Page 57 - Academic Press Encyclopedia of Physical Science and Technology 3rd Chemical Engineering

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Encyclopedia of Physical Science and Technology En001c-14 May 7, 2001 18:25

280 Aerosols

metastable liquid state. For example, sodium chloride has erties. Scattered light is affected by the morphology and
been observed to exist as a relatively stable subcooled liq- optical properties of the particle and the distribution of
uid at room temperature, hundreds of degrees below the optically active molecules within it. The light scattered
melting point of crystalline sodium chloride. and its angular distribution are quite different from those
Another example of an extrinsic deviation was found found when the molecules are distributed within the same
by examining absorption spectra from freshly formed material in bulk.
˚
aerosols composed of iron carbonyl, 30–200 A in size.
Among the spectra were some corresponding to excited
1. Cohesive and Adhesive Forces
states of carbon monoxide, as well as bands that were pos-
sibly associated with a molecular oxygen transition. The Particles in dusts or powders tend to stick together remark-
oxygen excitation had energy levels of 7–9 eV, suggest- ably well. The suspension of powders depends critically
ing that the excitation was not due to chemical reactions on the agglomeration characteristics. Once suspended, the
or incident photons only. It is possible that the spectral capability of particles to agglomerate after collision also
absorption was also related to gas molecules adsorbed on depends on the attractive forces of interaction after con-
the iron surface or to large surface energy of the small tact. It is difficult to break up aggregates of particles to
particles. When the small particles coagulate or surface produce clouds of nonagglomerated material. The capac-
crystallites are relocated, a large amount of energy may ity of particles to stick together indiscriminately is the re-
be released and transferred to gas molecules adsorbed on sult of weak attractive forces between molecules as well as
the particle surface. In this way, certain high excitation bipolar electrostatic forces. These forces have been named
levels may be populated in a manner differing from that cohesive and adhesive, depending on the heterogeneity of
predicted by thermal equilibrium. material at the boundary between particles. The distinc-
Other types of extrinsic deviation are found in the spe- tion between cohesion and adhesion in the literature on
cial properties imparted to small particles by the manner fine powders is somewhat fuzzy, but we shall adopt the
of their preparation. For example, production of small par- following conventions, which are consistent with classical
ticles by grinding in a mill alters the heat of adsorption of definitions in physics. Cohesion is the tendency for parts
gases by the particles. In general, the method of particle of a body of like composition to hold together. This im-
production may introduce defects or microscopic impuri- plies that cohesive forces arise between like molecules in a
ties that differ from what is found in the parent material. solid or between small particles of the same composition.
Often extrinsic and intrinsic deviations occur in the same Adhesion, on the other hand, refers to attraction across the
physical property, as in the example of supercooling of boundary or interface between two dissimilar materials.
sodium chloride cited above. This fact makes the study of Thus, adhesive forces are likely to be the most common
intrinsic deviations very difficult. attractive forces in all but artificially generated aerosols.
Intrinsic deviations are perhaps most widely known
through the effect of the radius of curvature of small par-
B. Particle Size Distribution
ticles on many physical properties such as vapor pressure,
freezing point, surface tension, heat of evaporation, and In practice, aerosols in nature and in technology cover a
others. Intrinsic deviations not directly associated with the broader size range than called for by their rigorous scien-
radius of curvature have been observed by X-ray crystal- tificdefinition. Normally, the range of interest is less than
lographic studies of very small crystallites with radii less 100 µm in diameter, extending to molecular dimensions.
than 0.01 µm. In these studies, the lattice spacings ob- A summary of particle dispersoids, methods of measure-
served in the small crystallites differed significantly from ment, gas cleaning equipment, and mechanical parame-
the lattice spacings observed for the bulk state of the parent ters is given in Fig. 1. A striking and important feature
material. The effect of such alterations on various physi- of aerosols is illustrated in the figure. Particles range over
cal properties has not been studied. In general, one expects five orders of magnitude in size. By analogy, this roughly
that for particles of radius less than ∼0.01 µm, intrinsic corresponds to a domain from sand grains to tall build-
deviations of this sort must occur; however, it has been ings in a city. Thus, the microscopic world of fine-particle
obviously very difficult to observe such deviations ex- suspensions should be as diverse and rich as our everyday
perimentally. Only recently has substantial progress been macroscopic environment. This range poses major chal-
made in characterizing unique properties associated with lenges to the scientist for developing theory, for measure-
the nanoparticle regime. ment, and for mechanical production and removal.
Another type of intrinsic property is derived from the The theory of particle clouds proceeds from consid-
theory of light scattering in particles. The phenomenon eration of the dynamics of the particle size distribution
of Raman and fluorescent scattering from molecules sus- function or its integral moments. This distribution can take
pended in small dielectric particles exemplifies such prop- two forms. The first is a discrete function in which particle