(614h) Phase Equilibria of Triangle-Well Fluids Confined inside Slit Pores: A Transition Matrix Monte Carlo Simulation Study

The effect of confinement on fluid phase behaviour is the focus of this work as confined fluids occur abundantly in nature (for example, fluids in porous rocks, at grain boundaries throughout the Earth’s crust, inside clays) and also find extensive industrial applications (such as lubricating fluid layers, adhesion, chromatography, enhanced oil recovery, membrane technology, porous catalytic materials, micro-fluidic devices, gas separation, purification, etc.). Computer simulation procedures are essential to study systems confined within narrow pores because performing experiments may not be practical in such cases. Thus, we have employed the grand canonical transition matrix Monte Carlo simulation technique [1], which is a powerful, computationally efficient and cost-effective tool, to examine how individual factors which characterize a pore (such as pore size and pore – fluid interactions) influence the phase behaviour of a confined fluid and have contrasted this behaviour with that predicted for the bulk TW fluids.

The novel features observed in the phase diagrams of confined fluids arise due to the complex interplay between fluid-fluid and pore-fluid interactions. To obtain a qualitative understanding of the effect of the pore characteristics on the fluid phase behaviour of confined fluids, we have assumed our model systems to be as simple as possible: the fluid-fluid interactions are modelled using the triangle-well (TW) potential [2] and the pore is modelled as a slit pore characterized by hard planar walls (without any atomistic features) with or without uniform attractive potential,[3] or with a square-well (SW) type [4] wall-fluid interaction. The TW potential is chosen to model the fluid-fluid interactions as this model provides a good compromise between realism and simplicity. As Hamada and co-workers [3] have noted, while more complex and realistic models provide data which is comparable to experiments; simpler and qualitatively accurate models are more suitable for our purpose of investigating the effects of the basic characteristics of a pore on the phase behaviour of confined TW fluids with variable well-width (λ_ff= 1.5, 1.75 and 2.0). For the bulk system, we observe that as the TW potential range (λ_ff) increases, the vapour-liquid coexistence curves shift towards a higher temperature range with the critical temperature and pressure increasing, and the critical density values decreasing. From the results of our simulations of the confined system, we find that the phase envelopes of the confined TW fluids progressively shift to a higher coexistence temperature range as the pore-fluid interactions change from pure confinement to SW type pores and finally to uniformly attractive pores of the same height. The vapour-liquid coexistence curves of the confined TW fluids under pure confinement are always found to be within the phase envelope of the corresponding bulk phase TW fluid (for all values of and slit height considered in this study). Thus, from this study, we note the anticipated shift in the coexistence temperature range inside the attractive pores (i.e. uniformly attractive pores and SW type pores) above that of the coexistence temperature range of fluids under pure confinement. Also, the phase envelopes of the TW fluids under pure confinement are within the corresponding bulk phase envelope. The critical temperature of the fluid inside the pore increases with an increase in slit height and with increasing range of attraction of the TW potential, irrespective of the type of pore-fluid interactions. Unlike the critical temperature values of the confined TW fluids, the critical density behaviour shows significant variation depending on the type of pore-fluid interactions considered. We find that for a TW fluid of given potential range (λ_ff), the critical density under pure confinement decreases as the slit height increases; whereas inside the SW type pores and inside the uniformly attractive pores, the critical density value increases with an increase in slit height. The critical density value inside the pore with a given slit height shows a decrease with an increase in of the TW fluid, irrespective of the type of pore-fluid interactions considered. These trends observed for the values of the critical temperature and the critical density of a confined TW fluid in a pore of constant slit height with a change in the value are consistent with those in the bulk phase TW fluid . The effect of confinement on the phase behaviour of the TW fluids is further explained by determining the density profiles along the axis perpendicular to the walls of the slit pore, which show variation with pore characteristics as well as the temperature conditions. Presently, our group is in the process of extending this work to the study of binary mixtures of TW fluids under confinement in weakly attractive TW pores.

References

[1] J.R. Errington, Phys. Rev. E, 2003. 67: p. 012102 (4 pages)

[2] T. Nagamiya, Proc. Phys.-Math. Soc. Japan, 1940. 22: p. 705 – 720

[3] Y. Hamada, K. Koga, H. Tanaka, J. Chem. Phys. 127 (2007) 084908 (10 pages)

[4] T.W. Rosch, J.R. Errington, J. Phys. Chem. B. 112 (2008) 14911–14919