Molecular simulation of phase equilibria for water-n-butane and water-n-hexane mixtures

Monte Carlo simulations were performed to obtain the Henry's law constants of n-butane and n-hexane in water and of water in n-hexane at different temperatures. Literature intermolecular potential functions optimized to the pure component vapor-liquid coexistence properties were used for the calculations, The Widom test-particle insertion technique was used for the infinite dilution properties of water in n-hexane. This methodology becomes impractical for large solute molecules in dense solvents. A combination of Widom insertions of a small weakly interacting molecule and expanded ensemble simulations was used to determine the Henry's law constants of n-butane and n-hexane in water. As the hydrocarbon size increased, simulation results were found to deviate from experimental data. These deviations are likely due to limitations of the potential models employed.