Molecular dynamics studies for the new refrigerant R152a with simple model potentials

Abstract

The new refrigerant R152a (CH₃-CHF₂) is modelled as a fluid of homonuclear two-centre Lennard-Jones molecules with a point dipole along the axis. This 2CLJD potential has four parameters which were determined by using results of previous and new molecular dynamics simulations for 2CLJD molecules of elongation L* = 0·505 and different reduced dipole moments μ*. For each of these dipole moments zero pressure liquid densities at two temperatures were taken to determine the Lennard-Jones parameters ϵ and σ from a fit to experimental saturated liquid densities. A subsequent comparison of calculated second virial coefficients with experimental values led to the conclusion that μ*² = 8 in combination with ϵ/k = 119·0K and σ = 3.845 Å is a reasonable choice. Further simulations with those parameters, all performed with vectorized codes on a CYBER 205, gave reasonable predictions of the thermodynamic properties. As the reduced dipole moment μ*² = 8 corresponds to 2·73 D compared to the experimental value of 2·27 D, the effect of rotating the dipole vector out of the molecular axis was also investigated. It is found that for a dipole vector forming some angle with the molecular axis a smaller value of the dipole moment is required to produce similar thermodynamic properties as for the case when the dipole is along the axis. Spherical harmonic expansion coefficients g_u′m of the pair correlation function are given for the model with and without the dipole and also for three different orientations of the dipole vector with respect to the molecular axis.