Page 232 - Adsorption by Powders and Porous Solids

C~pPTER 8. ASSESSMENT OF MICROPOROSITY 22 1

temperature. As we saw in Chapter 7, for the condensate in a mesoporous adsorbent
this assumption 0.e. the Gurvich rule) appears to be justified. The situation is quite
different, however, with a microporous material - particularly when the pore
dimensions are in the ultramicropore range.
Studies of the packing of molecules in cylindrical and slit-shaped pores have
the ~mportance of both the width and the shape of narrow pores (Carrott
,tal., 1987; Balbuena and Gubbins, 1994). An indication of the effect of pore size on
fie packing density of spherical molecules is given in Figure 8.2. Here, the degree of
in cylinders and slits is expressed as a percentage of the packing density in
fie corresponding close-packed state. Although this is an oversimplified picture
since it does not allow for the adsorption forces, it does illustrate the difficulty of
arriving at an unambiguous assessment of the accessible pore volume. Inspection of

TOP

Side

wid-3.00
packing density = 79% packing density = 44%

loo
packing density
slit

LO
(pore width) 1 (molecular diameter)
I I 1 I 1 I i I
1 2 3 L 5 6 7 8 9 1 0
Figure 8.2. Packing of spherical molecules in narrow cylinders and slits.