(395x) Molecular Simulation Study of Argon and Krypton Confined Inside Slit Pores

Molecular simulation study of Argon and Krypton
confined inside slit pores.

Angan
Sengupta^* and Jhumpa
Adhikari⁺

Department of Chemical
Engineering, Indian Institute of Technology, Bombay.

Mumbai ? 400076. India.

Grand
canonical Monte Carlo (GCMC) simulations have been used extensively in recent
years to study the adsorption and the phase behaviour of confined fluids inside
various pore geometries.  These molecular
simulation studies enable an easy fundamental understanding of the distinct
behaviour of a confined fluid which arises due to the presence of wall-fluid
interactions in addition to the fluid-fluid interactions. We have investigated
two mono-atomic fluids, viz. Argon and Krypton, confined inside the simplest
pore model available, slit pore. The wall fluid interactions are given by
equation 1^[1].

                                                                                                                                                 [1]

The
fluid-fluid interactions in the system, have been modelled using a simple pair
potential energy function called the triangle-well (TW) model (equation 2)^[2], which is more realistic
than the square-well model; and being segmental in nature this potential is computationally
faster than the Lennard-Jones model. Parameters of
the TW model are: core diameter, σ_ff; well width, λ; and well depth, ε_ff.

                                                                                                                                                 [2]

These
parameter values for Argon and Krypton have been obtained by fitting the
expression for the second virial coefficient determined
from the Mayer's equation with the experimental data reported in literature^[3,4]. Verification of the
parameters have been done by performing the isothermal-isobaric ensemble
simulations and determining the densities of both the fluids over a range of
temperatures and pressures which were found to be in excellent agreement with the
experimental data^[5,6].

In
the present work, GCMC simulations are carried out inside slit pores of heights
9 Å and 15 Å at 246 K and 300 K. The adsorption isotherms follow the same
trends as reported by Hung and co-workers^[7]
from experiments as well as simulations, where Ar  and Kr (modelled as LJ fluid) was confined
inside meso-porous silica. For a given slit height,
the change in the densities of both Ar and Kr; indicates a vapour ? liquid
phase transition (a discontinuity in the adsorption isotherms) at higher values
of ε_wf.
The density changes continuously at relatively lower values of ε_wf,
including for ε_wf
= 0 (hard wall pores). The value of ε_wf above which this sudden change from
vapour-like to liquid-like densities is observed depends on the type of fluid
(Ar or Kr), the temperature and the pore volume.  The density profiles show layering behaviour
to occur perpendicular to the pore walls at progressively lower pressures as the
slit height increases, at a given temperature. Also, noted is that the number
of layers for Ar is less than that for Kr at the same temperature, pressure and
pore height, though
. Inside the wider pores, enhanced adsorption increases the number
of layers for both the confined fluids. Moreover, layering is also observed
inside the hard wall pores at relatively higher pressures. Thus, the complex
behaviour (layering and phase change during adsorption) of confined Ar and Kr, due
to the effects of temperature, pressure, pore attractiveness and slit height are
duly addressed using molecular simulation with two of the simplest potential models
available in literature.

References:

1. Hamada, Y.; Koga, K. and Tanaka, H. Phase equilibria and interfacial tension of fluids confined in narrow pores. Journal of Chemical Physics, 127, 084908, 1 ? 9. (2007).

2.     
Feinberg, M. J. and Rocco, A. G. De.
Intermolecular forces: the triangle well and some comparisons with the square
well and Lennard-Jones. Journal of Chemical Physics, 41, 11, 3439 ? 3450. (1964).

3.     
Byrne, M. A., Jones, M. R. and Staveley, L. A. K. Second virial
coefficients of Argon, Krypton and Methane and their binary mixtures at low temperatures.
Transactions of the Faraday Society,
64, 1747 ? 1756. (1967).

4.     
Sevast'yanov,
R. M. and Chernyavskaya, R. A. Virial
coefficients of neon, argon, and krypton at temperatures up to 3000 K. Translated from Inzhenerno-Fizicheskii
Zhurnal, 52, 6, 974 ? 977. (1986).

5.     
Van, I.
A. and Verbeke, O. Density of liquid Nitrogen and
Argon as a function of pressure and temperature. Journal of Physica, 26, 931 ? 938.
(1960).

6.     
Trappeniers,
N. J.; Wassenaar, T. and Wolkers,
G. J. Isotherms and thermodynamic properties of Krypton at temperatures between
0^o and 150^oC and at densities up to 620 amagat. Journal of Physica, 32, 1503 ? 1520. (1966).

7.     
Hung, R. F.; Bhattacharya S.; Coasne, B.; Thommes, M. and Gubbins, K. E. Argon and Krypton adsorption on templated meso-porous silicas: molecular simulation and experiment. Journal of Adsorption, 13, 425 ? 437.
(2007).

^*
Presenting
author. E-mail address: angan.sengupta@gmail.com