Vapor-liquid equilibria simulation and an equation of state contribution for dipole-quadrupole interactions

A systematic investigation on vapor-liquid equilibria (VLE) of dipolar and quadrupolar fluids is carried out by molecular simulation to develop a new Helmholtz energy contribution for equations of state (EOS). Twelve two-center Lennard-Jones plus point dipole and point quadrupole model fluids (2CLJDQ) are studied for different reduced dipolar moments mu^{*2}=6,12, reduced quadrupolar moments Q^{*2}=2,4 and reduced elongations L^*=0, 0.505,1. Temperatures cover a wide range from about 55% to about 95% of the critical temperature of each fluid. The NpT + test particle method is used for the calculation of vapor pressure, saturated densities and saturated enthalpies.Critical data and the acentric factor are obtained from fits to the simulation data.On the basis of this data, an EOS contribution for the dipole-quadrupole cross-interactions of non-spherical molecules is developed. The expression is based on a third-order perturbation theory and the model constants are adjusted to the present 2CLJDQ simulation results. When applied to mixtures, the model is found to be in excellent agreement to results from simulation and experiment. The new EOS contribution is also compatible with segment-based EOS, like the various forms of the Statistical Associating Fluid Theory (SAFT) EOS.