Biomedical Engineering Reference
In-Depth Information
that the curves are quite close to each other, indicating a certain
similarity among the adsorption properties of the investigated
adsorbents. Some differences in the rate of sorption increase with
gas pressure on the initial part of isotherm curves for different
adsorbents is likely related to the differences in microporous
structures of the samples. Such structural differences, that can
be a consequence of different aggregation degree of the studied
samples, have been already noted for both fullerene [40, 41] and
astralene [29].
As a rule, the theory of volume filling of micropores developed
by Dubinin [19] is used to analyze the adsorption in the microporous
structure of the adsorbent or the process of volume filling of the
micropores. The Dubinin-Radushkevich equation was obtained
on the basis of this theory under the assumption of a Gaussian
distribution of adsorption potential:
2
(4.2)
W W
/ exp[-( / ) ]
E
0
a
0
where
ε
=
RT
ln(
P
/
P
),
W
is the adsorption at some
P
/
P
a
0
0
pressure, given in cm
3
of the normal liquid per adsorbent gram,
W
is the limiting adsorption value equal to the specific volume of
micropores,
0
P
is the vapor pressure of the saturated adsorbate;
0
E
is the characteristic energy of adsorption, increasing with the
average size of micropores decreasing, the
0
parameter describes
the adsorption properties of adsorbate used with respect to some
standard substance such as benzene at 293 K or nitrogen at 77 K.
The
β
[22]. Calculations
by the Dubinin-Radushkevich equation linearize the experimental
adsorption isotherm as
β
value is 0.33 for N
and 0.324 for O
2
2
ln( ) ln( )-[( / ) × (ln / ) ]
W
W
RT
E
2
P
P
2
(4.3)
0
0
0
The linearized nitrogen adsorption isotherms at 77 K for the
three types of adsorbents (activated carbon, fullerene C
, and
60
astralene) are shown in Fig. 4.11. The
-axis intercept, obtained
by extrapolating the linear part of the plot, is assumed equal to the
volume of micropores
y
is the density of adsorbate,
equal to the density of normal liquid at the test temperature.
The slope of the plots corresponds to (
W
=
a
/ρ
g
where
ρ
g
0
0
,
2
2
RT
/
βE
)
(ln(
p
/p
))
,
0
0
which allows
to be calculated. Figure 4.11 shows that the curves
of adsorption isotherms obtained for fullerene and astralene are
characterized by the presence of kinks.
βE
0
