Predicting and Rationalizing the Soret Coefficient of Binary Lennard‐Jones Mixtures in the Liquid State

Abstract

The thermodiffusion behavior of binary Lennard‐Jones mixtures in the liquid state is investigated by combining the individual strengths of non‐equilibrium molecular dynamics (NEMD) and equilibrium molecular dynamics (EMD) simulations. On the one hand, boundary‐driven NEMD simulations are useful to quickly predict Soret coefficients because they are easy to set up and straightforward to analyze. However, careful interpolation is required because the mean temperature in the measurement region does not exactly reach the target temperature. On the other hand, EMD simulations attain the target temperature precisely and yield a multitude of properties that clarify the microscopic origins of Soret coefficient trends. An analysis of the Soret coefficient suggests a straightforward dependence on the thermodynamic properties, whereas its dependence on dynamic properties is far more complex. Furthermore, a limit of applicability of a popular theoretical model, which mainly relies on thermodynamic data, was identified by virtue of an uncertainty analysis in conjunction with efficient empirical Soret coefficient predictions, which rely on model parameters instead of simulation output. Finally, the present study underscores that a combination of predictive models and EMD and NEMD simulations is a powerful approach to shed light onto the thermodiffusion behavior of liquid mixtures.

Modeling the Coupled Mass‐Heat Transport in Lennard–Jones‐Like Binary Mixtures by Approach‐to‐Equilibrium Molecular Dynamics

“…The negligible role of pressure is also supported by two features observed during the NVE run: i) no significant increase of the average temperature of the system, indicating a negligible conversion of (initial) mechanical to thermal energy, ii) direct inspection of the time evolution of average concentration differences reveal, in all the cases of study, a clear exponential decay. The robustness of the method is confirmed by the benchmark calculations we performed using as reference two different equimolar LJ mixtures, reported in Table 2, previously investigated by Zimmermann et al [14] and Bonella et al [8] In order to perform benchmark calculations, we adopted the same simulation parameters stated in the two studies, as well as matching, in the case of ref. [8] also the timestep and the LJ cutoff radius, which were set to 0.005 and 3.21, respectively.…”

“…is introduced, the constraint w 1 = 1 − w 2 must hold. Therefore, as also reported by Zimmermann, [14] the state of affairs expressed in Equation ( 6) is completely summarized by focusing on the study of just ⃗ J q,1 and ⃗ J m,1 while the dependence from chemical potential can be re-expressed in terms of the concentration of mass fraction, obtaining…”

“…while the off-diagonal elements 𝛽 ′ , 𝛽 ′′ are related to the Soret coefficient. In fact (as reported in literature [14] ), the Soret coefficient  T can be expressed as…”

“…where it is understood that Onsager off-diagonal coefficients are equal L 𝜇q = L q𝜇 . [6] The link existing between the experimental accessible quantities and the Onsager coefficients is expressed by [5,14,15]…”

Molecular Dynamics Simulation of the Thermal Diffusion Effect in n-Alkane Binary Mixtures

“…To provide a complete view of the thermal diffusion effect, more MD simulations of a wider range of n -alkane binary mixtures under different pressure and temperature conditions need to be performed. Second, different MD simulation procedures and settings were observed in various studies. ,, For a specific case (for example, a n C 5 – n C 10 binary mixture), the MD simulation results obtained by different researchers show large deviations due to the differences in MD settings. Our investigations show that the temperature gradient, size of the simulation box, number of molecules, and simulation procedures have a significant effect on the thermal diffusion factor or Soret coefficient.…”