Aziz Ghoufi

Surface tension of water and acid gases from Monte Carlo simulations

We report direct Monte Carlo (MC) simulations on the liquid-vapor interfaces of pure water, carbon dioxide, and hydrogen sulfide. In the case of water, the recent TIP4P/2005 potential model used with the MC method is shown to reproduce the experimental surface tension and to accurately describe the coexistence curves. The agreement with experiments is also excellent for CO(2) and H(2)S with standard nonpolarizable models. The surface tensions are calculated by using the mechanical and the thermodynamic definitions via profiles along the direction normal to the surface. We also discuss the different contributions to the surface tension due to the repulsion-dispersion and electrostatic interactions. The different profiles of these contributions are proposed in the case of water.

Multiple histogram reweighting method for the surface tension calculation.

The multiple histogram reweighting method takes advantage of calculating ensemble averages over a range of thermodynamic conditions without performing a molecular simulation at each thermodynamic point. We show that this method can easily be extended to the calculation of the surface tension. We develop a new methodology called multiple histogram reweighting with slab decomposition based on the decomposition of the system into slabs along the direction normal to the interface. The surface tension is then calculated from local values of the chemical potential and of the configurational energy using Monte Carlo (MC) simulations. We show that this methodology gives surface tension values in excellent agreement with experiments and with standard NVT MC simulations in the case of the liquid-vapor interface of carbon dioxide.

Entropy and enthalpy calculations from perturbation and integration thermodynamics methods using molecular dynamics simulations: applications to the calculation of hydration and association thermodynamic properties

Calculation of association thermodynamic properties using molecular simulation is essential in computational chemistry. In the case of good agreement with the experimental thermodynamic binding properties, this type of calculation may complement experimental works by providing a microscopic view of the association process. Whereas the calculation of the free energy of association is nowadays well controlled, the calculation of the enthalpy and entropy of association remains difficult in most cases, especially as the association involves hosts and guests of biological interest. A novel method for calculating the entropy change from a molecular dynamics simulation is described. Within the theoretical framework, we discuss the different approximations leading to the final stage of the operational expressions of ΔG and ΔH in the NpT ensemble and we establish an expression for ΔS using the Free Energy Perturbation (FEP) formalism in this statistical ensemble. Finally, we illustrate the theoretical considerations by calculations of the hydration entropy changes between cations of different masses and charges. We extend the study by calculating the changes in the thermodynamic properties of association of inorganic cations with a macrocycle of biological interest.

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.

Calculation of the absolute thermodynamic properties of association of host-guest systems from the intermolecular potential of mean force.

The authors report calculations of the intermolecular potential of mean force (PMF) in the case of the host-guest interaction. The host-guest system is defined by a water soluble calixarene and a cation. With an organic cation such as the tetramethylammonium cation, the calixarene forms an insertion complex, whereas with the Lanthane cation, the supramolecular assembly is an outer-sphere complex. The authors apply a modified free energy perturbation method and the force constraint technique to establish the PMF profiles as a function of the separation distance between the host and guest. They use the PMF profile for the calculation of the absolute thermodynamic properties of association that they compare to the experimental values previously determined. They finish by giving some structural features of the insertion and outer-sphere complexes at the Gibbs free energy minimum.

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.

Calculations of the potential of mean force from molecular dynamics simulations using different methodologies: an application to the determination of the binding thermodynamic properties of an ion pair

We report here the calculation of the potential of mean force (PMF) of the Ca2+…SO ion pair in the gas phase and in water using molecular dynamics simulations. The PMF calculation is performed using different free-energy perturbation (FEP) and force constraint (FC) methods. We pay attention to the FEP method for which we propose a new development specially adapted to systems with solvent molecules. The gas-phase interionic PMFs are compared with that calculated from the primitive model. We compare the shapes of the calculated PMFs in water and the resulting thermodynamic properties ( , and ) of complexation with the experimental ones. We also tend to give a molecular description of the environment of the ion pair for some regions of the PMFs.

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.