Influence of dipole moment and anisotropy on entropy scaling of Stockmayer and diatomic fluids

Abstract

It is shown that the shear viscosity, self-diffusion coefficient, and thermal conductivity of dipolar and anisotropic fluids in their entropy scaled form do not fall on a single univariate function. Instead, they depend on the dipole moment, elongation, and temperature. These insights result from an extensive equilibrium molecular dynamics (MD) simulation series of  36 dipolar two-center Lennard-Jones plus point dipole (2CLJD) model fluids across a large parameter space, covering the squared dipole moment 0 ≤ μ2/(4πɛ0ɛσ3) ≤ 20 and the elongation 0 ≤ L/σ ≤ 1. In total, 4830 thermodynamic state points are considered, spanning liquid and gaseous sub- and supercritical conditions. The MD data are correlated both with the plus entropy scaling (ES+) and residual entropy scaling (RES) frameworks. Correlations with 10–15 parameters for the three transport properties are developed, reproducing the underlying simulation data within 3.6%–12.4%. Zero-density limit transport properties, which are required for the RES approach, are obtained from MD extrapolations of the kinetic contribution to the transport properties, and subsequently modeled with a new 2CLJD correlation. The ES+ and RES correlations are assessed for five real fluids: R12, R123, R134a, R142b, and R152a. By employing a property-specific prefactor, the average absolute relative deviation of the ES+ and RES correlations to reference data for shear viscosity and thermal conductivity is 4.3% and 6.5% (ES+) and 12.0% and 16.4% (RES), respectively. The expected scale invariance for state points with a Pearson correlation coefficient R > 0.9 is disrupted, except at T/Tc = 10, where the plus-scaled transport properties of dipolar, anisotropic fluids coincide with those of non-polar, monatomic fluids.