Molecular interactions at vapor-liquid interfaces: Binary mixtures of simple fluids

Abstract

Properties of vapor-liquid equilibria and planar interfaces of binary Lennard-Jones truncated and shifted (LJTS) mixtures were investigated with molecular dynamics (MD) simulations, density gradient theory (DGT), and conformal solution theory (CST) at constant liquid phase composition and temperature. The results elucidate the influence of the liquid phase interactions on the interfacial properties (surface tension, surface excess, interfacial thickness, and enrichment). The studied mixtures differ in the ratios of the dispersion energies of the two components ε 2 /ε 1 and the binary interaction parameter ξ. By varying ξ and ε 2 /ε 1 , a variety of types of phase behavior is covered by the study. The dependence of the interfacial properties on the variables ξ and ε 2 /ε 1 reveals regularities that can be explained by conformal solution theory of the liquid phase. It is thereby shown that the interfacial properties of the mixtures are dominated by the mean liquid phase interactions whereas the vapor phase has only a minor influence.

Vapor-liquid interfacial properties of the system cyclohexane + CO2: Experiments, molecular simulation and density gradient theory

“…On the one side, studies on simple model systems can be carried out. We have recently systematically studied binary mixtures of Lennard-Jones fluids and have examined the influence of the variation of parameters, which describe the molecular interactions on the interfacial properties [4,16,17,21]. Similar studies have also been carried out by multiple other groups, e.g.…”

“…Since the relative adsorption and the enrichment both describe the surface excess, they are closely linked, but they do not contain the same information [4,16,20]. Adsorption may occur although no enrichment is present (E CO 2 = 1) -if the two component density profiles are shifted relative to each other [4].…”

“…Interesting phenomena have been observed in the nanoscopic interfacial region of binary and multicomponent mixtures, such as maxima in the density profiles of the low-boiling component at the interface [13][14][15][16]. The density of low-boiling components at such maxima can be several times higher than the bulk densities.…”

Characteristic Curves of the Lennard-Jones Fluid

“…It is probably the most frequently investigated monomer model fluid in molecular simulation [7]. The Lennard-Jones (LJ) potential can be favorably used for testing new theories and simulation methods, e.g., for mixtures, phase changes, non-equilibrium phenomena, and interfaces between phases [8][9][10][11][12][13][14][15][16][17][18][19]. Also, the Lennard-Jones potential is often used as a starting point for the development of many state-of-the-art force fields for complex molecules [20][21][22].…”

Characteristic Curves of the Lennard-Jones Fluid

“…The integrals in Eq. (10) were solved in this work using the method proposed by Hutem and Boonchui [80]. Equations ( 9) and ( 10) were implemented and numerically integrated to obtain exact results (within the significant digits used for the computer precision in the calculations) for B * and C * as well as their characteristic points using the LJ potential.…”