Mixtures of dipolar with nonpolar linear molecules

Abstract

Thermodynamic properties of mixtures of dipolar with nonpolar linear molecules are obtained from NVT-MD simulations and from a perturbation theory based on nonspherical reference systems. In the simulations, the interactions are all of the same two-centre Lennard-Jones (2CLJ) type, of elongation L = 0·505 and with an additional axial point dipole on the polar molecules. Simulation raw data are the pressures, internal energies and dipolar energies for a grid of dipole moments and compositions at the state point T* = 3·078 and ρ* = 0·466. From these raw data, first the excess values of the internal energy and the Helmholtz energy for mixing at constant density are obtained. Then the excess values of the Gibbs energy, of the enthalpy and of the volume for mixing at constant pressure are derived. In perturbation theory, the equivalent Kihara interactions yield results which are in good agreement with the simulation data. Neither simulations nor theory, however, show a liquid-liquid phase separation for the state conditions and dipole moments considered. In addition, a reanalysis of previous simulation data for the Lennard-Jones-Stockmayer system does not necessarily confirm the claim that demixing has already been found there. Hence, since very recently demixing was found by RHNC theory for the hard sphere-dipolar hard sphere system, it remains a challenge to search for such a phase separation caused solely by the dipolar forces in mixtures with realistic interactions.