A molecular dynamics study of a liquid–liquid interface: structure and dynamics (original) (raw)
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The Journal of Chemical Physics, 1999
By means of extensive equilibrium molecular dynamics simulations we have investigated, the behavior of the interfacial tension γ of two immiscible symmetrical Lennard-Jones fluids. This quantity is studied as function of reduced temperature T * = k B T ǫ in the range 0.6 ≤ T * ≤ 3.0. We find that, unlike the monotonic decay obtained for the liquid-vapor interfacial tension, for the liquidliquid interface, γ(T) has a maximum at a specific temperature. We also investigate the effect that surfactant-like particles has on the thermodynamic as well as the structural properties of the liquidliquid interface. It is found that γ decays monotonically as the concentration of the surfactant-like particles increases.
A molecular theory of liquid interfaces
Physical Chemistry Chemical Physics, 2005
We propose a site-site generalization of the Lovett-Mow-Buff-Wertheim integro-differential equation for the one-particle density distributions to polyatomic fluids. The method provides microscopic description of liquid interfaces of molecular fluids and solutions. It uses the inhomogeneous site-site direct correlation function of molecular fluid consistently constructed by nonlinear interpolation between the homogeneous ones. The site-site correlations of the coexisting bulk phases are obtained from the reference interaction site model (RISM) integral equation with our closure approximation. For illustration, we calculated the structure of the planar liquid-vapor as well as liquid-liquid interfaces of n-hexane and methanol at ambient conditions.
Physical review. E, Statistical, nonlinear, and soft matter physics, 2003
Direct molecular dynamics simulations of the liquid-liquid interface of water-n-alkane and water-methanol-n-alkane systems have been performed in order to study the interfacial properties of these systems. The simulations were carried out using the NERD revised force field of Nath et al. for the n-alkanes, the simple point charge extended model for water, and the optimized potential for liquid simulations model for methanol. In order to validate the model employed in this work for the n-alkanes we calculated the coexisting densities, surface tension, and thickness of the interface for pure n-pentane. For all the systems studied the interfacial tension and thickness were calculated at 298.15 K. Our results show that, by adjusting the number of molecules to reproduce the liquid densities in the direct simulation method of the liquid-liquid interface in multicomponent systems, we are able to reproduce available experimental data for interfacial tension. The interfacial thickness is und...
Molecular-dynamics studies of binary mixtures of Lennard-Jones fluids with differing component sizes
Fluid Phase Equilibria, 1984
In this paper, properties of binary mixture of polar portions of the crude oil are studied by classical molecular dynamics simulation. Particularly, the bulk and the interface of pyridine (and its alkyl derivatives) mixture with n-octane are studied, including density and structure in bulk as well as their molecular orientations at the liquid-vapor interface. Initial simulations of density profile and interfacial properties are to validate the force field applied. Additionally, the structure of solution of pyridine derivatives in bulk and at the interface is studied. As a result, the derivatives with long alky chain (of low surface tension) approaches to the surface while those with short chain (of high surface tension) aggregate in the solution and avoid approaching the surface. Finally, the molecular orientations of pyridine and alkyl derivatives are investigated in the bulk and at interface by the intrinsic bivariate orientational maps.
Computer simulations of liquid/vapor interface in Lennard-Jones fluids: Some questions and answers
The Journal of Chemical Physics, 1999
Canonical molecular dynamics ͑MD͒ and Monte Carlo ͑MC͒ simulations for liquid/vapor equilibrium in truncated Lennard-Jones fluid have been carried out. Different results for coexistence properties ͑orthobaric densities, normal and tangential pressure profiles, and surface tension͒ have been reported in each method. These differences are attributed in literature to different set up conditions, e.g., size of simulation cell, number of particles, cut-off radius, time of simulations, etc., applied by different authors. In the present study we show that observed disagreement between simulation results is due to the fact that different authors inadvertently simulated different model fluids. The origin of the problem lies in details of truncation procedure used in simulation studies. Care has to be exercised in doing the comparison between both methods because in MC calculations one deals with the truncated potential, while in MD calculations one uses the truncated forces, i.e., derivative of the potential. The truncated force does not uniquely define the primordial potential. It results in MD and MC simulations being performed for different potential models. No differences in the coexistence properties obtained from MD and MC simulations are found when the same potential model is used. An additional force due to the discontinuity of the truncated potential at cut-off distance becomes crucial for inhomogeneous fluids and has to be included into the virial calculations in MC and MD, and into the computation of trajectories in MD simulations. The normal pressure profile for the truncated potential is constant through the interface and both vapor and liquid regions only when this contribution is taken into account, and ignoring it results in incorrect value of surface tension.
Physical Review E, 2004
We have carried out extensive equilibrium molecular dynamics (MD) simulations to study the structure and the interfacial properties in the liquid-vapor (LV) phase coexistence of partially miscible binary Lennard-Jones (LJ) mixtures. By analyzing the structural properties as a function of the miscibility parameter, α, we found that at relatively low temperatures the system separates forming a liquid A-liquid B interface in coexistence with the vapor phase. At higher temperatures and, 0 < α ≤ 0.5, we found a temperature range, T * w (α) ≤ T * < T * cons (α), where the liquid phases are wet by the vapor phase. Here, T * w (α) represents the wetting transition temperature (WTT) and T * cons (α) is the consolute temperature of the mixture. However, for 0.5 < α < 1, no wetting phenomenon occurs. For the particular value, α = 0.25, we analyzed quantitatively the T * versus ρ * , and P * versus T * phase diagrams and found, T * c ≃ 1.25, and T * cons ≃ 1.25. We also studied quantitatively, as a function of temperature, the surface tension and the adsorption of molecules at the liquid-liquid interface. It was found that the adsorption shows a jump from a finite negative value up to minus infinity, when the vapor wets the liquid phases, suggesting that the wetting transition (WT) is of first order. The calculated phase diagram together with the wetting phenomenon strongly suggest the existence of a tricritical point. These results agree well
A critical study of simulations of the Lennard-Jones liquid-vapor interface
Fluid Phase Equilibria, 1992
The surface tension and li and two-component Lennard-P uid-gas density profile through the interface of oneits simulation techniques. ones fluids were calculated using Molecular Dynam-The system size, film thickness, interface area, intermolecular potential cut-off, composition, and temperature were varied. For the onecomponent system, the results were compared to previous work and some discreancies of the past work were resolved. By combining this work with correct resu ts P from and t R revious authors, the minimum system size, film thickness, equilibration time, e trade-off between cut-off and corn uter time were determined. Using configurations calculated for moderate cut-o ffp s, the surface tension was extrapolated to the full potential value by using a tail correction and the results compared to simulations performed with the longer cut-offs. The results showed the possibility of obtaining estimates of the surface tension for large cut-off simulattons from moderate cut-off simulations provided that the density profile does not change significantly with increase in cut-off. Using the cntenon for equrhbratron determined from the one-component systems, two-component systems at varying compositions and temperatures were simulated and the tail correction applied.
The Journal of Physical Chemistry B, 2005
Monte Carlo simulation of the vapor-liquid interface of water-methanol mixtures of different compositions, ranging from pure water to pure methanol, have been performed on the canonical (N, V, T) ensemble at 298 K. The analysis of the systems simulated has revealed that the interface is characterized by a double layer structure: methanol is strongly adsorbed at the vapor side of the interface, whereas this adsorption layer is followed at its liquid side by a depletion layer of methanol of lower concentration than in the bulk liquid phase of the system. The dominant feature of the interface has been found to be the adsorption layer in systems of methanol mole fractions below 0.2, and the depletion layer in systems of methanol mole fractions between 0.25 and 0.5. The orientation of the molecules located at the depletion layer is found to be already uncorrelated with the interface, whereas the methanol molecules of the adsorption layer prefer to align perpendicular to the interface, pointing straight toward the vapor phase by their methyl group. Although both the preference of the molecular plane for a perpendicular alignment with the interface and the preference of the methyl group for pointing straight to the vapor phase are found to be rather weak, the preference of the methyl group for pointing as straight toward the vapor phase as possible within the constraint imposed by the orientation of the molecular plane is found to be fairly strong. One of the two preferred orientations of the interfacial water molecules present in the neat system is found to disappear in the presence of methanol, because methanol molecules aligned in their preferred orientation can replace these water molecules in the hydrogen-bonding pattern of the interface. *
Journal of Molecular Liquids, 2013
Molecular dynamics simulations of the water-CCl 4 interface have been done in two different ways. In the first simulation the CCl 4 phase has been frozen in an equilibrium configuration, and only the water molecules have been allowed to move, whilst in the other one no such artificial freezing has been done. This way the effect of the fluid-like structure and fluid-like dynamics of the CCl 4 phase on the surface properties of the aqueous phase could be investigated separately. Due to the separate thermostatting of the two types of molecules in the simulations all the differences seen between the interfacial properties of water in the two systems can indeed be attributed to the rigid vs. fluid nature of the organic phase, and not to the thermal contact with a phase of zero temperature. The obtained results reveal that the rigidity of the opposite phase introduces an ordering both in the layering structure and orientation of the surface water molecules. The enhanced orientational ordering leads to a stronger lateral hydrogen bonding structure of the molecules within the subsequent molecular layers beneath the surface, and hence also to a slower exchange of the water molecules between the surface and the bulk aqueous phase.