ADSORPTION OF
GASES IN NANOSTRUCTURES
When a vapor is exposed to an adsorbent substrate, like a pore or a
surface, the molecules are attracted to the atoms in the substrate and “stick” to
it. When the molecules in the vapor and the adsorbent material are not reactive
(there is not a chemical bond formed) the attraction is only physical, or physisorption, by means of the van der
Waals forces. This is the case for example of the rare gases.
If the adsorption occurs on a surface, two situations may happen, wetting
or non-wetting, depending on the very delicate balance between cohersive and adhesive forces and the temperature. In many cases the vapor does not wet the
surface when it’s cold and it wets at a higher temperature. Then we are in the
presence of a “wetting transition” at a characteristic “wetting
temperature”. For example, theoretical
predictions indicate that Helium (the least interactive element) would have a wetting transition on Cesium at 2K and on Rubidium at a
temperature ~1.4 K, although confirmation has not been found experimentally yet.
Another more natural example is water, that
doesn't wet many surfaces at room temperature. For example, over graphite, it's
supposed to have a wetting transition that however has never been observed. We
estimated the critical temperature of this transition to be 474 K.
If
the molecules are adsorbed inside a pore, a “capillary condensation” phenomenon
may be observed, that is, condensation inside the pore below the saturated
vapor conditions. As well as for wetting, this depends on the temperature and
how attractive is the interaction but also on the size of the pores
relative to the adsorbate. For large pores, a thin
film would cover the interior of the pores before the condensation happens. For
small pores, one dimensional phase appears at the center of the pore. Similar
phenomena is observed in slabs.
link
to my articles about capillary condensation
A particularly interesting substrate is a bundle of Carbon Nanotubes. This material that consists of many parallel nanotubes has different sites of adsorption and offers the possibility of finding new phases and dimensionality crossover.
The interstitial channels or the interior of
the tubes are very long tubes that accept one-dimensional phases and eventually
capillary
condensation.
However, due to the heterogeneity of the bundle, the effective
dimensionality is higher with dramatic consequences in the properties of the adsorbate. For example, we predicted that if Helium or Hydrogen are adsorbed
a Bose Einstein Condensation transition, that would not be be possible in 1D or 2D, may occur.
The adsorption in the exterior provides very
exciting possibilities, from one-dimensional phases to 3-dimensional as long as
the coverage increases. The low coverage regime shows 1-D adsorption only in
the “grooves” between every two tubes. At higher
coverage, a monolayer is formed followed by multiple layers as in the case of a
flat surface.
link
to my articles about adsorption in carbon nanotubes