Homogeneous aerosols

Aggregation of Particles. Sinclair 91) discusses the coagulation in a homogeneous aerosol. For ordinary concentrations of aerosols the coagulation rate is low. Stirred settling of the aerosol produces faster rates. Equations applicable to these processes are given. 

Homogeneous Chemical similarity. A homogeneous aerosol is one in which all particles are chemically identical. In an inhomogeneous aerosol different particles have different chemical compositions. 

Observed aerosol number distributions are usually expressed as n°( ogDp), that is, using log Dp as an independent variable (see Chapter 8). However, it is often convenient to use alternative definitions of the particle size distribution to facilitate the calculations of its rate of change. The distribution can be expressed in several forms assuming a spatially homogeneous aerosol of uniform chemical composition. 

The Discrete Coagulation Equation A spatially homogeneous aerosol of uniform chemical composition can be fully characterized by the number densities of particles of various monomer contents as a function of time, Afc(f). The dynamic equation governing... 

Steel capillary with nitrogen gas flowing through the space between the two capillaries. Insertion of the miniaturized nebulizer into the end of the drift tube and its contact with the heated wall of the drift tube, the purpose of which is to preheat the nebulizer gas, allows faster system equilibration at startup and production of a more stable and homogeneous aerosol. 

Several instrument manufacturers al.so rtffer ulira.sonic nebulizers in which Ihe sample is pumped onto the surface of a piezoelectric crystal that vibrates at a frequency of 20 kllz to several megahertz. Ulira.sonic nebulizers produce more dense and more homogeneous aerosols than pneumatic nebulizers do. 1 hc.se devices have low cflicicncies with viscous solutions and solutions containing parliciilaies, however. 

Microemulsions are treated in a separate section in this chapter. Unlike macro- or ordinary emulsions, microemulsions are generally thermodynamically stable. They constitute a distinctive type of phase, of structure unlike ordinary homogeneous bulk phases, and their study has been a source of fascination. Finally, aerosols are discussed briefly in this chapter, although the topic has major differences from those of emulsions and foams. 

Solutions. To dehver a spray, the formulated aerosol product should be as homogeneous as possible. That is, the active ingredients, the solvent, and the propellant should form a solution. Because the widely used halocarbon and hydrocarbon propellants do not always have the desired solubiUty characteristics for all the components in the product concentrate, special formulating techniques using solvents such as alcohols (qv), acetone (qv), and glycols (qv), are employed. 

Finally, atmospheric chemical transformations are classified in terms of whether they occur as a gas (homogeneous), on a surface, or in a liquid droplet (heterogeneous). An example of the last is the oxidation of dissolved sulfur dioxide in a liquid droplet. Thus, chemical transformations can occur in the gas phase, forming secondary products such as NO2 and O3 in the liquid phase, such as SO2 oxidation in liquid droplets or water films and as gas-to-particle conversion, in which the oxidized product condenses to form an aerosol. 

W [Stefan flow], movement in a temperature gradient [thermophoresis j, and sprays), (. i steam condensation onto aerosols, and (4) homogeneous nudeation of water droplets. 

Other natural materials with expected intrinsic homogeneity properties that could be produced as CRMs include fractionated aerosols, conifer pollen, egg yolk or albumen, plankton of defined origin and size, and certain sediments. However, there are particular problems related to the collection and preparation, of large enough quantities of such matrices for their preparation as RMs. RM producers might initiate research and development to provide technical solutions to handle some of these problems. 

McMurray, P. H. and J. C. Wilson, Droplet Phase (Heterogeneous) and Gas Phase (Homogeneous) Contributions to Secondary Ambient Aerosol Formation As Functions of Relative Humidity, Atmos. Environ.. 

Dry powders must be able to flow readily in order to leave the capsule or powder reservoir, but must also generate a fine aerosol enabling the patient to inhale a proper dose. These two requirements are often difficult to achieve simultaneously. Fine powders tend to be eohesive and have poor flow properties. Blending with a carrier phase, pelletization, and other approaches have been used to overcome these limitations. The featmes of blends and homogeneous powders are compared in Table 4 from a DPI device perspective ... 

Homogeneous Advantages Little mouth and throat deposition patient does not feel inhaling an aerosol... 

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