Page 56 - 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

Aerosols 279

and considerable effort must be exercised sometimes to particles. The physical properties of small particles of a
prevent worker exposure to them. given substance can be quite different from the corre-
Public concern for the hazards of particle suspensions in spondingpropertiesofthesamesubstanceinthebulkstate.
the indoor and outdoor environment has produced regula- Deviations in behavior of small aerosol particles from
tions limiting particle concentrations and exposure levels. the bulk state are widely recognized for many physical
In the workplace, dust hazards are constrained by total properties such as vapor pressure, freezing point, and crys-
mass concentration as well as concentration of specific tal structure. Yet there has been a lack of a systematic clas-
toxic chemicals. In the ambient air, protection is stipulated sification of these deviations. Also, although deviations
in terms of total mass concentration of suspended parti- are expected to occur for small particles, there have been
clesandcertainchemicalspecies,namely,leadandsulfate. few experimental measurements of such deviations owing
Recently, measures of exposure have begun to distinguish partly to the difficulties of making such measurements.
between fine particles less than 2.5 µm and coarse parti- We classify a deviation from the bulk properties of the
cles between 2.5 and 10 µm. This separation relates to the properties of small aerosols as either extrinsic or intrinsic.
ability of particles to penetrate the human respiratory sys- Extrinsic deviations are associated with characteristics of
tem, and to different sources of fine and coarse particles. particles that are not inherent but are caused by external
One of the most common airborne suspensions known agents such as the mode of formation of the particle or the
to affect the respiratory system adversely is tobacco absence of phase-transition nuclei in the particles. Thus,
smoke. The chemicals in tobacco smoke include a num- extrinsic deviations are associated more with a lack of
ber of carcinogens, including nicotine and some of its control in the particle generation process than with any
derivatives, as well as poisonous gases including carbon fundamental cause. Intrinsic deviations may occur in sev-
monoxide and nitrogen dioxide. eral ways. One type of intrinsic deviation is associated
with the radius of curvature of small particles. For ex-
ample, it is well known that a liquid droplet with a given
II. PHYSICAL AND CHEMICAL radius has a higher vapor pressure than that found with an-
PROPERTIES other droplet of the same composition but of larger radius.
For sufficiently small particles, another type of intrinsic
Thegasesinwhichparticlesaresuspendedretaintheirnor- deviation may occur. Here, the intermolecular energy of
mal physical and chemical properties, taking into account interaction of molecules making up a very small particle
exchange processes with the particles. The suspended par- is altered by the fact that a given molecule does not inter-
ticles have properties that correspond to condensed mate- act with an extremely large number of other molecules,
rial. These include their surface, volume, mass, mass den- but instead can interact only with a limited number within
sity, surface energy, freezing point (if liquid), heat of va- the particle. Furthermore, if the aerosol particle is still
porization or sublimation, solubility, heat of adsorption for smaller, almost molecular size of order 0.01 µm, in the
gases, vapor pressure, viscosity or elastic properties, ther- “nanoparticle” range, molecular fluctuations will be so
mal conductivity, diffusivity of components, magnetic and large that it is probably no longer meaningful to speak of
electric properties, dielectric constant index of refraction, thephysicalpropertiesofaparticleofsuchsmallsizeinthe
chemical reactivity, radioactivity, and momentum and en- usual macroscopic sense. Examples of “near-molecular”
ergy properties. behavior in nano-particle formed ceramics is super plastic
behavior. Band gap energy levels in semiconductors are
also increased by quantum effects.
A. Critical Physical Properties
One type of extrinsic deviation is found in the lowering
Perhaps the properties most critical physically to aerosol of the freezing point or the raising of the boiling point
particles are those related to size and distribution of size for small liquid droplets from that for the bulk state. Such
in a cloud. effects are usually attributed to the absence of phase transi-
It is generally assumed that the substances making up tion nuclei. The absence of such nuclei stems from the fact
the aerosol particle possess the same characteristics as the that the bulk material from which the aerosol particles are
substance in macroscopic amounts, which is termed the formed probably contains only minute traces of foreign
bulk state. Then the various physical properties of the par- material (nuclei) per unit volume, so that there is only a
ticle, such as those mentioned above, are either known very small probability that any small aerosol particle will
or easily determined by standard techniques. These tech- contain even one nucleus. This circumstance results in the
niques generally do not involve direct measurements on situation that nearly all aerosol particles formed by va-
aerosol particles. The assumption of bulk state behavior, por condensation and subsequent cooling well below the
however, may not be justified generally for small aerosol melting point of the parent material are likely to be in a