Colloids, metal theory

At short interparticle distances, the van der Walls forces show that two metallic particles will be mutually attracted. In the absence of repulsive forces opposed to the van der Walls forces the colloidal metal particles will aggregate. Consequently, the use of a protective agent able to induce a repulsive force opposed to the van der Walls forces is necessary to provide stable nanoparticles in solution. The general stabihzation mechanisms of colloidal materials have been described in Derjaguin-Landau-Verway-Overbeck (DLVO) theory. [40,41] Stabilization of colloids is usually discussed... 

Early experiments of Berl and coworkers (15, 16), together with the findings of Withrow and Rassweiler (136), support the metallic theory of antiknock action. In Berl s work colloidal lead exhibited antiknock action, while colloidal lead oxide had no effect. Spectra of engine knocking combustion obtained by Withrow and Rassweiler showed the presence of metallic lead but not lead oxide or other lead oxides. The metallic theory is also supported by other investigators (96, 101, ) ... 

The theories that have been advanced to the effect that the decomposition products of the antiknock dopes and not the compounds themselves are the effective centers of the action has been tested by the use of metallic colloids, prepared in various ways, in the fuels by which engines were operated. The work on colloidal metal sols has been based on the theory that knocking is due to the spontaneous ignition of the unburned charge in an engine cylinder. By acting as catalysts for combustion these substances insure a slow, homogeneous combustion rather than a detonation. 

A somewhat similar theory10 postulates the formation of colloidal lead by the decomposition of lead tetraethyl, which deposits on sharp points, edges, and projections in the cylinder which would otherwise aid reaction to an extent that a detonation wave would result. The theory fails to explain the action of organic amines, of di- and tetravalent. selenium, of the colloidal metal sols, and fails to account for the immediate recurrence of knocking when the antiknock dope is discontinued in the gasoline feed. However, tubes coated with lead oxide have been found to reduce the oxidation of hexane more than lead tetraethyl vapors.1158 On the basis of this finding the hypothesis was advanced that oxidation occurs to some extent prior to compression and at the first contact of the gaseous mixture with the hot walls, aud that in the presence of the lead... 

The absorption spectra of spherical particles of colloidal dimensions can be calculated by Mie theory from a wavelength dependence of optical constants of the particles relative to the surrounding medium [7]. Spherical particles that do not interact with each other exhibit a single resonance as long as r T is valid, where r and A are the particle size and wavelength of the incident light, respectively. In this size regime the surface plasmon frequency is essentially size independent. Colloidal metallic particles produced by laser ablation have dimensions typically on the... 

J. S. Bradley. Clusters and Colloids From Theory to Applications, chapter The Chemistry of Transition Metal Colloids, pages 459-544. VCH, Weinheim, 1994... 

Bradley JS (1994) The Chemistry of Transition Metal CoUoids. In Schmid G (ed) Clusters and Colloids from Theory to Applications. VCH, Weinheim, pp 459-544... 

Bradley, J.S. 1994. The chemistry of transition metal colloids. In Clusters and Colloids From Theory to Applications, ed. G. Schmid, pp. 459-544. VCH Verlags gesells chaft mbH, Weinheim, Germany. 

Hirai, H. and Toshima, N. In Catalysis by Metal Complexes, Tailored Metal Catalysts, Iwasawa, Y., Ed., D. Reidel Publishing Conq)any Dordrecht, 1986. Bradley, J. S. In Clusters and Colloids. From Theory to Applications, Schmid, G., Ed., VCH Weinheim, 1994. 

One other model which was proposed primarily to explain the color of the solutions was the colloidal model by Kruger. According to this model, the metal is considered to be present as a colloid with negative charge, the colloidal metal particles exhibiting the observed blue color. However, the theory has met with little success in explaining the optical, magnetic, and other properties and will not be discussed any further. 

A theory to describe the optical characteristics of a colloid - metal surface system is the CPS-theory developed by hance, Prock and Silbey. Another theory more focusing on colloid films interacting with metal surfaces is the stratified medium theory (SMT). 

Adsorption of Ag on the surface of PdO is also an interesting option offered by colloidal oxide synthesis. Silver is a well-known promoter for the improvement of catalytic properties, primarily selectivity, in various reactions such as hydrogenation of polyunsaturated compounds." The more stable oxidation state of silver is -F1 Aquo soluble precursors are silver nitrate (halide precursors are aU insoluble), and some organics such as acetate or oxalate with limited solubility may also be used." Ag" " is a d ° ion and can easily form linear AgL2 type complexes according to crystal field theory. Nevertheless, even for a concentrated solution of AgNOs, Ag+ does not form aquo complexes." Although a solvation sphere surrounds the cation, no metal-water chemical bonds have been observed. 

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