The Nature of Fluid Flow - Fluid Properties at Nano/Meso Scale: A Numerical Treatment - page 39

molecules in the system is increased (hence increasing the density of the fluid).
This axial phase transition operates in a similar way to capillary condensation and
layering transition. The layering transitions are known to occur at higher temper-
ature, but the one-dimensionality of the system limits the transition to occur at
T 0 ◦ C. It has also been shown that a bundle of adsorbing tubes exhibits corre-

lation effects, which raises the transition temperature above zero [30, 34].
The work of Radhakrishnan and Gubbins [19] agrees with the above discussion
of confined phase change, but applied to slit shaped pores. As with the cylindrical
pores, fluids confined within slit shaped pores showed strong evidence of a third
phase close to the walls. They investigated the effect of the wall-fluid interaction
strength on the phase change, varying it from strongly attractive to repulsive, with
respect to the fluid-fluid interaction strength.
Previous work by Miyahara and Gubbins [35] had already found that the
strength of the interaction affects the hysteresis loop of the freezing temperature
relative to the bulk material. However, Maddox and Gubbins [36] also found that
the reduced confinement of the fluid in slit pores, as opposed to cylindrical pores,
leads to higher freezing temperatures.
The study found that for strongly attractive walls the layer of particles near-
est the wall froze at a higher temperature than those in the middle of the pore,
similar to many of the examples described above for cylindrical pores. However,
as the interaction swings the other way, becoming repulsive, the freezing effect
also switches so that the centre of the pore freezes before the layer in contact with
the walls. This implies that there must be a level of attraction or repulsion where
the fluid freezes at one temperature, making the intermediate shell phase meta-
stable, or disappear completely. The attractive/repulsive interaction potentials at
the walls represent the difference between graphite carbon/silica walls, as carbon
walls are strongly attractive and silica walls are weakly repulsive. However, most
silica-based porous materials have cylindrical pores.
Kim and Steele [37] also looked at phase change at solid boundaries, studying
the effect corrugation had on the monolayer of methane on graphite. Their small
scale simulations of 289 molecules showed that increased corrugation leads to
pre-transitional effects that are not present in solidification against smooth walls.

=

1.5.2 Adsorption/Desorption in Pores

Adsorption is the process by which a fluid adheres in a thin film to a solid or
liquid with which it has contact. As an example, the following discussion consid-
ers the effect of the conditions for filling and emptying of a silicate nanotube, as
studied by Gelb [38]. The first thing to remember is that classical statistical me-
chanics laws do not allow a first-order phase transition to take place within short-
range one-dimensional systems, even for the case of meso scale pores, despite
their three-dimensional structure. Bundles of pores or tubes add to the system's