On the phase and interface behavior along the three-phase line of ternary Lennard-Jones mixtures: A collaborative approach based on square gradient theory and molecular dynamics simulations

Abstract

This work focuses on the application of a two-way approach, where Molecular Dynamics (MD) simulations and the Square Gradient Theory (SGT) have been used for describing the phase and interface behavior of binary and ternary Lennard-Jones (LJ) mixtures, along a condition of three-phase equilibrium. The unequivocal correspondence between MD and SGT has been achieved by using the global phase diagram of binary mixtures composed by equally sized Lennard-Jones molecules, from which representative molecular parameters for Type-I, Type-II, and Type-III systems have been determined. The so selected binaries have been used then to scale the behavior of a ternary mixture characterized by complex phase equilibrium patterns. For the case of the theoretical SGT approach applied to the Lennard-Jones equation of state was used for predicting phase equilibrium and interfacial properties. In addition the corresponding MD simulations of these macroscopic properties have been conducted for the LJ potential by using equivalent molecular parameters and conditions than in the theoretical approach. Excellent agreement has been observed between the predictions obtained from theory and simulations. Particularly, our results concerning the characterization of the three phase line of a binary Type-III mixture indicate that the bulk liquid (α) and the bulk gas (G) regions are sharply separated by a bulk liquid region (β) for all the explored temperature, pressure, and concentration conditions. The structural analysis of these bulk phases reveals that a secondary liquid phase (β) perfectly wets the liquid-gas interface (α-G), as previously found for Type-II mixture [A. Mejía and L. F. Vega, J. Chem. Phys. 124, 244505 (2006)]. The exploration along the three-phase line for the ternary mixture shows good agreement between SGT and MD. Particularly, we observed the specific influence of a third component in the phase and interface behavior. From all the previous results, we conclude that the SGT applied to an EoS with appropriate mixing rules produces reliable predictions of the properties of ternary mixtures.

Probing the Interfacial Behavior of Type IIIa Binary Mixtures Along the Three-Phase Line Employing Molecular Thermodynamics

“…The molecular simulation methodology for the three-phase equilibrium has been described and applied in previous works for binary and ternary mixtures [18,19,23,24]. In this section, we retain the main technical aspects related to its initialization, equilibration, production as well as the calculation of the concentration profiles along three-phases interfacial region and their interfacial tensions.…”

“…As we established in the previous works [18,19,23,24], and following the standard procedure [35,37,41], to guarantee that the simulated systems are at a true equilibrium state, neither transient nor steady-state, we monitor the time evolution of the concentration profiles, the pressure tensor profiles, and the total energy of the system. Additionally, the fulfillment of the condition [P zz (z) -(P xx (z) + P yy (z))/2]dz = 0 within all bulk phases is verified.…”

“…In MD, the interfacial tension is calculated by the Hulshof integral [21], where the pressure along the interfacial region is described by the Irving-Kirkwood (IK) tensor [22]. This approach has been successfully used by us in previous works to describe the three-phase interfacial tension for Lennard-Jones [18,19,23], and coarse-grained Mie [24] fluids. In the case of MC, the interfacial tension can be calculated using similar approach or alternatively using the grand canonical ensemble (µ, T, V).…”

“…At this isothermal condition, γ VL1 ≈ 54.03 mN/m, γ L1L2 ≈ (99.44-54.03) mN/m ≈ 46.41 mN/m, and γ VL2 ≈ (108.56-99.44) mN/m ≈ 9.12 mN/m. Further details related to the technical implementation of the previous expressions and their evaluation at three-phase conditions have been discussed in our previous works [18,19,23,24]. Molecules 2020, 24, x FOR PEER REVIEW 10 of 17…”

Influence of dispersive long-range interactions on properties of vapour–liquid equilibria and interfaces of binary Lennard-Jones mixtures

“…They have recently received particular attention as in many systems an enrichment of components at this interface occurs, which is thought to have an influence on the mass transfer [38][39][40][41][42][43]. Vapour-liquid interfacial properties of Lennard-Jones mixtures have been investigated many times in the literature by molecular simulations, density gradient theory (DGT) or density functional theory [39,41,[44][45][46][47][48][49][50].…”