Vapour-liquid equilibria for Stockmayer fluids

Abstract

Results of Monte Carlo simulations in the Gibbs ensemble for the vapourliquid equilibria of Stockmayer fluids are presented. The vapour-liquid curves, critical temperatures and critical densities are calculated for dipolar strengths of #.2 = #2/err3 = 1'0 and 2.0. Comparison of these results shows that perturbation theory over-estimates the critical point.The Stockmayer potential is a convenient model to study the influence of dipolar interaction on the properties of polar fluids. Although the Stockmayer fluid has been frequently studied using computer simulation techniques [1][2][3][4][5][6][7], an accurate calculation of the vapour-liquid curves has yet to be published. Because the long-range interactions require substantially more computer time than, for example, the Lennard-Jones fluid, using the conventional techniques [8] the calculation of the phase diagram of a Stockmayer fluid would be an enormous task. However, a new simulation technique proposed by Panagiotopoulos [9][10][11], which samples the Gibbs ensemble, drastically reduces the amount of computer time to calculate the vapour-liquid curve. Using this elegant technique one can obtain from a single simulation data on the coexisting vapour and liquid phases. In this article we present the results of Gibbs ensemble simulations for the Stockmayer fluid for #.2 = 1.0 and 2.0. Furthermore, the results of the calculations are compared with the perturbation theory of 13].For most of the simulations we have used 216 particles. Close to the critical temperature and at low temperatures some simulations were performed with 512 particles. The Lennard-Jones potential was truncated at half the box size and the standard long-tail corrections were added. The long-range dipolar interactions were handled with the standard Ewald summation technique using 'tinfoil' boundary conditions [14]. The Gibbs ensemble simulations were performed in cycles, each cycle having three steps. In the first step the particles of both boxes were given successively a new position and new orientation in such a way that approximately 50 per cent of the new configurations were accepted. In the second step the volume of the sub-systems was changed (keeping the total volume constant) with an acceptance of 50 per cent. In the final step we have performed Ntry attempts to exchange particles between the two boxes.

Liquid-vapor interface of the Stockmayer fluid in a uniform external field

“…For E * = 0 and µ * = 1.0, T * c = 1.415 and ρ * c = 0.309, which agree well with GEMC results reported by van Leeuwen and Smit [47]: T * c = 1.41 ± 0.01 and ρ * c = 0.30 ± 0.01. Applying the external field parallel to the interface increases the critical temperature, while applying the field perpendicular to the interface decreases the critical temperature as shown in Figure 4, which is in agreement with DFT calculations [19].…”

The Boundary Condition in the Gibbs Ensemble Simulation of a Stockmayer Fluid under an Applied Field

“…The relation between the boundary condition, the external field and the Maxwell field of the system is well established [2,3,4,20]. The internal consistency between these three properties has been demonstrated by simulation [21,10].…”

A theoretical equation of state for real quadrupolar fluids

“…(4)(5)(6)(7)(8)(9)(10)(11) They are based mainly on the pioneering work of Stell et al (12,13) and tested against molecular simulations. (8,(14)(15)(16)(17)(18)(19)(20)(21) A perturbation theory for polar fluids developed by Benavides et al (22)(23)(24) uses an effective potential that takes into account overlap and dispersion forces via a square-well potential and the electrostatic forces arising from interactions between point multipoles at a To whom correspondence should be addressed (E-mail: alb@ifugl.ugto.mx). 4.55 (−4.65 ± 0.08), (33) (−4.67 ± 0.33), (2) (−4.67 ± 0.3), (2) (5.00 ± 0.5), (2) 5.00, (2) (−4.90 ± 0.3), (2) (5.00 ± 0.3), (2) 4.67, (2) 5.34 (2) CO 2 14.99 (−14.27 ± 0.61), (33) (−14.98 ± 0.5), (34) −14.54 (35) (−14.34 ± 0.7), (2) (−15.24 ± 1.0), (2) (14.68 ± 2.67), (2) 15.01, (2) 14.34, (2) −15.01, (2) (−14.98 ± 0.5) (2) the molecular centres.…”