Biomedical Engineering Reference
In-Depth Information
Also the activated carbon samples gave a Henry-Langmuir
adsorption isotherm (but less pronounced; see Fig. 12.7) with
maximum saturation (at pressures ≥35 MPa) of the order of
1.5 wt% [(H
≈ 0.09].
It corresponds to a relatively low reduced value of the sorption
capacity of about 0.5 wt% per 1000 m
/C)
2
m
2
1
. We can therefore assume
that only the sorption-active sections of the specific surface area
become saturated and, apparently, the percentage of these sections
is not higher than 50% of the experimental value.
The saturated local surface concentration in the single-wall
nanotube samples used in Ref. [74] at 30 MPa [(H
g
s
≈ 0.18] is
much lower than the carbohydride value (about 0.5). It differs from
the situation with single-wall nanotube samples in Ref. [77], where
a local adsorbate concentration on the interbundle surfaces reaches
the carbohydride value of (H
/C
)
exp
2
m
s
) ≈ 0.5 at 2-4 MPa and 80 K. As
the pressure grows to 12 MPa, it has been detected an anomalous
increase of hydrogen sorption, up to the values of a local adsorbate
concentration (H
/C
exp
2
s
), about 10 times higher than a carbohydride
value (see Fig. 2.16) [74].
In such a context, it is worth noticing that the adsorbate
concentration in single-wall nanotube samples in Ref. [72] at
77 K and 108 kPa reaches the value of (H
/C
exp
2
/C) ≈ 0.15 (Fig. 2.15b),
2
s
corresponding to the local concentration (H
) ≈ 0.76 (Eq. (2.33)),
which is higher than the carbohydride value. This is closer to the
situation of the single-wall samples studied in Ref. [77] than to that
of the samples studied in Ref. [74].
The process of anomalous hydrogen adsorption at 80 K and
pressures of about 2-12 MPa [77] can be related to the “intergrowth”
of a two-dimensional carbohydride phase from the interbundle
surfaces along the surface grooves depicted in Fig. 2.2 into the
inner regions of nanotube bundles, i.e., on the surface of constituent
nanotubes. This leads to disintegration of the bundles into separate
nanotubes coated by a carbohydride monolayer.
Another model (suggested by the results of the data [72]
analysis and Fig. 2.15) for such anomalous sorption may be related
to the physical multilayer adsorption (polylayer intercalation,
clusterization [31], and/or condensation [29, 30] of the adsorbate)
in the boundary nanoregions between the nanotube bundles,
occurring when suitable carbohydride concentrations on the bundle
surfaces are reached (the chemisorption stage).
/C
2
exp
