(222aj) Molecular Simulation Study of Triangle Well Fluids Confined in Slit Pores

Molecular simulation study of triangle well fluids
confined in slit pores.

Angan
Sengupta^*, Pratik
Behera^# and Jhumpa
Adhikari⁺

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

Mumbai ? 400076. India.

Fluids
confined inside geometries of molecular scale dimensions, do not obey macroscopic
laws due to the pronounced wall-fluid interactions in addition to the
fluid-fluid interactions. Adsorption and phase behaviour of such confined
fluids are the result of the collective effect of the pore geometry, the
interactions between the fluid particles, and the interactions between the
fluid particles and the confining surfaces. Grand canonical Monte Carlo (GCMC)
simulations have been performed in this study, to distinguish the effect of
each of the above factors on adsorption and phase behaviour of confined fluids.
To achieve this, we have selected a simple triangle well (TW) (equation 1)^[1]fluid confined inside the
simplest pore model available, the slit pore.

                                                                                                                                                [1]

Parameters
of the TW model are: core diameter, σ_ff; well width, λ; and well depth, ε_ff.

The
wall fluid interactions in the slit pore are given by equation 2^[2].

                                                                                                                                                [2]

In
literature, studies using hard sphere^[3],
square-well^[4] and Lennard-Jones^[5]
(LJ) fluids have been reported which isolate the effect of the pore geometry,
fluid-fluid interactions and wall-fluid interactions on adsorption, phase
behaviour and surface tension. The TW potential has been used here as it is
more realistic than the square-well model; and being segmental in nature this potential
also enables us to study the effect of well-width (λ);
which is not a parameter in the LJ model. The effect of slit height (pore
geometry), TW potential width λ
(fluid-fluid interactions) and ε_wf (wall-fluid interactions) on the adsorption
and the phase behaviour of confined fluids is
determined in this study. Verification of the in-house codes has been done by
performing bulk-phase molecular simulations to obtain densities and pressures
which show an excellent agreement with data reported by Betancourt-Cardenas and
co-workers^[6].

The
adsorption isotherms for the confined TW fluids are determined using the GCMC
simulations inside slit pores of reduced heights, H^* = 5, 7 and 9 at reduced temperature, T^* = 1, 1.5 and 2 and within a range of reduced pressure, P^* = 0.01 to 0.8.
The adsorption isotherms follow the same trends as proposed by Evans et al.^[7] and also as reported by Balbuena and Gubbins^[5]
for LJ fluids confined inside narrow slit pores. As is known from experiments,
we also observe that the density inside a given slit pore decreases with
increase in temperature. For a given pore volume, the change in the densities
of the TW fluids (with different values of λ);
indicate a vapour ? liquid phase transition 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 TW potential
width (λ) and the reduced pore height (H^*). The density profiles
perpendicular to the pore walls show that layering occurs at progressively
lower pressures as the slit height increases at a given temperature in
attractive pores. Layering is also observed inside the hard wall pores, though
at a relatively higher pressure as compared to that in attractive pores. At
lower pressures the fluid particles are uniformly distributed inside the hard
wall pores. Also noted is that as λ
increases; the number of layers formed inside the attractive pores remain
unaffected, while the height of the peaks in the density profiles increase. With
slit height increasing, the number of layers is found to be more because of the
enhanced adsorption inside the pore. We have, thus, been able to observe the
effect of each factor separately on the complex behaviour of confined fluids
via molecular simulations.

References:

1.     
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).

2. Hamada, Y.; Koga, K. and Tanaka, H. Phase equilibria and interfacial tension of fluids confined in narrow pores. Journal of Chemical Physics, 127, 8, 084908, 1 ? 9. (2007).

3.     
Mittal,
J; Errington, J. R. and Truskett,
T. M. Does confining the hard-sphere fluid between hard walls change its
average properties? Journal of Chemical
Physics, 126, 24, 244708, 1 ? 8. (2007).

4.     
Jana, S; Singh, J. K. and Kwak, S. K. Vapour-liquid
critical and interfacial properties of square-well fluids in slit pores. Journal of Chemical Physics, 130, 21,
214707, 1 ? 7. (2009).

5.     
Bulbuena, P. B.
and Gubbins, K. E. Classification of adsorption
behavior: simple fluids in pores of slit-shaped geometry. Journal of Fluid Phase Equilibria, 76, 21 ? 35. (1992).

6.     
Betancourt-Cardenas, F. F.;
Galicia-Luna, L. A.; Benavides, A. L.; Ramirez, J. A. and Scholl-Paschinger, E. Thermodynamics of a long-range triangle-well
fluid. Journal of Molecular Physics,
106, 1, 113 ? 126. (2008).

7.     
Evans,
R.; Marconi, U. M. B. and Tarazona, P. Fluids in
narrow pores: adsorption, capillary condensation, and critical points. Journal of Chemical Physics, 84, 5, 2376
? 2399. (1985).

⁺ Corresponding author. E-mail address: adhikari@iitb.ac.in

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

^# E-mail
address:
pratik2005@gmail.com