MONTE-CARLO STUDY OF THE THERMODYNAMIC AND STRUCTURAL-PROPERTIES OF 1-BUTANETHIOL PLUS N-HEPTANE MIXTURES

The ability to use computer simulations to predict mixture properties using potential models that have been optimized for the pure compounds is demonstrated. Since the potential models were optimized only for t he pure compounds, there is no guarantee that they will describe the i nteractions between dissimilar molecules in the mixture correctly. In this study, Monte Carlo simulations have been carried out in the isoth ermal-isobaric(NPT) ensemble to calculate the density and excess entha lpy for 1-butanethiol + n-heptane mixtures at 298.15 K and 1 atm. The OPLS potential-parameters developed by Jorgensen were used to describe the n-heptane molecule. Two models for the butanethiol molecule were employed: PES1 used the OPLS potential-parameters unaltered, while PES 2 used the OPLS parameters with slightly modified partial charges. Sim ulations were performed on mixtures with butyl mercaptan mole fraction s of 0.0, 0.23, 0.42, 0.62, 0.83, and 1.0. The average rms deviation b etween the calculated densities for PES1 and PESZ and the experimental results is 0.021 g/cc and 0.015 g/cc, respectively, while the average rms deviation for the excess enthalpies for PES1 and PES2 is 0.058 kc al/mol and 0.027 kcal/mol, respectively. We also compared our calculat ed densities with the COSTALD correlation. The extent of self-associat ion of the butanethiol molecules was found it to be small for all of t he mixtures for the PES2 model.