Liquid–Vapor Interfacial Tension in Alkane Mixtures: Improving Predictive Capabilities of Molecular Dynamics Simulations

Surface Tension of Two Near-Ideal Binary Liquid Mixtures and the Influence of Adjacent Vapors

The measured surface tension of a binary liquid is found to depend strongly on the constituents of the adjacent vapor and on whether equilibrium has been achieved, giving insight into the complex interfacial configuration. This dependence is quantified by three techniques that offer complementary insights: surface tension measurements with a constrained sessile drop surrounded by different vapors, surface tension measurements by surface light scattering spectroscopy in a sealed cell at equilibrium, and molecular dynamics simulations of the equilibrium surface tension and excess surface concentration. Ensuring homogeneity of the binary liquid, which is essential for surface light scattering, was found to be nontrivial and was assured by high-sensitivity Schlieren imaging. Two pairs of liquids, n-pentane with 2-methylpentane and n-pentane with n-hexane, were investigated. The first pair was motivated by the observed improvement in the effectiveness of binary fluids versus a single constituent in wickless heat pipes studied in microgravity. The second pair was used for comparison. Experimental evaluation of different volume fractions of the two liquids showed strong dependence of surface tension on the relative concentration of different molecules near the interfacial region. For the above pairs of liquids, which appear to form ideal mixtures in bulk, we present sufficiently precise surface tension measurements to indicate unexpectedly complex behaviors at interfaces.

Comprehensive Automated Routine Implementation, Validation, and Benchmark of the Anisotropic Force Field (AUA4) Using Python and GROMACS

Molecular simulation users are sometimes discouraged from using specific molecular models because of the inconvenience of finding the force field parameters and preparing and validating the topology files. To facilitate this process and make the accurate anisotropic force field AUA4 available to molecular dynamics users, we have created and validated an automated topology and coordinate file creation routine for the GROMACS molecular simulation software. In the present work, we describe the AUA4, explain its particularities and how it was implemented, thoroughly validating the implementation, and for the first time, perform a molecular dynamics benchmark for this transferable force field. Several properties were computed, namely, liquid density, vapor pressure, and vaporization enthalpy by conducting explicit vapor-liquid interface simulations. The results evidence the correct implementation showing slight deviations from the parametrization studies. The benchmark shows the superior predictive capability of the AUA4 in recreating liquid density (RMSD equal to 17.0 kg/m³) and vaporization enthalpy (RMSD equal to 1.3 kJ/mol) compared to other transferable force fields. In addition, its superior computational time performance doubles or even triples compared to an all-atom force field such as the OPLS, depending on whether the workstation counts with GPU.

Quantitative Predictions and Experimental Validation of Liquid-Vapor Interfacial Tension in Binary and Ternary Mixtures of Alkanes Using Molecular Dynamics Simulations

Liquid-vapor interfacial properties of alkane mixtures present a challenge for experimental determination, especially under conditions relevant to the energy industry processes. Molecular dynamics (MD) simulations can accurately predict interfacial tensions (IFTs) for complex alkane mixtures under virtually any conditions, thereby alleviating the need for difficult and costly experiments. MD simulations with the CHARMM force field and empirical corrections for the IFT and pressure were used to obtain the IFT for three binary mixtures of ethane (with n-pentane, n-hexane, and n-nonane) and a ternary system (ethane/n-butane/n-decane) under a variety of conditions. The results were thoroughly validated against experimental data from the literature, and new original IFT data were collected using the pendant drop method. The simulations are able to reproduce the experimental IFT to better than 0.5 mN/m or 5% on average and within 1 mN/m or 10% in the worst case. IFTs for the studied three binary and ternary alkane mixtures were predicted for wide ranges of conditions with no known experimental data. Finally, using the MD simulation data, the reliability of the widely used empirical parachor model for predicting IFT was reaffirmed, and the significance of the empirical parameters examined to establish an optimal balance between the accuracy and broad applicability of the model.