Chemistry Reference
In-Depth Information
molecular area of 17.5 Å
2
, but apparent value for the more oxidized surface of Spheron
was about 10.5 Å
2
per molecule. However, this procedure assumes that the dye mol-
ecules are present as monomers in the solution. The fatty acid adsorption method
has been used by many investigators. Pyridine adsorption has been used on various
oxides to obtain surface areas. In the adsorption data that followed the Langmuir
equation, the effective molecular area of pyridine is about 24 Å
2
per molecule.
In the literature, many different approaches have been proposed for estimating
the surface area of a solid. Surface areas may be estimated from the exclusion of
like charged ions from a charged interface. This method is intriguing in that no
estimation of either site or molecular area is needed. In general, however, surface
area determination by means of solution adsorption studies, while convenient exper-
imentally, may not provide the most correct information. Nonetheless, if a solution
adsorption procedure has been standardized for a given system by means of inde-
pendent checks, it can be very useful determining relative areas of a series of similar
materials. In all cases, it is also more real as it is what happens in real life.
5.6.2.1 experimental method
Typical procedure used was to take 1.0 g of alumina (as an example) powder and add
10 mL of solution of detergent with varying concentration. The mixture was shaken,
and the concentration of detergent was estimated by a suitable method. It was found
that equilibrium was obtained after 2-4 h. The detergent such as dodecylammonium
chloride was found to adsorb 0.433 mM/g of alumina with a surface area of 55 m
2
/g.
The surface area of alumina, as determined from stearic acid adsorption (and using
the area/molecule of 21 Å
2
from monolayer), gave this value of 55 m
2
/g. These data
can be analyzed in more detail:
Surface area = 55 m
2
/g
Amount adsorbed = 0.433 mM/g
= 0.433 10
−3
M 6 10
23
molecules
= 0.433 10
20
molecules
Area/molecule = 55 10
4
cm
2
10
16
Å
2
/ 0.433 10
20
molecule
= 55/0.433 Å
2
= 127 Å
2
The adsorption isotherms obtained for various detergents showed a characteristic
feature that an equilibrium value was obtained when the concentration of deter-
gent was over critical micelle concentration (CMC). The adsorption was higher
at 40°C than at 20°C. However, the shapes of the adsorption curves was the same
(Birdi, 2002).
One can also calculate the amount of a small molecule, such as pyridine (mol. wt.
100), adsorbed as a monolayer on charcoal with 1000 m
2
/g. In the following, these
data are delineated:
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