Kinetic stability of disperse systems

Kinetic Stability of Disperse Systems and General Stabilisation Mechanisms... 

Kinetic stability of disperse systems and the general stabilization mechanisms... 

The first example (Fig. 5.70a) involves dipping a solid into an aqueous salt solution, thus, creating a soHd-liquid interface. Either cations or anions will be preferentially adsorbed on the smface, and the result is an excess surface charge. The coimter-charge is distributed in the zone of the solution adjacent to the interface the extent of this zone is determined by the Debye length (see Eq. (5.203)). It is inversely proportional to the square root of charge carrier concentration in the bulk solution. A rigid double-layer is formed in a concentrated solution in dilute solution a diffuse layer is formed with appreciable extension (typical numbers are several tens of nanometres). Such electrostatic effects are responsible for the kinetic stability of dispersed systems in colloid chemistry . 

Specific chemical interactions affecting the stability of dispersed systems. Croatica Chem. Acta 42 223-245 Su, C. Puls, R.W (2001) Arsenate and arsenite removal by zerovalent iron Kinetics, Redox transformation, and implications for in situ groundwater remediation. Environ. Sd. 

The participation of colloidal particles in thermal motion (the entropic factor) was taken into consideration, mostly indirectly, in earlier studies dealing with the molecular-kinetic properties of disperse systems. Volmer was the first to realize the importance of the role that the thermal motion of colloidal particles played in controlling the formation and stabilization of disperse systems. However, the attempt to compare the work of surface formation and the entropic factor directly, undertaken by March, was not successful, since it was applied only to systems with high interfacial energy. 

The investigation of kinetics of thickness decrease in thin films carried out experimentally (see Chapter VIII, 2) allows one to obtain important information regarding the nature of forces acting in such fielms, and, consequently, regarding the stability of disperse systems. Expression (VII. 18), referred to as the Reynolds equation, is often written as... 

The term emulsification refers to the technique that involves mixing of two immiscible materials (usually liquids) in order to produce a homogeneous system. Usually, one of the two materials has an oily nature and the second is the water. In the emulsion, the liquid present in the larger proportion is called continuous phase, while the liquid in the smaller proportion, which disperses, is called dispersed phase. Depending on the dispersed phase, there are different types of emulsions, that is, oil-in-water (o/w) wherein oil is the dispersed phase, water-in-oil (w/o) wherein water is the dispersed phase, as well as multiple emulsions, like oil-in-water-in-oil (o/w/o), in which there are continuous layers of the two immiscible materials. Emulsification process includes the use of an emulsifying agent, which will be adsorbed on the interface of the two immiscible materials, in order to achieve the miscibility of them. The structure of these agents contains a polar and nonpolar part, and such molecules may be proteins, phospholipids, etc. Emulsification can also include the use of other colloidal macromolecules, which are able to form multilayer films on the interface, in order to achieve a better kinetic stabilization of the system. ... 

In such concentrated disperse systems three types of liquid films form foam films (G/L/G), water-emulsion films (O/W/O) and non-symmetric films (O/W/G). The kinetics of thinning of these films, their permeability as well as the energy barrier impeding the film rupture determine the stability of these systems. They might be subjected to internal collapse, i.e. coalescence of bubbles or droplets and increase in their average size, or to destruction as a whole, i.e. separation into their initial phases - gas, oil and water. 

It has been repeatedly emphasized that lyophobic disperse systems are thermodynamically unstable as compared to macroheterogeneous systems. The cause for this instability is a high excess of surface free energy at the interfaces. At the same time, many lyophobic colloids are stable towards aggregation and may maintain such stability for infinite periods of time. Let us now discuss the basic thermodynamic and kinetic factors that favor stabilization in disperse systems. In this sections we will restrict ourselves to just naming some of these factors, and will return to their detailed discussion later on. 

Adsorption of polymers onto colloidal particles is of great interest for the chemical industry [88] and physical chemistry of disperse systems in cormection with the fundamental problem of colloidal stability [89]. The structure of an adsorbed polymer layer can be probed by neutron [90] or X-ray [91] scattering, while the kinetics of adsorption can be followed by absorption spectroscopy [44]. 

It should be emphasized that stabihzation of metal nanoparticles by high-molecular compounds presents a major branch of polymer colloidal modem science. Modem polymer colloidal science studies generation regularities of dispersed systems with highly developed interfaces, their kinetic and aggregation stabilities, different surface phenomena arising at the interface, and adsorption of macromolecules from liquids on solid surfaces. The theory of improving stabihty of colloidal particles by polymers has been treated in detail elsewhere. This chapter focuses on basic questions that are connected with nanoparticles and nanocomposites. 

Chemical Kinetics, Tank and Tubular Reactor Fundamentals, Residence Time Distributions, Multiphase Reaction Systems, Basic Reactor Types, Batch Reactor Dynamics, Semi-batch Reactors, Control and Stability of Nonisotheimal Reactors. Complex Reactions with Feeding Strategies, Liquid Phase Tubular Reactors, Gas Phase Tubular Reactors, Axial Dispersion, Unsteady State Tubular Reactor Models... 

As with experimental work on polymer adsorption, experiments in the area of dispersion stability in the presence of polymers require detailed characterisation of the systems under study and the various controlling parameters (discussed above) to be varied in a systematic way. One should seek the answer to several questions. Is the system (thermodynamically) stable If not, what is the nature of the equilibrium state and what are the kinetics of flocculation If it is stable, under what critical conditions ( s, T, x> p etc.) can flocculation be induced ... 

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