Ions at interfaces: the central role of hydration and hydrophobicity (original) (raw)
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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.
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.
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.
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.
Specific Ion Adsorption and Surface Forces in Colloid Science
The Journal of Physical Chemistry B, 2008
Mean-field theories that include nonelectrostatic interactions acting on ions near interfaces have been found to accommodate many experimentally observed ion specific effects. However, it is clear that this approach does not fully account for the liquid molecular structure and hydration effects. This is now improved by using parametrized ionic potentials deduced from recent nonprimitive model molecular dynamics (MD) simulations in a generalized Poisson-Boltzmann equation. We investigate how ion distributions and double layer forces depend on the choice of background salt. There is a strong ion specific double layer force set up due to unequal ion specific short-range potentials acting between ions and surfaces.
On the Nature of Ions at the Liquid Water Surface
Annual Review of Physical Chemistry, 2006
Key Words surface enhancement, electrolytes, interfacial ions, second harmonic generation, Jones-Ray effect, Hofmeister series ■ Abstract A qualitatively new understanding of the nature of ions at the liquid water surface is emerging. Traditionally, the characterization of liquid surfaces has been limited to macroscopic experimental techniques such as surface tension and electrostatic potential measurements, wherein the microscopic picture then has to be inferred by applying theoretical models. Because the surface tension of electrolyte solutions generally increases with ion concentration, all inorganic ions have been thought to be repelled from the air-water interface, leaving the outermost surface layer essentially devoid of ions. This oversimplified picture has recently been challenged: first by chemical kinetics measurements, then by theoretical molecular dynamics simulations using polarizable models, and most recently by new surface sensitive experimental observations. Here we present an overview of the nature of the interfacial structure of electrolyte solutions and give a detailed description of the new picture that is emerging.
Journal of Physical Chemistry B, 2001
According to the TATB (tetraphenylarsonium tetraphenylborate) assumption, large isosterical ions of opposite charge have identical free energies of solvation in any solvent. In this context, we present a molecular dynamics study of the solvation of tetrahedral (AsPh 4 + vs BPh 4-) and large spherical (S + vs S-) ions in water using the recently developed TIP5P model. The results markedly differ from those obtained in TIP3P water and are in better agreement with the TATB hypothesis. According to free energy perturbation calculations, Sis better hydrated than S + , but the difference, ∆G +-, in hydration energies is much weaker in TIP5P (3.2 kcal/mol) than in TIP3P water (27.3 kcal/mol) which leads to an artifactually positive electrostatic potential at the center of the neutral S 0 species. BPh 4is better hydrated than AsPh 4 + and, contrary to the TATB assumption, ∆G +markedly depends on the details of charge distributions. The set8 charges equally diluted on all atoms lead to ∆G +of 4.3 kcal/mol only, much less than the value obtained with the set1 ESP charges (25 kcal/ mol). These values are much smaller than those obtained in TIP3P water, but still indicate some preference for the anion hydration. The differences are discussed from hydration patterns, electrostatic potentials and solute-solvent interactions. Simulations of AsPh 4 + and BPh 4at the (TIP5P)water-chloroform interface confirm the high surface activity of both ions, despite their "symmetrical structure". These results are important for our understanding of the influence of water models on calculated hydration and association of hydrophobic species in pure and mixed liquid environments.
Ion-specific interfacial behaviors of monovalent halides impact processes such as protein denaturation, interfacial stability, and surface tension modulation, and as such, their molecular and thermodynamic underpinnings garner much attention. We use molecular dynamics simulations of monovalent anions in water to explore effects on distant interfaces. We observe long-ranged ion-induced perturbations of the aqueous environment, as suggested by experiment and theory. Surface stable ions, characterized as such by minima in potentials of mean force computed using umbrella sampling MD simulations, induce larger interfacial fluctuations compared to nonsurface active species, conferring more entropy approaching the interface. Smaller anions and cations show no interfacial potential of mean force minima. The difference is traced to hydration shell properties of the anions, and the coupling of these shells with distant solvent. The effects correlate with the positions of the anions in the Hofmeister series (acknowledging variations in force field ability to recapitulate essential underlying physics), suggesting how differences in induced, nonlocal perturbations of interfaces may be related to different specific-ion effects in dilute biophysical and nanomaterial systems.
Atomistic simulation of ion solvation in water explains surface preference of halides
Proceedings of the National Academy of Sciences, 2011
Water is a demanding partner. It strongly attracts ions, yet some halide anions-chloride, bromide, and iodide-are expelled to the air/water interface. This has important implications for chemistry in the atmosphere, including the ozone cycle. We present a quantitative analysis of the energetics of ion solvation based on molecular simulations of all stable alkali and halide ions in water droplets. The potentials of mean force for Cl − ,Br − , and I − have shallow minima near the surface. We demonstrate that these minima derive from more favorable water-water interaction energy when the ions are partially desolvated. Alkali cations are on the inside because of the favorable ion-water energy, whereas F − is driven inside by entropy. Models attempting to explain the surface preference based on one or more ion properties such as polarizability or size are shown to lead to qualitative and quantitative errors, prompting a paradigm shift in chemistry away from such simplifications.