Hydrophobic Ions in TIP5P Water and at a Water−Chloroform Interface: The Effect of Sign Inversion Investigated by MD and FEP Simulations (original) (raw)
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Distribution of hydrophobic ions and their counterions at an aqueous liquid-liquid interface
HAL (Le Centre pour la Communication Scientifique Directe), 2004
We report a molecular dynamics study on the distribution of spherical hydrophobic ions S + and S-(radius ≈ 5.5 Å) and hydrophilic counterions (halide X-; alkali M +) at a water-"oil" interface, where "oil" is modeled by chloroform. The results reveal the surface activity of S + and S-, with marked counterion effects. The S + Ssalt fully adsorbs at the interface, which is electrically neutral, while in the S + Xseries, the anion concentration near the interface decreases in the Hofmeister order I-> Br-> Cl-> F-, thus increasing the change in interfacial electrostatic potential ∆φ. A similar effect is observed with the S-M + salts, when Cs + is compared to Na +. We also investigate the effect of ion charge sign reversal, and find a larger ∆φ for S + Nathan S-Na + salts, in relation with the higher hydration of the fictitious Naanion compared to the isosteric Na + cation. The effect of the magnitude of the ion charge is studied with the divalent S 2+ vs S 2ions and Navs Na + counterions. Despite their mutual repulsion, the S 2+ or S 2like-charged species tend to self-aggregate at the interface and in water as a result of hydrophobic association and, again, differences in distributions are observed upon sign reversal. With regard to the treatment of electrostatics, the Ewald and Reaction Field methods qualitatively yield similar trends, but the latter underestimates the repulsion between like ions at the interface and thus exaggerates the calculated difference in interfacial potential ∆φ. When compared to standard calculations, our results point to the importance of the treatment of cutoff boundaries on the distribution of hydrophilic counterions near the interface. Implications of these results concerning the mechanism of assisted ion transfer are discussed.
Journal of Physical Chemistry B, 2004
We report a molecular dynamics study on the distribution of spherical hydrophobic ions S + and S-(radius ≈ 5.5 Å) and hydrophilic counterions (halide X-; alkali M +) at a water-"oil" interface, where "oil" is modeled by chloroform. The results reveal the surface activity of S + and S-, with marked counterion effects. The S + Ssalt fully adsorbs at the interface, which is electrically neutral, while in the S + Xseries, the anion concentration near the interface decreases in the Hofmeister order I-> Br-> Cl-> F-, thus increasing the change in interfacial electrostatic potential ∆φ. A similar effect is observed with the S-M + salts, when Cs + is compared to Na +. We also investigate the effect of ion charge sign reversal, and find a larger ∆φ for S + Nathan S-Na + salts, in relation with the higher hydration of the fictitious Naanion compared to the isosteric Na + cation. The effect of the magnitude of the ion charge is studied with the divalent S 2+ vs S 2ions and Navs Na + counterions. Despite their mutual repulsion, the S 2+ or S 2like-charged species tend to self-aggregate at the interface and in water as a result of hydrophobic association and, again, differences in distributions are observed upon sign reversal. With regard to the treatment of electrostatics, the Ewald and Reaction Field methods qualitatively yield similar trends, but the latter underestimates the repulsion between like ions at the interface and thus exaggerates the calculated difference in interfacial potential ∆φ. When compared to standard calculations, our results point to the importance of the treatment of cutoff boundaries on the distribution of hydrophilic counterions near the interface. Implications of these results concerning the mechanism of assisted ion transfer are discussed.
Inorganica Chimica Acta, 2000
We report molecular dynamics studies on the interfacial distribution of ionic species of different size, shape and topology at a water/chloroform interface: hydrophilic K + Cl − , K + SCN − and K + Pic − ions, amphiphilic ammonium NTMA + cations and farnesylphosphate FPH − anions, tetrahedral hydrophobic AsPh 4 + and BPh 4 − ions, with different counterions. Contrasted distributions are observed. The K + Cl − and K + SCN − ions sit almost exclusively in the water phase, but SCN − is less 'repelled' than Cl − by the interface. The Pic − anions are partly adsorbed at the interface and dissolved in the water phase where they display remarkable p-stacking interactions. Amphiphilic NTMA + cations or FPH − anions adsorb and dilute at the interface. Less expected is the high surface activity of symmetrical AsPh 4 + and BPh 4 − ions, with marked counterion effects. The two ions fully adsorb at the interface in the AsPh 4 + BPh 4 − salt, while in the Na + BPh 4 − or AsPh 4 + Cl − salts, they display an equilibrium between the organic phase and the interface. Crossed comparisons between the different solutions reveal the important role of counterions on the distribution of a given ionic species. These results are discussed in relation to experimental data.
Ions at interfaces: the central role of hydration and hydrophobicity
Current Opinion in Colloid & Interface Science, 2016
It is increasingly being accepted that solvation properties of ions and interfaces (hydration of ions, hydrophobic or hydrophilic character of interfaces) play a fundamental role in ion-surface interaction in water. However, a fundamental understanding of the precise role of solvation in ionic specificity in colloidal systems is still missing, although an important progress has been made over the last years. We present in this contribution experimental evidences (including also ions not usually included in specific ion studies) together with Molecular Dynamics (MD) simulations that highlight the importance of the hydration of ions and surfaces in order to understand the origin of ionic specificity. We first show that both surface polarity and ion hydration determine the sorting of ions according to their ability to induce specific effects (the so-called Hofmeister series). We extend these classical series by considering the addition of the inorganic anions IO 3-, BrO 3 and ClO 3-, which present unusual properties as compared with the ions considered in classical Hofmeister series. We also consider big hydrophobic organic ions such as tetraphenylborate anion (Ph 4 Band nd tetraphenylarsonium cation (Ph 4 As +) that in the context of the Hofmeister series behave as super-chaotropes ions.
Physical Chemistry Chemical Physics, 2012
We present results from all-atom molecular dynamics simulations of large-scale hydrophobic plates solvated in NaCl and NaI salt solutions. As observed in studies of ions at the air-water interface, the density of iodide near the water-plate interface is significantly enhanced relative to chloride and in the bulk. This allows for the partial hydration of iodide while chloride remains more fully hydrated. In 1M solutions, iodide directly pushes the hydrophobes together (contributing −2.51 kcal/mol) to the PMF. Chloride, however, strengthens the water-induced contribution to the PMF by ~ −2.84 kcal/mol. These observations are enhanced in 3M solutions, consistent with the increased ion density in the vicinity of the hydrophobes. The different salt solutions influence changes in the critical hydrophobe separation distance and characteristic wetting/dewetting transitions. These differences are largely influenced by the ion-specific expulsion of iodide from bulk water. Results of this study are of general interest to the study of ions at interfaces and may lend insight to the mechanisms underlying the Hofmeister series.