Particles electrophoretic dispersion

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. 

In the above equation Xv and XQ are, respectively, the specific electric conductance of the disperse system as a whole, and of the dispersion medium u0 is the particle motion velocity v0/E is particle electrophoretic mobility, and q is the effective charge related to the separation of particles from portion of the diffuse layer of counterions. Assuming that such separation takes place at the plane of shear and using eqs. (V.30) and (V.31), for particles smaller than 1/k, one may write that... 

The results presented for spheres reject a general view lying at the basis of all schemes proposed for explanation for the low-frequency sign reversal in colloidal electro-optics — the necessary presence of anisotropic or deformable species in the disperse system as a condition for the detection of electro-optical responses. This is not a necessary condition for the observation of low-frequency negative electro-optical responses either. The essential role of particle electrophoretic mobility (giving rise to electroacoustic effects) for the latter is evident. The results presented concern a series of phenomena in different systems widely studied by researchers involved in the field of colloids, (bio)polyers, (bio)polyelectrolytes, etc. Below we turn our attention to some of these phenomena. 

The Penn State workplan is based on five tasks Task 1 to participate in round-robin particle electrophoretic mobility measurements, Task 2 to determine the nature of reactive sites on the silicon nitride surface, Task 3 to modify the chemistry of the silicon nitride interface using organic species, Task 4 to determine the rheological and dispersion properties of the nonaqueous silicon nitride suspensions, and Task 5 to ensure transfer technology to ORNL via meetings and reports. Each task is discussed in detail below. 

Figure 12. Relative effects on particle electrophoretic mobility and Zeta potential, of adsorbing anionic, amphoteric or cationic surfactants. The different kinds of rock particles are sandstone (SS), limestone (LS), and dolomite (Dolo), all dispersed in aqueous solution. (From Mannhardt et al. [152]. Copyright 1992 Elsevier Science Publishers, Amsterdam.)...

Preparation procedure of sols for the electrophoretic is as follows. Eirst, the particles are dispersed in 1 mass% NH4OH solution with stirring. The solution is placed in an ultrasonic water bath to disperse the particles homogeneously. Second, after the particles... 

Suspensions of oil in water (32), such as lanolin in wool (qv) scouring effluents, are stabilized with emulsifiers to prevent the oil phase from adsorbing onto the membrane. Polymer latices and electrophoretic paint dispersions are stabilized using surface-active agents to reduce particle agglomeration in the gel-polarization layer. 

Initial studies were made with the Rank Bros, electrophoresis unit, using the dilute supernatant suspension over a dispersion of 3.33g of carbon black per liter of dodecane equilibrated for 24 hours with the added 0L0A-1200. The electrophoretic mobility (u) of 1-3 pm clumps of particles was observed at a field of 100 volts per centimeter. The zeta-potentials ( ) were calculated... 

The colloidal particles are often deposited on metallic electrodes in the form of adsorbed coatings. Rubber and graphite coatings can be formed in this way, using solvent mixtures (water-acetone) as the dispersion media. The advantage of this method is that additives can firmly be codeposited with, for example, rubber latex. Thermionic emitters for radio valves are produced in a similar manner. The colloidal suspensions of alkaline earth carbonates are deposited electrophoretically on the electrode and are later converted to oxides by using an ignition process. 

Particle mobility and zeta potential can now be measured by more sophisticated techniques. With photoelectrophoresis, particle mobility is measured as a function of pH under the influence of ultraviolet radiation. At pH < 8, the electrophoretic mobility of irradiated hematite particles (A = 520 nm) was markedly different from that measured in the absence of UV irradiation. This was attributed to the development of a positive surface charge induced by photo-oxidation of the surface Fe-OH° sites to (Fe-OH) sites (Zhang et al., 1993). The electroacoustic technique involves generation of sound waves by the particles in the colloidal dispersion and from this data. 

Particles of Fe203 with an average diameter of 1 /un were dispersed in xylene containing 5 x 10 3 mole liter 1 of copper(I) oleate. These showed an electrophoretic mobility of 0.110 fim s V-1 cm. The conductivity of the solution was 4.7 x 10 10 ohm cm-1, indicating an ion... 

Upon illumination, semiconductor particles become charged, allowing even for electrophoretic mobility under an applied electrical field When appropriately prepared, colloidal TiO2 can apparently accumulate charge to effect directly multiple quanta redox reactions The efficiency of such charge accumulation is surely related to doping level for the doping level can alter band positions and may improve the efficiency of photoinduced electron transfer. For example, the dispersal of FcjOa... 

Electrophoretic mobilities of the quartz particles in cobalt (II) perchlorate solutions were determined with a calibrated Zeta-Meter apparatus. Coagulation sedimentation behavior was followed using a stop-flow type apparatus. The dispersion is pumped in a closed loop from an equilibration vessel through an optical cell located in the sample compartment of a recording spectrophotometer. From the optical densitytime curve obtained from the time the pump is switched off, the turbidity index (in arbitrary units) is obtained as the slope of the curve at zero time. 

The glass fibers and fused-silica glass (Thermal American Fused Quartz Co.) were crushed and then dispersed in water. The pH of this near-neutral suspension was varied using KOH or HNO,. In some experiments, a hydrolyzed solution of y-APS was added to this suspension. Here, the initial pH was 10. The electrophoretic mobilities of glass fragments suspended in these solutions were measured without any further treatment except for the addition of electrolyte (10-3 M KNO,). These analyses were performed using a Rank Brothers Particle Micro-Electrophoresis Apparatus Mark II or a Pen Kem System 3000 Automated Electrokinetics Analyzer. 

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 electrophoretic mobilities of particles in concentrated dispersion have been measured using (a) a relatively simple moving boundary technique185 and (6) a mass transport method186. The interpretation of such measurements may be complicated by electric double layer... 

Investigations of the electrophoretic behaviour of monodispersed carboxylated polystyrene latex dispersions as a function of particle size and electrolyte concentration by Shaw and Ottewill191 have confirmed, at least qualitatively, the existence of tea and relaxation effects. 

In the early work of Schulze ( 0, Linder and Picton (2) and Hardy (3) the sensitivity of colloidal dispersions to the addition of electrolytes was clearly demonstrated. Then in 1900 Hardy (4) showed that the stability of sols was connected with the electrophoretic mobility of the particles and he demonstrated, i) that the valency of the ion opposite in charge to that of the sol particles determined the ability of an electrolyte to coagulate a sol and that, ii) the effectiveness of the electrolyte increased rapidly with increase in valency of the counter-ion. These observations formed the basis of the so-called Schulze-Hardy rule. 

In electrophoresis an electric field is applied to a sample causing charged dispersed droplets, bubbles, or particles, and any attached material or liquid to move towards the oppositely charged electrode. Their electrophoretic velocity is measured at a location in the sample cell where the electric field gradient is known. This has to be done at carefully selected planes within the cell because the cell walls become charged as well, causing electro-osmotic flow of the bulk liquid inside the cell. From hydrodynamics it is found that there are planes in the cell where the net flow of bulk liquid is zero, the stationary levels, at which the true electrophoretic velocity of the particles can be measured. 

The electrostatic stabilization theory was developed for dilute colloidal systems and involves attractive van dcr Waals interactions and repulsive double layer interactions between two particles. They may lead to a potential barrier, an overall repulsion and/or to a minimum similar to that generated by steric stabilization. Johnson and Morrison [1] suggest that the stability in non-aqueous dispersions when the stabilizers are surfactant molecules, which arc relatively small, is due to scmi-stcric stabilization, hence to a smaller ran dcr Waals attraction between two particles caused by the adsorbed shell of surfactant molecules. The fact that such systems are quite stable suggests, however, that some repulsion is also prescni. In fact, it was demonstrated on the basis of electrophoretic measurements that a surface charge originates on solid particles suspended in aprotic liquids even in the absence of traces of... 

Beside of the progress in the theory of a particle movement in the zetameter measurement cell, there was progress in particle measurement techniques. New models of zetameters enable automatic measurement of electrophoretic mobility on the basis of the shift of light wave scattered on the particle that moves in the electric field [82]. This technique is called photon correlation spectroscopy (PCS). To increase the sensitivity of the measurement, it is supported by multiangle electrophoretic light scattering (ELS). This combination, allows one also to measure the particle size distribution of the dispersed phase [83]. 

As mentioned in Sec. I, the system in most applications of electrophoresis contains more than one single particle suspended in the electrolyte solution. The interactions between the particles should be taken into account unless the concentration of particles is very low. In this section, studies on particle interactions in electrophoresis will be reviewed. The results for spherical and nonspherical particles will be presented and discussed separately in two respective subsections. Then, we will show how to obtain the average electrophoretic velocity for a dispersion of particles from the interaction results. 

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... 

Electrophoresis — Movement of charged particles (e.g., ions, colloidal particles, dispersions of suspended solid particles, emulsions of suspended immiscible liquid droplets) in an electric field. The speed depends on the size of the particle, as well as the -> viscosity, -> dielectric permittivity, and the -> ionic strength of the solution, and it is directly proportional to the applied electric field. In analytical as well as in synthetic chemistry electrophoresis has been employed to separate species based on different speeds attained in an experimental setup. In a typical setup the sample is put onto a mobile phase (dilute electrolyte solution) filled, e.g., into a capillary or soaked into a paper strip. At the ends of the strip connectors to an electrical power supply (providing voltages up to several hundred volts) are placed. Depending on their polarity and mobility the charged particles move to one of the electrodes, according to the attained speed they are sorted and separated. (See also - Tiselius, - electrophoretic effect, - zetapotential). 

Sedimentation potential— (also called electrophoretic or Dorn potential) Potential difference established during sedimentation (caused, e.g., by gravitation or centrifugation) of small charged particles (suspended in solution dispersion of solid particles or emulsion of immiscible liquid droplets). 

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