Biomedical Engineering Reference
In-Depth Information
sorption monolayer on the interbundle (interface) surface agrees
with the carbohydride value (in the order of magnitude).
When the initial single-wall nanotube samples have been
saturated with hydrogen at higher pressures (from about 2 to
7 MPa; curve 1 in Fig. 2.16), an anomalous increase (with respect
to the Henry−Langmuir isotherm) of the adsorbate concentration
has been registered, reaching (H
/C) ≈ 0.52 (8.0 wt%) at 7 MPa. It
corresponds to the local concentration (H
2
s
) ≈ 4.7 (Eq. (2.33))
and 10 times higher than the carbohydride value. In the next rounds
of hydrogen saturation of single-wall nanotube samples with
pressure increasing from about 2 to 12 MPa (curves 3 in Fig. 2.16),
the anomalous increase of the adsorbate concentration up to (H
/C
exp
2
/
C) ≈ 0.47 (7.3 wt%) has also been registered, corresponding to the
anomalous value (H
2
s
) ≈ 4.3.
In their work [77], the authors assumed that in their single-wall
nanotube samples at about 2-4 MPa, the interbundle surfaces of
the nanotube bundles (Eq. (2.36) and Fig. 2.2) has been primarily
filled with adsorbate (within a monolayer). They also assumed
that at higher pressures (4−12 MPa), the adsorbate (the sorption
monolayer) structure undergoes a first-order phase transition, which
makes the bundles “fall apart,” so that only individual nanotubes
are present, with the physical (monolayer) adsorption of hydrogen
appearing on their surfaces (
/C
2
exp
th
th
, [29]). Using this model, the
indirect experimental value for the cohesion energy has been found
to be about 0.5 kJ mol
S
or
S
ext
tot
(C) for single-wall nanotubes in the bundles.
It has been assumed that the “falling apart” of bundles (with the
nanotubes becoming independent of each other) is initiated by the
high pressure of gaseous H
−1
(> 4 MPa) in the course of the transition
phase in the adsorbate structure at 80 K.
The authors of Ref. [77] noted that their experimental value of
hydrogen sorption for single-wall nanotubes ((H
2
/C) ≈ 0.47 at 11.7
MPa) corresponds to the maximum possible value of the adsorbate
concentration for any adsorbing carbon material, within the sorption
monolayer model. The absence of the maximum (Langmuir)
saturation on the sorption isotherms for their single-wall nanotube
samples has been explained by the specific features of the kinetics of
successive bundle disintegration.
Figure 2.17 [74] shows the isotherm of hydrogen adsorption
by clean single-wall nanotubes (
2
exp
2
S
≈ 800 m
g
) and by samples of
−1
activated carbon (
S
exp
≈ 2800 m
2
g
), at 294 K and pressures up to
−1