Ions at hydrophobic interfaces

Abstract

We review the present understanding of the behavior of ions at the air-water and oil-water interfaces. We argue that while the alkali metal cations remain strongly hydrated and are repelled from the hydrophobic surfaces, the anions must be classified into kosmotropes and chaotropes. The kosmotropes remain strongly hydrated in the vicinity of a hydrophobic surface, while the chaotropes loose their hydration shell and can become adsorbed to the interface. The mechanism of adsorption is still a subject of debate. Here, we argue that there are two driving forces for anionic adsorption: the hydrophobic cavitational energy and the interfacial electrostatic surface potential of water. While the cavitational contribution to ionic adsorption is now well accepted, the role of the electrostatic surface potential is much less clear. The difficulty is that even the sign of this potential is a subject of debate, with the ab initio and the classical force fields simulations predicting electrostatic surface potentials of opposite sign. In this paper, we will argue that the strong anionic adsorption found in the polarizable force field simulations is the result of the artificial electrostatic surface potential present in the classical water models. We will show that if the adsorption of anions would be as large as predicted by the polarizable force field simulations, the excess surface tension of NaI solution would be strongly negative, contrary to the experimental measurements. While the large polarizability of heavy halides is a fundamental property and must be included in realistic modeling of the electrolytes solutions, we argue that the point charge water models, studied so far, are incompatible with the polarizable ionic force fields when the translational symmetry is broken. The goal for the future should be the development of water models with very low electrostatic surface potential. We believe that such water models will be compatible with the polarizable force fields and can then be used to study the interaction of ions with hydrophobic surfaces and proteins.

Ion specific effects: decoupling ion–ion and ion–water interactions

“…The empirical polarizable energy as used in some molecular dynamics simulation 53, 70 was not considered. The experimentally measured values for Δ ψ for a two-state model as listed in Table 2, first column, are larger than the values from the simulations for surfaces whose solvent density or solvent dielectric constant is assumed to vary continuously across the surfaces 70–72 . For anions consisting of a single atom such as Cl − , Br − , and I − , the calculated Δ ψ matches the experimental Δ ψ value better when the image charge potential 44 at the surface and its hydrophobic hydration energy of the hydrated ion radius are considered.…”

Effective charges and zeta potentials of oil in water microemulsions in the presence of Hofmeister salts

“…[10][11][12][13][14][15][16][17][18] Ionic specificity was first suggested to be due to dispersion interactions between ions and water; 10 this, however, was later shown not to be the case. [19][20][21][22][23][24][25][26] Recently we developed a theory which allows us to calculate the excess surface tensions of electrolyte-air interface in very good agreement with the experimental data. 24,[26][27][28] In theory, the chaotropic anions are allowed to adsorb to the interface, while kosmotropic ions remain hydrated in the aqueous medium.…”

Protons and Hydroxide Ions in Aqueous Systems

“…168,169 In this model, proton adsorption is the result of a square well potential positioned at ∼1 HB length from the interface. Given this ansatz, a square well potential chosen so as to give a proton adsorption free energy of −7.5 kJ/mol results in calculated surface tensions that reproduce the experimental values for solutions of most hydrohalous acids.…”