The Hofmeister anion effect and the growth of polyelectrolyte multilayers

The influence of a variety of counteranions on the properties of polyelectrolyte multilayers deposited by layer-by-layer technique is studied by using ellipsometry and AFM. We found out that in thin dry multilayers (20-90 nm) ofpoly(4-styrenesulfonate) (PSS) and poly(diallyldimethylammonium) (PDADMA), the thickness follows reasonably well the position of the counteranion in the Hofmeister series. The polyelectrolyte-counteranion interaction is studied by means of viscosity measurements of semidilute solutions of PDADMA in the presence of different anions. The dynamic viscosities follow the Hofmeister series of anions and correlate with the thickness of multilayers. Two parameters describing the interaction of ions with water, the Jones-Dole viscosity B coefficient and the hydration entropy, are used to explain the anion effect on the developing multilayer thickness. Reasonably smooth and monotonic functional dependence is observed between the layer thickness and these two parameters.     

Hofmeister effect on the interfacial dynamics of single polymer molecules

The Hofmeister effect on interfacial dynamics has been discovered for single charged polymer molecules (sodium polystyrene sulfonate) adsorbed on a hydrophobic surface from an aqueous solution. The presence of ions in the aqueous solution affects the surface diffusivity, and its amplitudes and the surface friction follow the Hofmeister series-the kosmotropic ions slowed down the surface diffusivity and the chaotropic ions speeded it up. The amplitude of the surface friction exhibits a good correlation with the surface tension increment, indicating the interfacial feature of the Hofmeister effect.     

Boundary lubrication by sodium salts: a Hofmeister series effect

This article describes the preparation and the physico-chemical characterization of a new host-guest system consisting of zeolite beta nanoparticles as host and mitoxantrone as guest. The resulting host-guest system mitoxantrone@beta is characterized in terms of morphology (transmission electron microscopy, dynamic light scattering), structure (powder wide-angle X-ray diffraction, nitrogen sorption), surface charge (ξ-potential measurements), and optical properties (UV-visible absorption, steady-state fluorescence). Mitoxantrone@beta particles are monodisperse in size with a mean diameter centered around 100 nm. Mitoxantrone guest molecules are adsorbed at the micropore entrances of zeolite host. Resulting nanoparticles retrieve the interesting optical properties of guest molecules with a fluorescence emission band in the near-infrared region. Mitoxantrone loading is comparatively evaluated by three different means (elemental analysis, direct and indirect UV-visible absorption studies) showing a loading level of 6.8 μmol/g. Mitoxantrone@beta nanoparticles also show a noticeable cytotoxic effect when applied to cancer cells.     

Hofmeister anion effect on aqueous phase behavior of heptaethylene glycol dodecyl ether

The aqueous phase behavior of heptaethylene glycol dodecyl ether (C12E7) was investigated in the presence of sodium salts of Cl-, I-, and ClO4-. Pseudo binary T-X phase diagrams were constructed for these mixtures by means of differential scanning calorimetry. The salting-out electrolyte NaCl expanded the Lalpha region toward higher temperatures and shrank the H1 region toward lower temperatures compared with the salt-free system. On the contrary, the salting-in electrolytes NaI and NaClO4 induced shrinkage of the Lalpha region and an expansion of the H1 phase. The influence of these salts on the mesophase regions was more pronounced for the Lalpha phase than for the H1 phase, and area of the Lalpha phase region decreased in the sequence of NaCl > none > NaI > NaClO4, consist with the Hofmeister series of the anions. This salt effect on the mesophase stability in aqueous nonionic surfactant mixture would be qualitatively interpreted in terms of the salt effect on the hydration of the polyoxyethylene chain in the surfactant molecules.     

The Hofmeister series effect in adsorption of cationic surfactants--theoretical description and experimental results

Interfacial properties of cationic surfactants show strong dependence on the type of surfactant counterion or on the type of anion of a salt added to the surfactant solution. In the paper, the models of ionic surfactant adsorption that can take into account ionic specific effects are reviewed. Model of ionic surfactant adsorption based on the assumption that the surfactant ions and counterions undergo nonequivalent adsorption within the Stern layer was selected to describe experimental surface tension isotherms of aqueous solutions of a number of cationic surfactants. The experimental isotherms for: n-alkyl trimethylammonium cationic surfactants, namely: C(16)TABr (CTABr or CTAB), C(16)TACl, C(16)TAHSO(4), C(10)TABr and C(12)TABr as well as decyl- and dodecylpyridinium salts with and without various electrolyte anions as Cl(-), Br(-), F(-), I(-), NO(3)(-), ClO(4)(-) and CH(3)COO(-) were described in terms of the model and a good agreement between the theory and experiment was obtained for a wide range of surfactants and added electrolyte concentrations. A very pronounced Hofmeister effect in dependence of surface tension of cationic surfactants on the type of anion was found. Analysing this dependence in terms of the proposed model of ionic surfactant adsorption, strong correlation between "anion surface activity" (the model parameter accounting for ion penetration into the Stern layer), and the ion polarizability was obtained. That suggests that the mechanism related to the dispersive interaction of polarized ion with electric field at interface is responsible for Hofmeister series effects in surface activity of cationic surfactants. The same mechanism was proposed recently to explain the dependence of surface tension increase with electrolyte concentration on anion and cation type.     

Anion Complexation and The Hofmeister Effect

The ¹H NMR spectroscopic analysis of the binding of the ClO₄^? anion to the hydrophobic, concave binding site of a deep‐cavity cavitand is presented. The strength of association between the host and the ClO₄^? anion is controlled by both the nature and concentration of co‐salts in a manner that follows the Hofmeister series. A model that partitions this trend into the competitive binding of the co‐salt anion to the hydrophobic pocket of the host and counterion binding to its external carboxylate groups successfully accounts for the observed changes in ClO₄^? affinity.     

The Hofmeister series and protein-salt interactions

In order to understand the origin of the Hofmeister series, a statistical-mechanical analysis, based upon the Kirkwood-Buff (KB) theory, has been performed to extract information regarding protein hydration and water-mediated protein-salt interactions from published experimental data-preferential hydration and volumetric data for bovine serum albumin in the presence of a wide range of salts. The analysis showed a linear correlation between the preferential hydration parameter and the protein-cosolvent KB parameter. The same linear correlation holds even when nonelectrolyte cosolvents, such as polyethelene glycol, have been incorporated. These results suggest that the Hofmeister series is due to a wide variation of the water-mediated protein-cosolvent interaction (but not the change of protein hydration) and that this mechanism is a special case of a more general scenario common even to the macromolecular crowding.     

Hofmeister effects in micromolar electrolyte solutions

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.     

Unprecedented tunable hydrophobic effect and anion recognition triggered by AIE with Hofmeister series in water

An unprecedented tunable hydrophobic effect in self-assembly of a small cationic organic fluorophore(NI-TPy~+)-based with aggregation-induced emission(AIE) property was realized in aqueous solution.The amplification of hydrophobicity was found to be significantly dependent upon the increasing aggregate s of NI-TPy~+,which enable d the study of the hydrophobic binding of chaotropic anions with the Hofmeister series.     

Hofmeister series: The quantum mechanical viewpoint

It is suggested that electromagnetic quantum vacuum fluctuations are at the very deep root of the so-called “specific ions effects” in concentrated solutions or in living cells. A many-body quantum-mechanical frame of thinking is proposed based on the concept of quantum coherence taking into account explicitly density and excitation frequencies of molecules and/or ionic species. It is also proposed that Hofmeister phenomena could have a natural explanation in the harmonic relationships between sets of characteristic frequencies ruled by quantum mechanical laws. It then follows that physical chemistry of concentrated media and biology should be ruled more by a quantum “symphony” between indistinguishable constituents rather than localized two-body electrical interactions between molecular or ionic species.     

Hofmeister effects of anions on self-assembled thermogels

Thermogels are temperature-responsive soft biomaterials with numerous biomedical applications. They possess high water content and can spontaneously gelate by forming non-covalent physical crosslinks between their constituent amphiphilic polymers when warmed. However, despite the ubiquity of salts in biological fluids and buffer media, the influence of salts on thermogelling polymers and the overall physical properties of the resulting hydrogels are poorly understood. Herein, we elucidate the effects of common inorganic salts on the gelation and micellization properties of a thermogelling polymer containing poly(ethylene glycol), poly(propylene glycol), and poly(caprolactone) components. The identity of the salts' anions and their concentrations was found to exhibit significant effects on the thermogel properties, in some cases being able to decrease the sol-to-gel phase transition by up to 10 °C. We demonstrate that these notable influences are likely brought about by the changes in solvation of both the polymer's hydrophobic and hydrophilic segments, as well as by direct interactions of poorly hydrated anions with the hydrophobic polymer segments. Our findings show that the effects of salts on amphiphilic thermogelling polymers are non-negligible and hence need to be taken into account for engineering and optimization of thermogel properties for different biomedical applications.     

Understanding the Hofmeister effect in interactions between chaotropic anions and lipid bilayers: molecular dynamics simulations

A set of all-atom molecular dynamics simulations have been performed to better understand critical phenomena regarding a Hofmeister series of anions and lipid bilayers. The simulations isolate the effect of anion size and show clear differences in the interactions with the dipolar phoshpatidylcholine headgroup. Cl- anions penetrate into the headgroup region of the bilayer, but the simulations confirm theories which predict that larger anions penetrate more deeply, into a more heterogeneous and hydrophobic molecular region. That anion size leads to such differences in partitioning in the bilayer provides atomic-level support to hypotheses inspired by several experimental studies. The ability of larger anions to bury deep within the bilayer is correlated with a less well-structured hydration shell, shedding of which upon penetration incurs a smaller penalty for the larger anions than for Cl-.     

Hofmeister效应对生物和化学体系的影响

Hofmeister效应涉及到的问题范围十分广泛,对很多化学、生物体系都有影响。总的说来,Hofmeister效应会影响到溶液的冰点、沸点、黏度、偏摩尔体积、饱和蒸汽压、传导率、pH、表面张力。文章主要介绍了Hofmeister效应的主要影响因素,以及相关理论的建立与完善,并对不同体系中Hofmeister效应的影响进行了描述。     

On the molecular mechanism of ion specific Hofmeister series

Hofmeister series ranks the ability of salt ions in influencing a variety of properties and processes in aqueous solutions.In this review,we reexamine how these ions and some other small molecules affect water structure and thermodynamic properties,such as surface tension and protein backbone solvation.We illustrate the difficulties in interpreting the thermodynamic information based on structural and dynamic arguments.As an alternative,we show that the solvation properties of ions and proteins/small molecules can be used to explain the salt effects on the thermodynamic properties of the solutions.Our analysis shows that the often neglected cation-anion cooperativity plays a very important role in these effects.We also argue that the change of hydrogen donor/acceptor equilibrium by added cosolutes/cosolvents can be used to explain their effects on protein secondary structure denaturation/protection:those increase hydrogen donor concentrations such as urea and salts with strongly solvated cations/weakly hydrated anions tend to dissolve protein backbone acting as secondary structure denaturants,whereas those lack of hydrogen donors but rich in acceptors have the opposite effect.     

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.     

Synergy between Hofmeister effect and coupled water in proteins: Unusual dilational moduli of BSA at air/solution interface

This study relates interfacial interactions of bovine serum albumin (BSA) molecules in dilute solutions with its dilatational rheology. Dynamic surface tension and the associated dilational elastic modulus and viscosity for BSA and mixtures of BSA with Hofmeister electrolytes—NaCl, NaClO₄, Na₂SO₄, NaF and Na₂HPO₄ have been studied using a sinusoidal surface compression and expansion for frequencies ranging from 0.01 to 0.4 Hz. at solution/air interface. In all the BSA + electrolyte systems, both the elastic modulus and viscosity show unusually high values compared with pure BSA or pure electrolytes. In the presence of NaF and Na₂SO₄ the viscosity of protein increases almost by 50–80-fold and the corresponding elastic modulus also changes by 30–50-fold. Hydrated Hofmeister ions surely influence the measured rheological properties. In addition, the synergistic effect of the hydrated protein and the vicinal hydrated electrolytes possibly contribute to the high viscosity and elasticity due to change in dynamics of these assemblies. Thus the behavior of BSA is effected by salts in different ways, especially due to the dynamics and strength of the water molecules in the assembly.     

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.