Colloidal intermediates

Taden A, Landfester K, Antonietti M (2004) Crystallization of dyes by directed aggregation of colloidal intermediates a model case. Langmuir 20 957-961... 

In this section I will describe a small number of inorganic molecules from the large and diverse collection that have covalent bonds between their atoms. Molecules that have covalent bonds are used as precursors for synthesis of advanced solids either from the gas phase or from liquid solution using colloidal intermediates. 

Multimetallic catalysts, alloy catalysts, intermetallic compounds, fuel cell catalysts, colloidal intermediates, metal complexes, and metal clusters have received considerable attention [12-21] because the metal-metal cooperating bifunctional catalysts which can activate reactants simultaneously showed high catalytic activity and stereoselectivity under mild conditions [19]. In fact, there have been many bifunctional multimetallic catalysts in which multimetallic alloy- and electro-catalysts offer a way to fine-tune the catalytic properties of metals, atomic composition, and microstrucrnres [16-18, 20]. Cooperative multimetallic activation of oxidants via the multielectron transfer is also a common feature in biological oxidation catalysis [14]. Artificial multimetallic complexes with two or more metal atoms that contain... 

Gold compounds are all easily reduced in alkaline solution to metallic gold which may occur in colloidal form and so be red, blue or intermediate colours. Reduction to gold, followed by weighing of ihe metal precipitated, may be used in quantiiaiive analysis. 

Silica fouling is the accumulation of insoluble silica on anion resins. It is caused by improper regeneration which allows the silicate (ionic form) to hydrolyze to soluble silicic acid which in turn polymerizes to form colloidal silicic acid with the beads. Silica fouling occurs in weak-base anion resins when they are regenerated with silica-laden waste caustic from the strongbase anion resin unless intermediate partial dumping is done. 

For convenience, colloids are divided into two main groups, designated as lyophobic and lyophilic colloids. The chief properties of each class are summarised in Table 11.1, although it must be emphasised that the distinction is not an absolute one, since some gelatinous precipitates (e.g. aluminium hydroxide and other metallic hydroxides) have properties intermediate between those of lyophobic and lyophilic colloids. 

Plasma protein fractions include human plasma protein fraction 5% and normal serum albumin 5% (Albuminar-5, Buminate 5%) and 25% (Albuminar-25, Buminate 25%). Plasma protein fraction 5% is an IV solution containing 5% human plasma proteins. Serum albumin is obtained from donated whole blood and is a protein found in plasma The albumin fraction of human blood acts to maintain plasma colloid osmotic pressure and as a carrier of intermediate metabolites in the transport and exchange of tissue products. It is critical in regulating the volume of circulating blood. When blood is lost from shock, such as in hemorrhage, there is a reduced plasma volume. When blood volume is reduced, albumin quickly restores the volume in most situations. 

There are a number of industrial and technological areas in which nanoscale adhesion is important. One of the earliest fields concerned with adhesion on this scale was colloid science. Colloid particles lie in the intermediate region between macro and nano, with dimensions typically of the order of hundreds of nanometers up to a few microns. This means that their true contact areas he well within the nano-domain and are influenced by interactions on this length scale. Adhesion between such particles is important, due to its influence on mineral separation processes and on the aggregation of powders, for example, on the walls of machinery or in the forming of medical tablets. In an extraterrestrial context, such... 

Particles whose dimensions are between 1 nanometer and 1 micrometer, called colloids, are larger than the t3/pical molecule but smaller than can be seen under an optical microscope. When a colloid is mixed with a second substance, the colloid can become uniformly spread out, or dispersed, throughout the dispersing medium. Such a dispersion is a colloidal suspension that has properties intermediate between those of a true solution and those of a heterogeneous mixture. As Table 12-3 demonstrates, colloidal suspensions can involve nearly any combination of the three phases of matter. Gas-gas mixtures are the exception, because any gas mixes uniformly with any other gas to form a true solution. 

Small particles of metals in solution often behave like electrodes although they are not connected to a battery which determines their potential. However, when a chemical reaction occurs in the solution of such particles intermediate free radicals may transfer electrons to them. The particles are thus charged chemically and are able to act as a metal electrode on cathodic potential. Electron transfer reactions become possible at these micro-electrodes which cannot be brought about by the radicals in the absence of the colloidal catalyst. 

In studies of this kind, methods developed in radiation chemistry and photochemistry are often applied The methods of pulse radiolysis and flash photolysis allow one to investigate the mechanism of reactions in which free radicals, electrons and positive holes are the intermediates. In order to understand the mechanisms of processes that occur on colloidal particles it is important to know how free radicals... 

Methyl viologen (l,T-dimethyl-4,4 -bipyridylium dichloride, MV " ) promotes photoanodic dissolution in aerated CdS solution Figure 8 shows how the rate of dissolution depends on the concentration. The colloid has a weak fluorescence at 620 nm which is quenched by. The curves for fluorescence and dissolution in Fig. 8 are symmetric, which indicates that the two processes have a common intermediate that reacts with M. These effects are explained by the following mechanism ... 

As a rule, high quantum yields for two-electron transfer reactions are expected when the mechanism is one-electron/two-hole or two-electron/one-hole. In the cases of twQ-electron/two-hole or one-electron/one-hole efficient back reactions of the intermediates on the colloidal particles or in solution, respectively, will lead to a low yield of the final products. 

The hydrated electrons then react according to e + Cd Cd, and the Cd ions which have a strong absorption at 300 nm react with the colloidal particles after the pulse. It was observed that the same bleaching took place during this reaction as in the reaction of e with CkiS particles, and it was concluded from this result that Cd" transfers an electron to a CdS particle Cd" + (CdS), - Cd + (CdS) . These observations also are of interest for our understanding of the formation of Cd atoms in the photocathodic dissolution of CdS (see Sect. 3.4). Cd" cannot be the intermediate of the overall reaction 2e + Cd - Cd° as already pointed out in discussing the mechanism of Eqs. (35) and (36)... 

Instead of postulating Zn," as intermediate, as it has a highly negative potential and is possibly unstable in ZnO, one may write the above mechanism with Zn e pairs. The blue-shift in the absorption upon illumination was explained by the decrease in particle size. The Hauffe mechanism was abandoned after it was recognized that an excess electron on a colloidal particle causes a blue-shift of the absorption threshold (see Fig. 19). In fact, in a more recent study it was shown that the blue-shift is also produced in the electron transfer from CH2OH radicals to colloidal ZnO particles When deaerated propanol-2 solutions of colloidal ZnO were irradiated for longer times, a black precipitate of Zn metal was formed. In the presence of 10 M methyl viologen in the alcohol solution, MV was produced with a quantum yield of 80 %... 

CdjPj and CdjAs are low band gap semiconductors (0.5 and 0.1 eV, respectively). The bulk materials are black and start to absorb in the infrared. These materials have been prepared as colloids in alkaline solution by precipitation of Cd with phosphine and arsine Depending on the conditions of preparation, particles of different sizes (between about 2 and 10 nm) were obtained, which could also be recovered in the solid state after evaporation of the solvent. The color of these materials ranged from black to colorless with decreasing particle size, with all kinds of intermediate colors in the visible. 

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