Silver halide colloid formation

In kinetic studies of the formation of silver bromide, Meehan and Miller found that formation of colloidal material was complete within 6 msec or less. They further concluded that fast flocculation, in which every collision of particles results in an agglomeration, occurs during the mixing process and for a few seconds thereafter. Berry and Skillman and Berriman determined that the total number of silver halide crystals remained essentially unchanged after the first minute of doublejet precipitation. 

The formation of colloidal silver halide dispersions (photographic emulsions) was reviewed as a model system of colloids which are formed by precipitation of sparingly soluble salts. For such systems, models for crystal nucleation and growth were derived which were verified for the AgBr system. These models can probably be extended to the study of nucleation and growth of other highly insoluble colloidal systems. 

In the silver halides Mott and Gurney suggested a mechanism for the formation of colloidal Ag [167]. A conduction-band electron produced by irradiation is first trapped at a lattice imperfection which may be a silver atom or ion, a chemical impurity, or a trapping site along a dislocation. The trapped electron then attracts a Ag interstitial ion to form a Ag atom. Following this, electrons and Ag " interstitials are trapped at the site in proper sequence to cause the buildup of a colloidal silver particle. This mechanism requires the presence and mobility of silver ions, and it is further required that the hole motion be sufficiently small that trapped electrons are not annihilated by electron-hole recombinations. 

The suggestion of Mott [190], that photodecomposition of Ba(N3)2 occurs by the same mechanism in silver halides, was disputed by Tompkins and coworkers on the basis of additional observations [80,191,206]. In particular, the photoconductivity was found to be too small to account for the electron motion necessary for the formation of barium colloids [80]. More recently, Marinkas and Bartram were unable to detect photoconductivity in anhydrous crystals [49]. In addition, measurements of the dark conductivity indicated that if it is due to Ba ", it is much too small to account for the observed rate of photodecomposition [80,206]. As a further indication that the photodecomposition of Ba(N3)2 does not take place by the silver hahde process, the energy of formation of a barium interstitial was estimated and found to be much greater than the estimated energy for vacancy formation, thus indicating the possibility of Schottky disorder rather than Frenkel disorder as intrinsic to Ba(N3)2 [206]. Interstitial metal ions are required for the Mott-Gurney mechanism discussed above [167]. 

Decomposition models for silver azide are similar to those proposed for the silver halides. McLaren and Rogers [95] suggested that band to band transitions give rise to electrons and holes in accord with photoconductivity data. Trapped electrons attract interstitial silver atoms which eventually form colloids, and holes lead to the formation of the nitrogen through a less-clearly determined process, possibly a bimolecular reaction of neutral azide molecules near the surface. Such a process requires the presence of discrete band-gap acceptor states, such as cation vacancies, that would serve as reaction sites for holes. The details of the process, however, remain undetermined. 

Approaching the formation of colloids from the other end of the size range involves one of several growth mechanisms. Such processes are commonly employed for the production of dispersions and aerosols, and less commonly in the production of emulsions. Typical examples of important condensation processes include fog formation (both water and chemical), silver halide emulsions (really dispersions) for use in photographic products, crystallization processes, colloidal silica, latex polymers, etc. 

The photolytic method has, of course, a long and important history in the formation of photographic images from silver halide emulsions. Over the past twenty years, predominantly from the work of Henglein and of Belloni, a wide variety of colloidal metals has been prepared by this method encompassing both main group metals such as cadmium [87, 88], thallium [89, 90] and lead, [91] as well as other noble [92-98] and non-noble transition metals [99-101]. Radiolytic methods differ in the type of redudng spedes which is formed under irradiation, which is a function of solvent and any added solute. The radiolysis of aqueous solutions of metal ions produces solvated electrons which may either react with the dissolved... 

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