Biomedical Engineering Reference
In-Depth Information
in the sorption activity of different sections of external and internal
surfaces of the adsorbent (see Fig. 2.2) and the fact that the adsorbent
molecules may or not reach these surfaces. Each carbon atom in a
defect-free graphene layer of a single-wall nanotube contributes
to the external and internal surfaces of the tube (
th
th
S
≈
S
≈ 1.3 ×
ext
int
3
2
-1
10
[29]).
We can assume that the presence of defects in graphene
tubular layers allows carbon atoms from defective areas to give an
additional contribution to the specific surface area of material. This
is corroborated by the data in Eletskii's review [29], which provides
experimental data on the specific surface areas for activated carbon
(
m
g
th
exp
for single-wall nanotubes by 15-27%. Such
excess could be caused by a high concentration of defects in the
carbon samples and/or systematic errors inherent in the common
method of determining
S
) exceeding
S
tot
exp
S
from the Brunauer-Emmett-Teller (BET)
adsorption isotherm for N
at 77 K.
At the same time, the experimental values for the specific
surface area of single-wall nanotubes are usually and considerably
smaller than the theoretical value
2
th
[9, 29]. This can be due to the
differences in the surface activity and in the access to different areas
of the external and internal surfaces by the adsorbent (see Fig. 2.2).
The fraction of the surface carbon atoms corresponding to
the experimental value for the specific surface area of bundles
constituted by open single-wall nanotubes, whose internal surfaces
can be reached by the adsorbate, can be estimated, as an order of
magnitude, as following:
S
tot
s
exp
C
S
exp
.
s
exp
g
(2.33)
th
tot
C
S
th
th
For closed single-wall nanotubes,
S
instead of
S
in Eq. (2.33)
ext
tot
must be used.
Nevertheless, at the present, the high absolute values (about
5-10 wt%,
/C ≈ 0.3-0.7) of hydrogen sorption by single-wall
nanotubes [26] at fairly low pressures and temperatures of hydrogen
saturation have not been confirmed by other researchers. This fact
could be due to the difficulty of accounting for and/or separating
different effects, as very small samples were used in Ref. [26],
containing only about 0.1-0.2 wt% of single-wall nanotubes.
Such systematic errors could be partially compensated by the
use of relative adsorbate concentrations
X
= H
2
θ = X
/
X
, with the maximum
m
(carbohydride) adsorbate concentration
X
equal to about 0.3-0.7.
m
