Particles electrophoresis

Particle electrophoresis, also sometimes known as microscope electrophoresis or microelectrophoresis, is one of the easiest and most useful techniques for investigating the electrical properties of colloidal particles. If the system of interest is in the form of a reasonably stable dispersion of particle size observable by light microscopy (say, larger than 200 nm for practical application), the electrokinetic behavior of the system can be observed and measured directly. Several commercial instruments are available for the purpose. For smaller particles, laser scattering instruments are now readily available. 

Particle electrophoresis has proved to be very useful in many areas of theoretical and practical interface and colloid science, including model polymer latex and silver halide systems, and more practical problems related to water purification, detergency, emulsion science, the characterization of bacterial surfaces, blood cells, viruses, and so on. With the advent of more sophisticated computer data analysis and laser hght sources, the limits of resolution for particle sizes that can be analyzed has been, and is being, steadily reduced, so that with proper (and more expensive) instrumentation, the electrophoretic nature of particles in the size range of a few nanometers can be readily determined. 

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Douglas et al. [98] have measured protein (serum albumin, ovalbumin, and hemoglobin) mobilities over a range of pH values using a free-flow electrophoresis apparatus and a particle electrophoresis apparatus. They found good agreement between the two measurements however, they also found some differences between their measurements and those reported in the older literature. They attributed the differences to the use of moving-boundary electrophoresis methods in the early experimental work and to differences in... 

Particle electrophoresis studies have proved to be useful in the investigation of model systems (e.g. silver halide sols and polystyrene latex dispersions) and practical situations (e.g. clay suspensions, water purification, paper-making and detergency) where colloid stability is involved. In estimating the double-layer repulsive forces between particles, it is usually assumed that /rd is the operative potential and that tf/d and (calculated from electrophoretic mobilities) are identical. 

The Huckel equation is not likely to be applicable to particle electrophoresis in aqueous media for example, particles of radius 10 8 m suspended in a 1-1 aqueous electrolyte solution would require an electrolyte concentration as low as 10-5 mol dm-3 to give kq = 0.1. The equation, however, does have possible applicability to electrophoresis in non-aqueous media of low conductance. 

For well-dispersed colloid systems, particle electrophoresis has been the classic method of characterization with respect to electrostatic interactions. However, outside the colloidal realm, i.e., in the rest of the known world, the measurement of other electrokinetic phenomena must be used to characterize surfaces in this respect. The term electrokinetic refers to a number of effects induced by externally applied forces at a charged interface. These effects include electrophoresis, streaming potential, and electro-osmosis. 

One way to increase the range of measurement may be in the use of light scattering (Doppler electrophoresis) to determine particle velocities. Such methods are used in contemporary commercially available analytical particle electrophoresis apparatuses. However, presently available equipment is not designed for ready exchange (replacement) of chamber surfaces for electro-osmosis studies. 

Though the model presented and used does not give a complete account of the interface and the origin of measured electro-osmotic fluid mobility, it was proven useful in interpretation of surface properties. The range of electrolyte concentration that can be used in the manual particle electrophoresis chamber developed in this work is limited, and this limits the model of the origin of electro-osmosis that can be tested, such as inclusion of a Stern layer. 

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. 

Sestier C, Da-Silva MF, Sabolovic D, Roger J, Pons JN (1998) Snrface modification of snperparamagnetic nanoparticles (ferroflnid) stndied with particle electrophoresis Application to the specific targeting of cells. Enrophoresis 19 1220-1226... 

In the case of mobile charged particles (electrophoresis and sedimentation potential Figure 5.66c and 5.66d), we should identify 7, as the flux of particles, Jp, and F, as the gravity force, F Then, the Onsager relations read... 

The size (and charge polarity) of separated particles Electrophoresis (separation of colloids) Iontophoresis (separation of ions in true solutions) Cataphoresis (separation of cations) Anaphoresis (separation of anions)... 

Very finely divided particles suspended in a liquid carry an electrical charge which is equivalent to the charge on the particle itself plus the charge on the fixed portion of the double layer. If an electrical field is applied to such a suspension, the particles move in the field in the direction determined by the charge on the particle (electrophoresis). The diffuse part of the double layer, since it is mobile, has the opposite sign and is attracted to the other electrode. Conversely, if a suspension of particles is allowed to settle, they carry their charge toward the bottom of the vessel and leave the charge on the diffuse layer in the upper portion of the vessel. A potential difference, the sedimentation potential, develops between the top and bottom of the container. 

Hasan, Z. and Pieters, J., Subcellular fractionation by organelle electrophoresis Separation of phagosomes containing heat-killed yeast particles. Electrophoresis, 19, 1179-1184, 1998. 

Zhu J, Xiian X (2009) Particle electrophoresis and dielectrophoresis in curved microchannels. J Colloid Interface Sci 340 285—290... 

Electric Fields Electric field assembly can be employed with attractive and repulsive properties of tmcharged particles (electrophoresis, EP) or with dielectric properties of components (dielectrophoresis, DEP). Electric field-mediated assembly is usually utihzed with metal electrodes. The assembly can be accomplished either on top of the electrodes or at desired locations. Electric field-mediated assembly is mostly restricted to two-dimensional assembhes. 

Electrophoresis refers to the motion of a charged particle in a solution in response to an applied electric field. The electrophoresis technique has been widely used to characterize the electrokinetic properties of charged particle-liquid interfaces. In the electrophoresis method, fine particles (usually of 1 pm in diameter) are dispersed in a solution. Under an applied electric field, the particle electrophoresis mobility, vg, defined as the ratio of particle velocity to electric field strength, is measured using an appropriate microscopic technique. The particle -potential is determined from the measured electrophoresis mobility, ve, by using the Smoluchowski equation expressed as... 

Knox, R.J., Bums, N.L., van Alstine, J.M., Harris, J.M., and Seaman, G.V.F., Automated particle electrophoresis modelling and control of adverse chamber surface properties. Anal. Chem., 70, 2268-2279, 1988. 

An alternative to particle electrophoresis is moving-boundary electrophoresis. The technique is used to study the movement of a boundary formed between a colloidal sol or solution and the pure dispersion medium under the influence of the electric field. The technique has found some application for determining not only electrophoretic mobility, but also for small-scale separation of species from a mixture for further identification. It found early application in the study of proteins and other dissolved marcomolecules. 

Reactions at the Silicon Nitride - Solution Interface - A study was conducted to determine the extent of aqueous reactions at the silicon nitride-water interface. It was demonstrated that up to 27 days are required to stabilize reactions at the interface as indicated by Ph and particle electrophoresis measurements. A semiautomatic titrator was also purchased and set up to utilize acid-base titrations to study the silicon nitride-solvent interface. A particular emphasis of this work will be on the nonaqueous potentiometric and conductometric titration to determine the strength of acid and base sites on the silicon nitride surface. 

A schematic of a particle electrophoresis apparatus is shown in Fig. 4.17. The suspension is placed in a cell, and a dc voltage V is applied to two electrodes at a fixed distance I apart. The sign of the particle charge is obtained directly since it is opposite to that of the electrode toward which the particle is migrating. The particle velocity is measured by using a microscope, and the velocity per unit field strength (the electrophoretic mobility) is used to determine the -potential and the surface charge. 

In a particle electrophoresis experiment, the particles in a dilute suspension are observed to travel an average distance of 1 mm in 25 s when a potential difference of 100 V is applied to the electrodes that are 5 cm apart. Determine the -potential... 

The most common method employed for particle electrophoresis is the use of closed, free-fluid electrophoresis chambers (5). In this method, particles suspended in a fluid medium are electrophoresed in an enclosed electrode chamber. Particle mobility is determined optically through the chamber wall. However, if the chamber surface is charged, electroosmosis is induced at the chamber walls due to the applied electric field. A hydrodynamic circulatory flow in the chamber thus results. This hydro-dynamic flow complicates mobility determination since the flow will impose upon the particle s mobility. This hydrodynamic flow must be taken into account. Particle mobility must either be measured in regions of the cell where there is no significant fluid flow, or the fluid velocity must be measured and subtracted from the observed particle velocity. 

The hydrodynamics of fluid flow in micro-particle electrophoresis chambers are described by solutions to the Navier-Stokes equation for steady laminar fluid flow (equation (19.1)) with boundary values defined by the chamber geometry. When considering the dimensions of an experimental chamber compared to the thickness of the double-layer (mm to nm), fluid flow at the surface would appear to move at a constant velocity. In other... 

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