(367c) Enthalpy of Mixing and Liquid Structure of Primary and Secondary Alcohols

Recently, Mathias [Ind. Eng. Chem. Res. 58, 12465 (2019)] showed that the enthalpy of mixing needs to be included in the training data for activity coefficient models to capture their temperature dependence. An unusual behavior was observed for mixtures of short-chain alcohols: the mixing primary and secondary alcohols is found to be an exothermic process, whereas the mixing of two primary alcohols is endothermic. A common explanation for the exothermic behavior is favorable cross-association leading to substantial structural changes for the mixture. Here, two predictive approaches (molecular simulation with the TraPPE united-atom force field and the COSMO-SAC implicit-solvent/activity coefficient model) are applied to probe the mixing behavior of short-chain alcohols, including methanol mixed with 1-propanol, 2-propanol, 1-butanol, and 2-butanol. With standard parameterization, both models fail to capture the exothermic mixing for primary–secondary alcohol mixtures. To improve the predictive ability of force-field-based simulations, the partial charge on the oxygen atom of the secondary alcohols is increased by 10%, and a 5% larger van der Waals radius for their alpha-carbon is adopted. The proposed effective model is further evaluated through computation of liquid densities of both pure compounds and mixtures, vapor pressures at the experimental normal boiling points of pure alcohols, and dielectric constants at 293 K of pure alcohols. Structural analyses (radial distribution functions, number integrals, local composition enhancements, numbers of hydrogen bonds per alcohol molecule, and cluster size distributions) point to very subtle differences between primary–primary and primary–secondary alcohol mixtures and do not support the assumption of substantial structural changes to distinguish endothermic and exothermic mixing processes.