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.

Prediction of the concentration dependence of the surface tension and density of salt solutions: atomistic simulations using Drude oscillator polarizable and nonpolarizable models.

Molecular simulations using Drude oscillator polarizable and nonpolarizable models for water and ions are carried out to predict the dependence of the surface tension on salt concentration. The polarizable water and ion models are based only on the classical Drude oscillators. The temperature dependence of the surface tension of water is examined for different water models. The dependence of salt densities on salt concentration is investigated through the nonpolarizable and Drude oscillator polarizable models. Finally, the reproduction of the surface tension of salt solution over a large range of concentrations is analyzed through a number of combinations between ions and water force fields. The structure of the interface is then discussed as a function of polarization effects. We establish here the inability of the Drude oscillator polarizable force fields to reproduce the salt concentration dependence of surface tension of NaCl aqueous solutions.

Prediction of the temperature dependence of the surface tension of SO2, N2, O2, and Ar by Monte Carlo molecular simulations.

We report Monte Carlo simulations of the liquid-vapor interface of SO(2), O(2), N(2), and Ar to reproduce the dependence of the surface tension with the temperature. Whereas the coexisting densities, critical temperature, density, and pressure are very well reproduced by the two-phase simulations showing the same accuracy as the calculations performed using the Gibbs ensemble Monte Carlo technique (GEMC), the performance of the prediction of the variation of the surface tension with the temperature depends on the magnitude of the electrostatic and repulsive-dispersive interactions. The surface tension of SO(2) is very well reproduced, whereas the prediction of this property is less satisfactory for O(2) and N(2), for which the average intermolecular electrostatic interactions are several orders smaller than the dispersion interactions. For argon, we observe significant deviations from experiments. The representation of the surface tension of argon in reduced units shows that our calculations are in line with the existing surface tensions of the Lennard-Jones fluid in the literature. This underlines the difficulty of reproducing the temperature dependence of the surface tension of argon with interactions only modeled by the Lennard-Jones pair potential.