Vapor-liquid interfacial properties of fully flexible Lennard-Jones chains

Abstract

We consider the computation of the interfacial properties of molecular chains from direct simulation of the vapor-liquid interface. The molecules are modeled as fully flexible chains formed from tangentially bonded monomers with truncated Lennard-Jones interactions. Four different model systems comprising of 4, 8, 12, and 16 monomers per molecule are considered. The simulations are performed in the canonical ensemble, and the vapor-liquid interfacial tension is evaluated using the test area and the wandering interface methods. In addition to the surface tension, we also obtain density profiles, coexistence densities, critical temperature and density, and interfacial thickness as functions of temperature, paying particular attention to the effect of the chain length on these properties. According to our results, the main effect of increasing the chain length (at fixed temperature) is to sharpen the vapor-liquid interface and to increase the width of the biphasic coexistence region. As a result, the interfacial thickness decreases and the surface tension increases as the molecular chains get longer. The interfacial thickness and surface tension appear to exhibit an asymptotic limiting behavior for long chains. A similar behavior is also observed for the coexistence densities and critical properties. Our simulation results indicate that the asymptotic regime is reached for Lennard-Jones chains formed from eight monomer segments. We also include a preliminary study on the effect of the cutoff distance on the interfacial properties. Our results indicate that all of the properties exhibit a dependence with the distance at which the interactions are truncated, though the relative effect varies from one property to the other. The interfacial thickness and, more particularly, the interfacial tension are found to be strongly dependent on the particular choice of cutoff, whereas the density profiles and coexistence densities are, in general, less sensitive to the truncation.

On interfacial properties of tetrahydrofuran: Atomistic and coarse-grained models from molecular dynamics simulation

“…We follow the same analysis and methodology as in our previous works, [72][73][74][75] and consider di↵erent temperatures for all the models analyzed. In order to characterize the bulk phase and interfacial behaviour, density profiles are calculated by dividing the system in 250 slabs along the z direction.…”

Effect of molecular flexibility of Lennard-Jones chains on vapor-liquid interfacial properties

“…We consider N = 672, 403, and 336 for systems formed from 3, 5, and 6 monomers, respectively. As in previous studies, 14,48,52,53 this choice is made so as to have systems with the same total number of monomers irrespective of the monomers per molecule. We also compare the results obtained here with those published previously by us 14 corresponding to FF chains formed from 4 monomers (see Sec.…”

“…All of them exhibit intramolecular flexibility governed by bending and torsional potentials, that determine the molecular configurations the chains can adopt without overlaps. From a formal point of view, n-alkanes exhibit an intermediate behavior between those shown by two wellknown model systems, i.e., the fully flexible (FF) 14,[44][45][46][47][48] and rigid-linear (RL) Lennard-Jones (LJ) chain models. 49 The FF chain-like system has neither bending nor torsional potentials between the monomers in a chain.…”

“…As a consequence of this, both models exhibit completely different phase diagrams, as demonstrated several years ago by Galindo et al 49 A related topic which we do not address here is the influence of bond length flexibility in the phase diagram and interfacial properties, which has been recently studied by Chapela et al 50,51 During last five years, we have studied the interfacial properties of different simple chain-like molecular models using perturbative methods, such as the WIM and TA methodologies. 14,48,52,53 Very recently, 53 we have determined the interfacial properties of short RLLJ chains from direct simulation of the vapor-liquid interface, including the surface tension using the TA method. In most cases, we have used an improved version 14,53 of the Janecek's method 12 for calculating long-range corrections (LRCs) to the energy in systems that interact through spherically symmetric intermolecular potentials.…”

Excess equimolar radius of liquid drops

“…The surface tension is then immediately obtained from ∆A/∆a, since the distortion of the interface itself (as opposed to its increase in area) makes a negligible contribution to the free energy difference [15]. In analogy with the Widom test-particle method [54], this implementation of the variational route is also called the test-area method [36,55]. Following Sampayo et al [22], it can be applied to curved interfaces, where the affine transformation scales one of the cartesian axes by the factor 1/(1 + ξ) and the remaining ones by (1 + ξ) 1/2 .…”