Colloidal dispersions condensation method

The class of mechanical methods used for preparing colloidal dispersions in which particles or droplets are progressively subdivided. See also Condensation Methods. 

Non-porous Zr02 powders can be produced by high-temperature vapour phase condensation methods in this manner discrete spherical particles of c. 4 nm diameter have been obtained (Avery and Ramsay, 1973). It is also possible to prepare colloidal dispersions of sub-micron sized, spheroidal particles of basic salts such as Zr2(OH)6C03 and Zr2(0H)6SO4 with the aid of the carefully controlled sol-gel techniques developed by Matijevic (1988). 

The condensation method makes it possible to obtain more highly dispersed systems than the dispersion method, and true lyophobic sols are always prepared by this method. Colloidal solutions are obtained by the condensation method as a result of chemical reactions of nearly all known types. But it should be noted that sols are by no means always formed, but only in the case of certain concentrations of the original substances, order of their mixing, temperatures of interaction, and a combination of several other conditions. The main method of preparing sols of heavy hydroxides is hydrolysis of solutions of salts, which takes place more completely and more rapidly at high temperatures and in dilute solutions. 

Many productive methods have been developed for the preparation of silica sol including acidification/121 electrolysation-electrodialysis,[13] ion-exchange,[14] peptization/111 and hydrolysis of silicon compounds/101 which can be grouped into two main types. One is called the aggregation method that contains two steps the polymerization of silicate ions and the aggregation of these polysilicate anions via condensation reaction between the hydroxy groups of the particles. The other one is called the peptization method, i.e., dispersal of a precipitate of Si02 to form colloid. The acidification method will be discussed in detail below. 

Condensation Methods Used for preparing colloidal dispersions in which either precipitation from solution or chemical reaction is used to create colloidal species. The colloidal species are built up by deposition on nuclei that may be of the same or different chemical species. If the nuclei are of the same chemical species, the process is referred to as homogeneous nucleation if the nuclei are of different chemical species, the process is referred to as heterogeneous nucleation. See also Dispersion Methods. 

Nuclei As a solute becomes insoluble, the formation of a new phase has its origin in the formation of clusters of solute molecules, termed germs, that increase in size to form small crystals or particles, termed nuclei. One means of preparing colloidal dispersions involves precipitation from solution onto nuclei, which may be of the same or different chemical species. See also Condensation Methods. 

Two methods are used to prepare the colloid solution the dispersion method and the condensation method. In the dispersion method, the dispersoid and dispersant are ground repeatedly by a colloid grinder until they meet the required degree of dispersion. The condensation method includes two options. One is the chemical reaction option through hydrolysis or metathesis, and the other is the change solvent option. 

The conductivity of mesoscopic metals can be measured only by noncontact means. For this reason the particles were embedded in an insulating matrix. The manufacture of the (indium) particles was generally achieved by condensation from the gas phase in a rotating oil film [69]. This method yielded metal particles of about 20 nm that were (colloidally) dispersed in the oil matrix. By means of thermal coalescence, panicles with a diameter of up to several hundred nanometers were obtained. Thus the effective dielectric function (DF) of the heterogeneous oil-indium system was measured. At constant volumetric filling ratio it was possible to mea-... 

By lyophobic sols are meant colloidal dispersions of insoluble substances in a liquid medium, usually an aqueous solution. They can be prepared in several different ways. It is very typical for these systems that they can never be prepared, by simple contact or slight shaking of macrocrystals of the insoluble substance and the solvent. The roundabout ways by which lyophobic sols can be arrived at may be distinguished according to SvEDBERG into condensation methods and dispersion methods. 

In the condensation methods a molecular (ionic, atomic) distribution of the insoluble substance is prepared first which then by suitable coarsening may give rise to a colloidal dispersion. 

It is not always possible to make a sharp distinction between the two different types. The classical method of electric disintegration of metals (see p 61) for instance probably is at the same time a dispersion and a condensation method. In the electric arc particles of colloidal dimensions are broken away from the electrodes, but at the same time part of the metal vaporizes and then passes the atomar dispersion. Moreover by direct electrolysis ionar dispersion may occur as an intermediate stage. 

An analogy may be drawn between the phase behavior of weakly attractive monodisperse dispersions and that of conventional molecular systems provided coalescence and Ostwald ripening do not occur. The similarity arises from the common form of the pair potential, whose dominant feature in both cases is the presence of a shallow minimum. The equilibrium statistical mechanics of such systems have been extensively explored. As previously explained, the primary difficulty in predicting equilibrium phase behavior lies in the many-body interactions intrinsic to any condensed phase. Fortunately, the synthesis of several methods (integral equation approaches, perturbation theories, virial expansions, and computer simulations) now provides accurate predictions of thermodynamic properties and phase behavior of dense molecular fluids or colloidal fluids [1]. 

The methods of disintegration rely entirely upon increasing the dispersity of a solids which process can, at least theoretically, be stopped at any instant resulting in the formation of a suspension of definite dispersity but one that is not necessarily stable. The processes of suspension formation by methods of condensation on the other hand are more complicated, owing to the fact that unless the resulting colloidal suspension possesses at least some degree of stability the process of condensation once set in operation will not cease but proceed until the transformation to the macrocrystalline structure is complete. 

Definitions. Colloids are solid particles with diameters of 1 100 nanometers, A sol is a dispersion of colloidal particles in a liquid. A gel is an interconnected rigid network of sub-micrometer dimensions. A gel can be formed from an array of discrete colloidal particles (Method I) or the 3-D network can be formed from the hydrolysis and condensation of liquid meial alkoxide precursors (Methods 2 and 3). shown in Fig. 11. The metal alkoxide precursors used in Methods 2 and 3 are usually Si(OR)4 where R is CHj. C-Hj. or C3H7. The metal ions can be Si, Ti. Sn. Al, and so on,... 

Colloidal form of transport of ferric iron in the Precambrian, apparently, was less important than in later geologic epochs. On the basis of an examination of the known methods of formation of colloidal solutions, it can be assumed that condensation phenomena predominate in the geochemical processes, and dispersion and peptization play a secondary role. Consequently one of the conditions for obtaining colloidal solutions is the presence of ionic solutions as an obligatory intermediate stage in the cycle weathering transport deposition. 

Over the decades that have passed since La Mer s work numerous examples of monodispersed particles of various composition, morphologies and properties, as well as methods for their preparation (not limited to condensational formation), were described in the literature. Extensive studies in this area were carried out by E. Matijevic and T. Sugimoto. Examples of monodisperse systems formed by precipitation from homogeneous solutions include dispersions of uniform particles of simple composition having different morphologies, such as metal halides, sulfides, phosphates, (hydrous) oxides, etc, various composite particles, including particles of internally mixed composition and coated particles. Both crystalline and amorphous materials can be obtained. Electron micrographs of some characteristic examples of monodispersed colloids are shown in Fig. IV-14. 

Along with the methods based on investigation of the scattering of light by disperse system as a whole, there are also methods based on the scattering (the diffraction) of light on individual particles. One of such methods, ultramicroscopy, played an important role in the development of colloid science. Dark field optical systems, ultramicroscopes, and dark field condensers, such as those utilized in optical microscopes for side illumination,... 

Finally, the so called Sol-gel processes should be mentioned in which porosity, pore size, and polarity of products manufactured by this method can be controlled. Processing begins with a solution (= sol) which becomes a gel. The solution can be prepared from either inorganic salts or organic compounds. It is then hydrolized to a sol or condensed to form a gel. The process can be terminated in the sol-phase, where a dispersion of colloidal particles in a liquid exists, or continued to the gel-phase, the development of a three-dimensional, linked solid structure in which the pores are filled with a liquid. In the wet gel-phase, the pores are interconnected. 

Precipitation into submicron-sized particles is another direct approach. Precipitation by pH shifting can be an effective approach for dyes that have weak acid functionality. A number of different families of such dyes have been dispersed by acidification of weakly alkaline dye solutions, in the presence of stabilizers such as surfactants and polymers. Alternatively, solvent shifting has been demonstrated to be an effective method of preparing absorber dye dispersions. Recent work by Brick et al. (14) has shown how such dyes can be very effectively precipitated from a variety of water-miscible organic solvents. Finally, another approach for incorporation of absorber dyes is to precipitate or condense them on the surface of a high-surface-area carrier species, such as colloidal silica. Such preparations can be prepared by pH- and solvent-shifting processes, in the presence of the carrier particles. 

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