Biomedical Engineering Reference
In-Depth Information
This value characterizes adsorbate-adsorbent interaction. It should
be noted that a reliable determination of the adsorption heats
from isotherms can be possible only for physical adsorption. This
requirement is usually fairly satisfactorily fulfilled for the initial
isotherm portions.
Measurements of specific surface area of adsorbents are
usually based on the Brunauer-Emmett-Teller (BET) method,
i.e., on the measurement of adsorption isotherms portions in
the regions of mono- and polymolecular adsorption before the
beginning of capillary condensation (for instance, for N
at 77 K, the
2
measurements are usually performed for
from 0.05 to 0.35).
The data obtained are used to calculate values of adsorption,
p/p
0
a
,
m
corresponding to the monolayer sorptive capability in mol/g.
Then the specific surface area is calculated by the equation
A
is average
surface area per adsorbate molecule in the filled monolayer, and
=
a
ωΝ
, where
a
is determined by BET equation,
ω
BET
m
Α
m
N
A
2
is the Avogadro's number. Note that for N
ω
= 0.162 nm
, and for O
2
2
2
ω
. Although the BET method is widely criticized, the
difference between the
= 0.143 nm
values and the values obtained by other
methods does not exceed 20-30%. This allows us to consider the
BET method substantially acceptable for determination of
A
BET
a
and
m
A
from experimental isotherms.
BET
Figure 4.9
Calculated dependence of the adsorption heat on adsorption
value
for the experimental adsorption isotherms of oxygen
on fullerene shown in Fig. 4.4.
m
a
The data presented in Fig. 4.4 have been used in (1) to calculate
the isosteric heats of oxygen adsorption on fullerene at various
