**Computer
Simulation Study of Binary Symmetrical Yukawa Fluids**

Tamaghna
Chakraborti^{1}, Jhumpa Adhikari^{1}

^{1}Department
of Chemical Engineering, IIT Bombay,

Powai, Mumbai – 400076.

(E-mail : tamaghna.chakraborti@gmail.com, adhikari@che.iitb.ac.in )

*Abstract*
- Knowledge and understanding of fluid phase behavior is extremely important
from the industrial perspective as it aids the design and optimization of
various separation processes. Binary fluid mixtures display a wide array of
fluid phase behavior ranging from simple vapor liquid equilibrium diagrams to
more complicated topology like azeotropy and heteroazeotropy. Such behavior is
intricately related to the intermolecular forces acting between ‘like' and
‘unlike' molecules and a study of the molecular parameters governing bulk
macroscopic phenomena is pertinent to the study of different types of
equilibrium existing between different forms of matter. As a first step towards
such understanding the phenomena of equilibrium of binary molecular systems,
the properties of binary monoatomic fluids, with none of the complicated
internal coordinates of complex systems, have been studied in this work. An
additional simplification made in this work is that of a symmetrical binary
system where the ‘like' molecules of the different components interact via
similar potential, *u(r)* amongst themselves and the ‘unlike' molecules
interact via the potential, *du(r)* where *d* is a scalar
parameter. Theoretical investigations have revealed a wide repertoire of interesting
correlations existing between macroscopic phenomena and molecular interaction
parameters like *d*. Wilding et al^{1}
had reported the phase diagram of binary symmetrical systems interacting via
the square well potential. Schöll-Paschinger et al^{2}
had investigated binary symmetrical systems interacting via hard core Yukawa
potential using self-consistent Ornstein-Zernike approximation as well as grand
canonical Monte Carlo simulations. In the current work, we investigate the
effect of change of interaction range on the topology of the phase diagrams.
The molecules interact via hard core Yukawa interaction and ‘unlike' molecules
tend to dislike one another such that *d < *1. The value of *d*
was taken to be 0.75, which makes our work in the regime where the
mixing-demixing transition line intersects the liquid vapor phase diagram far
away from the liquid vapor critical point. Grand canonical transition matrix
Monte Carlo simulations, using the formalism as envisaged by Shen and Errington^{3},
were performed for values of the interaction range parameters *κ* equal
to 1.8, 2.4 and 3.0. One of the main concerns was to discern the conditions of
temperature and pressure at which the system exhibits azeotropy and
heteroazeotropy. Our results indicate that the densities of the mixture, as predicted
by the above simulation algorithm, at *µ*_{1} - *µ*_{2}
= 0 are in close agreement with self-consistent Ornstein Zernike approximation
results of Schöll-Paschinger et al^{2}.

**References**

1. Wilding, N., Schmid, F. & Nielaba, P. Liquid-vapor phase behavior of a symmetrical binary
fluid mixture. *Phys. Rev. E* **58,** 2201–2212 (1998).

2. Schöll-Paschinger,
E., Levesque, D., Weis, J.-J. & Kahl, G. Phase diagram of a binary
symmetric hard-core Yukawa mixture. *J. Chem. Phys.* **122,** 024507
(2005).

3. Shen, V. K. &
Errington, J. R. Determination of fluid-phase behavior using transition-matrix
Monte Carlo: binary Lennard-Jones mixtures. *J. Chem. Phys.* **122,**
064508 (2005).

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