Expressions for local contributions to the surface tension from the virial route

Importance of the Electrostatic Correlations in Surface Tension of Hydrated Reline Deep Eutectic Solvent from Combined Experiments and Molecular Dynamics Simulations

In this study, the surface tension and the structure of hydrated reline are investigated by using diverse methods. Initially, the surface tension displays a nonlinear pattern as water content increases, decreasing until reaching 45 wt %, then gradually matching that of pure water. This fluctuation is associated with strong electrostatic correlations present in pure reline, which decrease as more water is added. Changes in surface tension reflect a shift from charge layering in pure reline to an increased interfacial hydrogen bonding as the water content rises. This shift causes the segregation of urea molecules into the bulk phase and a gradual anchoring of water molecules to the air-reline interface. An interesting observation is the antisurfactant effect, where heightened interfacial anchoring results in an unexpected increase in real contribution of surface tension. This, along with weakened electrostatic correlations beyond 45 wt % due to reinforced interfacial hydrogen bonding, contributes to the complex behavior of surface tension observed in this study.

Water/oil interfacial tension reduction - an interfacial entropy driven process

The interfacial tension (IFT) of a fluid-fluid interface plays an important role in a wide range of applications and processes. When low IFT is desired, surface active compounds (e.g. surfactants) can be added to the system. Numerous attempts have been made to relate changes in IFT arising from such compounds to the specific nature of the interface. However, the IFT is controlled by an interplay of factors such as temperature and molecular structure of surface-active compounds, which make it difficult to predict IFT as those conditions change. In this study, we present the results from molecular dynamics simulations revealing the specific role surfactants play in IFT. We find that, in addition to reducing direct contact between the two fluids, surfactants serve to increase the disorder at the interface (related to interfacial entropy) and consequently reduce the water/oil IFT, especially when surfactants are present at high surface density. Our results suggest that surfactants that yield more disordered interfacial films (e.g. with flexible and/or unsaturated tails) reduce the water/oil IFT more effectively than surfactants which yield highly ordered interfacial films. Our results shed light on some of the factors that control IFT and could have important practical implications in industrial applications such as the design of cosmetics, food products, and detergents.

Learning Coarse-Grained Potentials for Binary Fluids

For a multiple-fluid system, CG models capable of accurately predicting the interfacial properties as a function of curvature are still lacking. In this work, we propose a new probabilistic machine learning (ML) model for learning CG potentials for binary fluids. The water-hexane mixture is selected as a typical immiscible binary liquid-liquid system. We develop a new CG force field (FF) using the Shinoda-DeVane-Klein (SDK) FF framework and compute parameters in this CG FF using the proposed probabilistic ML method. It is shown that a standard response-surface approach does not provide a unique set of parameters, as it results in a loss function with multiple shallow minima. To address this challenge, we develop a probabilistic ML approach where we compute the probability density function (PDF) of parameters that minimize the loss function. The PDF has a well-defined peak corresponding to a unique set of parameters in the CG FF that reproduces the desired properties of a liquid-liquid interface. We compare the performance of the new CG FF with several existing FFs for the water-hexane mixture, including two atomistic and three CG FFs with respect to modeling the interface structure and thermodynamic properties. It is demonstrated that the new FF significantly improves the CG model prediction of both the interfacial tension and structure for the water-hexane mixture.

Coarse-Graining the Liquid-Liquid Interfaces with the MARTINI Force Field: How Is the Interfacial Tension Reproduced?

We report two-phase coarse-grained (CG) simulations of organic-water liquid-liquid interfaces with the MARTINI force field. We discuss the ability of the CG force field to predict quantitatively the interfacial tension of alkanes-water, benzene-water, chloroform-water, and alcohol-water systems. The performance of the prediction of the interfacial tension is evaluated through its dependence on temperature and alkane length. This study contributes to the challenging discussion about the robustness and the transferability of the MARTINI force field to interfacial properties. We have also used the distributions of the molecules along the direction normal to the interface to investigate the composition of the interfacial region and to compare the simulated densities of the coexisting phases with experiments.

Computer modelling of the surface tension of the gas–liquid and liquid–liquid interface

This review presents the state of the art in molecular simulations of interfacial systems and of the calculation of the surface tension from the underlying intermolecular potential.

Anisotropy of local stress tensor leads to line tension

Line tension of three-phase contact lines is an important physical quantity in understanding many physical processes such as heterogeneous nucleation, soft lithography and behaviours in biomembrane, such as budding, fission and fusion. Although the concept of line tension was proposed as the excess free energy in three-phase coexistence regions a century ago, its microscopic origin is subtle and achieves long-term concerns. In this paper, we correlate line tension with anisotropy of diagonal components of stress tensor and give a general formula of line tension. By performing molecular dynamic simulations, we illustrate the formula proposed in Lennard-Jones gas/liquid/liquid and gas/liquid/solid systems, and find that the spatial distribution of line tension can be well revealed when the local distribution of stress tensor is considered.

Quantitative Predictions of the Interfacial Tensions of Liquid-Liquid Interfaces through Atomistic and Coarse Grained Models

We report molecular simulations of oil-water liquid-liquid interfaces by using atomistic and coarse grained (CG) MARTINI force fields. We also apply the electronic continuum (EC) model to the MARTINI force field for the calculation of the interfacial tension of oil/water-salt systems. In a first step, we propose to calculate the interfacial tensions using thermodynamic and mechanical definitions of hydrocarbon-water interfacial systems modified by the addition of salts and alcohol. We also establish here the order of magnitude of the long-range corrections to the interfacial tension in fluid-fluid interfaces. Whereas the atomistic models are able to reproduce quantitatively the interfacial tension and the coexisting densities of oil-water systems, the coarse-description shows some deviations in the prediction of the interfacial tensions. Nevertheless, the physical features of these liquid-liquid interfaces are well-captured by this CG description. The CG force field offers then a very challenging alternative that will require however a more developed calibration of the parameters on the basis of liquid-liquid properties.

Recent advances in Many Body Dissipative Particles Dynamics simulations of liquid-vapor interfaces

Many Body Dissipative Particles Dynamics (MDPD) simulation is a novel promising mesoscopic method to model the liquid-vapor interfaces. Based upon works of Paganobarraga and Frenkel (J. Chem. Phys. 15, 5015 (2001)) and Trofimov (J. Chem. Phys. 117, 9383 (2002)) and of Warren (Phys. Rev. E 68, 066702 (2003)) this method has been critically reviewed during this last decade. We propose here to give an overview of the Many Body Dissipative Particles Dynamic simulation within the framework of the liquid-vapor interfaces. We recall the theoretical background of MDPD and we present some recent results of systems of interest such as water liquid-vapor interfaces and salt effect on water surface tension. Additionally we discuss the ability of MDPD to capture the mechanisms at the mesoscopic scale through the formation of micelles and the coalescence of a nanodroplet water on water surface.

Excess equimolar radius of liquid drops

The curvature dependence of the surface tension is related to the excess equimolar radius of liquid drops, i.e., the deviation of the equimolar radius from the radius defined by the macroscopic capillarity approximation. Based on the Tolman [J. Chem. Phys. 17, 333 (1949)] approach and its interpretation by Nijmeijer et al. [J. Chem. Phys. 96, 565 (1991)], the surface tension of spherical interfaces is analyzed in terms of the pressure difference due to curvature. In the present study, the excess equimolar radius, which can be obtained directly from the density profile, is used instead of the Tolman length. Liquid drops of the truncated and shifted Lennard-Jones fluid are investigated by molecular dynamics simulation in the canonical ensemble, with equimolar radii ranging from 4 to 33 times the Lennard-Jones size parameter σ. In these simulations, the magnitude of the excess equimolar radius is shown to be smaller than σ/2. This suggests that the surface tension of liquid drops at the nanometer length scale is much closer to that of the planar vapor-liquid interface than reported in studies based on the mechanical route.

COMPUTER SIMULATION OF LIQUID–VAPOR INTERFACIAL TENSION: LENNARD-JONES FLUID AND WATER REVISITED

We review several commonly used simulation methods for computing liquid–vapor surface tension and associated theoretical treatments of the long-range correction for inhomogeneous systems. Prototype model systems considered in this review are the Lennard-Jones (LJ) fluid and the SPC/E model water. In addition, we examine a variety of factors that can affect calculation of the surface tension γ via the mechanical approach (i.e. using either KB or IK method). It is found that for the LJ fluid, the size of simulation box and the number of particles in the system can have notable effects on the computed surface tension. For SPC/E water, the Ewald parameters can influence computed surface tensions (γ) as well, e.g., very small Ewald parameters tend to overestimate γ. It is also found that the IK method consistently gives γ that are 0.6 - 0.9 mN/m greater than γ computed based on the KB method. When computing the first reciprocal–space contribution to the surface tension, the Ghoufi's strategy gives rise to more sensible profile of pressure difference PN(z)-PT(z) than the Alejandre's strategy although both strategies result in nearly the same average surface tension through the integration of PN(z)-PT(z).

Calculation of the surface tension from multibody dissipative particle dynamics and Monte Carlo methods

We report the calculation of the coexisting densities and surface tensions of the liquid-vapor equilibrium using the multibody dissipative particle dynamics and Monte Carlo (MMC) methods. We focus on the calculation of the surface tension by using the thermodynamic and mechanical routes. It is the first time that the test-area method is applied on the many-body conservative potential. We discuss the mechanical equilibrium of these two-phase systems by analyzing the profiles of the normal and tangential components of the pressure tensor using the Irving-Kirkwood and Kirkwood-Buff approaches. The profile of the configurational temperature is shown to establish the thermal equilibrium of these two-phase simulations carried out with large time steps. We complete this study to show the impact of the range of the many-body repulsive term of the conservative force on the surface tension. We conclude that the MMC method is an efficient sampling scheme to compute the interfacial properties of liquid-vapor interfaces using the multibody soft potential.

Molecular dynamics simulations of a reversibly folding beta-heptapeptide in methanol: influence of the treatment of long-range electrostatic interactions

Eight 100-ns molecular dynamics simulations of a beta-heptapeptide in methanol at 340 K (within cubic periodic computational boxes of about 6-nm edge) are reported and compared. These simulations were performed with three different charge-state combinations at the peptide termini, one of them with or without a neutralizing chloride counterion, and using either the lattice-sum (LS) or reaction-field (RF) scheme to handle electrostatic interactions. The choice of the electrostatic scheme has essentially no influence on the folding-unfolding equilibrium when the peptide termini are uncharged and only a small influence when the peptide is positively charged at its N-terminus (with or without inclusion of a neutralizing chloride counterion). However, when the peptide is zwitterionic, the LS scheme leads to preferential sampling of the high-dipole folded helical state, whereas the RF scheme leads to preferential sampling of a low-dipole unfolded salt-bridged state. A continuum electrostatics analysis based on the sampled configurations (zwitterionic case) suggests that the LS scheme stabilizes the helical state through artificial periodicity, but that the magnitude of this perturbation is essentially negligible (compared to the thermal energy) for the large box size and relatively polar solvent considered. The results thus provide clear evidence (continuum electrostatics analysis) for the absence of LS artifacts and some indications (still not definitive because of the limited sampling of the folding-unfolding transition) for the presence of RF artifacts in this specific system.

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

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.