Molecular Simulation of Transport Properties of Polar Fluids

This work presents a comprehensive molecular dynamics study of transport properties in polar fluids using the Green–Kubo method, covering self-diffusion, binary Maxwell–Stefan diffusion, shear and bulk viscosity, and thermal conductivity for pure fluids and binary mixtures. The fluids are represented with two-center Lennard-Jones models augmented by either a point quadrupole (2CLJQ) or point dipole (2CLJD). First, predictions based on literature models fitted only to vapor–liquid equilibrium data are benchmarked against experiments and correlations. Self-diffusion coefficients are obtained with ~3% statistical uncertainty and generally agree well with experiments, showing typical deviations of ~5% (gas) and ~10% (liquid) for simple Lennard-Jones fluids and broader but still reasonable deviations for various quadrupolar fluids. For mixtures, Maxwell–Stefan diffusion is computed and, where experimental data exist (Ar/Kr/Xe gas mixtures), deviations are about 10%; mixture self-diffusion data are also used to test diffusion relations, with Darken’s equation performing best. For liquid mixtures such as N₂+CO₂, N₂+C₂H₆, and CO₂+C₂H₆, Darken’s equation again yields the most accurate Maxwell–Stefan predictions (≈10% average deviation), while the Caldwell–Babb and Vignes expressions fail to capture the composition dependence. Shear viscosity and thermal conductivity are determined with ~10% uncertainty and match experimental correlations well for many quadrupolar and dipolar fluids, whereas bulk viscosity shows large discrepancies that may reflect experimental uncertainty. A practical correction is proposed for slow convergence of Green–Kubo integrals at high density by analytically extrapolating the autocorrelation tail, improving shear-viscosity estimates by up to ~20% without extra computation. Second, a systematic parameter study quantifies how anisotropy and polarity influence transport by scanning reduced elongation and quadrupole/dipole strengths over ranges representative of real fluids, yielding large state-point datasets along bubble lines and in homogeneous liquid regions. Across both model classes, transport properties depend predominantly on density: increasing density lowers self-diffusion and raises shear viscosity and thermal conductivity, while increasing temperature in the liquid generally increases self-diffusion and decreases viscosity. For 2CLJQ fluids, stronger quadrupolar interactions and greater elongation reduce self-diffusion and increase viscosity and thermal conductivity, with temperature effects on thermal conductivity being weak within uncertainty. For 2CLJD fluids, dipolar interactions have a stronger and more nuanced impact, including an increase of self-diffusion with dipole strength above a threshold for some cases and a clear dipole-driven increase in viscosity and thermal conductivity even for highly elongated molecules. Overall, the results provide detailed, physically interpretable trends for polar-fluid transport and offer a data basis for improving predictive theories and developing more physically grounded equations of state.