(49c) Modeling Adsorption in Complex Structures: Use of Finely-Discretized Lattice-Gas DFT to Study the Effects of Pore Length, Shape, and Surface Roughness On Adsorption of Simple Gases

It has been long
understood that pore surface geometry, pore length, and adsorbent
material structure play a large role in determining the
characteristics of gas adsorption and capillary phenomena in porous
materials. Computational models of gas adsorption often involve
idealizations that fail to accurately capture experimentally-observed
behavior. Density functional theory (DFT) is often used to model
adsorption of simple fluids in porous materials, but practically is
limited to idealized pores. Conventional DFT requires a very fine
discretization of the fluid density distribution to be both accurate
and numerically stable[1]. The resultant computational cost precludes
the use of DFT in systems with multiple directions of asymmetry.
Consequently, conventional DFT is not a practical tool for studying
finite-length pores or materials with explicitly rough surfaces.

We have developed a
version of nonlocal DFT applicable to finely-discretized lattice
fluids that retains elements of both the coarse-grained DFT of
Kierlik et al.[2-4] and off-lattice conventional DFT[5]. This DFT is
well-suited to the study of fluids in asymmetric materials and
small-scaled confinement that cannot be practically modeled with
either conventional DFT or coarse-grained lattice-gas DFT.
Additionally, it can quantitatively model light gas adsorption in
atomistic model of molecular adsorbents and properly describe
hysteresis in finite-length pores.

We apply this
finely-discretized lattice DFT to study the effect of various pore
geometries on adsorption in porous materials. We begin by revisiting
the problem of open-ended finite-length pores and discuss desorption
hysteresis, pore neck effects, and the oft-asked question of how long
a pore need be to be treated as "infinite." Following, we study
adsorption in pores with features not present in idealized slits and
cylinders, such as closed ends, variable-diameters, and beveled
openings. Finally, we go on to study adsorption in pores with rough
surfaces and examine the effect of surface roughness on monolayer
formation and structure.

[1]
Frink and Salinger, J. Comp. Phys., 159,
407 (2000)

[2]
Kierlik et al., Phys. Rev. Lett., 74,
4361 (1995)

[3]
Kierlik et al., Mol. Phys., 95,
341 (1998)

[4]
Kierlik et al., Phys. Rev. Lett., 87,
055701 (2001)

[5] Siderius and Gelb,
Langmuir, 25, 1296 (2009)