Bernard Duguay

Thermal diffusion sensitivity to the molecular parameters of a binary equimolar mixture, a non-equilibrium molecular dynamics approach

Abstract The goal of this study is to analyse how the thermal diffusion process is dependent on molecular parameters when describing a fluid mixture. To estimate the associated transport coefficient, which is the thermal diffusion factor αT, a non-equilibrium molecular dynamics algorithm has been applied on equimolar binary mixtures of Lennard–Jones (LJ) particles in supercritical conditions. Firstly, it is shown that this model is able to correctly estimate αT for simple alkane mixtures, provided there are a sufficient number of particles and long enough simulations. Then, using various mixing rules, the separate influences of the mass, the moment of inertia, the atomic diameter and the interaction strength have been studied. Results indicate that the molar fraction of the component, having the smallest mass and moment of inertia as well as the biggest radius and the strongest potential, tends to increase in the hot area. Elsewhere, simulations for various cross-interaction parameters show that αT is extremely sensitive to the intermolecular pair potential between unlike particles. Finally, results on methane/normal alkane mixtures indicate that a simple sum between the separate contributions provides a reliable evaluation of αT only when the molecular parameter ratios between the two components are close to 1.

On thermal diffusion in binary and ternary Lennard-Jones mixtures by non-equilibrium molecular dynamics

Molecular simulation appears to be an alternative to experiment for the estimation of transport and thermodynamics properties of fluid mixtures, which is of primary importance in the evaluation of the initial state of a petroleum reservoir. In this study, a non-equilibrium molecular dynamics algorithm has been applied to mixtures of Lennard-Jones spheres in order to compute the thermal diffusion process. The pertinence of such an approach to simple alkane mixtures is shown. The separate influences on the thermal diffusion of the molecular features in binary equimolar mixtures are then summarized. Simulations on binary non-equimolar mixtures have been performed as well. The results indicate an increase in the thermal diffusion process with increasing molar fraction of the lightest component. Moreover, this increase is enhanced with increasing difference in the number of carbons between the two alkanes. Then, a simple method, which yields results consistent with simulations, is proposed to predict thermal diffusion for the whole range of molar fractions starting only from the equimolar value. Finally, for ternary mixtures, the law of the corresponding states is shown to be valid when the appropriate mixing rules are applied, which allows the estimation of thermal diffusion in such mixtures from equivalent binary mixtures.

MASS EFFECT ON THERMODIFFUSION USING MOLECULAR DYNAMICS

Abstract The scope of this study is to improve the understanding of the thermal diffusion process on a microscopic scale by studying the mass effect on thermal diffusion factors. To achieve such a goal, non-equilibrium molecular dynamics simulations are performed on binary mixtures of simple Lennard–Jones spheres for a large range of thermodynamic states. Mixtures for which only the mass between species differs, up to mass ratios of 50, are analysed (isotope-like mixtures). In equimolar mixtures, it is shown that the link between the thermal diffusion factors and the ratio between the difference in masses and the sum of masses holds approximately for all states studied. In addition, it is found that this link strongly depends on density but weakly on temperature. In nonequimolar mixtures, results indicate that the effect of the mass ratio between species depends on the molar fraction. Using the data computed, a simple density-dependent correlation is proposed to quantify the mass effect in Lennard-Jones binary mixtures. Finally, it is shown that, taking into account only the mass effect, this correlation is able to provide a reasonable estimation of thermodiusion in n-pentane/n-decane mixtures, which underlines the intrinsic weakness of some of the usual thermodynamic models predicting thermodiffusion.

A molecular dynamics study of thermal diffusion in a porous medium

It is the main aim of this work to identify the microscopic origin of the perturbations brought about by a porous environment on the thermal diffusion (Soret effect) of a liquid mixture. For this purpose, nonequilibrium molecular dynamics simulations are carried out on model systems representing hydrocarbons in a porous medium. In keeping with previous simulations, a simplified model,i.e., Lennard-Jones spheres, is used for representing a methanedecane mixture, while the porosity is modeled by the inclusion of quasi-harmonic solids of various sizes and shapes. The model parameters are chosen to yield the proper order of magnitude for a silicate and its interactions with the alkanes. The model was first validated by investigating the equilibrium properties of the system. Then the thermal influence of the porous medium was evaluated and the adsorbing behaviour of the alkanes on the pores was characterized. It is found that the Soret coefficient of the equimolar mixture studied here is lowered by about 30% at 75% porosity. We find also that this reduction is strongly dependent on the structure of the porous medium.

Theoretical studies of [n]paracyclophanes and their valence isomers

SummaryThe valence isomerisations of benzene, [6]- and [7]paracyclophane to their Dewar benzene and prismane isomers are studied with the MNDO method using the unrestricted Hartree-Fock (UHF) and the configuration interaction (C.I.) approximations. The enthalpy of the reaction Dewar benzene → benzene is ΔH°r=−68.9 kcal/mol and the activation enthalpy is ΔH°‡=27.9 kcal/mol (with C.I.). The reaction path hasC2v symmetry.The determination of several points of the lowest potential energy surface of [6]- and [7]paracyclophanes leads to a minimum reaction path having the same topology as for the potential energy surface of the nonbridged benzene. The only difference is a quantitative change in the energy values of the aromatic isomers due to the deformation introduced by the alkyl chain. For [6]paracyclophane, the activation enthalpy is ΔH°‡=24.6 kcal/mol and the activation entropy is ΔS0‡=0.6 cal K−1 mol−1 calculated with C.I.The enthalpy of the reaction prismane → Dewar benzene is ΔH°r≈−32 kcal/mol and the activation enthalpy is ΔH°‡≈19 kcal/mol. The highest molecular symmetry group common to both molecules isC2v, whereas the symmetry group of the reaction path is lowered toCs. Along this reaction path is located a biradicaloid intermediate, separated by low activation barriers from the products. No significant changes of the potential energy surfaces are found for the bridged [n]prismanes and the [n]Dewar benzenes.All the calculated values, reaction enthalpies, activation enthalpies and entropies, are in a good agreement with literature experimental data.

Conformational analysis of [n]paracyclophanes and [n] Dewar benzenes by simulated annealing and local methods

Abstract In this work, we present the application of our recently developed simulated annealing algorithm and local search methods within semiempirical approaches to [ n ]paracyclophanes and their Dewar benzene isomers. The geometries of the most stable conformations of the [ n ]paracyclophanes agree with the known experimental X-ray data for the solid phase. Moreover, further conformations have been found which are expected to be present at room temperature only for long chains ( n > 7). However, several conformations of [ n ]Dewar benzenes are sufficiently close in energy to be present at room temperature. The entropies of the various conformations for a given value of n only vary slightly. In the case of [6]paracyclophane, a more detailed study of the configuration space has been done and conformations found which are not yet known experimentally. Substituents at the aromatic ring are expected to increase the percentage of these conformations. The transition states for the conversions between pairs of conformations are located. The barrier between the conformations which are known experimentally is found to be high (in agreement with experimental data).