About the TATB assumption: effect of charge reversal on transfer of large spherical ions from aqueous to non-aqueous solvents and on their interfacial behaviour
Résumé
We present a molecular dynamics study of the solvation properties of large spherical ions S(+) and S(-) of same size, in water, chloroform and acetonitrile solutions, and at a water-chloroform interface. According to the "extrathermodynamic" TATE hypothesis, such ions have identical free energies of transfer from water to any solvent. We find that this is not the case, because S(-) interacts better than S(+) with water (by about 20 kcal mol(-1)), while S(+) is better solvated by acetonitrile (by about 2 kcal mol(-1)) and chloroform (about 8 kcal mol(-1)) solvents. The importance of "long-range" electrostatic interactions on the charge discrimination by solvent is demonstrated by the comparison of standard vs corrected methods to calculate: (i) the electrostatic potential at the centre of the solute; (ii) the interaction energies between the ions and the solvents, and (iii) the free energies of charging the neutral sphere S(0) to S(+) and S(-), respectively. These conclusions are obtained with several solvent models and simulation conditions. The question of ion pairing for the S(+)S(-), S(+)Cl(-) and S(-)Na(+) pairs is also examined in the three solvents. Finally, simulations at a liquid-liquid water-chloroform interface represented explicitly, show that S(+) and S(-) are highly surface active, although they do not possess, like classical surfactants, an amphiphilic topology. Adsorption at the interface is found with different methodologies and at different ion concentrations. 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 liquid environments. (C) 2000 Elsevier Science B.V. All rights reserved.