Biomedical Engineering Reference
In-Depth Information
conditions, the increasing with a temperature decrease of kinetic
diameters of the adsorbed molecules, and the decreasing rate of
diffusion into inner sample cavities can significantly influence the
sorption value.
Figure 4.6
Isotherms of adsorption of (1) nitrogen and (2) oxygen at 77 K on
activated carbon.
At the same time it should be noted that the deviation of the
isobars from the exponential trend occurs at temperature of
260 K. At this temperature, the crystalline structure of fullerene
changes from face-centered cubic into cubic primitive one [36-
38]. The isobars remain exponential but with a different exponent
at temperatures from 250 to 150 K. The sorption saturation on
temperature decrease below 150 K may be related to a further phase
transition [38], when fullerenes are partially transformed into the
amorphous state.
The structural state of samples of porous and highly
dispersed materials is usually investigated by nitrogen adsorption
measurements at 77 K. Experimental nitrogen adsorption isotherms
are used to determine characteristic specific surface area, volume,
and pore size, as well as the characteristic adsorption energy. That is
why we have measured the sorption capabilities of the investigated
adsorbents at 77 K for oxygen in order to compare its characteristics
with the corresponding values for nitrogen. The experimental
results are shown in Figs. 4.6-4.8. Nitrogen and oxygen saturated
vapor pressures
P
at temperature of 77 K are 760 and 170 torr,
0
respectively.
For the sake of clarity, the experimental adsorption isotherms
are reported in Figs. 4.6-4.8 using both absolute (
P
) and relative
