Colloidal suspension preparation

A colloidal suspension prepared according to the method described in Section 13.2.3 was contacted with a porous alumina carrier to obtain a bimetallic palladium-tin catalyst. Evaluation of the catalytic properties of this system is detailed... 

Some preparation methods specific to the formation of nanoparticle suspensions are provided in References [20,62,63]. Many such methods are simply conventional colloidal suspension preparation methods that have been extended to produce smaller particle sizes, but others involve novel approaches. Some ofthese involve making nanoemulsions as a first step. For example, membrane, microfluidic and nanofluidic devices have been used to make nanoscale emulsions of all kinds, as already noted earlier, and the emulsion droplets so generated can be used in turn to make sohd microparticles and nanoparticles. If the nanoparticles are intended to encapsulate other materials, then a double emulsification technique can be used, at elevated temperature, to prepare a multiple emulsion (i.e. 

After preparation, colloidal suspensions usually need to undergo purification procedures before detailed studies can be carried out. A common technique for charged particles (typically in aqueous suspension) is dialysis, to deal witli ionic impurities and small solutes. More extensive deionization can be achieved using ion exchange resins. 

Chitosan can be spray-dried at neutral pH if a colloidal suspension is prepared with NaOH. Nevertheless, this preparation is prohibitively time-consuming due to the difficulties involved in washing and removal of excess alkali and salts. 

The solgel process uses a liquid reactive precursor material that is converted to the final product by chemical and thermal means. This precursor is prepared to form a colloidal suspension or solution (sol) which goes through a gelling stage (gel) followed by drying and consolidation. The process requires only moderate temperatures, in many cases less than half the conventional glass or ceramics... 

The topic of gold nanospheres attracted the interest of several famous nineteenth century scientists such as Michael Faraday, Richard Zsigmondy, and Gustov Mie [43]. Interest diminished in the mid-twentieth century although some excellent contributions were made by Turkevich [42, 44], Frens [45], and Brust [46] in that period regarding the controlled preparation of nearly monodisperse colloidal suspensions. 

The reduction of transition metal salts in solution is the most widely practiced method for synthesis of metal colloidal suspensions [7]. In the preparation process, polymer is often used in order to prevent the agglomeration of metal particles as well as to control their size. Ahmadi et al. [5] reported that the concentration of the capping polymer affects the shape of platinum particles obtained by salt reduction. This means that the addition of a... 

A colloidal suspension of conductive vanadium pentoxide [130] can be used to perform intercalation, adsorption or encapsulation of electroactive molecules or biomolecules for electrodes or biosensor realization [131]. Encapsulation of glucose oxidase in nanocomposite films made with polyvinyl alcohol and V205 sol-gel matrix or in ferrocene intercalated V2Os sol-gel [132] were envisaged to prepare glucose biosensors. 

The structures of four of the synthetic carotenoids (beta-carotene, canthaxanthin, beta-apo-8 -carotenol, beta-apo-8 -carotenoic acid) are shown in Fig. 8.2. By virtue of their conjugated double bond structure, they are susceptible to oxidation but formulations with antioxidants were developed to minimize oxidation. Carotenoids are classified as oil soluble but most foods require water soluble colorants thus three approaches were used to provide water dispersible preparations. These included formulation of colloidal suspensions, emulsification of oily solutions, and dispersion in suitable colloids. The Hoffman-LaRoche firm pioneered the development of synthetic carotenoid colorants and they obviously chose candidates with better technological properties. For example, the red canthaxanthin is similar in color to lycopene but much more stable. Carotenoid colorants are appropriate for a wide variety of foods.10 Regulations differ in other countries but the only synthetic carotenoids allowed in foods in the US are beta-carotene, canthaxanthin, and beta-8-carotenol. 

The reverse operation, i.e. the addition of palladium nitrate solution to soda, can also be used to prepare colloidal suspensions. Figure 13.8 presents the corresponding titration curve of a soda ([NaOH] = 0.2 N) by a palladium nitrate solution ([Pd] = 5 g/L). 

Thermohydrolysis is another reaction which can be used to prepare colloidal suspensions of palladium oxide particles, and has been proven for other oxides. " ... 

The colloidal suspensions obtained by the neutralization or thermohydrolysis procedures described previously have been used to prepare supported Pd catalysts. The support is impregnated with a volume of colloidal oxide suspension corresponding exactly to the porous volume of the solid, according the well-known incipient wetness impregnation. 

TABLE 13.4. Characteristics and catalytic activity of samples prepared from colloidal suspensions. 

The mode of synthesis of alumina membranes through the colloidal suspension route is given in Figure 2.6. The first step involves the preparation of a slip consisting of boehmite particles. These arc plate-shaped in the form of pennies with a diameter of 25-50 nm and a thickness of 3.5-5.5nm (Leenaars ct al. 1984,1985). The synthesis chemistry of the colloidal boehmite (y-AlOOH) solution is described in detail by Leenaars and Yoldas (1975) and to some extent by Anderson, Gieselman and Xu (1988) and by Larbot et al. (1987). 

Characteristic microstructural properties of TiOj membranes produced in this way are given in Table 2.5. Mean pore diameters of 4-5 nm were obtained after heat treatment at T < 500°C. The pore size distribution was narrow in this case and the particle size in the membrane layer was about 5 nm. Anderson et al. (1988) discuss sol/gel chemistry and the formation of nonsupported titania membranes using the colloidal suspension synthesis of the type mentioned above. The particle size in the colloidal dispersion increased with the H/Ti ratio from 80 nm (H /Ti = 0.4, minimum gelling volume) to 140 nm (H /Ti " — 1.0). The membranes, thus prepared, had microstructural characteristics similar to those reported in Table 2.5 and are composed mainly of 20 nm anatase particles. Considerable problems were encountered in membrane synthesis with the polymeric gel route. Anderson et al. (1988) report that clear polymeric sols without precipitates could be produced using initial water concentrations up to 16 mole per mole Ti. Transparent gels could be obtained only when the molar ratio of H2O to Ti is < 4. Gels with up to 12 wt.% T1O2 could be produced provided a low pH is used (H /Ti + < 0.025). 

From these data it is evident that a momentary suspension" stage is obtained for values of U equal to 8-25, but on attempting to increase the U values to obtain more definite persistence of the supension, a gel is formed thus to prepare a colloidal suspension of barium sulphate a medium must be employed in which U values of 8-25 may be obtained at lower concentrations, i.e. we must reduce the actual solubility of barium sulphate. By performing the double decomposition in alcohol water mixtures in which the solubility of barium sulphate is small a clear stable suspension may in fact readily be formed. 

We point out here that the colloid prepared by these methods is very clean, because the carrier gas used is usually high-purity grade at six-nine, the chamber is once evacuated to depress the extent of contaminating oxygen and moisture, and the liquids themselves are always purified by sublimation process except for the solution trap method. To transfer the colloidal suspension after preparation, a specially designed stock bottle with a Luer-lock syringe is normally used in order to enable the operations under Ar flow to avoid unexpected air contamination. Therefore, we can carry the suspension liquid away from the production chamber without exposure to air, which means that the surface of colloidal metal is very clean if it does not react with suspension liquids. 

So far, we have prepared and tested many kinds of colloids, mainly in nonaqueous suspensions with combinations of metals or alloys as a dispersed phase and organic liquids as the dispersion media, without the use of any dispersing agents these are listed in Table 9.4.1. We next give some examples of transmission electron micrographs of nanoparticles produced by an aerosol method. A sample for TEM measurement was obtained by dropping colloidal suspension onto a Cu mesh coated with an evaporated carbon film of 10 nm thickness. Many colloids were so unstable... 

Fig. 9.4.26 Optical absorption spectra of Au-UFP/2-propano suspension as a function of time. The numbers in the figure are the passage of time after the preparation of the sample. The. sample colloids were prepared by a gas flow-solution trap method. (Front Ref. 27.)...

The precipitation methods always have the disadvantage that the stoichiometry of the precipitate (s) may not be exact if one or more ions are left in solution. The solgel method overcomes this because the reactants never precipitate out. First, a concentrated solution or colloidal suspension of the reactants, the sol , is prepared, which is then concentrated or matured to form the gel . This homogeneous gel is then heat-treated to form the product. The main steps in the sol-gel process are outlined in Figure 3.3. 

Amphotericin is an amphoteric polyene macrolide (polyene = containing many double bonds macrolide = containing a large lactone ring of 12 or more atoms). It is nearly insoluble in water and is therefore prepared as a colloidal suspension of amphotericin and sodium desoxycholate for intravenous injection. Several new formulations have been developed in which amphotericin is packaged in a lipid-associated delivery system (Table 48-1 and Liposomal Amphotericin B). 

Periodically, these mixtures were centrifuged and an aliquot of the clay-free supemate taken for counting analysis. Two naturally-occurring clays were selected for the experiments one was labeled kaolin (for the mineral kaolinite) while the second was referred to as attapulgite (or polygorshite). Both were obtained from the Source Clay Mineral Repository (3) as standard clays representative of each class of clay and were used as received. Stable, colloidal suspensions of each were prepared by ultrasonically dispersing weighed quantities of each clay in triple-distilled water. 

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