BASIC CONCEPTS AND AN OVERVIEW - Foundations of Nanotechnology: Pore Size in Carbon-Based Nano-Adsorbents - page 138

1.3.2.8 DUAL-CONTROL-VOLUME GRAND CANONICAL
MOLECULAR DYNAMIC (DCV-GCMD) METHOD FOR PORE SIZE
EFFECTS ANALYSIS ON DIFFUSION IN SWCNTS
Dual-control-volume grand canonical molecular dynamics simulations were used
to study the diffusion mechanisms of counter diffusing CH/CF 4 mixtures in cylin-
drical model Pores. It was found that in the Pores two different diffusion mecha-
nisms occur independently of each other. Therefore, the pores were divided for
purposes of analysis into two regions,a wall region close to the pore wall where
most of the fluid molecules are located and an inner region where fewer mol-
ecules are Present but from where the main contribution to the flux comes. The
dependence of the transport diffusion coefficients on pore radius and temperature
were analyzed separately for these two regions. The varying contributions from
fluid-fluid and fluid-wall collisions to the diffusion mechanism could be dem-
onstrated. Whereas in the wall region surface diffusion takes Place, in the inner
region diffusion occurs in the transition regime between molecular and Knudsen
diffusion. Depending on different factors such as the pore size, or the tempera-
ture different diffusion mechanisms apply. There exist several simulation stud-
ies where these different diffusion mechanisms are investigated by Monte Carlo
(MC) methods, by equilibrium molecular dynamics (EMD) simulations, and non-
equilibrium molecular dynamics (NEMD) simulations. Several reviews exist for
molecular simulation of diffusion in zeolites. A simulation method that allows the
direct simulation of transport diffusion is the dual-control-volume grand canoni-
cal molecular dynamic (DCV-GCMD) method. Here, the chemical potential in
two control volumes is kept constant by periodically performing grand canonical
Monte Carlo (GCMC) insertions and deletions. BY assigning two different val-
ues to the chemical potential in the control volumes, a gradient in the chemical
potential, the driving force for transport diffusion can be established. The move-
ment of the fluid molecules is described by MD steps where Newton's equations
of motion are integrated to get a physical description of the particle movement. In
general, diffusion mechanisms are classified according to the interactions of the
fluid molecules with the pore wall. If the pore diameter is large in comparison to
the mean free path of the fluid molecules, collisions between diffusing molecules
occur far more than between the molecules and the Dore walls. The diffusion
mechanism is the same as in the bulk and is called molecular diffusion. DCV-
GCMD simulations of counter diffusing CH 4 /CF 4 mixtures in cylindrical model
pores have been carried out. The diffusion mechanisms governing transport in
these pores with different radii and at different temperatures. The outline of this
paper is as follows. In the following section, the simulation methods as well as the
fluid model and the Dore model are introduced. In the third section, the simulation
results are presented and analyzed in order to determine the underlying diffusion
mechanism. DCV-GCMD simulations of binary CH 4 /CF 4 mixtures were carried
out in the cylindrical model. Model pore used in simulations. Each pore consists