Microstructure of nonideal methanol binary liquid mixtures.

The nonideality of binary mixtures is often related to the nature of the interactions between both liquids and of the heterogeneity at the nanoscale-named microstructure. When one of the liquids is a hydrogen bonds former and the second is aprotic, the progressive diluting of the hydrogen-bonding network leads to a clustering and nanophases. By considering two mixtures, toluene-methanol and cyclohexane-methanol, the nonideality and its connection with the structure at the nanoscale and the intermolecular interactions are numerically investigated. Contrary to the toluene that is fully miscible in methanol, cyclohexane presents a high range of immiscibility which makes it a relevant system to study the nucleation (local segregation) and its propagation. In both mixtures, the deviation from the ideal behavior is observed. In the case of the toluene-methanol mixture, the initial hydrogen-bonding network corresponding to a homogenous structure is locally broken due to the favorable toluene-methanol interactions leading to the spatial heterogeneity at the origin of the nonideality. In the range of miscibility of the cyclohexane-methanol mixtures, the formation of hydrophobic nanophases of larger size is observed due to the unfavorable interactions between both components leading to a self-organizing of cyclohexane molecules. The immiscibility of cyclohexane and methanol are then correlated to the formation of nanophases and their propagation, which are also at the origin of the spatial heterogeneity. In the pure methanol, we highlight the disconnection between the clustering and the heterogeneity. We shed light on the fact that the prepeak observed in the structure factor is independent of the degree of heterogeneity, but is connected to the presence of cyclic clusters.

Test-area surface tension calculation of the graphene-methane interface: Fluctuations and commensurability.

The surface tension (γ) of methane on a graphene monolayer is calculated by using the test-area approach. By using a united atom model to describe methane molecules, strong fluctuations of surface tension as a function of the surface area of the graphene are evidenced. In contrast with the liquid-vapor interfaces, the use of a larger cutoff does not fully erase the fluctuations in the surface tension. Counterintuitively, the description of methane and graphene from the Optimized Potentials for Liquid Simulations all-atom model and a flexible model, respectively, led to a lessening in the surface tension fluctuations. This result suggests that the origin of fluctuations in γ is due to a model-effect rather than size-effects. We show that the molecular origin of these fluctuations is the result of a commensurable organization between both graphene and methane. This commensurable structure can be avoided by describing methane and graphene from a flexible force field. Although differences in γ with respect to the model have been often reported, it is the first time that the model drastically affects the physics of a system.

Physical Properties and Hydrogen-Bonding Network of Water-Ethanol Mixtures from Molecular Dynamics Simulations.

While many numerical and experimental works were focused on water-ethanol mixtures at low ethanol concentration, this work reports predictions of a few physical properties (thermodynamical, interfacial, dynamical, and dielectrical properties) of water-ethanol mixture at high alcohol concentrations by means of molecular dynamics simulations. By using a standard force field a good agreement was found between experiment and molecular simulation. This was allowed us to explore the dynamics, structure, and interplay between both hydrogen-bonding networks of water and ethanol.

Surface tension and long range corrections of cylindrical interfaces.

The calculation of the surface tension of curved interfaces has been deeply investigated from molecular simulation during this last past decade. Recently, the thermodynamic Test-Area (TA) approach has been extended to the calculation of surface tension of curved interfaces. In the case of the cylindrical vapour-liquid interfaces of water and Lennard-Jones fluids, it was shown that the surface tension was independent of the curvature of the interface. In addition, the surface tension of the cylindrical interface is higher than that of the planar interface. Molecular simulations of cylindrical interfaces have been so far performed (i) by using a shifted potential, (ii) by means of large cutoff without periodic boundary conditions, or (iii) by ignoring the long range corrections to the surface tension due to the difficulty to estimate them. Indeed, unlike the planar interfaces there are no available operational expressions to consider the tail corrections to the surface tension of cylindrical interfaces. We propose here to develop the long range corrections of the surface tension for cylindrical interfaces by using the non-exponential TA (TA2) method. We also extend the formulation of the Mecke-Winkelmann corrections initially developed for planar surfaces to cylindrical interfaces. We complete this study by the calculation of the surface tension of cylindrical surfaces of liquid tin and copper using the embedded atom model potentials.

Tunable dielectric constant of water at the nanoscale.

Using molecular dynamics simulations, the influence of the surface charge density of a nanotube on the static dielectric permittivity ε of confined water was reported. Whereas the dielectric anisotropy between the radial and axial directions of water confined in hydrophilic and hydrophobic membranes and the increase in axial dielectric permittivity with respect to the bulk value have previously been described, we found that an increase in the surface charge density leads to a drastic decrease in ε into the axial direction. The decrease in ε is accompanied by a strong slowdown in the rotational dynamics of water molecules. We show that this effect is due to the strong orientation of water molecules induced by the surface charge. Thus, by controlling the surface charge in nanotubes and nanocavities, it is possible to tune the dielectric permittivity of confined fluids at the nanoscale.

Communication: Slab thickness dependence of the surface tension: toward a criterion of liquid sheets stability.

Microscopic Monte Carlo simulations of liquid sheets of copper and tin have been performed in order to study the dependence of the surface tension on the thickness of the sheet. It results that the surface tension is constant with the thickness as long as the sheet remains in one piece. When the sheet is getting thinner, holes start to appear, and the calculated surface tension rapidly decreases with thickness until the sheet becomes totally unstable and forms a cylinder. We assume here that this decrease is not due to a confinement effect as proposed by Werth et al. [Physica A 392, 2359 (2013)] on Lennard-Jones systems, but to the appearance of holes that reduces the energy cost of the surface modification. We also show in this work that a link can be established between the stability of the sheet and the local fluctuations of the surface position, which directly depends on the value of the surface tension. Finally, we complete this study by investigating systems interacting through different forms of Lennard-Jones potentials to check if similar conclusions can be drawn.

Surface tension of spherical drops from surface of tension.

The determination of surface tension of curved interfaces is a topic that raised many controversies during the last century. Explicit liquid-vapor interface modelling (ELVI) was unable up to now to reproduce interfacial behaviors in drops due to ambiguities in the mechanical definition of the surface tension. In this work, we propose a thermodynamic approach based on the location of surface of tension and its use in the Laplace equation to extract the surface tension of spherical interfaces from ELVI modelling.

Solvation effects on self-association and segregation processes in tert-butanol-aprotic solvent binary mixtures.

The present study reveals that the fully miscible binary mixtures consisting of tert-butanol with aprotic solvents form well-defined ordered supermolecular structures, which have been characterized on different length scales. Three different types of microstructures have been determined. They are separated by distinct crossovers that appear as a function of the dilution rate, going from "correlated clusters" to "diluted clusters" and "diluted monomer" microstructures. These observations have been made possible by the combination of Raman vibration spectroscopy, (1)H NMR, and neutron diffraction that probe, respectively, the cluster formation (self-association) and the intercluster correlations (cluster segregation). The solvation effects on both the cluster formation and the intercluster correlations have been assessed by tuning the alcohol-solvent interaction, i.e., changing the chemical nature of the diluting solvent from a purely inert alkane to a weakly interacting aromatic system.

Molecular modeling of the liquid-vapor interfaces of a multi-component mixture: Prediction of the coexisting densities and surface tensions at different pressures and gas compositions.

Two-phase molecular simulations are performed in order to report the interfacial tensions and the coexisting densities of a multicomponent mixture (nitrogen + methane) + water for five gas compositions in the pressure range of 1-30 MPa at 298 K. The interfacial tensions are calculated using different definitions and the long range corrections of the surface tensions are considered using expressions designed for multicomponent mixtures and each definitions. We can conclude that the agreement with experiments is quantitative with deviations smaller than 5% for the interfacial tensions and 2% for the densities. The interfacial region is described in terms of specific arrangements of the gas components at the water surface.

Anomalous dielectric behavior of nanoconfined electrolytic solutions.

We report an anomalous dielectric effect of electrolytes under cylindrical nanoconfinement. In bulk phase, the decrease in the water dielectric constant (ε) with increasing salt concentration is well known, and is due to dielectric saturation. From molecular dynamic simulations of confined water and NaCl solutions, we show a dielectric anisotropy and an unexpected increase in ε(perpendicular) of NaCl solutions with respect to the confined pure liquid until a critical concentration is reached. We infer that this striking dielectric behavior results from the interplay between the effect of confinement and that of ions on the water hydrogen bonding network.

Calculation of the surface tension and pressure components from a non-exponential perturbation method of the thermodynamic route.

Surface tension is probably the most important interfacial property and a large number of techniques have been devoted to its calculation. Usually, this calculation is carried out using mechanical or thermodynamic definitions. The mechanical route uses an arbitrary choice to affect the contribution of the pairwise force. To overcome this arbitrariness, a thermodynamic route based on the area perturbation (test-area (TA) method) has been developed for the calculation of surface tension. The volume perturbation (VP) method provides an original route to compute the components of the pressure tensor. These two routes are developed from the perturbation theory leading to working expressions using exponential averages of energy. The use of exponential averages makes the calculation strongly dependent on the occurrence of low values of ΔU. Additionally, the decomposition of the energy to obtain local surface tension is nontrivial. From the explicit derivation of the partition function the exponential average is avoided providing an interesting alternative to TA, VP, and mechanical methods. To make a consistent comparison, we study the profiles of the surface tension along the direction normal to the surface for the different definitions and techniques in the cases of liquid-vapor interfaces of acids gases, binary, and apolar systems.

The non-Gaussian dynamics of glycerol.

We have combined incoherent quasielastic neutron scattering experiments and atomistic molecular simulations to investigate the microscopic dynamics of glycerol moving away from the hydrodynamic limit. We relate changes in the momentum transfer (Q) dependence of the relaxation time to distinct changes of the single-particle dynamics. Going from small to large values of Q, a first crossover at about 0.5 Å(-1) is related to the coupling of the translational diffusion dynamics to the non-Debye structural relaxation, while the second crossover at a Q-value near the main diffraction peak is associated with the Gaussian to non-Gaussian crossover of the short-time molecular dynamics, related to the decaging processes. We offer an unprecedented extension of previous studies on polymeric systems towards the case of the typical low-molecular-weight glass-forming system glycerol.

Mesoscale modeling of the water liquid-vapor interface: a surface tension calculation.

We report a mesoscale modeling of the liquid-vapor interface of water. A mesoscopic model of water has been established in dissipative particle dynamics (DPD) to reproduce the interfacial properties of water. The surface tension and coexisting densities are compared between atomistic and mesoscopic simulations. Simple scaling relations have been established to link the atomistic and mesoscopic length and time scales. Our study demonstrates the capability of the DPD method to explore the interfacial properties of a planar water liquid-vapor interface and a water nanodroplet. This constitutes an important step toward the calculation of the surface tension of larger and more complex interfacial systems.

Surface tension of water-alcohol mixtures from Monte Carlo simulations.

Monte Carlo simulations are reported to predict the dependence of the surface tension of water-alcohol mixtures on the alcohol concentration. Alcohols are modeled using the anisotropic united atom model recently extended to alcohol molecules. The molecular simulations show a good agreement between the experimental and calculated surface tensions for the water-methanol and water-propanol mixtures. This good agreement with experiments is also established through the comparison of the excess surface tensions. A molecular description of the mixture in terms of density profiles and hydrogen bond profiles is used to interpret the decrease of the surface tension with the alcohol concentration and alcohol chain length.

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.

Adsorption of light hydrocarbons in the flexible MIL-53(Cr) and rigid MIL-47(V) metal-organic frameworks: a combination of molecular simulations and microcalorimetry/gravimetry measurements.

The adsorption of short linear alkanes has been explored in the highly flexible MIL-53(Cr) porous metal-organic framework by means of molecular simulations based on configurational bias grand canonical Monte Carlo. The unusual shape of the adsorption isotherms with the existence of steps has been successfully modelled by creating a (narrow pore, large pore) phase mixture domain, the composition of which varies with pressure. A further step consisted of combining our computational approach with several experimental tools including microcalorimetry, gravimetry and in situ X-ray diffraction, to fully characterize the adsorption behaviour of the isostructural MIL-47(V) rigid MOF, i.e. the preferential arrangement of each type of alkane inside the pores and the resulting interaction energy. Finally, relationships are established between the adsorption enthalpies and both alkyl chain length and polarisability of the alkanes that can be further utilised to predict the energetics of the adsorption process for longer alkane chains.

Toward a Coarse Graining/All Atoms Force Field (CG/AA) from a Multiscale Optimization Method: An Application to the MCM-41 Mesoporous Silicates.

Many interesting physical phenomena occur on length and time scales that are not accessible by atomistic molecular simulations. By introducing a coarse graining of the degrees of freedom, coarse-grained (CG) models allow ther study of larger scale systems for longer times. Coarse-grained force fields have been mostly derived for large molecules, including polymeric materials and proteins. By contrast, there exist no satisfactory CG potentials for mesostructured porous solid materials in the literature. This issue has become critical among a growing number of studies on confinement effects on fluid properties, which require both long time and large scale simulations and the conservation of a sufficient level of atomistic description to account for interfacial phenomena. In this paper, we present a general multiscale procedure to derive a hybrid coarse grained/all atoms force field CG/AA model for mesoporous systems. The method is applied to mesostructured MCM-41 molecular sieves, while the parameters of the mesoscopic interaction potentials are obtained and validated from the computation of the adsorption isotherm of methanol by grand canonical molecular dynamic simulation.

Monte Carlo calculation of the methane-water interfacial tension at high pressures.

Monte Carlo simulations have been performed in the Np(N)AT statistical ensemble to study the methane-water mixture as a function of pressure. The interfacial tensions are calculated with different definitions and are reported for pressures from 1 to 50 MPa. The interfacial tensions, coexisting densities, and composition of the methane and water phases are shown to be in good agreement with the corresponding experimental properties. The interfacial region has been described through the profiles of the number of hydrogen bonds, the coordination number of each species, and the different energy contributions. We complete this study by a theoretical investigation of the thermal and mechanical equilibria in the binary mixture. We have also examined the profile of the intrinsic and long range correction parts of the interfacial tension along the normal to the water surface.

Monte Carlo simulations of the pressure dependence of the water-acid gas interfacial tensions.

We report two-phase Monte Carlo (MC) simulations of the binary water-acid gas mixtures at high temperature and high pressure. Simulations are performed in the Np(N)AT ensemble in order to reproduce the pressure dependence of the interfacial tensions of the water-CO(2) and water-H(2)S mixtures. The interfacial tension of the binary water-CO(2) mixture is determined from 5 to 45 MPa along the isotherm T = 383 K. Water-H(2)S interfacial tensions are computed along one supercritical isotherm (T = 393 K) in a pressure range of 1-15 MPa. The temperature and pressure conditions investigated here by the MC simulations are typical of the geological storage conditions of these acid gases. The coexisting densities and the compositions of the water-rich and acid-gas-rich phases are compared with experiments and with data calculated from Gibbs ensemble Monte Carlo (GEMC) simulations.