Micromeritics ASAP 2020 Confirm Operator's Manual page 417

ASAP 2020 Confirm
Figure F-2. Model Isotherm for Argon at 87.3 K in a 40 Å Slit in a Carbon Substrate
The profiles show the density distribution from one wall to the center of the slit; the other half
of the distribution is a mirror image of the profile shown.
As the pressure is first increased from zero, almost all the adsorbed atoms occupy a position
close to the surface.
• Inset a shows the profile corresponding to point a on the isotherm where the surface is
about half covered.
• At point b, the first layer is so full that it is more favorable for atoms to start a new
layer.
• At point c, a third layer is forming. Point c, for this size slit, is the critical pore-filling
pressure. In inset c, the profile shows the density decreasing to near zero (actually the
bulk gas density) at 4 or 5 molecular diameters from the surface.
• Inset d shows the profile converging on a density similar to that of bulk liquid argon in
the center of the pore, indicating a phase transition.
Note that the adsorption isotherms for pores larger than the one shown in Figure F-2 is iden-
tical up to point c. The lower branch of the isotherm simply continues to a higher pressure for
larger pores. This trend is illustrated in Figure F-3, where isotherms for some larger size pores
are shown. It is clear that pore size is uniquely characterized by a corresponding critical pore-
filling pressure. At large pore sizes, density functional theory produces results for the critical
filling pressures that are in good agreement with those produced by the Kelvin equation.
02-42811-01 - Mar 2011
Appendix F
F-5