Colloid properties ordered suspensions

The colloidal state of matter is distinguished by a certain range of particle size, as a consequence of which certain characteristic properties become apparent. Colloidal properties are in general exhibited by substances of particle size ranging between 0 2 /an and 5 nm (2 x 10"7 and 5 x 10"9 m). Ordinary filter paper will retain particles up to a diameter of 10-20/an (1-2 x 10" 5 m), so that colloidal solutions, just like true solutions, pass through an ordinary filter paper (the size of ions is of the order of 0-1 nm = 10 10 m). The limit of vision under the microscope is about 5-10 nm (5-10 x 10 9 m). Colloidal solutions are therefore not true solutions. Close examination shows that they are not homogeneous, but consist of suspension of solid or liquid particles in a liquid. Such a mixture is known as a disperse system the liquid (usually water in qualitative analysis) is called the dispersion medium and the colloid the disperse phase. 

A suspension is a dispersion of solid particles in a liquid. A colloidal suspension is a sol. Colloidal properties become significant when the size of the parhcles is of the order of a few micrometer or less. In suspensions of large particles, for example, of some 10 pm or higher, hydrod5mamic interactions dominate the suspension flow properties emd particle packing behaviour. In colloidal suspensions interaction forces between the particles as well as hydro-dynamic interactions play a role in determining the flow and particle packing properties. 

The formation of primary radicals governs the rate of initiation and particle population. Because radical generation occurs in the aqueous phase, whereas radical termination occurs in the polymer particles, the polymerization rate and molecular weight can be increased at the same time. In vinyl chloride emulsion polymerization, the emulsifier greatly affects the polymerization kinetics and the physicochemical and colloidal properties of the polymer. The average polymer particle size is of the order 0.1-0.3 p,m, which is the size of primary particle nuclei in bulk and suspension polymerizations. The following is a summary of the typical kinetic features of batch vinyl chloride emulsion polymerization [61] ... 

Polymer brushes were found to minimize adsorption of proteins by the soft or steric repulsion of the flexible yet immobihzed macromolecules [179], although a generally valid explanation of the protein resistant properties of some hydrophihc brushes is not available. A similar explanation can be formulated for the improvement of the colloidal stability of particle suspensions, when polymer brush-type layers are bound to small particles. This and other intriguing features of polymer brushes prompted a remarkable experimental and theoretical research activity in order to understand and exploit the unique properties of polymer brushes. 

Monte Carlo techniques were first applied to colloidal dispersions by van Megen and Snook (1975). Included in their analysis was Brownian motion as well as van der Waals and double-layer forces, although hydrodynamic interactions were not incorporated in this first study. Order-disorder transitions, arising from the existence of these forces, were calculated. Approximate methods, such as first-order perturbation theory for the disordered state and the so-called cell model for the ordered state, were used to calculate the latter transition, exhibiting relatively good agreement with the exact Monte Carlo computations. Other quantities of interest, such as the radial distribution function and the excess pressure, were also calculated. This type of approach appears attractive for future studies of suspension properties. 

Around 1975, investigations of photoelectrochemical reactions at semiconductor electrodes were begun in many research groups, with respect to their application in solar energy conversion systems (for details see Chapter 11). In this context, various scientists have also studied the problem of catalysing redox reactions, for instance, in order to reduce surface recombination and corrosion processes. Mostly noble metals, such as Pt, Pd, Ru and Rh, or metal oxides (RUO2) have been deposited as possible catalysts on the semiconductor surface. This technique has been particularly applied in the case of suspensions or colloidal solutions of semiconductor particles [101]. However, it is rather difficult to prove a real catalytic property, because a deposition of a metal layer leads usually to the formation of a rectifying Schottky junction at the metal-semiconductor interface (compare with Chapter 2), as will be discussed below in more... 

There is a constant challenge for improved techniques in order to make accurate predictions on the colloidal stability of various sytems. In this section we demonstrate how dielectric spectroscopy can be applied as a technique to follow the breakdown of water-in-oil emulsions and to monitor the sedimentation of particle suspensions. Dielectric spectroscopy, combined with statistical test design and evaluation, seems to be an appropriate technique for the study of these problems. However, one should continue to seek satisfactory theoretical models for the dielectric properties of inhomogeneous systems. 

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