Colloidal suspensions system

Eq.(17) shows how the viscosity of a liquid changes with the temperature and other physical parameters. It also indicates that the constant A in Eq.(lO) is dependent on the temperature. The free volume concept is important and will be used again to treat the colloidal suspension systems later. 

The viscosity of a liquid containing charged particulate—the colloidal suspension system has been found to increase to a considerable extent even without an external electric field. This can be demonstrated by comparing Eq. (91), the viscosity equation for charged spherical colloidal suspensions, with Hq. (54), the viscosity equation for uncharged spherical colloidal... 

In practice, e.g., in nature or in fonnulated products, colloidal suspensions (also denoted sols or dispersions) tend to be complex systems, consisting of many components that are often not very well defined, in tenns of particle size for instance. Much progress has been made in the understanding of colloidal suspensions by studying well defined model systems, which allow for a quantitative modelling of their behaviour. Such systems will be discussed here. 

The remainder of this contribution is organized as follows. In section C2.6.2, some well studied colloidal model systems are introduced. Methods for characterizing colloidal suspensions are presented in section C2.6.3. An essential starting point for understanding the behaviour of colloids is a description of the interactions between particles. Various factors contributing to these are discussed in section C2.6.4. Following on from this, theories of colloid stability and of the kinetics of aggregation are presented in section C2.6.5. Finally, section C2.6.6 is devoted to the phase behaviour of concentrated suspensions. 

An important step in tire progress of colloid science was tire development of monodisperse polymer latex suspensions in tire 1950s. These are prepared by emulsion polymerization, which is nowadays also carried out industrially on a large scale for many different polymers. Perhaps tire best-studied colloidal model system is tliat of polystyrene (PS) latex [9]. This is prepared with a hydrophilic group (such as sulphate) at tire end of each molecule. In water tliis produces well defined spheres witli a number of end groups at tire surface, which (partly) ionize to... 

Even when well defined model systems are used, colloids are ratlier complex, when compared witli pure molecular compounds, for instance. As a result, one often has to resort to a wide range of characterization teclmiques to obtain a sufficiently comprehensive description of a sample being studied. This section lists some of tire most common teclmiques used for studying colloidal suspensions. Some of tliese teclmiques are discussed in detail elsewhere in tliis volume and will only be mentioned in passing. A few teclmiques tliat are relevant more specifically for colloids are introduced very briefly here, and a few advanced teclmiques are highlighted. 

Our main focus in this chapter has been on the applications of the replica Ornstein-Zernike equations designed by Given and Stell [17-19] for quenched-annealed systems. This theory has been shown to yield interesting results for adsorption of a hard sphere fluid mimicking colloidal suspension, for a system of multiple permeable membranes and for a hard sphere fluid in a matrix of chain molecules. Much room remains to explore even simple quenched-annealed models either in the framework of theoretical approaches or by computer simulation. 

Colloidal suspensions are systems of small mesoscopic solid particles suspended in an atomic liquid [1,2]. We will use the term colloid a little loosely, in the sense of colloidal particle. The particles may be irregularly or regularly shaped (Fig. 1). Among the regular shapes are tiny spherical balls, but also cylindrical rods or flat platelets. As the particles are solid, fluctuations of their form do not occur as they do in micellar systems. Not all particles in a suspension will, in general, have the same form. This is an intrinsic effect of the mesoscopic physics. Of course in an atomic system, say silicon, all atoms are precisely similar. One is often interested in the con-... 

Colloidal suspensions are, per definition, mixtures of mesoscopic particles and atomic liquids. What happens if there are several different species of particles mixed in the solvent One can invent several different sorts of mixtures small and large particles, differently charged ones, short and long rods, spheres and rods, and many more. Let us look into the literature. One important question when dealing with systems with several components is whether the species can be mixed or whether there exists a miscibility gap where the components macroscopically phase-separate. 

Any real sample of a colloidal suspension has boundaries. These may stem from the walls of the container holding the suspension or from a free interface towards the surroundings. One is faced with surface effects that are small compared to volume effects. But there are also situations where surface effects are comparable to bulk effects because of strong confinement of the suspension. Examples are cylindrical pores (Fig. 8), porous media filled with suspension (Fig. 9), and thin colloidal films squeezed between parallel plates (Fig. 10). Confined systems show physical effects absent in the bulk behavior of the system and absent in the limit of extreme confinement, e.g., a onedimensional system is built up by shrinking the size of a cylindrical pore to the particle diameter. 

FIG. 10 A colloidal suspension between two parallel plates. There is strong confinement perpendicular to the plates, but an infinite system in the lateral orientations. 

Colloidal particles experience kicks from the surrounding atoms or molecules of the solvent. This leads to Brownian dynamics in colloidal suspensions (Fig. 14). The study of dynamics is challenging as, of course, first the equilibrium of the system has to be understood. One often knows the short-time dynamics that govern the system and is interested in long-time properties. 

Chapters 15 through 17 are devoted to mathematical modeling of particular systems, namely colloidal suspensions, fluids in contact with semi-permeable membranes, and electrical double layers. Finally, Chapter 18 summarizes recent studies on crystal growth process. 

The viscosity of a fluid arises from the internal friction of the fluid, and it manifests itself externally as the resistance of the fluid to flow. With respect to viscosity there are two broad classes of fluids Newtonian and non-Newtonian. Newtonian fluids have a constant viscosity regardless of strain rate. Low-molecular-weight pure liquids are examples of Newtonian fluids. Non-Newtonian fluids do not have a constant viscosity and will either thicken or thin when strain is applied. Polymers, colloidal suspensions, and emulsions are examples of non-Newtonian fluids [1]. To date, researchers have treated ionic liquids as Newtonian fluids, and no data indicating that there are non-Newtonian ionic liquids have so far been published. However, no research effort has yet been specifically directed towards investigation of potential non-Newtonian behavior in these systems. 

Ultrafiltration of heterogenous colloidal suspensions such as citrus juice is complex and many factors other than molecular weight contribute to fouling and permeation. For example, low MW aroma compounds were unevenly distributed in the permeate and retentate in UF in 500 kd MWCO system (10). The authors observed that the 500 kd MWCO UF removed all suspended solids, including pectin and PE. If PE is complexed to pectate in an inactive complex, then it is conceivable that release of PE from pectin with cations will enhance permeation in UF. At optimum salt concentration, less PE activation was observed at lower pH values than at higher pH (15). In juice systems, it is difficult to separate the effect of juice particulates on PE activity. Model studies with PE extracts allows UF in the absence of large or insoluble particulates and control of composition of the ultrafilter. In... 

Most descriptions of the dynamics of molecular or particle motion in solution require a knowledge of the frictional properties of the system. This is especially true for polymer solutions, colloidal suspensions, molecular transport processes, and biomolecular conformational changes. Particle friction also plays an important role in the calculation of diffusion-influenced reaction rates, which will be discussed later. Solvent multiparticle collision dynamics, in conjunction with molecular dynamics of solute particles, provides a means to study such systems. In this section we show how the frictional properties and hydrodynamic interactions among solute or colloidal particles can be studied using hybrid MPC-MD schemes. 

The methodology discussed previously can be applied to the study of colloidal suspensions where a number of different molecular forces and hydrodynamic effects come into play to determine the dynamics. As an illustration, we briefly describe one example of an MPC simulation of a colloidal suspension of claylike particles where comparisons between simulation and experiment have been made [42, 60]. Experiments were carried out on a suspension of AI2O3 particles. For this system electrostatic repulsive and van der Waals attractive forces are important, as are lubrication and contact forces. All of these forces were included in the simulations. A mapping of the MPC simulation parameters onto the space and time scales of the real system is given in Hecht et al. [42], The calculations were carried out with an imposed shear field. 

The above model assumes that both components are dynamically symmetric, that they have same viscosities and densities, and that the deformations of the phase matrix is much slower than the internal rheological time [164], However, for a large class of systems, such as polymer solutions, colloidal suspension, and so on, these assumptions are not valid. To describe the phase separation in dynamically asymmetric mixtures, the model should treat the motion of each component separately ( two-fluid models [98]). Let Vi (r, t) and v2(r, t) be the velocities of components 1 and 2, respectively. Then, the basic equations for a viscoelastic model are [164—166]... 

Finally, these particles generated in ionic liquids are efficient nanocatalysts for the hydrogenation of arenes, although the best performances were not obtained in biphasic liquid-liquid conditions. The main importance of this system should be seen in terms of product separation and catalyst recycling. An interesting alternative is proposed by Kou and coworkers [107], who described the synthesis of a rhodium colloidal suspension in BMI BF4 in the presence of the ionic copolymer poly[(N-vinyl-2-pyrrolidone)-co-(l-vinyl-3-butylimidazolium chloride)] as protective agent. The authors reported nanoparticles with a mean diameter of ca. 2.9 nm and a TOF of 250 h-1 in the hydrogenation of benzene at 75 °C and under 40 bar H2. An impressive TTO of 20 000 is claimed after five total recycles. 

Although several noble-metal nanoparticles have been investigated for the enantiomeric catalysis of prochiral substrates, platinum colloids remain the most widely studied. PVP-stabilized platinum modified with cinchonidine showed ee-values >95%. Several stabilizers have been also investigated such as surfactants, cinchonidinium salts and solvents, and promising ee-values have been observed. Details of a comparison of various catalytic systems are listed in Table 9.16 in one case, the colloid suspension was reused without any loss in enantioselectiv-ity. Clearly, the development of convenient two-phase liquid-liquid systems for the recycling of chiral colloids remains a future challenge. 

Siloxane compounds, in vitreous silica manufacture, 22 414 Siloxane materials, 20 240 Siloxane oligomers, in silicone polymerization, 22 555-556 Siloxanols, silylation and, 22 703 Silsesquioxane hybrids, 13 549 Silsesquioxanes, 15 188, 22 589-590 SilvaGas process, 3 696, 697 Silver (Ag), 22 636-667. See also Silver compounds. See Ag entries Argentothiosulfate complexes Batch desilverizing Lead-silver alloys Palladium-silver alloy membranes analytical methods for, 22 650-651 applications of, 22 636-637, 657-662 as bactericide, 22 656, 657, 660 barium alloys with, 3 344 in bimetallic monetary system, 22 647-648 in cast dental gold alloys, 8 307t coke formation on, 5 266 colloidal precipitation color, 7 343t colloidal suspensions, 7 275 color, 7 334, 335... 

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