Chemical Potential of Benzene Fluid from Monte Carlo Simulation with Anisotropic United Atom Model

The profile of chemical potential of benzene fluid has been investigated using Anisotropic United Atom (AUA) model. A Monte Carlo simulation in canonical ensemble was done to obtain the isotherm of benzene fluid, from which the excess part of chemical potential was calculated. A surge of potential energy is observed during the simulation at high temperature which is related to the gas-liquid phase transition. The isotherm profile indicates the tendency of benzene to condensate due to the strong attractive interaction. The results show that the chemical potential of benzene rapidly deviates from its ideal gas counterpart even at low density. Keywords: Benzene, Anisotropic United Atom, chemical potential, Monte Carlo simulation INTRODUCTION Benzene is an attractive subject from both scientific and industrial perspectives. It represents the interaction of aromatic π-systems which immensely control the stability of protein and nucleic acid [1]. Benzene is widely used in laboratory, petrochemical, and other industry either as additive, solvent, or raw materials. The extent of benzene usage increases the risk of benzene exposure to the environment. US Enviromental Protection Agency classified benzene as a carcinogen, and recommended activated carbon adsorption as a control to prevent benzene intrusion to the environment [2]. Grand Canonical Monte Carlo (GCMC) simulation is useful to study the adsorption of a compound at equilibrium condition, in which the chemical potential of component in the adsorbed phase is equivalent to the bulk phase [3]. Nguyen and co-workers had investigated the adsorption of benzene in graphitic slit pore in the presence of water molecules [4], and Klomkiang had reported the multilayer adsorption of benzene on a graphite surface [5]. Both used GCMC simulation with TraPP-EH model of benzene [6] which consists of 12 Lennard Jones and Coulomb interaction sites; thus the calculation of potential energy of a system with N molecules of benzene would involve 24×N(N−1) pair-interactions. The chemical potential of benzene was treated by assuming benzene fluid as an ideal gas in both works. Contreras and co-workers had reported the usage of Anisotropic United Atoms (AUA) model of benzene in their Monte Carlo simulation [7]. In this model, an interaction site is located between carbon and hydrogen atom, making a total of six interaction sites in benzene. The number of pair interaction that occurs between two molecules of benzene is then 6×N(N−1), which is 1/16 times fewer than TraPP-EH model; thus reducing the J. Pure App. Chem. Res., 2013, 2 (3), 115-­‐121 1 December 2013 X The journal homepage www.jpacr.ub.ac.id ISSN : 2302 -­‐ 4690 116 computational resources needed to carry out the simulation. The model has been reported to accurately reproduce the physical properties of benzene [7]. Similarly, the rigid body model of water, for example, is known to reproduce many important properties of water [8] with less computational cost as compared to some flexible models of water [9]. In case of cyclic organic compound model, the inclusion of pseudorotation in a twist-model of tetrahydrofuran has been reported to give a negligible difference on thermodynamic data obtained with a simpler planar model [10]. While the AUA is a promising model to be utilized in the study of benzene system, the chemical potential of benzene fluid (gas and liquid) using this model is not known. The chemical potential information is essential to carry out a GCMC simulation for the study of adsorption, for example. Even though some works [4, 5] had reported the treatment of chemical potential of benzene using ideal gas assumption, it is intriguing to question the density limit of which benzene fluid can be fairly treated as an ideal gas. In this work, we provide a realistic chemical potential of benzene fluid of AUA model using simple calculation from the isotherm data which were priorly obtained using Monte Carlo simulation in canonical ensemble. The characteristic of potential energy and chemical potential of this model is also discussed. We show that the chemical potential of benzene fluid rapidly deviates from ideal state even at low density due to the strong intermolecular interaction. SIMULATION METHOD Interaction model The potential energy of system Φ is described as the sum of pair-interaction φij between a pair of benzenes (noted here as i and j) in the absence of any external field