VAPOR-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 b y two methods. The first is an extension of Gubbins-Pople-Stell pertur bation theory (PT) of polar liquids in the presence of an applied elec tric field. This paper proposes a new simulation technique, the NpTE p lus test particle method, developed to determine the vapour-liquid equ ilibrium of polar fluids in the presence of an applied field. It is ba sed on the three-dimensional Taylor series expansion of the thermodyna mic function (y) over tilde (this is the Legendre transformation of th e chemical potential g with respect to the polarization m:(y) over til de = 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 li quid phase raw point. The condition of the equilibrium is the equality of the functions (y) over tilde in the two phases. Vapour-liquid equi libria 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 die lectric constants decrease, while the liquid density increases with in creasing applied field strength at fixed temperatures. The PT reproduc es the simulation results qualitatively in most of the cases. The phen omena of electrostriction and dielectric saturation (e.g., the variati on of the density and the dielectric constant with the field strength at constant pressure) are also studied and a quadratic field dependenc e is detected. Both methods show that the critical temperature increas es quadratically with the field strength.