Vapour-liquid equilibrium of Stockmayer fluids in applied field - Application of the NpTE plus test particle method and perturbation theory

The influence of a static homogeneous applied electric field E on the vapour-liquid phase equilibrium of Stockmayer fluids is investigated by two methods. The first is an extension of Gubbins-Pople-Stell perturbation theory (PT) of polar liquids in the presence of an applied electric field. This paper proposes a new simulation technique, the NpTE plus test particle method, developed to determine the vapour-liquid equilibrium of polar fluids in the presence of an applied field. It is based on the three-dimensional Taylor series expansion of the thermodynamic function (y) over tilde (this is the Legendre transformation of the chemical potential g with respect to the polarization m:(y) over tilde = beta (g) over tilde = beta(g-mE)) as a function of the intensive parameters beta, p and E up to third order around a suitably selected raw point (beta(0)p(0), E(0)). The zero order term comes from the test particle method; the first- and the higher-order coefficients of the series can be derived from running averages and fluctuation formulas, respectively, by performing Monte Carlo simulations for a gas and a liquid phase raw point. The condition of the equilibrium is the equality of the functions (y) over tilde in the two phases. Vapour-liquid equilibria of the Stockmayer fluids with reduced dipole moments mu*(2) = 1 and 2 are studied at four different reduced electric field strengths. It is found that the vapour pressure, the vapour density and both dielectric constants decrease, while the liquid density increases with increasing applied field strength at fixed temperatures. The PT reproduces the simulation results qualitatively in most of the cases. The phenomena of electrostriction and dielectric saturation (e.g., the variation of the density and the dielectric constant with the field strength at constant pressure) are also studied and a quadratic field dependence is detected. Both methods show that the critical temperature increases quadratically with the field strength.