Dispersed aerosols

An interesting example of a large specific surface which is wholly external in nature is provided by a dispersed aerosol composed of fine particles free of cracks and fissures. As soon as the aerosol settles out, of course, its particles come into contact with one another and form aggregates but if the particles are spherical, more particularly if the material is hard, the particle-to-particle contacts will be very small in area the interparticulate junctions will then be so weak that many of them will become broken apart during mechanical handling, or be prized open by the film of adsorbate during an adsorption experiment. In favourable cases the flocculated specimen may have so open a structure that it behaves, as far as its adsorptive properties are concerned, as a completely non-porous material. Solids of this kind are of importance because of their relevance to standard adsorption isotherms (cf. Section 2.12) which play a fundamental role in procedures for the evaluation of specific surface area and pore size distribution by adsorption methods. 

Another variant (the cone spray) allows the sample solution to flow down the sides of an inverted cone and through a hole at the bottom of which flows a fast stream of argon gas. As the liquid film meets the gas, it is ripped apart into a finely dispersed aerosol (Figure 19.15). 

Aerosol, monodisperse An aerosol with a size-distribution function described by a geometrical standard deviation less than 1.15. If the deviation is between 1.15 and 1.5, it is classified as a quasi-mono-disperse aerosol. 

See also Axial dispersion aerosols, 1 774-775 aqueous, 18 292 behavior of, 15 685-690 chemical processing aids, 8 705-711 chromatic, 11 134 classification, 8 698-699 colorants for plastics, 7 360-361 donor-acceptor interactions, 8 707-708 electrostatic repulsion, 8 732-734 in filled networks, 22 572 of filled polymers, 11 307-308 flow, 8 726-730 flushing, 8 711... 

Hinds, W.C. (1980). Dry dispersion aerosol generators. In Generation of Aerosols and Facilities for Exposure Experiments (Willeke, K., Ed.). Ann Arbor Science, pp. 171-187. 

Holma, B. (1967). Lung clearance of mono- and di-disperse aerosols determined by profile scanning and whole body counting A study on normal and S02 exposed rabbits. Acta Med. Scand. (Suppl.) 473 1-102. 

Willoughby, R.C. Browner, R.F. Mono-disperse Aerosol Generation Interface for Combining Liquid Chromatography with Mass Spectroscopy. Anal. Chem. 1984,56, 2625-2631. 

The average number of eontaet points between partieles inereases as bulk density increases, and the interparticulate forces at these contact points must be overcome to produce a dispersed aerosol cloud. Therefore, a powder of low bulk density may be more easily dispersed as an aerosol than an otherwise identical powder of high bulk density. 

Diffusion of the gas to the surface (rg). As described by Fuchs and Sutugin (1970, 1971) in their comprehensive treatment of highly dispersed aerosols, the rate of transfer of mass to the surface of a spherical particle by diffusion of a gas is described by... 

Fuchs, N. A., and A. G. Sutugin, Highly Dispersed Aerosols, Ann Arbor Science Publishers, Inc., Ann Arbor, MI, 1970. 

This chapter treats those aerosol phenomena that are known or believed to be important in atmospheric chemistry. For treatment of related, but specialized, topics, a number of excellent references are available. The classic works on aerosol physics are The Mechanics of Aerosols by the late N. A. Fuchs (1964) and Highly Dispersed Aerosols (Fuchs and Sutugin, 1970). The... 

Dust dispersion—Aerosols of mineral particles and a few other materials may be prepared by dust dispersal techniques. A number of dust dispersal devices have been constructed. A commonly used device is the Wright Dust Feeder. In this device the dust is compressed into a cylindrical compact, which is placed in an electronically driven mechanism that rotates it and drives it against a fixed knife blade at a fixed rate. Material is scraped off the compact by the blade, entrained in an air stream, and blown against a metal plate to break up particle aggregates. 

In order to test the validity of the VELUT model it was used to generate predicted isokinetic values of VE differential sampling efficiency for values of MMAD and GSD corresponding to experimental values reported in the literature. A comparison of the experimental data obtained by Carson and Lynch (10) using mono-disperse aerosols of dioctylphthalate, with the predictions of VELUT for the same conditions is given in Table I. This limited data is in excellent agreement with the predictions of VELUT. 

Martonen, T. B., and Katz, I. M. Deposition patterns of poly-disperse aerosols within human lungs. J. Aerosol Med., 6 251—274, 1993. 

Colloidal-sized particles in the atmosphere are called aerosols. Those formed by grinding up bulk matter are known as dispersion aerosols, whereas particles formed from chemical reactions... 

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. 

Odor perception takes place only when molecules of the odorous material enter the nasal cavity and physically touch the olfactory receptors located there. In nearly all instances (the exception being finely dispersed aerosols), this means that perfume materials have to pass into the vapor phase in order to be perceived. The process of evaporation therefore is of central importance to perfumery. The volatility of perfume materials, that is, their readiness to pass into the vapor state, is one of their most important properties from the perfumer s point of view (Appell 1964). In fact two famous perfumers have proposed methods of perfume composition based upon volatility (Poucher 1955 Carles 1961). 

The organic component is also present in marine aerosol. It is also characterized by a complicated chemical composition. Over remote oceanic regions the concentration of organic matter in the rough-disperse aerosol fraction can reach 2-4 mg/m [41], but in other cases it does not exceed 0.15-0.47 mg/m [18]. 

The MDI may provide up to several hundred actuations, each containing typically from about 10 to 500 pg of drug dispersed in a 25 to 100 pi metered volume of liquid. The discharged liquid undergoes flash evaporation of the propellant to produce a finely dispersed aerosol spray. The deposition, and hence the clinical efficacy, are critically dependent on the mass of inhaled particles, which must have an appropriate aerodynamic size, typically less than 5 pm, to be deposited in the lungs (the respirable fraction). ... 

Inhalation exposure of rats to mancozeb, exposed everyday for 4 months indicated an increase in irritation of the mucous membrane of the upper respiratory tract and concentration-related nonspecific changes to the liver and kidneys. Exposure was in the form of dispersed aerosols at concentrations ranging from 2 to 135 mg m. At the lower concentrations, there were no observable effects. In animals exposed repeatedly to high doses of mancozeb (dust) equivalent to 150-250 times the acceptable exposure limit (AEL), reduced body weight, inflammation of the lungs, and abnormal thyroid function were observed. 

FIG. 21 -7 Dry disperser RODOS with vibratory feeder VIBRI creating a fully dispersed aerosol beam from dry powder. Courtesy of Sympatec GmhH.)... 

There are few methods suitable for on-line chemical analysis of aerosol particles. Raman spectroscopy offers the possibility of identifying the chemical species in aerosol particles because the spectrum is specific to the molecular. structure of the material, especially to the vibrational and rotational modes of the molecules. Raman spectra have been obtained for individual micron-sized particles placed on surfaces, levitated optically or by an eiectrodynamic balance, or by monodisperse aerosols suspended in a flowing gas. A few measurements have also been made for chemically mixed and poly disperse aerosols. The Raman spectrum of a spherical particle differs from that of the bulk material because of morphology-dependent resonances that re.su It when the Raman scattered photons undergo Mie scattering in the particle. Methods have been developed for calculating the modified spectra (McNulty el al., 1980). 

Fuchs. N. A and Sutugin, A. G. (1971) High-Dispersed Aerosols, in Hidy, G. M and Brock, J. R. (Eds,), Topics in Current Aerosol Research, Pergamon. New York. 

Condensation is the main route leading to the formation of finely dispersed aerosols in nature and industry. The formation of cumulus (composed of water droplets) and cirrus (composed of ice crystals) clouds mainly starts with heterogeneous nucleation on fine dusts and microcrystals of salt. These microcrystals form when splashes of sea water are dried and raised to high layers of atmosphere by convection air streams. 

Both LaMer s and the falling film aerosol generators yield only small quantities of products. Much larger amounts of aerosols can be produced by dispersing liquids with the help of various mechanical devices, e.g. rotating disks or ultrasonic nozzles [6]. These techniques, however, usually yield aerosols with broad distribution of droplet sizes and thus lead to polydisperse systems. The dispersion aerosol generators can, consequently, be used... 

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