Elsevier

Fluid Phase Equilibria

Volume 510, 15 April 2020, 112495
Fluid Phase Equilibria

Liquid-liquid phase transition in simple Lennard-Jones nano-confined fluids

https://doi.org/10.1016/j.fluid.2020.112495Get rights and content

Abstract

In this study, the phase transition of confined argon in slit by Lennard-Jones (12-6) potential for fluids-fluids and walls-fluids interactions is investigated. The effects of temperature, pressure, and distance between the two walls on the phase transition were investigated by using the molecular dynamics simulation and LAMMPS software. It was found that wall distances had a significant effect on phase transition. Although the molecules were spherical and the walls structureless, liquid-liquid phase transition was observed when wall distances were approximately 3σ; that is, confinement had a determining role in phase transition in confined fluids.

Introduction

Recently, the behavior of confined fluids in the nanometer-size group of systems has been of interest to researchers because of abundant applications in the fields of nanoscience and nanotechnology. Therefore, it is important to know the properties of the confined fluids in nano-systems, which are significantly different from macroscopic fluids [1].

Thermodynamic properties of the confined fluids, because of the limitations of nanopores as well as interactions between the walls and particles are very different from those of macroscopic fluids. The effects of temperature and pressure on the confined fluids of argon and krypton in nanopores with poor adsorption and those with hard walls have been investigated [2]. Also, a similar study was conducted with amorphous silica walls using the Monte Carlo simulation [3].

So far, many studies have been carried out on fluids by molecular dynamics simulation [[4], [5], [6], [7], [8], [9]] and theoretical methods [[10], [11], [12], [13], [14], [15], [16], [17], [18], [19]] aimed at understanding how to change the phase diagram by confinement as well as at evaluating the confined fluid structure. The phase behaviors of vapor-solid, liquid-vapor, and liquid-solid with argon in graphite nanopores of sizes 1–8 nm were studied by using Monte Carlo simulation and meso-canonical ensemble [20].

A solid-solid phase transition occurs only in the solid phase and differs from crystallization and melting processes. By changing temperature or pressure, a crystalline solid can be transformed into another form of crystalline solid without entering the liquid phase, which results in the pleomorphic form of the material [21]. Changes in the triangular structure and square structure, as well as the solid-solid phase transition, were addressed by confining the Lennard-Jones soft sphere in nanopores using Monte Carlo simulation [20]. Then, by applying the DFT theory, solid-solid phase transition was investigated in triangular and square crystalline structures for thin layers with graphite surfaces and the obtained data were compared with the results of simulation [22,23]. Furthermore, by using Monte Carlo simulations in nanopores with wall distances of 2–6 times of molecular diameter, surveying the solid-liquid and solid-solid phase transitions, and considering various types of structures such as fcc and hcp, crystals and solids in this system were investigated and the results were made consistent by the Clapeyron equation for the nanopores [24].

Recently, experimental and simulation results showed crystal-like arrangements in the liquid phase. The theory of liquid states declares that the arrangement of atoms in the liquid is likely to be in the following two forms [25]:

  • -

    The major portion of the atoms appertain to a multiply linked district of the “perfect matter; ”

  • -

    The remainder of the atoms appertains to spot-like and linear defects, which are the complement of the “perfect matter” district with respect to the complete space.

The structure of the main region is almost crystalline and a change in the arrangement of atoms can lead to the formation of a liquid with different densities, which is referred to as liquid-liquid transition [25]. The liquid-liquid phase transition between two Lennard-Jones fluids, which can almost be dissolved, has been studied [26]. In addition to studies on liquid-liquid phase transition for water [27], some investigations have carried out into the liquid-liquid phase transition for water confined in nanopores of silica [28]. However, so far, liquid-liquid phase transitions have not been investigated for confined argon in the slit. Thus, this study has been conducted for confined argon in slit with amorphous walls to show that even in simple systems, the liquid-liquid phase transition is observable. This means that it does not occur only in complex systems.

In this work, we investigated the pressure and temperature effects on the confined fluid phase transition. In order to study liquid-liquid phase transition, the effect of distance between two walls was investigated on argon confined in a slit in the liquid phase.

Section snippets

Simulation method

There are not many theories to investigate the behavior of confined fluid because of their complexity [24], so using simulation to study these systems is usual. Today, due to advances in the computer technology, and while simulation methods are safe, controllable and low cost, relative to the experimental methods, are of great advantage. Usually, the results of the simulation are in agreement with the experimental results, which makes it a good way to test the systems. LAMMPS is one of the

Results and discussion

In order to investigate the effect of pressure on confined fluid phase transition, the density-temperature diagram was drawn in three distances of walls, namely H = 2.8σ, 3σ, and 3.2 with reduced pressures (in x and y direction) of 0.0016 (0.68 atm, which is the pressure of the triple point of the bulk), 0.024 (10 atm), 0.049 (20 atm), and 1.00 (400 atm) and reduced temperature range of 0.2–1.4 (24–165 K), as shown in three dimensions in Fig. 1. From Fig. 1, it can be deduced that a small

Conclusion

In this study, we investigated the effects of temperature, pressure, and distance between two walls on phase transition of confined argon in slit. An increase in temperature led to a change of density, causing a phase transition of confined argon in the slit. In the investigation into the effect of pressure and slit size, H, results showed that a small change in the distance between walls would lead to a tangible change in the phase transition at reduced pressures of 0.0016 (0.68 atm), 0.024

CRediT authorship contribution statement

Narjes Sheibani: Writing - original draft, Data curation. Mohammad Kamalvand: Writing - original draft, Data curation, Supervision.

Declaration of competing interest

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

Acknowledgment

The authors wish to thank the support from Yazd University, Iran.

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