ABSTRACT LONG, YUN. Pressure Tensor of Adsorbate in ...

using the IK definition, but it is exactly the same as that obtained by the Harasima definition,
and is a constant due to the hydrostatic equilibrium (Sec. 3.2).
8.2.3 Results and Discussion
The normal pressure of adsorbate (argon) is a constant across the pore for a given pore width
and thus this value is also equal to the normal pressure acting on the wall. This normal
pressure usually oscillates between positive and negative as the empty pore width, H * e ,
increases due to oscillations in the average density of the adsorbate, as shown in Figure 8.2.
For example, for a pore width of H * e = 2.0 only two layers of argon can be accommodated,
and further increases in H * e only increases the distance between the two layers, which causes
the average density and the normal pressure to decrease, and places the adsorbate inside the
pore in a state of tension. However, at H e *
= 2.4 the pore becomes wide enough to
accommodate the formation of an additional layer of argon, which results in a rapid increase
in density and compression of the fluid in the normal direction. The density peaks at H * e =
2.5 when this additional layer is completely filled, and further increase in pore width leads to
another increase in interlayer distance and decrease in density and normal pressure, until
another additional layer of argon can start to form. Such oscillations in the normal pressure,
P N , are well known and are observed in surface force measurements [4, 5] as well as in
simulations and theoretical calculations [39]. It is worth noting that at a temperature as high
as 300.0 K, due to the large kinetic contribution (see Eqn. (3.15)), the normal pressure is
almost always positive, except for very small pores (H * e < 2.4), where the configurational
adsorbate-carbon contribution is very large.
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