Colloidal particles electrical charges

Colloidal suspensions are stable because all of the particles of the colloid are either positively or negatively charged. This charge results from cations or anions that are bound to the surface of the particles. We can easily demonstrate that colloidal particles are charged by observing their migration when placed in an electrical field. The process by which ions are retained on the surface of a solid is known as adsorption. 

In a solution with colloidal particles, a charged double layer will surround each particle, and the particle may be regarded as a macro-ion. The colloidal particles free to migrate contribute to the solution s electrical DC conductance, and they may be regarded as a colloidal electrolyte. Particles are called colloidal when two of the dimensions are in the range of 1 nm to 1 pm (the third dimension does not have this constraint e.g., a very thin string). 

Consider first the electrophoresis of the colloidal particle of charge Q and radius a depicted in Fig. 3. As mentioned above, the particle dissociates into a single large, charged species (the macroion) and a number of small counterions. Since these species have different charges, they will move in opposite directions with respect to the electric field. Determining the electrophoretic mobility of a charged colloidal particle for an... 

If a colloidal particle is charged, electric fields can have a profound effect on the flow behaviour of the dispersion. This gives rise to a number of electrokinetic phenomena. In electrophoresis measurements, the velocity of... 

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]. 

In electrophoresis, the motion of charged colloidal particles under the influence of an electric field is studied. For spherical particles, we can write... 

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). 

Ion Any particle of less than colloidal size possessing either a positive or a negative electric charge. 

Coliform bacteria Non-pathogenic microbes found in fecal matter that indicate the presence of water pollution are thereby a guide to the suitability for potable use. Colloids Very small, finely divided solids (particles that do not dissolve) that remain dispersed in a liquid for a long time due to their small size and electrical charge. 

Based on the application of the established theory of colloid stability of water treatment particles [8,85-88], the colloidal particles in untreated water are attached to one another by van der waals forces and, therefore, always tend to aggregate unless kept apart by electrostatic repulsion forces arising from the presence of electrical charges on the particles. The aggregation process... 

Electrophoresis motion of charged colloidal particles immersed in a liquid driven by an electric field. 

The stability of lyophobic colloids is intimately associated with the electrical charge on the particles. Thus in the formation of an arsenic(III) sulphide sol... 

Extremely small particle, typically 10-5 to 10 7 cm in diameter. Colloidal solutions or hydrosols contain colloidal particles that are electrically negatively charged, which contributes to their fine dispersion and the difficulty of sedimentation and clarification. Coagulation is usually carried out by causing the particles to adsorb positively charged ions, such as aluminum from alum. 

Observation of the electrode under examination being exposed to an electric field may yield information about the value of E. Any charge on the electrode, which can be a wire or a colloidal particle, will result in a movement in the external field. Assuming that the movement is due to charges being present on the electrode the rate of the movement should pass through a minimum at E i.c provided that specific adsorption is absent. (Data obtained with this method are labelled ED). 

Loeb, AL Overbeek, JTG Wiersema, PH, The Electrical Double Layer Around a Spherical Colloid Particle, Computation of the Potential, Charge Density, and Free Energy of the Electrical Double Layer Around a sperical Colloid Particle M.I.T. Press Cambridge, MA, 1961. Lorentz, HA, Wied, Ann. 11, 70, 1880. 

Hydrophobic colloidal particles move readily in the liqnid phase under the effect of thermal motion of the solvent molelcnles (in this case the motion is called Brownian) or under the effect of an external electric field. The surfaces of such particles as a rule are charged (for the same reasons for which the snrfaces of larger metal and insnlator particles in contact with a solution are charged). As a result, an EDL is formed and a certain valne of the zeta potential developed. 

One of the most obvious properties of a disperse system is the vast interfacial area that exists between the dispersed phase and the dispersion medium [48-50]. When considering the surface and interfacial properties of the dispersed particles, two factors must be taken into account the first relates to an increase in the surface free energy as the particle size is reduced and the specific surface increased the second deals with the presence of an electrical charge on the particle surface. This section covers the basic theoretical concepts related to interfacial phenomena and the characteristics of colloids that are fundamental to an understanding of the behavior of any disperse systems having larger dispersed phases. 

The influence of interionic fores on ion mobilities is twofold. The electrophoretic effect (occurring also in the case of the electrophoretic motion of charged colloidal particles in an electric field, cf. p. 242) is caused by the simultaneous movement of the ion in the direction of the applied... 

Several additional instrumental techniques have also been developed for bacterial characterization. Capillary electrophoresis of bacteria, which requires little sample preparation,42 is possible because most bacteria act as colloidal particles in suspension and can be separated by their electrical charge. Capillary electrophoresis provides information that may be useful for identification. Flow cytometry also can be used to identify and separate individual cells in a mixture.11,42 Infrared spectroscopy has been used to characterize bacteria caught on transparent filters.113 Fourier-transform infrared (FTIR) spectroscopy, with linear discriminant analysis and artificial neural networks, has been adapted for identifying foodbome bacteria25,113 and pathogenic bacteria in the blood.5... 

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