Why Do Liquids Mix? The Mixing of Protic Ionic Liquids Sharing theSame Cation Is Apparently Driven by Enthalpy, Not Entropy

We study hydrogen bond (HB) redistribution in mixtures of two protic ionicliquids (PILs) sharing the same cation: triethylammonium methanesulfonate ([TEA][OMs])and triethylammonium trifluoromethanesulfonate ([TEA][OTf]). The mixtures exhibit largenegative energies of mixing. Based on results obtained from atomic detail moleculardynamics (MD) simulations, we derive a lattice model, discriminating between HB andnonspecificintermolecular interactions. We demonstrate that due to the ordered structure ofthe PILs, mostly the HB interactions contribute to the mixing energy. This allows to us toconnect the equilibrium of HBs to each of the two anion species with the correspondingexcess energies and entropies. The entropy associated with HB redistribution is shown to benegative, and even overcompensating the positive entropy associated with a statisticaldistribution of the ions in the mixture. This is strongly suggesting that the mixing process isdriven by enthalpy, not entropy