Computational investigation of the hydration of alkyl diammonium chlorides and their effect on THF/water phase separation.

The solvation behavior of alkyl diammonium chlorides of varying alkyl chain length and the molecular details of their effect on the salting-out of organic molecules in aqueous phase have been investigated by classical molecular dynamics simulations. More specifically, systems containing water, tetrahydrofuran (THF), and their mixtures with α,ω-alkyl diammonium chlorides [H3N(CH2)nNH3]Cl2 (n = 2, 4, 6, 8, and 10) were simulated at ambient temperature and pressure. Various force fields were tested and one was chosen based on its ability to reproduce the physical properties of the pure THF solution and its mixture with water. Structural and thermodynamic analyses of the simulated salt-solvent mixtures reveal different extents of hydration of the dications depending on the alkyl chain length and indicate that the hydrophobic interactions between the dication alkyl chain and organic molecules play a key role in the solvation of the latter species. In fact, shorter dications are shown to promote THF/water phase separation, in agreement with previous experimental findings.