About the TATB hypothesis: solvation of the Asϕ4+ and Bϕ4- ions and their tetrahedral and spherical analogues in aqueous/nonaqueous solvents and at a water–chloroform interface

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Abstract
Based on molecular dynamics (MD) and free energy (FEP) simulations, we investigate the effect of +/- charge on the solvation properties of large "‘symmetrical’' ions in water, acetonitrile and chloroform solutions. The nearly isostructural Asφ4 + and Bφ4 - tetrahedral ions, which have been assumed to display identical energies of transfer from water to any solvent ("‘TATB hypothesis’'), are found to display marked differences in solution. The anion interacts more than the cation with water, chloroform and acetonitrile, due to short-range electrostatic interactions, in relation to the solvent granularity and shape of the ion. The importance of charge distribution is demonstrated by the simulations on four different models of Asφ4 + and Bφ4 -, on fictitious Bφ4 + and Asφ4 - ions, and on neutral Bφ4 0 and Asφ4 0 analogues. The role of ion shape is demonstrated by MD and FEP simulations on isovolumic spherical S+ and S- ions, which also display marked differences in solvation properties, but opposite to those of Asφ4 + and Bφ4 -. In water, S+ is much better hydrated than S-, due to clathrate-type hydration around S+, while in acetonitrile, S+ and S- display similar solvation energies. The question of ion pairing is also examined in the three solvents. At a liquid–liquid water–chloroform interface represented explicitly, the Bφ4 - anion is found to be more surface active than Asφ4 +. A number of methodological issues are addressed in the paper. These results are important in the context of the TATB hypothesis and for our understanding of solvation of large hydrophobic ions in pure liquids or in heterogeneous environments like aqueous interfaces.

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