Computer Simulation Study of Binary Symmetrical Yukawa Fluids
Tamaghna Chakraborti1, Jhumpa Adhikari1
1Department of Chemical Engineering, IIT Bombay,
Powai, Mumbai – 400076.
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 al1 had reported the phase diagram of binary symmetrical systems interacting via the square well potential. Schöll-Paschinger et al2 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 Errington3, 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 µ1 - µ2 = 0 are in close agreement with self-consistent Ornstein Zernike approximation results of Schöll-Paschinger et al2.
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).