We report computer simulations of the phase behavior of dipolar (ferro) fluids. We consider a model in which the dispersive interactions can be varied independently from the dipolar (magnetic) interactions. The simulation results show that a minimum amount of dispersive energy is required to observe liquid-vapor coexistence. If the dispersive energy is below this threshold, as for example in the dipolar hard-sphere fluid, the system forms chains of dipoles aligning nose to tail. Our simulations did not give any evidence that these "polymerlike" systems phase separate into a liquid and vapor phase.PACS numbers: 64.70.Fx, 64.60.Cn, 75.50.Mm, 82.20.Wt The seminal work of Alder and Wainwright has shown that hard-sphere interactions alone are sufficient to observe a fluid-solid phase transition [1], For a vapor-liquid phase transition attractive forces are required to provide the cohesive energy to stabilize the liquid phase [2]. Since all common molecular fluids have a liquid phase, it is tempting to assume that, if attractive forces are present, vapor-liquid equilibria will always be observed at sufficiently low temperature. This point of view is, however, too simplistic. For example, experiments on colloidal suspensions have shown that "liquid-vapor" phase coexistence is only observed when the range of the attractive forces is sufficiently large as compared to the diameter of the colloidal particles [3]. If the range is too small, the (metastable) critical temperature is lower than the triple temperature and hence no vapor-liquid equilibrium is observed.In this Letter, we consider the effects of dipolar inter-where JJ, is the dipole moment and \i i is the orientation of the dipole of particle i, r^ is the distance between the particles, and A controls the strength of the dispersive interactions. Note that we focus on a ferrofluid in zero magnetic field. It is interesting to consider some limiting cases of this model. For A = 1, the model is identical to the Stockmayer fluid [2] for which the vapor-liquid curve has been calculated using computer simulations [5,6]. For A = 0, the model reduces to a dipolar soft-sphere fluid which is similar to the dipolar hard-sphere fluid. For the dipolar hard-sphere fluid, the existence of a liquid phase also appears to be well established. Since the orientationally averaged interaction between two dipoles is a van der Waals-like 1/r 6 attraction, de Gennes and Pincus conjectured a vapor-liquid coexistence similar to that of a conventional van der Waals fluid [7]. Kalikmanov [8] actions on the vapor-liquid curve. In particular, we pose the question whether dipolar interactions are sufficient to stabilize a liquid phase. Since all polar fluids have a liquid phase, one might be inclined to assume that the answer to this question is affirmative. However, in real polar fluids the dispersive van der Waals forces can never be ignored and hence we cannot conclude that dipolar forces alone suffice to stabilize the liquid phase. The effects of dipolar interactions can also be investig...

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Several parameter sets have been presented in the literature for a three-site united-atom model for methanol.We compare the Gibbs ensemble Monte Carlo simulation results for the prediction of vapor-liquid equilibrium for the various sets. Furthermore, we present a new parameter set, which predicts phase coexistence properties of methanol with higher accuracy over a wide range of temperatures and densities.

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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.