Effect of the Range of Interactions on the Properties of Fluids. Phase Equilibria in Pure Carbon Dioxide, Acetone, Methanol, and Water Matthias Kettler, ² Ivo Nezbeda,* ,‡ Ariel A. Chialvo, §,‡ and Peter T. Cummings §,| Institute of Thermodynamics and Thermal Process Engineering, UniVersity of Stuttgart, 70550 Stuttgart, Germany, Department of Chemical Engineering, UniVersity of Tennessee, KnoxVille, Tennessee 37996-2200, Chemical Sciences DiVision, High-Temperature Aqueous Chemistry Group, Oak Ridge National Laboratory, Oak Ridge, Tennessee 37831-6110, and Department of Chemical Engineering, Chemistry, and Computer Science, UniVersity of Tennessee, KnoxVille, Tennessee 37996-2200 ReceiVed: January 16, 2002; In Final Form: April 23, 2002 The effect of the long-range Coulombic interactions on the vapor-liquid equilibria properties of polar and associating fluids has been investigated, by considering typical representatives of these classes of fluids, namely, carbon dioxide, acetone, methanol, and water, defined by realistic intermolecular pair potential models. Using the same decomposition of realistic potential models into a short-range part and a residual part as in previous papers [Kolafa, J.; Nezbeda, I. Mol. Phys. 2000, 98, 1505-1520. Kolafa, J.; Nezbeda, I.; Lısal, M. Mol. Phys. 2001, 99, 1751-1764], we carried out Gibbs ensemble simulations on both the full and short- range models to determine the thermodynamic properties of the considered compounds along the vapor- liquid coexistence curve. In addition, we also considered methanol in two homogeneous phases, liquid and supercritical, to determine its structure and thermodynamic properties. We have found that the long-range interactions affect all considered properties only marginally and that the short-range system provides a reasonably accurate and reliable zeroth-order approximation. A simple theoretical analysis has also been made to explain and estimate the effect of the long-range interactions on the thermodynamic properties both in the homogeneous phase and at phase equilibrium. 1. Introduction To find a relation between the observed properties of fluids and the intermolecular interactions acting between their mol- ecules is one of the most important goals of statistical mechanics. Knowledge of such relations helps us not only to understand and predict the properties of fluids, but also makes it possible to develop simple model Hamiltonians (simple models) upon which a theory, and hence workable equations, may be developed. As a typical example consider the various hard-body fluid models, whose origin lies in the recognition that the structure of normal (i.e., nonpolar) fluids is determined primarily by strong short-range repulsive interactions. 1 However, the concept of a simple decomposition of the total pair potential into a repulsive part and an attractive part is not generally thought to be applicable to strongly polar and associating fluids. These fluids are characterized by the presence of the long-range Coulombic interactions which have been believed to be the main contributing factor to their properties. Consequently, these interactions have explicitly been incorpo- rated into simple models designed to capture the essence of physics of polar of associating fluids. 2 Although the theory for such models has reached remarkable results, 3-6 their application to real fluids is rather cumbersome and in the case of associating fluids it encounters fundamental difficulties. Following the idea that it is rather the range of interactions which matters more than the interaction itself, 7 recently we have investigated in detail the effect of the long- and short-range forces on the structure and thermodynamic properties of nonsimple fluids. 8-11 We have considered three classes of fluidssquadrupolar, dipolar, and associatingsand a typical representative within each classscarbon dioxide for the first class, acetone for the second, and water for the third. Starting from realistic Hamiltonians, we constructed short-range models [see eq 5 below], and surprisingly, we found that the long-range Coulombic interactions play, in fact, only a marginal role. The conclusions which can be drawn from these investigations can be summarized as follows: 1. In general, in all cases the effect of the long-range forces on the spatial arrangement of the molecules is very small. Specifically, the structure of the realistic fluids and their short- range counterparts, described by the set of the site-site correlation functions, is very similar (nearly identical). 2. As a consequence of finding (1), the thermodynamic properties of the realistic fluids are very well estimated by those of the short-range models: the internal energy of the short- range models accounts for at least 95% of the total internal energy of the full realistic fluids. 3. The long-range forces affect only details of the orientational correlations and hence, to a certain extent, also pressure. However, integral quantities, such as, e.g., the dielectric constant, remain unaffected. All above findings are important from the theoretical point of view because they open the possibility of developing a fast * To whom correspondence should be addressed. E-mail: IvoNez@ icpf.cas.cz. On leave of absence from E. Ha´la Laboratory of Thermodynam- ics, Academy of Sciences, 165 02 Prague 6, and Department of Physics, Purkyneˇ University, 496 00 UÄ stı n. Lab., Czech Republic. ² Institute of Thermodynamics and Thermal Process Engineering. ‡ Department of Chemical Engineering. § Chemical Sciences Division. | Department of Chemical Engineering, Chemistry, and Computer Science. 7537 J. Phys. Chem. B 2002, 106, 7537-7546 10.1021/jp020139r CCC: $22.00 © 2002 American Chemical Society Published on Web 07/10/2002