Page 291 - Adsorption by Powders and Porous Solids

CHAPTER 9. ADSORPTION BY AWE CARBONS

Figure 9.27. Hydrogen-bonded structure of water in a slit-shaped pore (Carrott et al., 1991).

Furthermore, with at least some systems, it has been found that E and N do not vary
appreciably with temperature and therefore the temperature-invariance condition is
satisfied (Stoeckli et al., 1994b). On the other hand, so far it has not been possible to
define the exact meaning of these terms.
Recently, Iiyama et al. (1995) have obtained a series of distinctive X-ray diffrac-
tion patterns for water adsorbed in the 1.3 nm slit-shaped pores of a supermicro-
porous carbon. The diffraction patterns and derived electron distribution functions
appear to confirm the existence of some long-range order in the structure of the
adsorbed water. These interesting results are consistent with the fact that water can be
easily accommodated in narrow slit-shaped pores of carbon in contrast to the pore-
filling hydrophobicity of the tubular pores of Silicalite (Carroa et al., 1991). As indi-
cated in Figure 9.27, a thin slab of hydrogen-bonded water can be placed in a slit of
width greater than a0.5 nm with very little distortion of its structure. Moreover, we
can begin to see why the size of the hysteresis loop is dependent on pore size as well
as surface chemistry. It seems likely that the adsorbate in the central layer will
become more liquid-like as the pore width is increased. Therefore, the filling and
emptying of a very narrow pore occurs reversibly in one step, whereas a wider pore
is filled/emptied in two stages.
A novel method for determining the location of the primary water adsorbing sites
has been developed by Bailey et al., (1995). This approach involves the pre-adsorp-
tion of naphthalene, which was chosen because of its planar molecular shape and
immiscibility with water. With some activated carbons it was found that the growth
of the H-bonded water clusters was inhibited by the presence of naphthalene, while
in other cases there was very little effect. It was thought that sites in larger micro-
pores were prone to obstruction by the pre-adsorbed naphthalene. It is too early to
judge the success of this interesting approach, which may turn out to be a useful
alternative to pre-adsorption by n-nonane.

9.6. Immersion Microcalorimetry and Adsorption From Solution

9.6.1. Immersion microcalorimetry
Immersion calorimetry can be used to study either the surface chemistry or the
texture of active carbons. A sensitive Tian-Calvet microcalorimeter is adaptable for
either purpose, the main difference being in the choice of wetting liquids.