Characteristic Curves of Polar Fluids: (I) The Two-Center Lennard–Jones Plus Dipole Fluid

Abstract

Brown’s characteristic curves of polar fluids were studied using molecular simulation and molecular-based equation of state. The focus was on elucidating the influence of dipole interactions and the molecule elongation on the characteristic curves. This was studied using the symmetric two-center Lennard–Jones plus point dipole (2CLJD) model fluid class. This model class has two parameters (using Lennard–Jones reduced units), namely the elongation and the dipole moment. These parameters were varied in the range relevant for real substance models that are based on the 2CLJD model class. In total, 43 model fluids were studied. Interestingly, the elongation is found to have a stronger influence on the characteristic curves compared to the dipole moment. Most importantly, the characteristic curve results for the 2CLJD fluid are fully conform with Brown’s postulates (which were originally derived for simple spherical dispersive fluids). The independent predictions from the computer experiments and the theory are found to be in reasonable agreement. From the molecular simulation results, an empirical correlation for the characteristic curves of the 2CLJD model as a function of the model parameters was developed and also applied for modeling real substances. Additionally, the intersection points of the Charles and Boyle curve with the vapor-liquid equilibrium binodal and spinodal, respectively, were studied.