Colloidal Electrokinetic Processes

Charged coUoids allow one to visually investigate these systems under dynamic conditions. 

In the following, let us consider what happens if the charged particle or surface is under dynantic motion of some kind. Furflier, there are different systems under which electrokinetic phenomena are investigated. 

These systans are as follows (Adamson and Cast, 1997 Birdi, 2010a)  

Electrophoresis This system refers to the movement of the colloidal particle under an applied electric field. In biology, different proteins exhibit different charges and thus can be separated nsing this property. 

Negatively charged particle moves toward the positive electrode. 

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The electrokinetic processes can actually be observed only when one of the phases is highly disperse (i.e., with electrolyte in the fine capillaries of a porous solid in the cases of electroosmosis and streaming potentials), with finely divided particles in the cases of electrophoresis and sedimentation potentials (we are concerned here with degrees of dispersion where the particles retain the properties of an individual phase, not of particles molecularly dispersed, such as individual molecules or ions). These processes are of great importance in particular for colloidal systems. 

Two nucleation processes important to many people (including some surface scientists ) occur in the formation of gallstones in human bile and kidney stones in urine. Cholesterol crystallization in bile causes the formation of gallstones. Cryotransmission microscopy (Chapter VIII) studies of human bile reveal vesicles, micelles, and potential early crystallites indicating that the cholesterol crystallization in bile is not cooperative and the true nucleation time may be much shorter than that found by standard clinical analysis by light microscopy [75]. Kidney stones often form from crystals of calcium oxalates in urine. Inhibitors can prevent nucleation and influence the solid phase and intercrystallite interactions [76, 77]. Citrate, for example, is an important physiological inhibitor to the formation of calcium renal stones. Electrokinetic studies (see Section V-6) have shown the effect of various inhibitors on the surface potential and colloidal stability of micrometer-sized dispersions of calcium oxalate crystals formed in synthetic urine [78, 79]. 

Figure 4.12. Apparent electrokinetic potential as a function of the outer Helmholtz plane potential, when the slip process Is determined by the viscoelectrlc effect. J = 10.2 X 10 V m c Is the concentration of the (1-1) electrolyte In M. (Redrawn from J. Lyklema. Colloids Surf. A92 (1994) 41.)...

Singh, B.P, Bhattacharjee, S., and Besra, L., Influence of surface charge on maximizing solids loading in colloidal processing of alumina, Mater Lett.. 56, 475. 2002. Bhattacharjee, S., Singh, B.P, and Besra, L., Effect of additives on electrokinetic properties of colloidal alumina suspensions, J. Colloid Interf. Sci., 254, 95, 2002. Boily, J.-F. and Fein, J.B., Experimental study of cadmium-citrate co-adsorption... 

Phenomena that arise in these materials include conduction processes, mass transport by convection, potential field effects, electron or ion disorder, ion exchange, adsorption, interfacial and colloidal activity, sintering, dendrite growth, wetting, membrane transport, passivity, electrocatalysis, electrokinetic forces, bubble evolution, gaseous discharge (plasma) effects, and many others. 

The term electrophoresis encompasses the process of electrokinetic movement of colloidal or molecular-disperse particles in solution in the presence of an electric field (Figure 5.11). Colloids that carry either positive or negative surface charges move either to the cathode (cataphoresis) or the anode (anophoresis). An analogous process occurs when the dispersing medium moves relatively to the dispersed phase at rest. This process is called electroosmosis. 

His research interests have included many aspects of colloid and interface science applied to the petroleum industry, including research into mechanisms of processes for the improved recovery of light, heavy, or bituminous crude oils, such as in situ foam, polymer or surfactant flooding, and surface hot water flotation from oil sands. These mostly experimental investigations have involved the formation and stability of dispersions (foams, emulsions, and suspensions) and their flow properties, electrokinetic properties, interfacial properties, phase attachments, and the reactions and interactions of surfactants in solution. 

Closely related to this is the general problem in the whole chemical industry of the separation of solids from liquids. The processes of thickening, flocculation, dewatering, and filtration are all intimately controlled by the forces and structures arising in colloidal systems. The improvements brought about by the use of electroseparation processes depend on the exploitation of the electrokinetic effects discussed in Chapter 6. 

Protein adsorption on solid surfaces is discussed from a colloid chemical and thermodynamic point of view. Information is mainly obtained from adsorption isotherms, (proton)titrations, electrokinetics and calorimetry. The adsorption behavior of human plasma albumin and bovine pancreas ribonuclease at various surfaces is studied. The differences in behavior between the two proteins are related to differences in the structural properties. Furthermore, the essential role of the low molecular weight electrolytes in the overall protein adsorption process is stressed. 

It is evident that elucidation of the interfacial behavior of proteins is not a simple matter and requires contributions from several disciplines. In recent years considerable progress has been made in applying spectroscopic techniques to proteins in the adsorbed state (e.g., 7,8,9). In such studies a (small) part of the molecule is analyzed in detail. In our laboratory we study protein adsorption from a more classical, colloid-chemical point of view. Arguments are derived from experimental data referring to whole protein molecules or to layers of them. Information is obtained from adsorption isotherms, proton titrations and both electrokinetic and thermochemical measurements. Recently, topical questions such as reversibility of the adsorption process and changes in the protein structure have been considered. This more holistic approach has produced some insights that could not easily be obtained otherwise. 

These processes can be described by a set of nonlinear, coiqiled electrokinetic and convective diffusion equations for ion densities, in combination widi Q Navier-Stokes equations for the mass current, indicating that colloidal dynamics. 1 are nonlinear ( 71-72). 

The zeta potential (Q is thought to be the same as the Stem potential which is defined at the plane dividing the Stem layer and the diffuse layer of the EDL. Zeta potential is an experimentally measurable electrical potential that characterizes the EDL, and it plays an important role in many apphcations such as stability of colloidal dispersion, characterization of biomedical polymers, electrokinetic transport of particles, and capillary electrophoresis, etc. In addition, zeta potentials of the particles and the channel wall are cmcial to the design and process control of microfluidic devices. A review on measuring the zeta potential of microfluidic substrates was provided by Kirby and Hasselbrink [3]. 

A number of additional processes of practical importance involving colloids and surface interactions can be identified that require some knowledge of the electrical properties of the system involved. Investigators are always well advised to spend a little time assuring themselves that the proper electrokinetic information is available in order to avoid possible problems at some later date. 

Stana-Kleinschek K, Ribitsch V (1998) Electrokinetic properties of processed celulose fibers. Colloids Surf. A 140 127-138. 

Marian von Smoluchowski (1872-1917). .. was a Polish physicist whose research on discrete state matter is still highly valued in modem science. He is particularly acknowledged for his theory on Brownian motion, which he developed independently of Einstein and which laid the foundation for the theory of stochastic processes. A similar rank is deserved by his discovery of density fluctuations in liquids and gases and their relevance for macroscopic scattering— most prominently explained by the phenomenon of critical opalescence. Both works proved veiy influential for the understanding of colloidal suspensions. Furthermore, he did pioneering work on the quantification of particle aggregation as well as in the field of electrokinetic phenomena. 

In recent years, LBM has been applied to study many microscale and nanoscale transport phenomena and processes, including gas flows, electroosmotic flows, interfacial phenomena, and colloid suspensions. In the following, we briefly review major advances of LBM in the description of electrokinetics and interfacial phenomena. 

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