Biomedical Engineering Reference
In-Depth Information
centered) configuration, are the least thermodynamically favorable
and water adsorption is the most thermodynamically favorable
among the adsorption species tested. When the gas coverage is
doubled (Fig. 7.14b) the size dependence does not change, but
the thermodynamic stability of the different species is affected. At
low diameters the adsorption become more thermodynamically
favorable, especially in the case of water, but at large diameters it
becomes less thermodynamically favorable. The adsorption of O
or N is no longer stable at any size. When the coverage is increased
still further (Fig. 7.14c) the adsorption of H is only stable at very
small diameters ( less than 0.5 nm) or large diameters more than
1.6 nm, and the adsorption of water becomes even more sensitive
to the radius of curvature of the tube [58]. At 8% coverage, which is
still quite modest, adsorption of H is stable only for the narrowest of
CNTs, and adsorption of water becomes so efficient as to potentially
provide a new diameter-dependent separation technique: those
below 1 nm in diameter versus those above.
7.5.2
Humid Air
Using these results and the known chemical composition of air
the formation enthalpy can also be calculated for a range of CNTs
with different coverage of dry and humid air. These results, for
Θ = 2%, 4%, 6%, and 8% are shown in Fig. 7.14. A clear indication
from Fig. 7.14 is that a sparse coverage of water is always
thermodynamically favorable, even under ambient condition (with
respect to these alternative adsorbates tested), as the formation
enthalpy is always exothermic. If water molecules are present, the
interaction of water and the CNT is more likely than the interaction
of H, O, or N with the same structure. However, this is not the same as
immersing a CNT in liquid water, and is instead related to the
relative
humidity
, which can be directly translated to the relative fraction of
adsorbed water with respect to the dry air alternatives. Therefore,
results are shown in Fig. 7.15, for a range of Θ with 0% (dry air),
20%, 40%, 60%, and 80% relative humidity.
There are two important comparisons to consider when
assessing these results. Firstly, there is the direct comparison of the
air covered CNTs with the pristine counterparts. In general, these
results show that for diameters below ~0.7 nm the formation of CNTs
is enhanced by humid air, but this critical diameter decreases with
