Micromeritics ASAP 2460, Operator'S Manual

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ASAP 2460 Operator’s Manual Appendix F
246-42800-01 - Aug 2013 F-5
Figure
D
-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 D-2 is identical 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 D-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 agree-
ment with those produced by the Kelvin equation.