Molecular Dynamics Study of the Transfer of Iodide across Two Liquid/Liquid Interfaces

Abstract

This work focuses on the study of the properties of two liquid/liquid interfaces, the H2O/2-heptanone and the H2O/iso-octane interfaces, and on the transfer of the iodide ion across them. A detailed study of the properties of the first interface was already reported (J. Phys. Chem. B, 1999, in press). The iso-octane liquid is a hydrophobic analog of the very hydrophilic 2-heptanone, and the properties of the H2O/iso-octane interface are analyzed here and compared with the ones obtained for the H2O/2-heptanone system. It is shown that the basic features characterizing the interface structure (such as the non-existence of a mixed solvent region or the broadening of the sharp interface by capillary waves) are almost unaffected by the change of the hydrophilic nature of the organic solvent. A new method is proposed to calculate more accurately properties which depend on the distance to the interface. In the case of density profiles, the application of this method reveals that both liquids are packed in layers against the interface. This structural pattern, not detectable using classical methods, allows us to understand the reason for the oscillations in the density profiles calculated perpendicularly to the interfacial plane, an unsolved problem for more than one decade. The free energy profiles for the transfer of iodide across the two interfaces are computed and compared. In both cases they show a monotonous decrease in the free energy as the ion moves from the organic solvent into water. The value obtained for the Gibbs free energy of transfer is in good agreement with the available experimental data. In addition, the mechanism of the ion transfer is investigated. The process of transfer from the water phase to the organic one and the reverse process involve, in both cases, the formation of a water cone that connects the hydration sphere of the ion to the water phase. This water cone is a chain of molecules that can be as long as 10 Å. After the disruption and retraction of the water cone, the ion in the organic solvent retains part of its first hydration shell. The mechanism of the transfer through both interfaces is, in qualitative terms, very similar, although the ion transfer free energies are very different, as expected considering the relative hydrophilicity of the present solvents.