Colloidal silver, formation

They do not occur in homogeneous solution, as two radicals encountering each other dimerize or disproportionate. However, the formation of by short-lived radicals can be catalysed by colloidal metals The most detailed investigation has been carried out with colloidal silver Figure 1 shows the mechanism in a... 

The generation of Oj in suspensions of ZnO containing Ag" ions, which has been known for a long time has again been studied in recent years 210,211) experiments on colloidal solutions have also been carried out in our laboratory. Hada et al. measured the quantum yield of silver formation as a function of Ag concentration They found that the quantum yield q> could be expressed as... 

Nagata Y, Watanabe Y, Fujita S, Dohmaru T, Taniguchi S (1992) Formation of colloidal silver in water by ultrasonic irradiation. J Chem Soc Chem Commun 1620-1622... 

Yonezawa Y, Sato T, Kuroda S, Kuge KJ (1991) Photochemical formation of colloidal silver peptizing action of acetone ketyl radical. J Chem Soc Faraday Trans 87 1905-1910... 

Since we could not prepare a stable solution of the ester, we attempted its preparation in the styrene solution to be polymerised. Silver perchlorate was dissolved in this and the reaction was started by the crushing of a phial containing 1-phenylethyl bromide (under our conditions styrene was not polymerised by the silver perchlorate alone). The solutions became cloudy because of the formation of colloidal silver bromide, but no colour formation could be observed until the end of the polymerisation then the solutions became yellow, very like the reaction mixtures in which perchloric acid had been used as catalyst. The ester was found to be as effective a catalyst as anhydrous perchloric acid. Equal concentrations of the ester and the acid produced very similar polymerisations as shown in the Figure. The accelerating parts of the curves obtained with the ester as catalyst are readily explained by the fact that the reaction between silver perchlorate and 1-phenylethyl bromide is not instantaneous and therefore a steady increase in catalyst concentration characterises the first part of the polymerisation. 

Moreover, according to this general schematic mechanism, a more detailed picture of the particle formation has been tentatively given in two particular cases (1) synthesis of colloidal silver particles, and (2) formation of bimetallic ferromagnetic metal particles. 

It is interesting to note that Ag+ reacts with (RuII) on silica leading to a long lived, (several seconds) bleaching of RuII and to the formation of colloidal silver. The reaction is ... 

Silvert, P., Herrera-Urbina, R., and Tekaia-Elhsissena, K. (1997) Preparation of colloidal silver dispersions by the polyol process. Part 1— Mechanism of particle formation. J. Mater. Chem. 7 293-299. 

The catalytic activity toward hydrogenation reactions has been indeed observed in such bimetallic colloidal systems. The SERS spectrum of p-nitrobenzoate (PNBA) adsorbed on colloidal silver and the SERS spectra observed on Ag/Pd nanoparticles, immediately after the addition of PNBA and after 1 week, are shown in Fig. 20.7 A, B, and C, respectively. In the bimetallic colloid, instead of the SERS spectrum of PNBA (spectrum A), a different spectrum is obtained (spectrum B) that slowly evolves toward a different spectral feature, which becomes predominant after a week (spectrum C). This modification may be related to the initial formation of p-aminobenzoate as a result of the catalytic reduction of the nitro group, followed by slow oxidation to azodibenzoate by atmospheric oxygen (see Fig. 20.8). 

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. 

Silver in the metallic state is not readily eliminated from the body. Following intramuscular implantation, silver will complex with sulfur-containing amino acids or proteins and may remain permanently immobilized at the site of the implant (Luckey and Venugopal 1997). Implantation of silver foil has been demonstrated to induce fibrosarcoma in rats at the implant site, particularly when implanted subcutaneously (Oppenheimer etal. 1956). Subcutaneous injection of colloidal silver has likewise been shown to induce tumor formation. Autometallography is used to highlight... 

A dissolution of the complex in ethanol (Fig. 2a) and DMF (Fig. 2b) (1 mg/ml) results in its destruction accompanied by oxidation of pyrocatechol and reduction of silver with formation of colloid. Silver particles were adsorbed on the positively charged surface of the substrate. Particle size can be varied between —20 nm in ethanol and -50 nm in DMF. 

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. 

Cuba, V., Nemec, M., Gbur, T., John, J., Pospisil, M., Mucka, V. 2010b. Radiation formation of colloidal silver... 

A. Henglein, M. Giersig. Formation of Colloidal Silver Nanoparticles Capping Action of Citrate, J. Phys. Chem. BC 1999, v. 103,9533-9539. 

Henglein, A., 1999. Formation of colloidal silver nanoparticles capping action of citrate. J. Pbys. Cbem. B 103, 9533-9539. 

Protected Colloids. — Combinations of metal colloids and protective colloids have often been classed with metal colloids. They will be dealt with later, but are of sufficient interest to warrant a few remarks here. To this class belongs Lea s colloidal silver, and Paal s colloidal metals t that show the interesting catalytic effect in reduction reaction with hydrogen for instance, the formation of succinic acid from fumaric acid and the preparation of stearic acid from oleic acid. 

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