Solid Surfaces—Adsorption - Surface and Colloid Chemistry: Principles and Applications - page 122

adsorption, the amount of dye in solution is measured by any convenient spectro-
scopic method (VIS or UV or fluorescence).
5.6.2 S of l I d S u r f a c e a r e a d e T e r m I n a T I o n
In all applications where finely divided powders are used (such as talcum, cement, char-
coal powder), the property of these will depend mainly on the surface area per gram
(varying from a few square meters [talcum] to over 1000 m 2 /g [charcoal]). For example,
if one needs to use charcoal to remove some chemical (such as coloring substances)
from wastewater, then it is necessary to know the amount of absorbent needed to fulfill
the process. In other words, if a 1000 m 2 area is needed for adsorption when using
charcoal, then 1 g of solid will be required. In fact, under normal conditions, swallow-
ing charcoal would be considered dangerous because it would lead to the removal of
essential substances from the stomach lining (such as lipids and proteins).
The estimation of the surface area of finely divided solid particles from solu-
tion adsorption studies is subject to many of the same considerations as in the
case of gas adsorption, but with the added complication that larger molecules
are involved whose surface orientation and pore penetrability may be uncertain.
A first condition is that a definite adsorption model is obeyed, which in prac-
tice means that area determination data are valid within the simple Langmuir
Equation 5.23 relation. The constant rate is found, for example, from a plot of the
data, according to Equation 5.23, and the specific surface area then follows from
Equations 5.21 and 5.22. The surface area of the adsorbent is generally found eas-
ily in the literature.
In the case of gas adsorption where the BET method is used, it is reasonable to
select the van der Waals area of the adsorbate molecule; moreover, being small or
even monoatomic, surface orientation is not a major problem. In the case of adsorp-
tion from solution, however, the adsorption may be chemisorption.
In the literature, fatty acid adsorption has been used for surface area estimation
since fatty acids are known to pack perpendicular to the surface (self-assembly
monolayer formation) and without the close-packed area per molecule of 20.5 Å 2 .
This seems to be true for adsorption on such diverse solids as carbon black and not
too electropositive m and for TiO 2 . In all of these cases, the adsorption is probably
chemisorption in involving hydrogen bonding or actual salt formation with surface
oxygen. Even polar solvents are used to avoid multilayer formation on top of the
first layer, the apparent area obtained may vary with the solvent used. In the case
of stearic acid on a graphitized carbon surface, Graphon, the adsorption, while
obeying the Langmuir equation, appears to be physical, with the molecules flat on
the surface.
In another example, the adsorption of surfactants on polycarbonate indicated that,
depending on the surfactant and concentration, the adsorbed molecules might be
lying flat on the surface perpendicular to it, or might form a bilayer.
A second widely used class of adsorbates is that of dyes. Methods using these are
appealing because of the ease with which analysis may be made colorimetrically.
The adsorption generally follows the Langmuir equation. Graham found an appar-
ent molecular area of 19.7 Å 2 for methane blue on Graphon or larger than the actual