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.

Molecular Dynamics Study of the Interface between Water and 2-Nitrophenyl Octyl Ether

“…This is not straightforward. Several previous studies 10, [13][14][15][16][17][18] have computed the total width of the interface, usually from the mean square deviation of the interface position, and applied it directly in equation (6) together with rough estimates of ξ (ranging from 0.4 to 0.9 nm). The rather loose interpretation of these parameters has led to frequent disagreement with interfacial tensions calculated by the virial route.…”

“…This picture was confirmed in nearly all subsequent simulation studies of realistic liquid/liquid interfaces, and led to attempts to model these systems using capillary wave theory (CWT) 11,12 . Several authors 10,[13][14][15][16][17][18] have attempted to calculate the interfacial tension based on the original form of CWT, but quantitative agreement with experiment and with values calculated from the simulations by alternative routes was not always obtained.…”

“…The most obvious example of this is the density profile, which exhibits a transition from one bulk density to the other across a relatively narrow interfacial region 9,10,[13][14][15][16][17][18][19][20][21] . Other properties have been calculated for several water/organic interfaces, including radial distributions functions (RDFs) 10,15,17,19 , hydrogen bonds 9,10,[13][14][15][17][18][19]21 , molecular orientations 9,10,[13][14][15][17][18][19][20][21][22][23][24] and self-diffusion coefficients 10,[13][14][15]23 .…”

“…In an effort to circumvent this problem, Fernandes et al 16 have proposed a procedure for decoupling fluctuations of the interfacial plane from those occurring perpendicularly to the interface. Using this method, they calculated density profiles that showed pronounced oscillations in the vicinity of the interface, which died out as the bulk phases were approached.…”

Calculation of the intrinsic solvation free energy profile of methane across a liquid/liquid interface in computer simulations

“…these methods have been widely used for a number of non-ionic [19][20][21][22][23][24][25][26][27][28] and ionic penetrants [29][30][31][32][33][34][35][36][37][38][39][40][41][42][43][44] at various fluid interfaces.…”