Pa. Fernandes et al., Molecular dynamics study of the transfer of iodide across two liquid/liquid interfaces, J PHYS CH B, 103(42), 1999, pp. 8930-8939
This work focuses on the study of the properties of two liquid/liquid inter
faces, the H2O/2-heptanone and the H2O/iso-octane interfaces, and on the tr
ansfer 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 N2O/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 (s
uch 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 i
nterface. In the case of density profiles, the application of this method r
eveals that both liquids are packed in layers against the interface. This s
tructural pattern, not detectable using classical methods, allows us to und
erstand 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 monoto
nous 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 mecha
nism 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 cas
es, 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 b
e as long as 10 Angstrom. After the disruption and retraction of the water
cone, the ion in the organic solvent retains part of its first hydration sh
ell. The mechanism of the transfer through both interfaces is, in qualitati
ve terms, very similar, although the ion transfer free energies are very di
fferent, as expected considering the relative hydrophilicity of the present
solvents.