Page 101 - Partition & Adsorption of Organic Contaminants in Environmental Systems

92 ADSORPTION OF VAPORS ON MINERALS AND OTHER SOLIDS

large micropore volumes, and thus very high BET surface areas, in reflection
of their microporous structures. As expected, there is good positive correla-
tion between the measured BET areas and the micropore volumes of the
solids.

6.4 IMPROPER SURFACE-AREA MEASUREMENT

We now turn to a subject of special relevance to the surface-area determina-
tion by use of vapor data for natural solids. As stated earlier, a suitable adsor-
bate must be chemically inert, not subject to molecular sieving, and confined
only to the exterior of the solid. Whereas the molecular-sieving effect can be
realized more intuitively, the effect of adsorbate reaction and penetration into
a bulk solid often escapes our immediate detection. Thus, if the vapor-uptake
isotherm of an adsorbate, other than an inert vapor (e.g., N 2 or Kr), is to be
used, particularly at room temperature, the monolayer capacity obtained with
either the BET equation or other methods could deviate greatly from the BET
monolayer capacity of, say, N 2, measured at the liquid N 2 temperature. Exam-
ples from the soil science literature are: (1) the monolayer capacities of water
on clays based on the BET plot (Mooney et al., 1952); and (2) the monolayer
capacities of polar liquids, such as ethylene glycol (EG) (Dyal and Hendricks,
1950; Bower and Gschwend, 1952) or ethylene glycol monoethyl ether
(EGME) (Carter et al., 1965; Heilman et al., 1965; Eltantawy and Arnold,
1973), on clays and organic matter achieved under some evacuated conditions.
Although recently, the BET method with inert gases (e.g., N 2) has also fre-
quently been employed for surface-area determination in soil science, the ter-
minology that has emerged to distinguish the different surface-area values by
these different methods has become increasingly confusing, if not chaotic.
In soil science it has been assumed empirically that the amount of a polar
solvent retained by a clay mineral or even organic matter when the solvent–
solid mixture is evacuated to below a certain pressure over a certain length
of time represents an approximate monolayer-adsorption capacity on the
solid. By this estimated monolayer capacity and the molecular area of the
solvent, the total surface of the solid is then calculated. For distinction,
the surface area obtained by standard BET-N 2 method is considered to be the
external surface. The difference between the two is portrayed to be the inter-
nal surface of the solid. Since the internal surface as derived is clearly imper-
vious to inert gases (e.g., N 2 ), it deserves critical scrutiny. Although the same
term has long been used in allied surface science, the implied internal surface
there is nonetheless accessible to inert gases. As rationalized below, the inter-
nal surface adopted in soil science is largely an artifact of the calculation rather
than a true surface in that the extensive solvent penetration into bulk solids
is taken uncritically as formation of internal monolayers. In his classic book
published in 1945, Brunauer made the following insightful and pertinent com-
ments on the internal surface and external surface of a solid: