Solvation of "big" spherical solutes in room temperature ionic liquids and at their aqueous interface: A molecular dynamics simulation study

We report a molecular dynamics study of the solvation of spherical solutes S-q of ca. 5 angstrom radius in ionic liquids (ILs) based on the PF6- anion and the 1-butyl-3-methylimidazolium cation (BMI+) or 1-octyl-3-methylimidazolium cation (OMI+), as a function of the sign and magnitude of the charge q=0, +/-1, +/-2 and 4 of S-q. The simulations highlight the dual solvation properties of the ILs, namely the importance of polar moieties of the ILs to solvate the most charged solutes and of the apolar imidazolium alkyl chains to solvate the less charged or neutral solutes. For a given sign of the q charge, the amount of IL counterions in the first solvation shell of S-q increases with the magnitude of q. Furthermore, comparison of the "dry" and "humid" forms of the [BMI][PF6] liquid shows that the humidity has a larger impact on the solvation of the "hydrophilic", compared to the "hydrophobic" solutes, and the number of coordinated H2O molecules increases with the magnitude of the charge of S-q. Finally the distribution of S+ versus S- ions with different counterions (S- or Cl-, and S+ or Na+, respectively) and of S-0 neutral analogues is investigated at the water/[BMI][PF6] and water/[OMI][PF6] interfaces, showing that the "big" spherical solutes, although lacking the amphiphilic topology, are surface active. Their interfacial distribution depends on the sign of the charge of S-q, as well as on the hydrophilic/hydrophobic character of the counterions. The results are compared with those obtained at classical aqueous interfaces (e.g. with chloroform) allowing us to better understand the specificity of the interfaces with ionic liquids, with important implications as far as the mechanism of liquid-liquid ion extraction is concerned. (C) 2006 Elsevier B.V. All rights reserved.