Ion-Specific Aggregation of Hydrophobic Particles

Although many properties of electrolyte solutions can be successfully described by theories at the McMillan Mayer level of approximation, there are other phenomena that cannot be explained without taking into account the explicit nature of solvent molecules. One of these that have received much attention is the Hofmeister effect that describes the influence of different types of ions on the solubility of hydrophobic molecules in water. In this work we use two simple water models, the 'fused-spheres' and the two-dimensional 'Mercedes-Benz' models to study ion solvation in water, and test suppositions about their effect on hydrophobicity. Both models give good qualitative agreement with experiment, such as Samoilov ion hydration activation energies, and Setchenow coefficients, which describe the salt concentration dependence of the solubilities of hydrophobic solutes. The results suggest that the interactions of ions with water are governed mostly by the ionic charge densities. Water structure is determined by the balance of electrostatic forces and the tendency for hydrogen bond formation. Ions with a high charge density bind water molecules very tightly and therefore exclude the hydrophobe from their first shell, leading to salting-out. The effect decreases with decreasing charge density of the ion. ᮊ

Journal of the American Chemical Society, 2007

The parameters used to describe ammonium and the alkyl-ammonium ions are based upon those of protonated lysine in the OPLS-AA force field (Ref 60 in main text). The van der Waals parameters were left unchanged; the partial charges of the constituent atoms of ammonium and alkyl-ammonium cations were assigned as listed below:

2010

Experimental evidence of a fully reversible aggregation process due to ionic specificity is presented for the first time. The results indicate the existence of repulsive structural forces between particles coming from the specific accumulation of poorly hydrated ions around hydrophobic surfaces. This result emphasizes the key role that ionwater structure near interfaces plays in Hofmeister effects, as recently suggested by molecular dynamics simulations.

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.

Quarterly reviews of biophysics, 1997

Advances in experimental and computational methodologies have led to a recent renewed interest in the Hofmeister series and its molecular origins. New results are surveyed and assessed. Insights into the underlying mechanisms have been gained, although deeper molecular understanding still seems to be elusive. The principal reason appears to be that the Hofmeister series emerges from a combination of a general effect of cosolutes (salts, etc.) on solvent structure, and of specific interactions between the cosolutes and the solute (protein or other biopolymer). Hence every system needs to be studied individually in detail, a state of affairs which is likely to continue for some time. A deeper understanding of the Hofmeister series can be an extraordinarily valuable guide to designing experiments, including not only those probing the series per se, but also those designed to elucidate the adsorption, aggregation and stabilization phenomena which underlie so many biological events. The ...

Advances in Colloid and Interface Science, 2019

Highlight  Ionic hydration stiffens the H-O and meanwhile softens the O:H phonon cooperatively.  Hydrating H2O dipoles full-screen a cation to keep its constant hydration volume.  Inter-solute repulsion limits an anionic hydration volume in a f(C) 1-exp(-C/C0) way.  Ionic negativity, polarity, radius, shape, and valence discriminate the performance of an ion.  Inter-solute repulsion, and ionic polarization stem the properties of a solution.

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.

Measurements of pH with a glass electrode that reveal a strong dependence on the ion pair of the background electrolyte and on the salt concentration are presented. Although such phenomena are well known, they are inexplicable in the classical theory based on the Nernst equation. It is shown that this (Hofmeister) effect can be understood once neglected ionic dispersion potentials acting between ions and the glass-water interface are taken into account. At high concentrations, those of interest in biology (g0.1 M), co-ions, are shown to play a profound and previously overlooked role in pH changes near glass-water interfaces. Measurements of pH via the response of oxide glasses is a long-established and commonly used technique. 1,2 Some controversy exists as to the true origin of the pH-glass electrode potential. The phenomena at the interface and within the glass electrode are very complex and are not yet fully understood. However, for a long time, it has been taken as axiomatic that the Nernst equation can be used to interpret the experimental results. 3,4 According to this equation, a potential is set up across a glass electrode that separates a reference electrolyte solution and an unknown electrolyte. This potential difference depends on the concentrations (or, strictly speaking, the activities) of hydronium ions in the different compartments. It can then be used to deduce the pH of the unknown electrolyte solution. The Nernst equation does not predict any dependence on the ion pair of the background salt or on its concentration. Consequently, few experiments have focused on these specific ion effects, which, where they occur, are generally assigned to solid-state glass properties. We report pH measurements that depend strongly on both the choice of salt solution and concentration. We then show that ionic dispersion forces acting between ions and interfaces can account for the phenomenon. The measured pH will in general also be influenced by structural changes in the glass electrode and ion-specific changes in the bulk hydronium activity coefficient. Hofmeister effects that are common in biology have presented a mystery to physical chemistry for more than 100 years. 5 A large number of applications in biology and physical chemistry depend strongly on the supposedly irrelevant choice of background salt and especially on the co-ion. Examples include double-layer forces, 6,7 the surface-tension increment with added salt at an air-water interface 8 and at an oil-water interface, 9 bubble-bubble interactions, 10 and pH measurements. 3,11,12 Experiments on colloid interactions have furthermore recently revealed an important and previously ignored role of co-ion specificity 13 and dissolved gas. 14 Since the only ionic property included in theories of double layers was the ionic charge, all monovalent salt solutions are expected to give the same result. However, this is not what is seen experimentally. It is clear that electrostatics is not the full story. It is often the case that dispersion forces acting on ions dominate, particularly at biological salt concentrations. High-frequency correlations that give rise to ion-specific dispersion effects are only weakly screened, and above 0.1 M, salt can dominate electrostatic

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.

The Journal of Chemical Physics, 2012

Ions induce both specific (Hofmeister) and non-specific (Coulomb) effects at aqueous interfaces. More than a century after their discovery, the origin of specific ion effects (SIE) still eludes explanation because the causal electrostatic and non-electrostatic interactions are neither local nor separable. Since direct Coulomb effects essentially vanish below ∼10 μM (i.e., at >50 nm average ion separations in water), we decided to investigate whether SIE operate at, hitherto unexplored, lower concentrations. Herein, we report the detection of SIE above ∼0.1 μM in experiments where relative iodide/bromide populations, χ = I − /Br − , were determined on the surface of aqueous (NaI + NaBr) jets by online electrospray mass spectrometry in the presence of variable XCl (X = H, Na, K, Cs, NH 4 , and N(C 4 H 9) 4) and NaY (Y = OH, Cl, NO 3 , and ClO 4) concentrations. We found that (1) all tested electrolytes begin to affect χ below ∼1 μM and (2) I − and Br − are preferentially suppressed by co-ions closely matching their interfacial affinities. We infer that these phenomena, by falling outside the reach of even the longest ranged electrostatic interactions, are dynamical in nature.