The electrostatic origins of specific ion effects: quantifying the Hofmeister series for anions

Life as we know it is dependent upon water, or more specifically salty water. Without dissolved ions, the interactions between biological molecules are insufficiently complex to support life. This complexity is intimately tied to the variation in properties induced by the presence of different ions. These specific ion effects, widely known as Hofmeister effects, have been known for more than 100 years. They are ubiquitous throughout the chemical, biological and physical sciences. The origin of these effects and their relative strengths is still hotly debated. Here we reconsider the origins of specific ion effects through the lens of Coulomb interactions and establish a foundation for anion effects in aqueous and non-aqueous environments. We show that, for anions, the Hofmeister series can be explained and quantified by consideration of site-specific electrostatic interactions. This can simply be approximated by the radial charge density of the anion, which we have calculated for commonly reported ions. This broadly quantifies previously unpredictable specific ion effects, including those known to influence solution properties, virus activities and reaction rates. Furthermore, in non-aqueous solvents, the relative magnitude of the anion series is dependent on the Lewis acidity of the solvent, as measured by the Gutmann Acceptor Number. Analogous SIEs for cations bear limited correlation with their radial charge density, highlighting a fundamental asymmetry in the origins of specific ion effects for anions and cations, due to competing non-Coulombic phenomena.

Analysis of ions’ radial charge densities reveals they correlate with many specific ion effects, and provides a new basis to explain and quantify the 130-year-old Hofmeister series for anions.     

Chlorophyll a self-assembly in polar solvent-water mixtures

The conversion of chlorophyll a (Chl a) monomers into large aggregates in six polar solvents upon addition of water has been studied by means of absorption, fluorescence spectroscopy and fluorescence lifetime measurements for the purpose of elucidating the various environmental factors promoting Chl a self-assembly and determining the type of its organization. Two empirical solvent parameter scales were used for quantitative characterization of the different solvation properties of the solvents and their mixtures with water. The mole fractions of water f1/2 giving rise to the midpoint values of the relative fluorescence quantum yield were determined for each solvent, and then various solvent-water mixture parameters for the f1/2 values were compared. On the basis of their comparison, it is concluded that the hydrogen-bonding ability and the dipole-dipole interactions (function of the dielectric constant) of the solvent-water mixtures are those that promote Chl a self-assembly. The influence of the different nature of the non-aqueous solvents on the Chl aggregation is manifested by both the different water contents required to induce Chl monomer-->aggregate transition and the formation of two types of aggregates at the completion of the transition: species absorbing at 740-760 nm (in methanol, ethanol, acetonitrile, acetone) and at 667-670 nm (in pyridine and tetrahydrofuran). It is concluded that the type of Chl organization depends on the coordination ability and the polarizability (function of the index of refraction) of the organic solvent. The ordering of the solvents with respect to the f1/2 values--methanol < ethanol < acetonitrile < acetone < pyridine < tetrahydrofuran--yielded a typical lyotropic (Hofmeister) series. On the basis of this solvent ordering and the disparate effects of the two groups of solvents on the Chl a aggregate organization, it is pointed out that the mechanism of Chl a self-assembly in aqueous media can be considered a manifestation of the Hofmeister effect, as displayed in the lipid-phase behavior (Koynova et al., Eur. J. Biophys. 25, 261-274, 1997). It relates to the solvent ability to modify the bulk structure and to distribute unevenly between the Chl-water interface and bulk liquid.     

Interfacial Polycondensation in Aqueous and Non-Aqueous Systems

Abstract

Interfacial polycondensation in aqueous and non-aqueous systems has been investigated in order to synthesize various polyamides having functional groups. Rigid aromatic polyamides having pyridine moieties (PPy) were synthesized by interfacial polycondensation using an aqueous system and the solution properties of PPy in concentrated sulfuric acid were investigated in terms of solution viscosity and lyotropic behavior. Interfacial polycondensation in a non-aqueous system using two immiscible solvents was found to be useful for the synthesis of aromatic polyesters and copolyesters.     

Dissolution of cellobiose in the aqueous solutions of chloride salts: Hofmeister series consideration

The effects of chloride salts on the dissolution of cellobiose in aqueous solution were investigated using calorimetry and ¹H NMR. The dissolution of cellobiose in salt solutions is a typical entropy-driven process. The activity of ZnCl₂ and LiCl hydrated ions is enhanced as the hydration number decreases with increasing temperature. Zn²⁺ and Li⁺ hydrates can interact with the oxygen atoms at the O5 and O6 positions of cellobiose and associate with the Cl^? anions, leading to the breakage of cellobiose hydrogen bonds. We found that the solubility of cellobiose in aqueous solutions is on the order of ZnCl₂ > LiCl > NaCl > H₂O > KCl > NH₄Cl, which is consistent with the Hofmeister series. For the first time, we recognized the specific ionic effects of the Hofmeister series on the dissolution of cellobiose in salt aqueous solutions. This finding is helpful for understanding the dissolving mechanism of cellulose in aqueous solvents with salts and providing fundamental knowledge for finding and designing new cellulose solvents.     

Chaotropic salts interacting with soft matter: Beyond the lyotropic series

The effects of the chaotropic ions of the Hofmeister series on many systems and phenomena are typically quite pronounced. What happens, however, when one uses chaotropic ions beyond SCN⁻, ClO₄⁻, or guanidinium, which are the usual limiting ions of the lyotropic series considered in most investigations? This review focuses on the extensive but scattered literature that discusses how larger hydrophobic ions and hydrotropic ions interact with soft matter. There are many similarities between hydrophobic and hydrotropic ions; they differ in the fact that the hydrotropes are intrinsically asymmetric with respect to aqueous solvation. Strong specific effects of these ions with a common denominator are found in diverse systems: Hydrophobic ions “stick” to hydrophobic surfaces, or intercalate within soft matter interfaces, becoming a basic component of the structure and often inducing disruption or phase change. In other situations, hydrophobic ions act indirectly by failing to provide adequate screening of electrostatic interactions because of their large size. The hydrophobic and hydrotropic ions discussed here constitute the link between the lyotropic series and the surfactant domain. It is pointed out that, despite the size and breadth of the literature, there is still much work to be done to clarify how these ions interact with soft matter. Many important applications can result from the control of soft matter structure that can be achieved with these ions.     

Evaluation of Hofmeister effects on the kinetic stability of proteins

Dissolved salts are known to affect properties of proteins in solution including solubility and melting temperature, and the effects of dissolved salts can be ranked qualitatively by the Hofmeister series. We seek a quantitative model to predict the effects of salts in the Hofmeister series on the deactivation kinetics of enzymes. Such a model would allow for a better prediction of useful biocatalyst lifetimes or an improved estimation of protein-based pharmaceutical shelf life. Here we consider a number of salt properties that are proposed indicators of Hofmeister effects in the literature as a means for predicting salt effects on the deactivation of horse liver alcohol dehydrogenase (HL-ADH), alpha-chymotrypsin, and monomeric red fluorescent protein (mRFP). We find that surface tension increments are not accurate predictors of salt effects but find a common trend between observed deactivation constants and B-viscosity coefficients of the Jones-Dole equation, which are indicative of ion hydration. This trend suggests that deactivation constants (log k(d,obs)) vary linearly with chaotropic B-viscosity coefficients but are relatively unchanged in kosmotropic solutions. The invariance with kosmotropic B-viscosity coefficients suggests the existence of a minimum deactivation constant for proteins. Differential scanning calorimetry is used to measure protein melting temperatures and thermodynamic parameters, which are used to calculate the intrinsic irreversible deactivation constant. We find that either the protein unfolding rate or the rate of intrinsic irreversible deactivation can control the observed deactivation rates.     

Structural and dielectric behaviour of non-aqueous lyotropic mixtures: influence of amphiphile chain lengths and counter ions

This study highlights the effects of amphiphile chain length and counter ions on the self-assembly and dielectric behaviour of non-aqueous lyotropic liquid crystals. Two-dimensional hexagonal mesophase is seen for short-chain length sodium dodecyl sulphate, while lamellar and multiwall lamellar mesophases are noticed for long-chain length cetyltrimethylammonium bromide and polyoxyethylene (20) sorbitan monolaurate amphiphiles in the non-aqueous domains of ethylene glycol. A strong influence of amphiphile counter ions is seen on static dielectric constant, loss factor, relaxation frequency and relaxation time of these lyotropic mixtures. Refractive indices of these lyotropic phases are also highlighted.     

Dramatic specific-ion effect in supramolecular hydrogels

We report on a pronounced specific-ion effect on the intermolecular and chiral organization, supramolecular structure formation, and resulting materials properties for a series of low molecular weight peptide-based hydrogelators, observed in the presence of simple inorganic salts. This effect was demonstrated using aromatic short peptide amphiphiles, based on fluorenylmethoxycarbonyl (Fmoc). Gel-phase materials were formed due to molecular self-assembly, driven by a combination of hydrogen bonding and π-stacking interactions. Pronounced morphological changes were observed by atomic force microscopy (AFM) for Fmoc-YL peptide, ranging from dense fibrous networks to spherical aggregates, depending on the type of anions present. The gels formed had variable mechanical properties, with G'?values between 0.8?kPa and 2.4?kPa as determined by rheometry. Spectroscopic analysis provided insights into the differential mode of self-assembly, which was found to be dictated by the hydrophobic interactions of the fluorenyl component, with comparable H-bonding patterns observed in each case. The efficiency of the anions in promoting the hydrophobic interactions and thereby self-assembly was found to be consistent with the Hofmeister anion sequence. Similar effects were observed with other hydrophobic peptides, Fmoc-VL and Fmoc-LL. The effect was found to be less pronounced for a less hydrophobic peptide, Fmoc-AA. To get more insights into the molecular mechanism, the effect of anions on sol-gel equilibrium was investigated, which indicates the observed changes result from the specific-ion effects on gels structure, rather than on the sol-gel equilibrium. Thus, we demonstrate that, by simply changing the ionic environment, structurally diverse materials can be accessed providing an important design consideration in nanofabrication via molecular self-assembly.     

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.     

Tetraalkylammonium Ions in Aqueous and Non-aqueous Solutions

Property data for tetraalkylammonium cations, [H(CH₂) _n ]₄N⁺, are reviewed. They pertain to the isolated cations and their transfer from the gas phase into aqueous solutions. Various properties of these cations in aqueous and non-aqueous solutions and data for their transfer between these are also reviewed. Emphasis is placed on the dependence of data on the length n of the alkyl chains rather than on the absolute values. Most of the data are available only for the first four members of the series. The properties of the isolated ions increase linearly with the chain length. Molar enthalpies of formation of the gaseous and aqueous cations, and absolute standard molar enthalpies of hydration, are derived. Standard molar entropies of dissolution of several salts in water are obtained from their solubilities and enthalpies of solution. The molar entropies of the crystalline iodides of the first four members of the series then give the standard partial molar entropies of the aqueous cations and their molar entropies of hydration. The standard partial molar volumes in aqueous and non-aqueous solutions are quite linear with n and in non-aqueous solutions the molar volume hardly depends on the nature of the solvent. On transfer from water to non-aqueous solvents the volume of Me₄N⁺ suffers some shrinkage, that of Et₄N⁺ appears to be unaffected, but from Pr₄N⁺ onwards an increasing expansion takes place. This unexpected result is tentatively explained by hydrophobic intra-molecular association of pairs of alkyl chains in aqueous solutions, resulting in a tightening of the structure. The transfer of the R₄N⁺ cations from water into non-aqueous solvents is governed by a large positive entropy change, outweighing the smaller positive enthalpy change. The transport properties of the aqueous R₄N⁺ cations are non-linear with n. A major impediment to movement is thus the sticking of the water molecules to the ice-like hydrophobic hydration sheaths of the larger cations. The number of water molecules affected by the hydrophobic cations is open to widely differing estimates resulting from various approaches, and constitute an open issue.     

Do hydration forces play a role in thin film drainage and rupture observed in electrolyte solutions?

The mechanism that controls bubble coalescence in electrolyte solutions remains unresolved. The problem is difficult as sensitive dynamic thin film processes are critical. Here we discuss the relationship between film dynamics, specific-ion effects and the combining rules that codify electrolyte effects on bubble coalescence. The relationship with Hofmeister effects is explored, revealing that these very different manifestations of specific ion effects ultimately have the same origin, being the interfacial positioning of ions, which for the air–water interface correlates with the empirically derived α and β assignments used in the combining rules. Ion hydration is important as it strongly influences the interfacial positioning of ions and therefore ultimately bubble coalescence, however dynamic events determine if a collision results in coalescence and therefore we conclude that hydration forces play no role in bubble coalescence in electrolyte solutions.     

X-ray diffraction studies on mesophases of cetyl- and dodecyltrimethylammoniumbromide in liquid ammonia

We have studied solutions of the surfactants cetyltrimethylammoniumbromide (CTAB) and dodecyltrimethylammoniumbromide (DTAB) in liquid ammonia with respect to the formation of lyotropic phases. For this purpose, a set-up for performing X-ray scattering experiments at temperatures up to 120 degrees C on samples containing liquid ammonia has been developed. Both systems form hexagonal and monoclinic lyotropic phases above the dissolving temperature of the surfactant, thus representing the first examples for lyotropic phases in liquid ammonia, and for monoclinic phases in nonaqueous solvents. The phase diagrams of CTAB/liquid NH(3) and DTAB/liquid NH(3) show similarities to their respective aqueous systems. However, the regions of existence of monoclinic phases are much larger in the ammonia system, while the cubic phases, as observed in the water based systems, do not seem to exist. The liquid-crystalline phases found provide potentiality for preparing mesoporous, nitride-based solids.     

The influence of Hofmeister series ions on hyaluronan swelling and viscosity

The dissolution of hyaluronan in water leads to its degradation, and as a result its molecular weight decreases. The degradation of hyaluronan is mainly influenced by temperature, solution composition, and also its pH. This study describes the influence of Hofmeister series ions on hyaluronan behaviour and hyaluronan film swelling by solutions of these ions. It was found that Hofmeister ions show lyotropic effects influencing the entanglement of hyaluronan coils and their expansion from solid polymer films into swollen gel state. The hydrophobic and hydrophilic interactions in the structure of hyaluronan macromolecules are represented by the mutual diffusion coefficient D(c), the mean mutual diffusion coefficient D(s), the expansion work of coil swelling RA(delta,s), the activation enthalpy of diffusion connected with swelling H(D,s) and kinematic viscosity of hyaluronan-ions solutions nu.     

The 2017 Luckhurst-Samulski Prize

The investigation addressed in the title was carried out on a total of 17 members of the two disc-shaped series 1 or 2 of radial hexaesters differing only in their cyclic C6-core units. Their lyotropic behaviour appeared to be dependent on the intracolumnar order of stacking of the respective hexaester molecules. With linear alkanes in mixtures with the phenylene centred hexaesters (series 1 derived from hexahydroxybenzene) no lyomesomorphism occurs, whereas with most of the cyclohexane centred analogues (series 2, scyllitol derivatives) even two lyomesophases were observed: a hexagonal type and, in 49 cases, a nematic columnar (induced) phase. Here, the lyotropy demonstrates its dependence on the length of chains both of the alkanoyl groups of 2 and the solvent (the linear alkanes) used. In the case of the selected cyclic solvents, three factors proved to govern the lyotropic mesomorphism of all these disc-like hexaesters: (1) their degree of saturation, (2) their molecular size/volume, and (3) their stereostructure. Interestingly, with benzene and other (less unsaturated) monocyclic solvents, both series of hexaesters exhibit only a hexagonal type of mesophase which is induced in four cases. Saturated monocyclic hydrocarbons act on members of the aromatic centred series 1 in the same way; with cyclohexane however, two more cases of induction of a hexagonal type of lyomesophase have been observed. To our surprise, binary mixtures of most hexaesters of series 2 (scyllitol derivatives) with saturated monocyclic hydrocarbons show the same polymorphic (nematic and hexagonal) situation as that found here with linear hydrocarbons; moreover, in comparison with the results with linear alkanes, four more cases of induced phases appear with these cycloalkanes. These results, in particular the many cases of lyomesophase induction, relate to the first examples of lyotropic mesomorphism in binary systems of disc-like materials with saturated cyclic solvents/hydrocarbons. Finally, in some examples presented here it was found for the first time that bulkiness/space-filling or the stereostructure of the saturated cyclic solvents/hydrocarbons, e.g. cis- or trans-decalin, also play an important role in the lyotropic mesomorphism of their mixtures with the radial hexaesters of the two series 1 and 2. By means of models shown here for the first time in figures 4 to 6, ideas are put forward about peculiarities due to microsegregation in relation to the (formally) very similar members of both hexaester series 1 and 2 as a key to understanding the drastic difference in their thermotropic as well as their lyotropic states.     

Comparative lyotropy study of homologous hexaesters of hexahydroxy-benzene and -cyclohexane (scyllitol) in linear and cyclic hydrocarbons: microsegregation and mesophase formation of discotics [1]

The investigation addressed in the title was carried out on a total of 17 members of the two disc-shaped series 1 or 2 of radial hexaesters differing only in their cyclic C6-core units. Their lyotropic behaviour appeared to be dependent on the intracolumnar order of stacking of the respective hexaester molecules. With linear alkanes in mixtures with the phenylene centred hexaesters (series 1 derived from hexahydroxybenzene) no lyomesomorphism occurs, whereas with most of the cyclohexane centred analogues (series 2 , scyllitol derivatives) even two lyomesophases were observed: a hexagonal type and, in 49 cases, a nematic columnar (induced) phase. Here, the lyotropy demonstrates its dependence on the length of chains both of the alkanoyl groups of 2 and the solvent (the linear alkanes) used. In the case of the selected cyclic solvents, three factors proved to govern the lyotropic mesomorphism of all these disc-like hexaesters: (1) their degree of saturation, (2) their molecular size/volume, and (3) their stereostructure. Interestingly, with benzene and other (less unsaturated) monocyclic solvents, both series of hexaesters exhibit only a hexagonal type of mesophase which is induced in four cases. Saturated monocyclic hydrocarbons act on members of the aromatic centred series 1 in the same way; with cyclohexane however, two more cases of induction of a hexagonal type of lyomesophase have been observed. To our surprise, binary mixtures of most hexaesters of series 2 (scyllitol derivatives) with saturated monocyclic hydrocarbons show the same polymorphic (nematic and hexagonal) situation as that found here with linear hydrocarbons; moreover, in comparison with the results with linear alkanes, four more cases of induced phases appear with these cycloalkanes. These results, in particular the many cases of lyomesophase induction, relate to the first examples of lyotropic mesomorphism in binary systems of disc-like materials with saturated cyclic solvents/hydrocarbons. Finally, in some examples presented here it was found for the first time that bulkiness/space-filling or the stereostructure of the saturated cyclic solvents/hydrocarbons, e.g. cis- or trans-decalin, also play an important role in the lyotropic mesomorphism of their mixtures with the radial hexaesters of the two series 1 and 2. By means of models shown here for the first time in figures 4 to 6, ideas are put forward about peculiarities due to microsegregation in relation to the (formally) very similar members of both hexaester series 1 and 2 as a key to understanding the drastic difference in their thermotropic as well as their lyotropic states.     

Counterion specificity in the phase behavior of tetradecyldimethylamine oxides at different degrees of protonation

Effects of counterion species on the aggregate structure of tetradecyldimethylamine oxide (C14DMAO) accompanying the protonation were examined by viscoelastic properties and phase behavior observations. Different extents of the synergetic behavior between protonated and non-protonated amine oxides were observed depending on the counterion species, which follow the Hofmeister, or the lyotropic, series. The efficacy of monovalent anions with respect to the increasing the surfactant packing parameter was as follows: salicylate>perchlorate>nitrate>bromide>chloride, formate. Vesicle formation was found in the case of salicylate and perchlorate anions and elongation of rodlike micelles was observed for nitrate, bromide, sulfate, tartronate and tartrate anions but little effects in the case of chloride and formate anions. The observed ionic specificity is well correlated with the free energy of hydration of counterions.     

Non-aqueous solvents for DNP surface enhanced NMR spectroscopy

A series of non-aqueous solvents combined with the exogenous biradical bTbK are developed for DNP NMR that yield enhancements comparable to the best available water based systems. 1,1,2,2-tetrachloroethane appears to be one of the most promising organic solvents for DNP solid-state NMR. Here this results in a reduction in experimental times by a factor of 1000. These new solvents are demonstrated with the first DNP surface enhanced NMR characterization of an organometallic complex supported on a hydrophobic surface.     

The hydration of anions in nonaqueous media

A very simple isopiestic method based on that of S. Christian is used for measuring the salting-in of water into nonpolar, low-volatility solvents by tetraalkylammonium salts. The quantity of excess water which is dissolved in such solvents is directly proportional to the salt concentration and is sharply dependent on the nature of the anion but is nearly insensitive to that of the R₄N⁺ cation. The hydration ratioH, which we define as the moles of excess solubilized water per mole of R₄N⁺ X^–, is directly relatable to the enthalpy of hydration of the anion X^– in several solvents and in the gas phase. The quantityH is also correlated with many free-energy terms including those for the Hofmeister lyotropic series, for the ability of the anions to salt nonelectrolytes out of water, for the free-energy terms for separation of these ions by reverse osmosis membranes, and for their nucleophilicities. A surprising (but not unprecedented) feature of the hydration ratio is that it, rather than its logarithm, behaves as a free-energy term. It is proposed that all these properties have in common the free energy of hydration of the anions, and this notion is supported by a close correspondence between the anionic hydration ratio and their hydrogen-bonding energies with proton donors in aprotic solvents. The results support scattered observations by other workers that isolated water molecules do not have an unusual inherent affinity for anions. Accordingly, large anionic hydration energies in bulk aqueous media reflect extensive cooperative interactions in the solvent. Implications for nucleophilic activity in phase transfer catalysis and enzyme activity are mentioned.     

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.     

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.