Thermodynamic Analysis of n-Hexane/Ethanol Binary Mixtures Using Kirkwood-Buff Theory.

A complete thermodynamic analysis of mixtures consisting of molecules with complex chemical constitution can be rather demanding. Kirkwood-Buff theory of solutions allows the estimation of thermodynamic properties which cannot be directly extracted from atomistic simulations, such as the Gibbs energy of mixing (Δmix G). In this work we perform molecular dynamics simulations of n-hexane/ethanol binary mixtures in the liquid state under two temperature-pressure conditions and at various mole fractions. Based on the recently published methodology of Galata et al. [ Fluid Phase Equilib. 2018, 470, 25-37], we first calculate the Kirkwood-Buff (KB) integrals in the isothermal-isobaric ( NpT) ensemble, identifying how system size affects their estimation. We then extract the activity coefficients, excess Gibbs energy, excess enthalpy, and excess entropy for the n-hexane/ethanol binary mixtures we simulate. We employ two approaches for quantifying composition fluctuations: one based on counting molecular centers of mass, and a second one based on counting molecular segments. Results from the two approaches are practically indistinguishable. We compare our results against predictions of vapor-liquid equilibria obtained in a previous simulation work using the same force field, as well as with experimental data, and find very good agreement. In addition, we develop a simple methodology to identify the hydrogen bonds between ethanol molecules and analyze their effects on mixing properties.