Computer simulation of molecular fluid mixtures: Results and two new one‐fluid models

Abstract

(Sowers and Sandler, 1991, 1992 1 for pure site-site Lennard-Jones fluids from a combination of perturbation theory and a three-parameter corresponding states principle are extended to mixtures using simple one-fluid mixing rules. These simple mixing rules allow us to make satisfactory predictions of the compressibilities computed from simulation without the use of binary interaction parameters. We also show that the effect of composition on the perturbation free energy @A,/N not accounted for by one-fluid mixing rules is quite small, and therefore a more complicated mixing rule is not necessary in our equations of state for the mixtures studied. IntroductionIn previous work (Sowers and Sandler, 1991, 1992), we presented new simulation results for pure fluids of nonspherical molecules, and two new analytic equations of state that correlate the simulation data with good accuracy. These equations are based on a three-parameter corresponding states principle similar to that used in perturbed-hard-chain theory. Here, we consider binary mixtures of nonspherical molecules that differ both in size and shape. Because only a limited quantity of simulation results for mixtures of molecular fluids has been published in the literature, we present new simulation data for mixtures of spheres with diatomics and linear triatomics, and for mixtures of diatomics with different bond lengths, all interacting with the site-site Lennard-Jones potential. In each mixture, the ratio of the molecular volumes of the two species varied from one to six, and results were obtained for the compressibility 2 = P/pkT and configurational internal energy of the mixture over a wide range of densities. Also, we use one-fluid mixing rules in our equations of state to make predictions for the model fluids and evaluate different mixing rules by comparing the predictions with simulation data. We find that the compressibility can be predicted with an average error of approximately 5% using one-fluid mixing rules with no binary interaction parameters. By dividing the configurational Helmholtz free energy into reference and perturbation parts and directly calculating the value of the perturbation contribution using the simulation results, we show that for the mixtures studied here the deviation of the perturbation free energy from that predicted using a one-fluid model is quite small.Before proceeding further, we briefly review the available simulation and theoretical results for mixtures of nonspherical molecules. Binary mixtures of hard diatomics were studied using Monte Carlo simulation by Aviram and Tildesley (1978) and Wojcik and Gubbins (1983). Mixtures of hard spheres with hard diatomics and linear hard triatomics were simulated by Nezbeda et al. (1985). This study was extended to mixtures of nonlinear hard triatomics with several different types of molecules by Vortler et al. (1989). These simulations of mixtures of nonspherical hard molecules showed that the compressibility of the mixture is influenced substantially by the AIChE JournalApril 1993...

An accurate Van der Waals-type equation of state for the Lennard-Jones fluid

“…One reason for equations of this form is that they allow a highly accurate description of real fluids with only a few substance-specific parameters [6]. For mixtures, we believe in agreement with Sowers and Sandler [7] that special mixing rules are required for F n and F A and, finally, also for the polar contributions to F [8]. To achieve a satisfying mixing rule for F A one needs a rather accurate equation for this term.…”

Ισορροπια Φασεων Σε Ασυμμετρα Συστηματα:απο Απλα Ρευστα Μεχρι Πολυμερη

“…Η δεύτερη μέθοδος για την αξιολόγηση των κανόνων συνδυασμού ξεκινά από τον τρόπο ελέγχου της ποιότητας των κανόνων ανάμιξης με βάση δεδομένα μοριακής προσομοίωσης για την ογκομετρική συμπεριφορά ή την ισορροπία φάσεων μιγμάτων (π.χ. : • Takamiya και Nakanfshi, 1988, 1994α• Miyano, 1991• Koutras et ai, 1992• Sowers και Sandler, 1993• Georgoulaki et ai, 1994. Αυτό είναι δυνατό επειδή οι διαμοριακές αλληλεπιδράσεις ορίζονται πριν από την εκτέλεση της προσομοίωσης και επομένως Κανόνες συνδυασμού σε μίγματα με ασύμμετρα συστατικά 3-5 της διαμοριακής απόστασης, από το οποίο καταλήγουμε στο δεύτερο δραστικό συντελεστή (Harismiadis και Szleifer, 1994).…”

“…Mixing rules are conveniently tested against molecular simulation data, either of the volumetric behavior or the phase equilibria of mixtures (see for example Harismiadis et ai, 1991Harismiadis et ai, , 1994Miyano, 1991;Koutras et ai, 1992;Sowers and Sandler, 1993;Georgoulaki et ai, 1994). This is possible because the interactions between the molecules are defined before performing a simulation.…”