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.

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.

Calculation of the surface tension of cyclic and aromatic hydrocarbons from Monte Carlo simulations using an anisotropic united atom model (AUA).

We report the calculation of the surface tension of cycloalkanes and aromatics by direct two-phase MC simulations using an anisotropic united atom model (AUA). In the case of aromatics, the polar version of the AUA-4 (AUA 9-sites) model is used. A comparison with the nonpolar models is carried out on the surface tension of benzene. The surface tension is calculated from different routes: the mechanical route using the Irving and Kirkwood (IK) and Kirkwood-Buff (KB) expressions; the thermodynamic route by using the test-area (TA) method. The different operational expressions of these definitions are presented with those of their long range corrections. The AUA potential allows to reproduce very well the dependence of the surface tension with respect to the temperature for cyclopentane, cyclohexane, benzene and toluene.

Calculation of the surface tension from Monte Carlo simulations: does the model impact on the finite-size effects?

We report two-phase Monte Carlo simulations of the liquid-vapor interface of the Lennard-Jones (LJ) fluids in order to study the impact of the methodology used for the energy calculation on the oscillatory behavior of the surface tension with the system sizes. The surface tension values are illustrated through the LJ parameters of methane. The first methodology uses a standard truncated LJ potential, the second one adds a long range correction (LRC) contribution to the energy into the Metropolis scheme, and the third one uses a LJ potential modified by a polynomial function in order to remove the discontinuities at the cutoff distance. The surface tension is calculated from the mechanical and thermodynamic routes and the LRCs to the surface tension are systematically calculated from appropriate expressions within these definitions. The oscillatory behavior has been studied as a function of the size of the interfacial area and of the length of the dimension perpendicular to the surface. We show that the methodology has an important effect on the oscillatory variation in the surface tension with the system size. This oscillatory variation in the surface tension with the system size is investigated through its intrinsic and LRC contributions. We complete this work by studying the dependence of the surface tension with respect to the cutoff distance when the LRC part to the energy is considered into the Metropolis scheme.

Surface tensions of linear and branched alkanes from Monte Carlo simulations using the anisotropic united atom model.

The anisotropic united atoms (AUA4) model has been used for linear and branched alkanes to predict the surface tension as a function of temperature by Monte Carlo simulations. Simulations are carried out for n-alkanes ( n-C5, n-C6, n-C7, and n-C10) and for two branched C7 isomers (2,3-dimethylpentane and 2,4-dimethylpentane). Different operational expressions of the surface tension using both the thermodynamic and the mechanical definitions have been applied. The simulated surface tensions with the AUA4 model are found to be consistent within both definitions and in good agreement with experiments.