, microelectrophoresis was used to describe the measurement techniques in which electrophoretic mobilities are determined by observation through a microscope. The recommended term for these latter techniques is now microscopic electrophoresis (see reference 1). 

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