Specific ion effects in physicochemical and biological systems: Simulations, theory and experiments

doi:10.1016/j.colsurfa.2007.03.040

Colloids and Surfaces A: Physicochemical and Engineering Aspects

Volume 303, Issues 1–2, 1 August 2007, Pages 110-136

https://doi.org/10.1016/j.colsurfa.2007.03.040 Get rights and content

Abstract

Charged surfaces and ion–water interactions at an interface play a decisive role in many physico-chemical and biological processes. The classical treatment of ions at charged interfaces is the Poisson–Boltzmann (PB) theory. Despite severe simplifying assumptions it describes surprisingly well univalent ions not too close to the interface for low electrolyte concentrations in the mmol regime. However, it breaks down in the vicinity of the interface at higher surface charge densities. Consequently the list of decorations and modifications of the original PB equation is long aiming for a more realistic picture. One striking deficiency of the treatment on the pure electrostatic level is the prediction that ions of the same valence produce the same results, independent of their chemical nature. In contrast, experiments reveal pronounced differences between different ions. Specific ion effects can be found everywhere in chemistry and biology and there are many reports of pronounced differences in the properties of charged monolayers, micelles, vesicles, dispersions or polyelectrolyte multilayers using different identically charged counterions. The so-called “counterion effect” is usually discussed in terms of the Hofmeister series for cations or anions which are the result of a subtle balance of several competing evenly matched interactions. The complex interplay of electrostatics, dispersion forces, thermal motion, polarization, fluctuations, hydration, ion size effects and the impact of interfacial water structure makes it hard to identify a universal law. The diversity of specific ion effects is a direct consequence of this subtle interplay of forces and imposes a true challenge for the theories. The decisive information for an assessment of the theories is knowledge of the prevailing ion distribution. Hence a considerable amount of work has been applied to develop suitable model systems and to push surface characterization tools such as (resonant) X-ray reflectivity, total reflection X-ray fluorescence or X-ray standing waves to new limits. These techniques give direct information on the ions and on the interfacial architecture. A second alternative to complement these studies is infrared–visible sum frequency spectroscopy allowing to record surface vibrational spectra of the water as it is perturbed in the presence of the salts. The paper is organized in sections describing various facets of ion specific effects discussed within the network.

Introduction

Charged surfaces are omnipresent in nature and ion–water interactions at an interface play a decisive role in various physico-chemical [1], [2], [3] and biological processes [4]. Consequently, the distribution of ions at a charged interface defines a central theme of Colloid and Interface Science. Gouy and Chapman were the first who tackled this problem in a quantitative fashion [5], [6]. The ions were treated as point charges embedded in a continuum with given dielectric constants while the surface charge was considered to be continuously smeared out. The prevailing charge distribution generates a mean electrical potential in which the ions adopt a Boltzmann distribution. The combination of the Boltzmann distribution with the Poisson equation leads to a non-linear second order differential equation for the electric potential $Φ$ . The solution of the so-called Poisson–Boltzmann (PB) equation yields the number density of the counterions as a function of the distance to the interface. The oversimplification of the Gouy–Chapman approach was obvious from the beginning and Stern was the first who pointed out that this theory predicts unrealistic high concentrations of counterions in the vicinity of the interface due to a neglect of the geometrical dimensions of the ions [7]. Since then, many extension of the theory have been put forward to account for the finite size of the ions [8], image forces [9] and the dependence of the dielectric constant on the electric field [10] or ion correlation [11]. One striking deficiency of the treatment on the pure electrostatic level is the prediction that ions of the same valence produce the same results, independent of their chemical nature. In contrast, experiments reveal pronounced differences between different ions and any realistic theory must account for this experimental fact.

Specific ion effects can be found everywhere in chemistry and biology. The catalytic activity of proteins depends strongly on the nature of the dissolved ions. For example, Pinna et al. measured the hydrolytic activity of Aspergillus Niger Lipase within various aqueous sodium salt solutions. The anions chloride and bromide increase the specific activity while nitrate and perchlorate decrease the reactivity of the Lipase [12]. The solubility of bovine serum albumin (BSA) is affected in a highly specific manner in the presence of salts [13]. It is traditional to arrange ions in series on the basis of their effects on the solubility of proteins (Hofmeister, 1888) and small organic molecules in water in the so called Hofmeister series. There are many reports of pronounced differences in the properties of charged monolayers [14], [15], [16], [17], [18], [19], micelles [20], [21], small vesicles [22] and dispersions [23], [24] using different identically charged counterions. The so-called “counterion effect” is usually discussed in terms of the Hofmeister series for cations or anions [24], [25], [26], [27], [28]. If specific chemical interactions can be excluded, the effect of counterions on the properties of a system with charged interfaces can result only from their effect on the structure and energy of the electrical double layer (EDL). This effect contradicts the GC model which predicts the same properties of the EDL for any counterion of a given charge which are treated as idealized “point charges”.

The most simple ion specific effect manifest in the surface tension of simple aqueous electrolyte solutions. In general, ions increase the surface tension in a specific manner. The effects are not dramatic, however, due to the simplicity of this system it is crucial for testing the theories. The traditional picture of the interface of an aqueous electrolyte solution is based on a thermodynamic analysis of the equilibrium surface tension isotherm. The increase in the equilibrium surface tension is then interpreted as an interfacial zone depleted by ions [29]. Recently this picture has been challenged by molecular dynamics (MD) simulations using polarizable force fields which predicted that soft ions such as halides are enriched at the interface with a non-monotonic ion profile [30]. This finding is supported by certain atmospheric reactions and has a strong impact on the understanding of heterogeneous tropospheric chemistry [3]. The key to an understanding of this apparent contradiction lies in a reconsideration of the meaning of thermodynamics. There is no a priori prediction of a profile and thermodynamics can accommodate several conflicting interfacial models provided that the integral excess or depletion is in accordance with Gibbs equation. Therefore, direct experimental observations of molecular structure and energetics of ions in the interfacial region are required [31].

The investigation of specific ion effects in physicochemical and biological systems defines a key project within the “French–German-Network complex fluids: from 3d to 2d” network. This paper reviews some of the findings of the various groups within the network. The experimental key to understanding ion specific effects is the determination of the prevailing ion profile. Hence a considerable amount of work has been dedicated to further develop model systems and surface analytical tools for that purpose.

The contribution of Shapovalov, Brezesinski, Möhwald discusses the Langmuir monolayers of long chain alkylsulfates on subphases containing different alkali metal counterions. This is an appealing model system due to the tuneable and high charge density and low complexing ability of the headgroup. Surface potential, surface pressure and total reflection X-ray fluorescence (TRXF) prove deviations from the Gouy–Chapman (GC) model due to finite size of counterions. Specific salt effects can also be found on the thickness and roughness of polylelectrolyte multilayers [32]. The contribution of Daillant, Guenoun, Klitzing and Schollmeyer discusses internal architecture of polyelectrolyte multilayers. The absolute values of the concentration and its concentration profile have been determined by an X-ray standing wave field produced by interference of the incident and reflected wave above a mirror substrate. The effect of the sign of the charge of the outermost layer and the influence of washing with water post preparation is studied and provides valueable insights in the underlying physics. Giewekemeyer and Salditt use X-ray reflectivity for the determination of the bromide counterion distribution near a solid-supported monolayer of variable surface charge in the case of vanishing bulk electrolyte concentration. A special focus is the development of new strategies for the data analysis. Two independent analysis approaches, one based on a conventional box model with an additional counterion contribution, and the other on an unbiased “free-form” approach are discussed and applied to the charged interface. The contribution of Motschmann, Möhwald, Viswanath and Koelsch uses water as an interfacial tool for addressing the local surface potential. The idea is simple. Water is a dipole and adopts a preferential orientation within the local electric field produced by all charges. In a first approximation, the distribution is given by a Langevin function [33]. Hence, the polar order of the water is a sensitive probe of the electric field and provides insights in the prevailing ion distribution. Infrared–visible sum frequency spectroscopy was used to study the structure of the interfacial water. Under electric dipole approximation, the resonant signal arises from the non-centro symmetric arrangement of the molecules at the interface and is absent in the centro-symmetric bulk. Hence, the intensity of the SFG band can be linked to the number of oriented water molecules. The section of Horinek and Netz focusses on ion adsorption at a water/solid hydrophobic interface. The surprising observation that hydrophobic substrates appear to be negatively charged upon contact with pure water at neutral pH is related to specific ion adsorption. It was observed by electrokinetics for hydrophobic diamond surfaces [34] and self-assembled monolayers [35] and is well known for liquid oil/water interfaces, where the negative charge could be assigned to the adsorption of OH⁻ by its effect on the bulk pH [36]. A microscopic explanation of specific OH⁻ adsorption was furnished by quantum chemical density functional theory [37] and solvent-induced effects have been addressed by molecular dynamics simulations with classical force fields [38]. Hydrophobic interfaces are special insofar that the effects occurring at the interface are not necessarily caused by the detailed chemical structure of the solid but can also have their origin in the structure of the interfacial water itself. Thus, all hydrophobic interfaces share similar properties, which are in fact quite similar to properties of the water/air interface [39]. The contribution discusses classical molecular dynamics simulations performed on a diamond/hydrophobic interface as a representative solid hydrophobic/water interface. Potentials of mean force for ion adsorption are calculated by MD. Adsorption of OH⁻ is even found without an explicit account for polarization.

The diversity of specific ion effects as manifested by the Hofmeister series of ions is a direct consequence of a subtle interplay of electrostatics, dispersion forces, thermal motion, fluctuations, hydration, ion size effects and the impact of interfacial water. The contribution within the French–German network shed some light in the underlying relations.

Section snippets

Ion size effects in Langmuir monolayers

Langmuir monolayers of behenylsulfate (BS), which have a high charge density and low complexing ability, were examined on subphases containing different alkali metal counterions. The BS head groups are strongly acidic. At pH values of 5–6 and salt concentrations above 1 mM they are completely deprotonated, i.e. possess a single negative charge. The maximum packing density is close to 25 Å² molecule⁻¹. Therefore, their surface charge density reaches $e / (25 Å^{2}) = 0.64$ C m⁻².

BS monolayers on subphases

Background

When an X-ray beam impinges on a flat interface separating two materials that have different indices of refraction, part of the energy in the travelling wave is reflected and the remainder is transmitted. X-rays undergo total reflection for incidence angles $θ_{i}$ less than the critical angle $θ_{c}$ , because the index of refraction is less than unity for X-rays. The index of refraction is expressed as $n = 1 - δ - i β$ , with refraction $δ = N_{e} r_{e} λ^{2} / 2 π$ and absorption $β = μ λ / 4 π$ . Using Snell’s law the critical angle

A planar charged interface with counterions only studied by X-ray reflectivity

The bromide counterion distribution near a solid-supported monolayer of variable surface charge was determined in the case of vanishing bulk electrolyte concentration by X-ray reflectivity. The surface charge concentration $σ_{s}$ was varied by using different molar ratios of the cationic surfactant di-octadecyl-di-methyl-ammonium-bromide (DODAB) and the neutral phospholipid. 1,2-Dipalmitoyl-sn-glycero-3-phosphocholine (DPPC). Two independent analyis approaches, one based on a conventional box model

Interfacial water—a sensitive probe for the distribution of counterions

In the following we provide direct experimental evidence for an unusual phase transition in an adsorption layer of a cationic surfactant (1-dodecyl-4-dimethyl-amino-pyridinium bromide) at the air–water interface. Both surface phases differ in the distribution of the ions. At lower concentration the majority of counterions is spread out in a diffuse layer, whereas at higher concentration the counterions are condensed. Such a phase transition is not captured by Poisson–Boltzmann type mean-field

Acknowledgements

We thank our many skillful and motivated co-workers for their contributions and help during this collaboration. The authors wish to thank CNRS, CEA, DFG and MPG, “French–German-Network complex fluids: from 3d to 2d” for financial support.

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Specific ion effects in physicochemical and biological systems: Simulations, theory and experiments

Abstract

Introduction

Section snippets

Ion size effects in Langmuir monolayers

Background

A planar charged interface with counterions only studied by X-ray reflectivity

Interfacial water—a sensitive probe for the distribution of counterions

Acknowledgements

Curr. Opin. Colloid Interface Sci.

J. Colloid Interface Sci.

Thin Solid Films

J. Colloid Interface Sci.

J. Colloid Interface Sci.

Curr. Opin. Colloid Interface Sci.

J. Colloid Interface Sci.

E. Chem. Phys. Lett.

Mater. Sci. Eng. C

J. Colloid Interface Sci.

Curr. Opin. Colloid Interface Sci.

Chem. Phys. Lipids

Biophys. J.

Thin Solid Films

Curr. Opin. Colloid Interface Sci.

Chem. Rev.

Phys. Rev. Lett.

Chem. Rev.

J. Phys.

Philos. Magn.

Z. Elektrochem.

Discuss. Faraday Soc.

J. Chem. Soc. Faraday Trans. II

Recl. Trav. Chim. Pays-Bas

J. Chem. Phys.

J. Phys. Chem. B

Biophys. Chem.

J. Colloid Interface Sci.

Langmuir

Phys. Chem. Chem. Phys.

J. Phys. Chem. B

J. Phys. Chem. B

Langmuir

Langmuir

J. Phys. Chem. B

Langmuir

J. Am. Chem. Soc.

J. Chem. Phys.

J. Phys. Chem. B

Statistical Thermodynamics

Langmuir

Ang. Chemie Int. Ed.

J. Am. Chem. Soc.

J. Am. Chem. Soc.

Chem. Rev.

J. Phys. Chem.

Macromolecules