Biomedical Engineering Reference
In-Depth Information
The data obtained in Refs. [94, 95] on the sorption capacity
and the kinetics of process
in GNF and single-wall nanotubes are
described quite well by the sorption isotherm, thermodynamic and
diffusion characteristics corresponding to chemisorption process II,
as done in the analytical studies [10, 97] for the experimental data
presented in Refs. [12, 14, 61, 96].
Process
β
(II) can be related (see item (6) in the above list of facts)
to the observed release of hydrocarbons by single-wall nanotubes
[95] subjected to prolonged heating, if compared to the duration of
thermal desorption heating carried out during multistage treatments
(three subsequent heat steps of 3 h each at 373, 473, and 673 K, in
vacuum condition) [12, 14, 61, 96]. It should be kept in mind that
the enthalpy −∆
β
(characteristic of chemisorption
process II) of desorption or detachment of two H atoms from a
C atom in a zigzag-like edge position (see Fig. 2.8, model H) is much
higher than the enthalpy
H
≈ 570 kJ mol
−1
(12)II
of detachment of a
C atom from the two nearest C atoms. This suggests that in a process
of desorption II, the hydrocarbons formation may dominate within
certain temperature and temporal ranges.
In such a context, it is worth noticing the experimental data
in Refs. [98, 99] on the release of CH
−
∆
H
≈
485 kJ mol
C-C
-1
and molecular hydrogen by
graphite materials, which can be interpreted as the manifestation
of chemisorption process II. We also note that the accompanying
release of hydrocarbons cannot manifest itself in chemisorption
process III. It is due to the fact that for this process, the characteristic
enthalpy -∆
4
-1
of desorption or detachment of one
hydrogen atom from a carbon atom in a grapheme layer (see Fig. 2.8,
model F
H
≈ 243 kJ mol
(3)III
*
) is three times lower than the enthalpy -3
/
2∆
H
≈
730
C-C
kJ mol
of detachment of a carbon atom from three nearest carbon
atoms in a graphene layer. A similar situation happens also in case of
chemisorption processes IV and I.
Based on the experimental facts reported above, as done in
Refs. [10, 17, 97] we can compare process
-1
in the GNF and single-
wall nanotube samples [94, 95], which process provides not less than
60% of the sorption capacity, with chemisorption process III, which
under specific conditions can be described by the Sievert's sorption
isotherm (Eq. (2.5a)). This, in particular, agrees with the item (1) of
the above list.
But this does not explain the fundamental fact (item (4)) of the
anomalous increase (up to 40%) in the interplanar spacing between
γ
