Monte Carlo Study of the Thermodynamic and Structural Properties of 1-Butanethiol + N-Heptane Mixtures

Abstract

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 the pure compounds, there is no guarantee that they will describe the interactions between dissimilar molecules in the mixture correctly. In this study, Monte Carlo simulations have been carried out in the isothermal-isobaric (NPT) ensemble to calculate the density and excess enthalpy 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 PES2 used the OPLS parameters with slightly modified partial charges. Simulations were performed on mixtures with butyl mercaptan mole fractions of 0.0, 0.23, 0.42, 0.62, 0.83, and 1.0. The average rms deviation between the calculated densities for PES1 and PES2 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 kcal/mol and 0.027 kcal/mol, respectively. We also compared our calculated densities with the COSTALD correlation. The extent of self-association of the butanethiol molecules was found it to be small for all of the mixtures for the PES2 model.