Colloidal aerosol preparation

The polydivinybenzene colloids prepared by the aerosol technique were carbonized to yield uniform porous spheres of carbon of relative high specific surface areas (69). 

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... 

Evaporative decomposition erf solutions and spary pyrolysis have been found to be useful in the preparation of submicrometer oxide and non-oxide particles, including high temperature superconducting ceramics [819, 820], Allowing uniform aerosol droplets (titanium ethoxide in ethanol, for example) to react with a vapor (water, for example) to produce spherical colloidal particles with controllable sizes and size distributions [821-825] is an alternative vapor phase approach. Chemical vapor deposition techniques (CVD) have also been extended to the formation of ceramic particles [825]. 

Emulsions and suspensions are colloidal dispersions of two or more immiscible phases in which one phase (disperse or internal phase) is dispersed as droplets or particles into another phase (continuous or dispersant phase). Therefore, various types of colloidal systems can be obtained. For example, oil/water and water /oil single emulsions can be prepared, as well as so-called multiple emulsions, which involve the preliminary emulsification of two phases (e.g., w/o or o/w), followed by secondary emulsification into a third phase leading to a three-phase mixture, such as w/o/w or o/w/o. Suspensions where a solid phase is dispersed into a liquid phase can also be obtained. In this case, solid particles can be (i) microspheres, for example, spherical particles composed of various natural and synthetic materials with diameters in the micrometer range solid lipid microspheres, albumin microspheres, polymer microspheres and (ii) capsules, for example, small, coated particles loaded with a solid, a liquid, a solid-liquid dispersion or solid-gas dispersion. Aerosols, where the internal phase is constituted by a solid or a liquid phase dispersed in air as a continuous phase, represent another type of colloidal system. 

These films can be prepared by a variety of routes, only a few of which are mentioned here. The original references should be consulted for more practical details. Titanium dioxide is used as an illustrative example below. First, colloidal solutions are prepared, e.g., from titanium isopropoxide. The resultant sol is concentrated under vacuum at room temperature until its viscosity increases. Then it is spin-coated on to suitable supports (e.g., conducting glass) and fired in an oven. The firing temperature critically controls the morphology of the resultant film as discussed elsewhere [300-303]. Films up to several micrometers thick can be prepared by this simple version of the sol-gel technology [304]. Aerosol or spray pyrolysis is a somewhat related approach [305, 306]. 

Generation of solid colloidal particles in aerosols has certain advantages over precipitation from homogeneous solutions described in Chapter IV. During precipitation from solutions it is usually impossible to predict a priori the shape of the resulting particles, while particles prepared by the aerosol methods are usually spherical because of the natural shape of liquid droplets dispersed in gas. Also, it was pointed out earlier (see Chapter IV) that in the case of particles of internally mixed composition, the molar ratio of constituents in the solid phase differs from that in solution [13], while in the case of aerosol technique the content of resulting solid particles is determined by the molar ratio of components in solution that is dispersed in the gas phase to form aerosol droplets. ... 

Making use of constrained polymerisation of divinylbenzene on surfactant-modified colloid silica, Jang and Lim prepared carbon nanocapsules and mesocellular foams. Later, they reported that mesoporous carbons with highly uniform and tunable mesopores were fabricated by one-step vapour deposition polymerisation using colloidal silica nanoparticles as template and polyacrylonitrile as carbon precursor. Hampsey et al. recently reported the synthesis of spherical mesoporous carbons via an aerosol-based, one-step approach using colloidal silica particles and/or silicate clusters as template. ... 

Practical aerosols exhibit size ranges from molecular clusters in the nanoscale (1 nm and larger) to dusts and clouds containing aerosol droplets that exceed the classical colloidal size range limits given earlier, easily ranging to about 100 pm (see Tables 1.5,9.4 and 9.5). Section 7.1 describes the preparation of aerosols. Subsequent chapters (especially Chapters 8, 9,13, and 15) provide many examples of aerosols in industry and everyday life. 

Obviously, knowing how colloids can be stabilized provides an invaluable tool for the preparation of many useful systems. It also can provide clues to how an unwanted colloid can be destabilized and removed. The above-mentioned ideas, at times in a slightly different guise, will appear again in the following chapters on emulsions, foams, aerosols, and similar compounds. 

Aerosol techniques on-line sizing of colloidal nanoparticles, 20-40 ultrafine powder synthesis, 64 Ag particles, synthesis method, 128 Aluminum nitride powder prepared via chemical synthesis, characterization using Fourier-transform IR spectroscopy, 312-332... 

Dispersion processing (aqueous or nonaqueous). Precursor fumed silica/silica soot dispersed in polymerizing gels or potassium sihcate aqueous solutions colloidal silica for dispersion can be prepared from aerosols as soot or fumed silica by flame hydrolysis, or other suitable methods. Examples of fumed silica are Cab-O-Sil (Cabot (7orp.) and Aerosil OX-50 (Degussa Corp.). Process pore sizes 100 to 3(X) nm. 

An aerosol is a colloidal dispersion of liquid droplets in a vapor. Aerosols can be used to make oxide powders in a variety of ways. For example, an aqueous sol can be sprayed into alcohol, where the aerosol droplets gel to produce spheres [110]. Subsequent firing produces perfect dense oxide spheres whose size depends on the concentration of the initial sol. Schwartz et al. [4] prepared powders of lead lanthanum zirconate titanate by spraying a salt solution into ammonium hydroxide and then spray-dried the resulting powder to produce uniform spheres 0.5-2/rm in diameter. In such complex compositions, aerosols are advantageous, because each droplet is a small reaction vessel, and heterogeneity cannot occur on a scale larger than the size of the droplet. 

III.l Experimental Procedure The samples were made of variable amounts of hydrophobic colloidal silica (Aerosol R972 Degussa) mixed with an index matching solvent (ethanol + ethanolamine) containing the tracer molecule (fluorescein isothiocyanate. Molecular Probes). We have checked that tracer adsorption on the silica was negligible. Further details on the sample preparation can be found in reference [16]. 

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