Thermodynamic regimes over which homologous alkane fluids can be treated as simple liquids

Abstract

We compare the extent to which the properties of simple liquids are pertinent to short to moderate chain n-alkanes ranging from ethane (C2) to dodecane (C12). Explicit geometric features such as bonds, angles and dihedral potentials in currently available models of n-alkanes make these systems more realistic, and distinct from the generalized Lennard-Jones chain fluids. Our study confirms that the presence of these flexible geometric constraints completely suppresses the energy-virial correlation in these systems. However, they are found to have a strong energy-virial correlation in high density region of their phase diagram when the contribution from these geometric constraints are excluded. For fluids having simple liquid like behavior, semi-quantitative relationships between structure, dynamics, and thermodynamics are well established. Range of state points is explored to test the applicability of such relationships for n-alkanes. Considering the collection of monomer beads as the reference state, thermodynamic and structural entropic measures are systematically compared with and without intramolecular contributions. We show that the pair entropy computed with explicit intramolecular geometric constraints correlates well with thermodynamic excess entropy. Both the thermodynamic and pair entropy have strong isochore dependence with reduced diffusivity. Intermolecular pair entropy correlates well with reduced diffusivity. The role of multiparticle correlations is highlighted for predicting thermodynamic and transport properties in these chain systems. Triplet correlations in addition to pair correlations are used as an attempt to improve the structural contribution to excess entropy. We show that if the three particle contribution is included in the computation of structural entropy, the resulting value overestimates the excess entropy.