Colloids condensation methods

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,... 

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

Condensation methods are those in which the substances forming a true solution are converted into colloidal form. These are as follows ... 

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. 

Thus the condensation methods of formation of colloidal solutions, mainly from true ionic or molecular solutions, are most important in nature. The sols obtained nearly always are contaminated by various impurities, usually electrolytes, including components of the original true solutions. 

The condensation method begins with molecular units, and the particles are built-up by a process of nucleation typical example is the preparation of polymer lattices, in which case the monomer (e.g., styrene or methylmethacrylate) is emulsified in water using an anionic or nonionic surfactant (e.g., sodium dodecyl sulphate or alcohol ethoxylate). A polymeric surfactant is also added to ensure the long-term colloid stabiHty of the resulting latex. An initiator such as potassium persulphate is then added and, when the temperature of the system has increased, initiation occurs that results in formation of the latex [polystyrene or poly(methylmethacrylate)]. 

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. 

Condensation methods, i.e. nudeation and growth. Here, one starts with molecular units that are condensed to form nuclei that grow further to produce the particles. An example of such a process is the formation of particles by a predpita-tion technique, e.g. production of colloidal Agl particles by reaction of AgNOa with KI. Many suspensions are produced by addition of a solution of the chemical in a suitable solvent, which is then added to another miscible solvent in which the drug is insoluble. A third example of condensation methods is the production of polymer particles from their monomers by a suitable polymerisation technique. 

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. 

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. 

The reduction of K2TaF7 can also be performed using sodium vapors [584]. This process is conducted at an Na pressure as low as 0.1 torr, which enables the removal of interferring gases such as N, O and H20. The interaction begins at 350°C. The temperature further increases up to 800°C to prevent the condensation of sodium and the formation of colloidal tantalum powder. The product of the interaction is removed from the reactor after cooling and treated with boiled HC1 and HF solutions. The method enables the production of coarse grain tantalum powder with 99.5% purity. 

The Stober method is also known as a sol-gel method [44, 45], It was named after Stober who first reported the sol-gel synthesis of colloid silica particles in 1968 [45]. In a typical Stober method, silicon alkoxide precursors such as tetramethylorthosili-cate (TMOS) and tetraethylorthosihcate (TEOS), are hydrolyzed in a mixture of water and ethanol. This hydrolysis can be catalyzed by either an acid or a base. In sol-gel processes, an acidic catalyst is preferred to prepare gel structure and a basic catalyst is widely used to synthesize discrete silica nanoparticles. Usually ammonium hydroxide is used as the catalyst in a Stober synthesis. With vigorous stirring, condensation of hydrolyzed monomers is carried out for a certain reaction time period. The resultant silica particles have a nanometer to micrometer size range. 

The Stober method can be used to form core-shell silica nanoparticles when a presynthesized core is suspended in a water-alcohol mixture. The core can be a silica nanoparticle or other types of nanomaterials [46, 47]. If the core is a silica nanoparticle, before adding silicon alkoxide precursors, the hydroxysilicates hydrolyzed from precursors condense by the hydroxide groups on the surface of the silica cores to form additional layers. If the core is a colloid, surface modification of the core might be necessary. For example, a gold colloid core was modified by poly (vinylpyrrolidone) prior to a silica layer coating [46]. 

Transition-metal nanopartides are of fundamental interest and technological importance because of their applications to catalysis [22,104-107]. Synthetic routes to metal nanopartides include evaporation and condensation, and chemical or electrochemical reduction of metal salts in the presence of stabilizers [104,105,108-110]. The purpose of the stabilizers, which include polymers, ligands, and surfactants, is to control particle size and prevent agglomeration. However, stabilizers also passivate cluster surfaces. For some applications, such as catalysis, it is desirable to prepare small, stable, but not-fully-passivated, particles so that substrates can access the encapsulated clusters. Another promising method for preparing clusters and colloids involves the use of templates, such as reverse micelles [111,112] and porous membranes [106,113,114]. However, even this approach results in at least partial passivation and mass transfer limitations unless the template is removed. Unfortunately, removal of the template may re-... 

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. 

Methods of condensation in which protective colloids are employed, thus effecting condensation in the presence of a disintegrating agent, are largely employed for the preparation of stable suspensions, thus the precipitation of gold, platinum and palladium in thepresence of gum arabic or the protalbic and lysalbic acids of Paal by means of reducing agents such as hydroxylamine, hydrazine, or formaldehyde readily results in the formation of remarkably stable suspensions. 

Ray Kapral came to Toronto from the United States in 1969. His research interests center on theories of rate processes both in systems close to equilibrium, where the goal is the development of a microscopic theory of condensed phase reaction rates,89 and in systems far from chemical equilibrium, where descriptions of the complex spatial and temporal reactive dynamics that these systems exhibit have been developed.90 He and his collaborators have carried out research on the dynamics of phase transitions and critical phenomena, the dynamics of colloidal suspensions, the kinetic theory of chemical reactions in liquids, nonequilibrium statistical mechanics of liquids and mode coupling theory, mechanisms for the onset of chaos in nonlinear dynamical systems, the stochastic theory of chemical rate processes, studies of pattern formation in chemically reacting systems, and the development of molecular dynamics simulation methods for activated chemical rate processes. His recent research activities center on the theory of quantum and classical rate processes in the condensed phase91 and in clusters, and studies of chemical waves and patterns in reacting systems at both the macroscopic and mesoscopic levels. 

Many approaches have been taken to prepare colloidal doped semiconductor nanocrystals. For example, hot-injection methods have been used to synthesize colloidal Mn2+-doped CdSe (47, 48), ZnSe (49), and PbSe (50) colloidal nanocrystals. Colloidal ZnO DMS-QDs doped with Co2+, Ni2+, and Mn2+ have been prepared by low-temperature hydrolysis and condensation (51-54). Sol-gel methods have been used to prepare colloidal doped TiC>2 (55-57) and Sn02 (58-62) nanocrystals. Inverted micelle methods have been used for preparation of a range of doped II-VI sulfide DMS-QDs at low temperatures (63-68). A high-temperature lyothermal single-source method was used to synthesize Co2+- and Eu3+-doped CdSe nanocrystals (69, 70). Autoclaving has occasionally been used to induce crystallization at lower temperatures than reached under atmospheric pressures while retaining colloidal properties, for... 

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