Molecular Mechanism for Azeotrope Formation in Ethanol/Benzene Binary Mixtures through Gibbs Ensemble Monte Carlo Simulation

Abstract

Azeotropes have been studied for decades due to the challenges they impose on separation processes but fundamental understanding at the molecular level remains limited. Although molecular simulation has demonstrated its capability of predicting mixture vapor-liquid equilibrium (VLE) behaviors, including azeotropes, its potential for mechanistic investigation has not been fully exploited. In this study, we use the united atom transferable potentials for phase equilibria (TraPPE-UA) force-eld to model the ethanol/benzene mixture, which displays a positive azeotrope. Gibbs ensemble Monte Carlo (GEMC) simulation is performed to predict the VLE phase diagram, including an azeotrope point. The results accurately agree with experimental measurements. We argue that the molecular mechanism of azeotrope formation cannot be fully understood by studying the mixture liquid-state stability at the azeotrope point alone. Rather, azeotrope occurrence is only a re ection of the changing relative volatility between the two components over a much wider composition range. A thermodynamic criterion is thus proposed based on the comparison of partial excess Gibbs energy between the components. In the ethanol/benzene system, molecular energetics shows that with increasing ethanol mole fraction, its volatility initially decreases but later plateaus, while benzene volatility is initially nearly constant and only starts to decrease when its mole fraction is low. Analysis of the mixture liquid structure, including a detailed investigation of ethanol hydrogen-bonding con gurations at di erent composition levels, reveals the underlying molecular mechanism for the changing volatilities responsible for the azeotrope.

An atomistic explanation of the ethanol–water azeotrope

“…While we find several studies dedicated to low ethanol molecular percentages, only two studies, both simulations, have been dedicated to ethanol molecular percentages close to that of the azeotrope (up to 90%). 30,31 On the microscopic scale, the azeotropic behaviour of ethanol-water mixtures has been attributed to concentration-dependent changes in binary cluster-like structures in the bulk, 11,12 but this picture is criticized 17 for a positive azeotrope. However, as discussed above, it is the surface composition that really matters for the gas-liquid equilibrium.…”

“…While the results differ in details, most studies agree that the two components tend to form cluster-like structures in the mixture. However, it has also been ascertained 17 that, so far, no direct experimental evidence of stoichiometric mixed clusters in the azeotrope is available. Second, a critical aspect of the vapour/liquid equilibrium is how the vapour composition depends on the liquid composition.…”

Driver versus navigator causation in biology: the case of insulin and fasting glucose

“…If one starts from a non-equilibrium mixture, the rate of heating affects the time required to reach the azeotropic equilibrium but does not affect the equilibrium composition. This is because the molecular dynamics that decides the equilibrium has more complex elements than simple evaporation ( Li et al, 2020 ) It is possible that in a given system some causal factors affect both steady and perturbed states where as others affect only one of them. For example, stirring increases the rate of dissolution of a solute, but does not affect the saturation concentration.…”

Peer Review #1 of "Driver versus navigator causation in biology: the case of insulin and fasting glucose (v0.1)"

“…If one starts from a non-equilibrium mixture, the rate of heating affects the time required to reach the azeotropic equilibrium but does not affect the equilibrium composition. This is because the molecular dynamics that decides the equilibrium has more complex elements than simple evaporation(D. Li et al, 2020) It is possible that in a given system some causal factors affect both steady and perturbed states where as others affect only one of them. The differences between driver and navigator causes may also be specific to the context.…”