Page 58 - Partition & Adsorption of Organic Contaminants in Environmental Systems

SURFACE AREAS OF SOLIDS 49

ensures that the adsorbate solubilization into the solid matrix is minimized.
Although N 2 vapor at its normal boiling point (ca. 77K) is used most fre-
quently as the adsorbate, the choice is by no means restricted to N 2, and
use of a wide variety of other inert vapor adsorbates (e.g., krypton) should
yield similar results. The surface area, considered as the solid–vapor or
solid–vacuum interfacial area, which is external to solid material, is assumed
to predate the experiment and to be unchanged by the experiment. The
surface area is therefore a property of the solid; that is, within the precision of
the measurement method, it should be independent of the choice of any suit-
able adsorbates used.
For highly porous solids, the term internal surface is frequently used to refer
to the surface associated with the walls of pores that have narrow openings,
which extend inward from the granule surface to the interior of the granule.
On the other hand, the term external surface is used to refer to the surface
from all prominences and those cracks that are wider than they are deep
(Gregg and Sing, 1982). It is understood that the internal surface is restricted
to open-ended pores and does not apply to sealed-off pores (i.e., those having
no openings to the exterior of the granule). Although these two kinds of sur-
faces are somewhat operational in their definitions, it is understood that the
internal surface is nonetheless external to the material and accessible to gases,
as is measured in surface-area determination. Thus, as long as the adsorbate
does not penetrate the field of force that exists between the atoms, ions, or
molecules inside the solid, it is considered to be on the external surface, despite
the fact that it may adsorb on the solid’s internal surface (Brunauer, 1945).
For a highly microporous solid such as activated carbon, one may then say that
the solid has a very high surface area, as determined by the BET method,
because it has a large internal surface.
It is unfortunate that the term internal surface practiced in soil science
literature gives a confusing implication to the surface area. The confusion
initiates from the use of the amounts of some polar solvents (e.g., water and
ethylene glycol) retained by a unit mass of the soil or mineral sample under
certain evacuating conditions for determining the total surface area of the
sample. By taking the surface area from the BET method using an inert gas
(e.g., N 2) as the external surface area, the difference between the thus deter-
mined total surface area and the external surface area is considered to be the
internal surface area of the sample. Evidently, the analytical method leading
to the internal surface area does not comply with the accepted criterion in
surface area determination, and consequently, this internal surface area is
mainly an artifact of the method. As mentioned earlier, appropriate adsor-
bates for surface-area determination must be chemically inert so that they
neither alter the structure of the solid nor penetrate the molecular network of
the solid. Because some polar solvents can potentially alter the solid structure,
such as by a solvation process with some clay minerals, or penetrate the soil
organic matter by dissolution, the resulting internal surface area is often a
measure of phenomena other than physical adsorption. As pointed out by