Influence of the Hofmeister series of anions on the molecular organization of positively ionized monolayers of a viologen derivative

The effects of the Hofmeister series of ions are ubiquitous in chemistry and biology. In this paper specific ion effects on the surface behavior of a viologen dication, namely 1,1(')-dioctadecyl-4,4(')-bipyridilium, are shown. Surface pressure and surface potential vs area isotherms were obtained on aqueous subphases containing potassium salts with several representative counterions in the Hofmeister series (C6H5O3-7, SO2 -4, HPO2-4, Cl-, Br-, NO-3, I-, and ClO-4). The parameters obtained from the compression isotherms (area per molecule, phase transitions, Young modulus, initial surface potential, and variation of the surface potential upon compression) are dependent on the nature of the counterion, indicating ion specificity. Aqueous subphases containing C6H5O3-7, SO2-4, and HPO2-4 anions yield more expanded viologen monolayers and these anions do not effectively penetrate into the monolayer. Brewster angle microscopy was used to map the different phases of the viologen monolayers at the air-water interface. The Langmuir films were also characterized by UV-vis spectroscopy, with quantitative analysis of the reflection spectra supporting an organizational model in which the viologen chromophore undergoes a gradual transition to a more vertical position with respect to the water surface upon compression. A comparison of the tilt angles of the viologen on the different subphases indicates that anions that can more easily penetrate in the monolayer permit the viologen moieties to adopt a slightly more vertical position with respect to the water surface.     

Specific ion effects on interfacial water structure near macromolecules

We investigated specific ion effects on interfacial water structure next to macromolecules with vibrational sum frequency spectroscopy (VSFS). Poly-(N-isopropylacrylamide) was adsorbed at the air/water interface for this purpose. It was found that the presence of salt in the subphase could induce the reorganization of water adjacent to the macromolecule and that the changes depended greatly on the specific identity and concentration of the salt employed. Ranked by their propensity to orient interfacial water molecules, sodium salts could be placed in the following order: NaSCN > NaClO4 > NaI > NaNO3 approximately NaBr > NaCl > pure water approximately NaF approximately Na2SO4. This ordering is a Hofmeister series. On the other hand, varying the identity of the cation exhibited virtually no effect. We also showed that the oscillator strength in the OH stretch region was linearly related to changes in the surface potential caused by anion adsorption. This fact allowed binding isotherms to be abstracted from the VSFS data. Such results offer direct evidence that interfacial water structure can be predominantly the consequence of macromolecule-ion interactions.     

Surprising Hofmeister Effects on the Bending Vibration of Water

The Hofmeister series, which originally described the specific ion effects on the solubility of macromolecules in aqueous solutions, has been a long‐standing unsolved and exceptionally challenging mystery in chemistry. The complexity of specific ion effects has prevented a unified theory from emerging. Accumulating research has suggested that the interactions among ions, water and various solutes play roles. However, among these interactions, the binding between ions and solutes is receiving most of the attention, whereas the effects of ions on the hydrogen‐bond structure in liquid water have been deemed to be negligible. In this study, attenuated‐total‐reflectance Fourier transform infrared spectroscopy is used to study the infrared spectra of salt solutions. The results show that the red‐ and blue‐shifts of the water bending band are in excellent agreement with the characteristic Hofmeister series, which suggests that the ions’ effects on water structure might be the key role in the Hofmeister phenomenon.     

Watching the low-frequency motions in aqueous salt solutions: the terahertz vibrational signatures of hydrated ions

The details of ion hydration still raise fundamental questions relevant to a large variety of problems in chemistry and biology. The concept of water "structure breaking" and "structure making" by ions in aqueous solutions has been invoked to explain the Hofmeister series introduced over 100 years ago, which still provides the basis for the interpretation of experimental observations, in particular the stabilization/destabilization of biomolecules. Recent studies, using state-of-the-art experiments and molecular dynamics simulations, either challenge or support some key points of the structure maker/breaker concept, specifically regarding long-ranged ordering/disordering effects. Here, we report a systematic terahertz absorption spectroscopy and molecular dynamics simulation study of a series of aqueous solutions of divalent salts, which adds a new piece to the puzzle. The picture that emerges from the concentration dependence and assignment of the observed absorption features is one of a limited range of ion effects that is confined to the first solvation shell.     

Specific ion effects on the water solubility of macromolecules: PNIPAM and the Hofmeister series

Aqueous processes ranging from protein folding and enzyme turnover to colloidal ordering and macromolecular precipitation are sensitive to the nature and concentration of the ions present in solution. Herein, the effect of a series of sodium salts on the lower critical solution temperature (LCST) of poly(N-isopropylacrylamide), PNIPAM, was investigated with a temperature gradient microfluidic device under a dark-field microscope. While the ability of a particular anion to lower the LCST generally followed the Hofmeister series, analysis of solvent isotope effects and of the changes in LCST with ion concentration and identity showed multiple mechanisms were at work. In solutions containing sufficient concentrations of strongly hydrated anions, the phase transition of PNIPAM was directly correlated with the hydration entropy of the anion. On the other hand, weakly hydrated anions were salted-out through surface tension effects and displayed improved hydration by direct ion binding.     

Ion pairing as a possible clue for discriminating between sodium and potassium in biological and other complex environments

For a series of biologically relevant anions, we present free energy changes upon replacing potassium with sodium in a contact ion pair. Calculations performed using a combination of molecular dynamics simulations and ab initio methods demonstrate the ordering of anions in a Hofmeister series. Small anionic groups such as carboxylates preferentially pair with sodium, while intermediate cases such as chloride or monovalent phosphate exhibit almost no specificity, and large anions (e.g., methylsulfonate) prefer potassium over sodium. These results can rationalize different behavior of Na+ versus K+ at the surface of hydrated proteins, DNA, and reversed micelles.     

How ions affect the structure of water

We model ion solvation in water. We use the MB model of water, a simple two-dimensional statistical mechanical model in which waters are represented as Lennard-Jones disks having Gaussian hydrogen-bonding arms. We introduce a charge dipole into MB waters. We perform (NPT) Monte Carlo simulations to explore how water molecules are organized around ions and around nonpolar solutes in salt solutions. The model gives good qualitative agreement with experiments, including Jones-Dole viscosity B coefficients, Samoilov and Hirata ion hydration activation energies, ion solvation thermodynamics, and Setschenow coefficients for Hofmeister series ions, which describe the salt concentration dependence of the solubilities of hydrophobic solutes. The two main ideas captured here are (1) that charge densities govern the interactions of ions with water, and (2) that a balance of forces determines water structure: electrostatics (water's dipole interacting with ions) and hydrogen bonding (water interacting with neighboring waters). Small ions (kosmotropes) have high charge densities so they cause strong electrostatic ordering of nearby waters, breaking hydrogen bonds. In contrast, large ions (chaotropes) have low charge densities, and surrounding water molecules are largely hydrogen bonded.     

Sensitivity of cationic surfactant templates to specific anions in liquid interface crystallization

We investigated the specific effects of potassium salts of various anions upon the interfacially templated crystal nucleation of K(2)SO(4). Previously, we have shown that the presence of several salts at low concentrations could induce changes in important crystallization characteristics templated by 1-octadecylamine at the liquid-liquid interface, and that these changes depended greatly on the specific identity of the salt. In this work we extend our surfactant monolayers to include dimethyldioctadecylammonium bromide (DODAB) and hexadecyltrimethylammonium bromide (CTAB). Addition of 10 mM of various potassium salts results in a diminution in efficiency of the templating capability of CTAB monolayer, as evidenced by higher C(onset) values and polycrystalline habit. The ability of the anions to perturb these values varied in a manner consistent with a Hofmeister series. However, DODAB maintained its templating effectiveness regardless of the nature of the salt or concentration. DODAB and CTAB are likely to be undergoing different reordering effects in the monolayer upon binding with chaotropic anions: a combined reduction in surface charge with different monolayer ordering results in a differing template ability. These studies have provided significant insights into the understanding of the interaction of ordered surfactant arrays with salts, and provide exciting possibilities for crystal engineering and materials design.     

Reversed Hofmeister series—The rule rather than the exception

Over recent years, the supposedly universal Hofmeister series has been replaced by a diverse spectrum of direct, partially altered and reversed series. This review aims to provide a detailed understanding of the full spectrum by combining results from molecular dynamics simulations, Poisson–Boltzmann theory and AFM experiments. Primary insight into the origin of the Hofmeister series and its reversal is gained from simulation-derived ion–surface interaction potentials at surfaces containing non-polar, polar and charged functional groups for halide anions and alkali cations. In a second step, the detailed microscopic interactions of ions, water and functional surface groups are incorporated into Poisson–Boltzmann theory. This allows us to quantify ion-specific binding affinities to surface groups of varying polarity and charge, and to provide a connection to the experimentally measured long-ranged electrostatic forces that stabilize colloids, proteins and other particles against precipitation. Based on the stabilizing efficiency, the direct Hofmeister series is obtained for negatively charged hydrophobic surfaces. Hofmeister series reversal is induced by changing the sign of the surface charge from negative to positive, by changing the nature of the functional surface groups from hydrophobic to hydrophilic, by increasing the salt concentration, or by changing the pH. The resulting diverse spectrum reflects that alterations of Hofmeister series are the rule rather than the exception and originate from the variation of ion-surface interactions upon changing surface properties.     

Specific-ion effects in non-aqueous systems

It is widely acknowledged that specific-ion effects are ubiquitous in aqueous systems and undoubtedly are essential to the fundamental processes of life, although a deep fundamental understanding of specific-ion effects remains an important challenge. Specific-ion effects in non-aqueous solvents are known but have attracted far less attention, yet knowledge of specific-ion effects in non-aqueous systems is likely to provide important information for guiding, evaluating and testing our theories of specific-ion effects. Here, the literature on specific-ion effects in non-aqueous solvents is surveyed with a view to determining if the Hofmeister series or lyotropic series so universally observed in aqueous systems is widely evident in non-aqueous systems. Particular attention has been applied to experiments on non-aqueous systems that are known to exhibit Hofmeister series in aqueous systems with the aim of determining if a consistent ion ordering in the strength of specific-ion effects is observed in other solvents. We find that specific-ion effects are ubiquitous in non-aqueous solvents, that both Hofmeister and lyotropic series are widely observed, although not necessarily for the same class of experiment. Moreover, we find that Hofmeister and lyotropic series are observed in non-aqueous solvents even for experiments in which these series are not observed for water. Additionally, series reversal is seen for a given experiment when the solvent is changed. All this poses significant challenges for our understanding of specific-ion effects in aqueous and non-aqueous systems and also provides guideposts for future investigations.     

Hofmeister effects in surface tension of aqueous electrolyte solution

The surface tension of electrolyte solutions shows marked specific ion effects. We here show an important role for both ionic solvation energies and ionic dispersion potentials in determining this ion specific surface tension of salt solutions. The ion self-free energy changes when an ion moves from bulk solution into the interfacial region, with its decreasing water density profile. We will show that the solvation energies of different ions correlate very well with the surface tension of salt solutions. Inclusion of this distance-dependent self-free energy contribution brings qualitative agreement with experiments and the right Hofmeister series. This is so not only for surface tension changes but also for measured surface potentials. The inclusion of ionic dispersion interaction potentials further improves the agreement with experiments. We discuss how further progress in the theory of the surface tension of salts can be achieved.     

Hofmeister effects on poly(NIPAM) microgel particles: macroscopic evidence of ion adsorption and changes in water structure.

The term Hofmeister effects is broadly used to refer to ionic specificities in many different physical, chemical and biological phenomena. The origin of this ionic specificity is sought in two interdependent microscopic sources: 1) the peculiarities of the solvent structure near surfaces and around the ions, and 2) specific ion adsorption-exclusion mechanisms near a surface. In this work, Hofmeister effects on poly(N-isopropylacrylamide) [poly(NIPAM)]-based microgels are examined. Poly(NIPAM) particles are thermally sensitive microgels exhibiting volume-phase transitions with temperature. This temperature-sensitive system seems to be suitable for the independent observation of the two microscopic sources of Hofmeister effects. On the one hand, volume-phase transition, evaluated by photon correlation spectroscopy (PCS), gives information about how the presence of ions changes the water structure around the poly(NIPAM) chains. On the other hand, electrokinetic studies show relevant data about ionic adsorption-exclusion phenomena at the polymer surface.     

Possible origin of the inverse and direct Hofmeister series for lysozyme at low and high salt concentrations

Protein solubility studies below the isoelectric point exhibit a direct Hofmeister series at high salt concentrations and an inverse Hofmeister series at low salt concentrations. The efficiencies of different anions measured by salt concentrations needed to effect precipitation at fixed cations are the usual Hofmeister series (Cl(-) > NO(3)(-) > Br(-) > ClO(4)(-) > I(-) > SCN(-)). The sequence is reversed at low concentrations. This has been known for over a century. Reversal of the Hofmeister series is not peculiar to proteins. Its origin poses a key test for any theoretical model. Such specific ion effects in the cloud points of lysozyme suspensions have recently been revisited. Here, a model for lysozymes is considered that takes into account forces acting on ions that are missing from classical theory. It is shown that both direct and reverse Hofmeister effects can be predicted quantitatively. The attractive/repulsive force between two protein molecules was calculated. To do this, a modification of Poisson-Boltzmann theory is used that accounts for the effects of ion polarizabilities and ion sizes obtained from ab initio calculations. At low salt concentrations, the adsorption of the more polarizable anions is enhanced by ion-surface dispersion interactions. The increased adsorption screens the protein surface charge, thus reducing the surface forces to give an inverse Hofmeister series. At high concentrations, enhanced adsorption of the more polarizable counterions (anions) leads to an effective reversal in surface charge. Consequently, an increase in co-ion (cations) adsorption occurs, resulting in an increase in surface forces. It will be demonstrated that among the different contributions determining the predicted specific ion effect the entropic term due to anions is the main responsible for the Hofmeister sequence at low salt concentrations. Conversely, the entropic term due to cations determines the Hofmeister sequence at high salt concentrations. This behavior is a remarkable example of the charge-reversal phenomenon.     

Hofmeister anionic effects on hydration electric fields around water and peptide

Specific ion effects on water dynamics and local solvation structure around a peptide are important in understanding the Hofmeister series of ions and their effects on protein stability in aqueous solution. Water dynamics is essentially governed by local hydrogen-bonding interactions with surrounding water molecules producing hydration electric field on each water molecule. Here, we show that the hydration electric field on the OD bond of HOD molecule in water can be directly estimated by measuring its OD stretch infrared (IR) radiation frequency shift upon increasing ion concentration. For a variety of electrolyte solutions containing Hofmeister anions, we measured the OD stretch IR bands and estimated the hydration electric field on the OD bond to be about a hundred MV∕cm with standard deviation of tens of MV∕cm. As anion concentration increases from 1 to 6 M, the hydration electric field on the OD bond decreases by about 10%, indicating that the local H-bond network is partially broken by dissolved ions. However, the measured hydration electric fields on the OD bond and its fluctuation amplitudes for varying anions are rather independent on whether the anion is a kosmotrope or a chaotrope. To further examine the Hofmeister effects on H-bond solvation structure around a peptide bond, we examined the amide I' and II' mode frequencies of N-methylacetamide in various electrolyte D(2)O solutions. It is found that the two amide vibrational frequencies are not affected by ions, indicating that the H-bond solvation structure in the vicinity of a peptide remains the same irrespective of the concentration and character of ions. The present experimental results suggest that the Hofmeister anionic effects are not caused by direct electrostatic interactions of ions with peptide bond or water molecules in its first solvation shell. Furthermore, even though the H-bond network of water is affected by ions, thus induced change of local hydration electric field on the OD bond of HOD is not in good correlation with the well-known Hofmeister series. We anticipate that the present experimental results provide an important clue about the Hofmeister effect on protein structure and present a discussion on possible alternative mechanisms.     

Structure, energetics, and dynamics of water adsorbed on the muscovite (001) surface: a molecular dynamics simulation

Molecular dynamics (MD) computer simulations of liquid water adsorbed on the muscovite (001) surface provide a greatly increased, atomistically detailed understanding of surface-related effects on the spatial variation in the structural and orientational ordering, hydrogen bond (H-bond) organization, and local density of H2O molecules at this important model phyllosilicate surface. MD simulations at constant temperature and volume (statistical NVT ensemble) were performed for a series of model systems consisting of a two-layer muscovite slab (representing 8 crystallographic surface unit cells of the substrate) and 0 to 319 adsorbed H2O molecules, probing the atomistic structure and dynamics of surface aqueous films up to 3 nm in thickness. The results do not demonstrate a completely liquid-like behavior, as otherwise suggested from the interpretation of X-ray reflectivity measurements and earlier Monte Carlo simulations. Instead, a more structurally and orientationally restricted behavior of surface H2O molecules is observed, and this structural ordering extends to larger distances from the surface than previously expected. Even at the largest surface water coverage studied, over 20% of H2O molecules are associated with specific adsorption sites, and another 50% maintain strongly preferred orientations relative to the surface. This partially ordered structure is also different from the well-ordered 2-dimensional ice-like structure predicted by ab initio MD simulations for a system with a complete monolayer water coverage. However, consistent with these ab initio results, our simulations do predict that a full molecular monolayer surface water coverage represents a relatively stable surface structure in terms of the lowest diffusional mobility of H2O molecules along the surface. Calculated energies of water adsorption are in good agreement with available experimental data.     

Thermodynamic origin of hofmeister ion effects

Quantitative interpretation and prediction of Hofmeister ion effects on protein processes, including folding and crystallization, have been elusive goals of a century of research. Here, a quantitative thermodynamic analysis, developed to treat noncoulombic interactions of solutes with biopolymer surface and recently extended to analyze the effects of Hofmeister salts on the surface tension of water, is applied to literature solubility data for small hydrocarbons and model peptides. This analysis allows us to obtain a minimum estimate of the hydration b1 (H2O A(-2)), of hydrocarbon surface and partition coefficients Kp, characterizing the distribution of salts and salt ions between this hydration water and bulk water. Assuming that Na+ and SO4(2-) ions of Na2SO4 (the salt giving the largest reduction in hydrocarbon solubility as well as the largest increase in surface tension) are fully excluded from the hydration water at hydrocarbon surface, we obtain the same b1 as for air-water surface (approximately 0.18 H2O A(-2)). Rank orders of cation and anion partition coefficients for nonpolar surface follow the Hofmeister series for protein processes, but are strongly offset for cations in the direction of exclusion (preferential hydration). By applying a coarse-grained decomposition of water accessible surface area (ASA) into nonpolar, polar amide, and other polar surface and the same hydration b1 to interpret peptide solubility increments, we determine salt partition coefficients for amide surface. These partition coefficients are separated into single-ion contributions based on the observation that both Cl- and Na+ (also K+) occupy neutral positions in the middle of the anion and cation Hofmeister series for protein folding. Independent of this assignment, we find that all cations investigated are strongly accumulated at amide surface while most anions are excluded. Cation and anion effects are independent and additive, allowing successful prediction of Hofmeister salt effects on micelle formation and other processes from structural information (ASA).     

Hofmeister series: the hydrolytic activity of Aspergillus niger lipase depends on specific anion effects

The specific activity of lipase A (Aspergillus niger) toward the hydrolysis of p-nitrophenyl acetate (p-NPA) is shown to increase as a result of sodium salt addition according to specific ion effects of the Hofmeister series. This shows explicitly that the Hofmeister effect is due to the different specific interactions between anions and the enzymatic surface.     

Hofmeister effects in the restabilization of IgG--latex particles: testing Ruckenstein's theory

The hydration interaction is responsible for the colloidal stability observed in protein-coated particles at high ionic strengths. The origin of this non-DLVO interaction is related not only to the local structure of the water molecules located at the surface but also to the structure of those molecules involved in the hydration of the ions that surround the colloidal particles. Ruckenstein and co-workers have recently developed a new theory based on the coupling of double-layer and hydration interactions. Its validity was contrasted by their fitting of experimental data obtained with IgG-latex particles restabilized at high salt concentration. The theory details the important role played by the counterions in the stability at high salt concentrations by proposing an ion pair reaction forming surface dipoles. These surface dipoles are responsible of repulsive interactions between two approaching surfaces. This paper checks the theory with recent data where some ions associated with the Hofmeister series (NO(3)(-), SCN(-) and Ca(2+)) restabilize the same kind of IgG-latex systems by means of hydration forces. Surprisingly, these ions induce stability acting even as co-ions, likely by modifying the water structure at the surface, but not forming surface ion pairs. Therefore, this experimental evidence would question Ruckenstein's theory based on the surface dipole formation for explaining the observed restabilization phenomena.     

Anion effects on calixarene monolayers: a Hofmeister series study

Due to their amphiphilic structure, calixarenes adsorb at the air/water interface and form stable Langmuir films. We have explored the effect of salts on calix[6]- and calix[8]arene spreading isotherms at the air/water interface. A wide range of different potassium salts was used in the subphase: KCl, KI, KBr, KSCN, KNO(3), CH(3)COOK, K(2)SO(4), and K(3)PO(4). The differences in Langmuir isotherms are due to the presence of different anions in the subphase, to the different conformations of the ligands at the interface, and to the different complexing affinities of calix[6]- and calix[8]arene for potassium ions. The two systems show a significant specific ion effect that can be discussed in terms of Hofmeister series. Characteristic monolayer parameters, e.g., limiting area (A(lim)), collapse pressure (pi(coll)), modulus of compressibility (C(s)(-1)), and surface potential (DeltaV), are discussed in terms of some physicochemical parameters that reflect dispersion forces: in particular, anion polarizabilities, lyotropic number (N), molar surface tension increment (sigma), and partial molar volume (nu(s)).     

Calcium-induced phospholipid ordering depends on surface pressure

The effect of sodium and calcium ions on zwitterionic and anionic phospholipids monolayers is investigated using vibrational sum-frequency generation in conjunction with surface pressure measurements and fluorescence microscopy. Sodium ions only subtly affect the monolayer structure, while the effect of calcium is large and depends strongly on the surface pressure. At low surface pressures (approximately 5 mN/m), the presence on Ca2+ results in the unexpected appearance of ordered domains. For pressures between approximately 5 and approximately 25 mN/m, Ca2+ ions induce disorder in the monolayer. For pressures exceeding 25 mN/m, calcium cations expand the monolayer, while simultaneously ordering the lipid chains. Interestingly, effects are similar for both zwitterionic lipids and negatively charged lipids. In both vibrational sum-frequency generation and surface tension measurements, the molecular signature of the association of Ca2+ with the lipids is evident from Ca2+-induced changes in the signals corresponding to area changes of 4 A2/lipid-precisely the surface area of a Ca2+ ion, with evidence for a change in lipid Ca2+ complexation at high pressures.