Phase transitions of quadrupolar fluids

Gibbs ensemble simulations are reported for Lennard-Jones particles with embedded quadrupoles of strength Q*=Q/(εσ5)1/2=2.0 where ε and σ are the Lennard-Jones parameters. Calculations revealing the effect of the dispersive forces on the liquid–vapor coexistence were carried out by scaling the attractive r−6 term in the Lennard-Jones pair potential by a factor λ ranging from 0 to 1. Liquid–vapor coexistence is observed for all values of λ including λ=0 for Q*=2.0, unlike the corresponding dipolar fluid studied by van Leeuwen and Smit et al. [Phys. Rev. Lett. 71, 3991 (1993)] which showed no phase transition below λ=0.35 when the reduced dipole moment μ*=2.0. The simulation data are analyzed to estimate the critical properties of the quadrupolar fluid and their dependence on the strength λ of the dispersive force. The critical temperature and pressure show a clear quadratic dependence on λ, while the density is less confidently identified as being linear in λ. The compressibility is roughly linear in λ.