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.     

Ions at interfaces: the central role of hydration and hydrophobicity

It is increasingly being accepted that solvation properties of ions and interfaces (hydration of ions, hydrophobic or hydrophilic character of interfaces) play a fundamental role in ion-surface interaction in water. However, a fundamental understanding of the precise role of solvation in ionic specificity in colloidal systems is still missing, although important progress has been made over the last years. We present in this contribution experimental evidences (including also ions not usually included in specific ion studies) together with Molecular Dynamics (MD) simulations that highlight the importance of the hydration of ions and surfaces in order to understand the origin of ionic specificity. We first show that both surface polarity and ion hydration determine the sorting of ions according to their ability to induce specific effects (the so-called Hofmeister series). We extend these classical series by considering the addition of the inorganic anions IO₃⁻, BrO₃⁻ and ClO₃⁻, which present unusual properties as compared with the ions considered in classical Hofmeister series. We also consider big hydrophobic organic ions such as tetraphenylborate anion (Ph₄B⁻) and tetraphenylarsonium cation (Ph₄As⁺) that in the context of the Hofmeister series behave as super-chaotropes ions.     

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.     

Hofmeister effects at low salt concentration due to surface charge transfer

We present a theoretical comparison of the surface forces between two graphite-like surfaces at salt concentrations below 10 mM with surfaces charged by various mechanisms. Surface forces include a surface charging or chemisorption contribution to the total free energy. Surfaces are charged by charge regulation (H⁺ binding), site competition (H⁺ and cation binding) and redox charging with electrodes coupled to a countercell. Constant surface charge is also considered. Surface parameters are calibrated to give the same potential when isolated. Nonelectrostatic physisorption energies of the potential determining ions provide a specific and significant contribution to the charging energy. Consequently ion specificity is found in the surface forces at concentrations of 1–10 mM, which is not observed under constant charge conditions. The force between redox electrodes continues to show Hofmeister effects at 0.01 mM. We refer to this low concentration Hofmeister effect as “Hofmeister charging”, and suggest that the more common high concentration ion specific effects may be known as “Hofmeister screening”. Hofmeister series are considered over LiCl, NaCl, KCl and NaNO₃, NaClO₄, NaSCN with the cations (or H⁺) being the potential determining ions. A K⁺ anomaly is attributed to the small size of the weakly hydrated chaotropic K⁺ ion, with Li⁺ and Na⁺ explicitly modelled as strongly hydrated cosmotropes.     

The inevitable accumulation of large ions and neutral molecules near hydrophobic surfaces and small ions near hydrophilic ones

The present contribution offers a unified explanation to three central phenomena in physical chemistry of interfaces in contact with aqueous solution: (1) Accumulation of large anions at the air/water interface. (2) Accumulation of neutral gas molecules near hydrophobic surfaces and the resulting hydrophobic interaction between two such surfaces, and (3) The Hofmeister effect, namely, the enhanced propensity of small ions to hydrophilic surfaces and large ions to hydrophobic surfaces. The common thread linking these phenomena is the free energy balance between ion or molecule hydration in solution and the cost of localizing these objects at the water-surface interface. Comparing the results of an abstract lattice-gas model to force spectroscopy data collected by AFM we reveal the underlying principles and demonstrate their universality.     

Study of specific ion-amino acid interactions through the use of local correlation methods

Specific ion effects, related to the hydration of ions and ion-solute interactions, play a fundamental part in many processes in chemistry and biology. Although intensively studied since the seminal studies of Franz Hofmeister and co-workers, their molecular origin has only recently started to be unveiled. In this work, we have investigated the interaction between halide anions and a selected set of amino acid residues in an attempt to identify the forces behind ion specificity. Two-dimensional potential energy surfaces have been calculated with the use of local second order M?ller-Plesset perturbation theory (LMP2), coupled with the COSMO model to describe solvent effects. The results show in great detail the impact of dispersion interactions, in particular for the heavier anions (Br(-) and I(-)). The obtained potential energy surfaces also hint at a greater mobility of iodide in the vicinity of a residue, which correlates well with its placing in the Hofmeister series.     

Analysis of oxidizer salt mixtures by electrospray ionization mass spectrometry

Binary aqueous mixtures of NaNO3, KNO3 and NaClO4 oxidizers were analyzed using electrospray ionization mass spectrometry. Sodium nitrate solutions were observed to form doubly charged clusters of the type [(NaNO3)n2Na]2+ and [(NaNO3)n2NO3]2-, where n = 11, 13, 15, etc., in addition to singly charged cluster ions that have been reported previously. The identity of the doubly charged clusters was determined by tandem mass spectrometry. Two-component NaNO3-KNO3 salt solutions were observed to form cluster ions of the type [(NaNO3)i(KNO3)jNO3]- in the negative ion mode and [(NaNO3)i(KNO3)jNa]+ and [(NaNO3)i(KNO3)jK]+ in the positive ion mode, where i + j = 1, 2, 3 ... 10. Two-component solutions of KNO3-NaClO4 formed ions of the type [(KNO3)i(NaClO4)j(KClO4)k(NaNO3)lK](+) and [(KNO3)i(NaClO4)j(KClO4)k(NaNO3)lNa]+ in the positive ion mode, where i + j + k + l = 1, 2, 3 ... 10. Similar clusters containing excess nitrate and perchlorate to provide the charge are formed in the negative ion mode. In each case, the maximum number of spectral lines for a cluster of size n can be calculated as the number of combinations of n(th) order (where n = i + j) of N different cation-anion pairs taken with replication and without regard for the ordering of the N cation-anion pairs. The actual number of lines observed may be reduced due to degeneracy of nominal m/z values for some ions.     

The Hofmeister Anion Effect on Ionophore‐based Ion‐selective Nanospheres Containing Solvatochromic Dyes

Recently reported ionophore‐based ion‐selective nanospheres contained pH‐independent and positively charged solvatochromic dyes. Here, we evaluate systematically the effect of anions to the fluorescence response of the nanospheres. The anion interference was found significant for anion concentrations above 10 mM. The sensor responses in the presence of various anion background was studied. While target ion (K⁺) causes the fluorescence of the nanospheres to decrease, increasing anion background also leads to lower fluorescence intensity. Lipophilic anions such as ClO₄^?, SCN^?, and I^? exhibited much more interference than hydrophilic anions (e. g., NO₃^?, Cl^?, F^?, SO₄^2?). The trend of the anion interference followed the Hofmeister series. A theoretical model was also demonstrated based on anion adsorption on the surface of the nanospheres.     

Finite volume solution of the modified Poisson-Boltzmann equation for two colloidal particles

The double layer forces between spherical colloidal particles, according to the Poisson-Boltzmann (PB) equation, have been accurately calculated in the literature. The classical PB equation takes into account only the electrostatic interactions, which play a significant role in colloid science. However, there are at, and above, biological salt concentrations other non-electrostatic ion specific forces acting that are ignored in such modelling. In this paper, the electrostatic potential profile and the concentration profile of co-ions and counterions near charged surfaces are calculated. These results are obtained by solving the classical PB equation and a modified PB equation in bispherical coordinates, taking into account the van der Waals dispersion interactions between the ions and both surfaces. Once the electrostatic potential is known we calculate the double layer force between two charged spheres. This is the first paper that solves the modified PB equation in bispherical coordinates. It is also the first time that the finite volume method is used to solve the PB equation in bispherical coordinates. This method divides the calculation domain into a certain number of sub-domains, where the physical law of conservation is valid, and can be readily implemented. The finite volume method is implemented for several geometries and when it is applied to solve PB equations presents low computational cost. The proposed method was validated by comparing the numerical results for the classical PB calculations with previous results reported in the literature. New numerical results using the modified PB equation successfully predicted the ion specificity commonly observed experimentally.     

Electrolyte Anion Affinity and Its Effect on Oxyanion Adsorption on Goethite

The influence of various types of background electrolytes (NaCl, NaNO(3), and NaClO(4)) on the proton adsorption and on the adsorption of sulfate and phosphate on goethite have been studied. Below the PZC the proton adsorption on goethite decreases in the order Cl>NO(3)>ClO(4). The decreasing proton adsorption affects the adsorption of oxyanions on goethite. Anion adsorption of strongly binding polyvalent anions is lower in the studied electrolytes in the order Cl相似文献   

Measurement of Colloidal Stability in Solutions of Simple, Nonadsorbing Polyelectrolytes

The stability of a solution of charged polystyrene particles in the presence of nonadsorbing polyelectrolyte macromolecules is measured using optical light scattering. The particles were negatively charged polystyrene latex spheres (0.5–1 μm diameter) while the macromolecules were simulated using negatively charged colloidal silica spheres (5–7 nm diameter). Because of the electrostatic repulsion between the particles, the solution is found to be stable against primary flocculation (irreversible flocculation into a primary energy minima). However, because of long-range attractive depletion forces, reversible secondary flocculation of the particles occurs into a local potential energy minimum. As observed with uncharged macromolecules, the polyelectrolyte first induces flocculation at a critical flocculation concentration (v*), but later restabilizes the system at a critical restabilization concentration (v**). These critical concentrations are found to decrease with decreasing macromolecule size and increasing particle size. The restabilized solutions are found to remain suspended for periods greater than 20 days. Comparison of the measured flocculation and restabilization results to predictions made using a recently developed force-balance model show qualitative agreement.     

The effect of surface dipoles and of the field generated by a polarization gradient on the repulsive force

Double-layer and hydration interactions have been coupled into a single set of equations because both are dependent on the polarization of the water molecules. The coupled equations involve the electric fields generated by the surface charge and surface dipoles, as well as the field due to the neighboring dipoles in water. The dipoles on the surface are generated through the counterions' binding to sites of opposite charge. The equations obtained were employed to explain the restabilization observed experimentally at large ionic strengths for colloidal particles on which protein molecules were adsorbed. Polar molecules adsorbed on a charged surface of colloidal particle can generate a field either in the same direction as that generated by the charge or in the opposite direction. The effect of the sign of the dipole of the adsorbed polar molecules on the interaction between surfaces was also examined.     

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.     

Hofmeister离子对水溶液中热响应聚合物的局域环境的影响

The Hofmeister ion effect is a very interesting but elusive phenomenon, the importance of which is revealed in self-assembly, ion recognition, and protein folding regulation. With an increasing number of studies suggesting that interactions between ions and solutes play a role in the Hofmeister ion effect, the nature of the Hofmeister phenomenon becomes more debatable. Yet, it is not clear whether the Hofmeister ion effect is a local effect or bulk effect that can reach beyond many hydration shells, where specific interactions between ions and solutes play key roles. In order to further explore this, we applied proton nuclear magnetic resonance (¹H-NMR) spectroscopy to study the effects of specific ions on the local environment around N, N-dimethylpropionamide (NDA) and N-isopropylisobutyramide (NPA), which are the model compounds for poly(2-ethyl-2-oxazoline) and poly(N-isopropylacrylamide), respectively. These polymers are important bio-engineering materials that possess thermoresponsive properties and are also subject to specific ion effects. By correlating the changes in chemical shifts of the two methyl groups on either side of the amide bond, it was found that the Hofmeister ion effects on NPA were more anisotropic than on NDA, and that the cationic effects were more anisotropic than the anionic effects on NPA. These results indicated that the effects of specific ions were almost identical for all methyl groups of NDA. On the other hand, NPA is a larger molecule; thus, not all of its methyl groups were subjected to the specific ion effects to the same extent. The calculation of the electrostatic potential surfaces of NDA and NPA suggested that these observations on the Hofmeister ion effects might be due to steric hindrance, and that the observations on the cationic effects might be due to the interactions between cations and NPA being stronger than the interactions between anions and NPA. This would explain why the highly charged cations caused a significant anisotropicity. Additionally, we found that the chemical shift of the water protons (Δδ_H2O) of conventional kosmotropic anions was larger than zero, which suggested a stronger HB and more charge transfer between water and these anions. The Δδ_H2O of conventional chaotropic anions was less than zero. Despite the different solutes, the results were indifferent in both NDA and NPA solutions. Surprisingly, the Δδ_H2O of Cl^- at concentrations lower than 1 mol∙L^-1 was zero, thus becoming the benchmark between chaotropes and kosmotropes. These results suggested a quantitative measurement of kosmotropicity/chaotropicity, where the anion would be kosmotropic if its Δδ_H2O were higher than that of Cl^- and chaotropic for the opposing condition. Moreover, the results showed that the effects of the cations on the water structure were minimal, which was consistent with minimal charge transfer between the cations and water. The overall results of this study suggest that the Hofmeister ion effect is a global effect, while local interactions of ions with solutes also play a key role.     

Light-regulated electrostatic interactions in colloidal suspensions

The net charge of a colloidal particle was controlled using light and a new photocleavable self-assembled monolayer (SAM). The SAM contained a terminal ammonium group and a centrally located carboxylic acid group that was masked with an ortho-nitrobenzyl functionality. Once exposed to UV light, the 2-nitrobenzyl group was cleaved, therefore transforming the colloidal particle from a net positive (silica-SAM-NH3+) to a net negative (silica-SAM-COO-) charge. By varying the UV exposure time, their zeta potential could be tailored between +26 and -60 mV at neutral pH. To demonstrate a photoinduced gel-to-fluid phase transition, a binary colloidal suspension composed of silica-SAM-NH3+ and negatively charged, rhodamine-labeled silica particles was mixed to form a gel. Exposure to UV light rendered all of the particles negative and therefore converted the system into a colloidal fluid that settles to form a dense sediment.     

Electrostatic interactions between amphoteric latex particles and proteins

The electrostatic interactions between amphoteric polymethyl methacrylate latex particles and proteins with different pI values were investigated. These latex particles possess a net positive charge at low pH, but they become negatively charged at high pH. The nature and degree of interactions between these polymer particles and proteins are primarily controlled by the electrostatic characteristics of the particles and proteins under the experimental conditions. The self-promoting adsorption process from the charge neutralization of latex particles by the proteins, which have the opposite net charge to that of the particles, leads to a rapid reduction in the zeta potential of the particles (in other words colloidal stability), and so strong flocculation occurs. On the other hand, the electrostatic repulsion forces between similarly charged latex particles and the proteins retard the adsorption of protein molecules onto the surfaces of the particles. Therefore, latex particles exhibit excellent colloidal stability over a wide range of protein concentrations. A transition from net negative charge to net positive charge, and vice versa (charge reversal), was observed when the particle surface charge density was not high enough to be predominant in the protein adsorption process.     

The effect of electrolyte on the colloidal properties of poly(N-isopropylacrylamide-co-dimethylaminoethylmethacrylate) microgel latexes

Monodisperse cationic thermosensitive latex microgels prepared by radical-initiated precipitation copolymerization of N-isopropylacrylamide (NIPAM), methylenebisacrylamide, and dimethylaminoethylmethacrylate (DMAEMA) have been reported (Zha LS, Hu JH, Wang CC, Fu SK, Elaissari A, Zhang Y 2002 Colloid Polym Sci 280:1) and we suggested (Zhang Y, Zha LS, Fu SK J Appl Polym Sci) that the polyelectrolyte chains are rich in their expanded shell layers. The effect of a range of electrolytes on several colloidal properties of these cationic latexes (such as particle size, zeta potential and colloidal stability) has been investigated. The ability of the anions to induce the particle deswelling and flocculation is related to their position in the Hofmeister series. Owing to the DMAEMA-rich layer on the latex particles, the ionic-strength dependence of the particle hydrodynamic size and the zeta potential become more profound with increasing amount of DMAEMA incorporated into the microgel. It is suggested that the effect of electrolytes on the colloidal properties of the copolymer microgel latexes is attributed to the dehydration of the poly(NIPAM) segment and the screening of the electrostatic interaction between the charged DMAEMA units induced by electrolytes.     

Co-ion and ion competition effects: ion distributions close to a hydrophobic solid surface in mixed electrolyte solutions

We consider within a modified Poisson-Boltzmann theory an electrolyte, with different mixtures of NaCl and NaI, near uncharged and charged solid hydrophobic surfaces. The parametrized potentials of mean force acting on Na+, Cl-, and I- near an uncharged self-assembled monolayer were deduced from molecular simulations with polarizable force fields. We study what happens when the surface presents negative charges. At moderately charged surfaces, we observe strong co-ion adsorption and clear specific ion effects at biological concentrations. At high surface charge densities, the co-ions are pushed away from the interface. We predict that Cl- ions can also be excluded from the surface by increasing the concentration of NaI. This ion competition effect (I- versus Cl-) may be relevant for ion-specific partitioning in multiphase systems where polarizable ions accumulate in phases with large surface areas.     

Hofmeister effects on the colloidal stability of poly(ethylene glycol)-decorated nanoparticles

The colloidal stability of poly(ethylene glycol)-decorated poly(methyl methacrylate), PMMA/Tween-20, particles was investigated by means of phase separation measurements, in the presence of sodium fluoride (NaF), sodium chloride, sodium bromide, sodium nitrate, or sodium thiocyanate (NaSCN) at 1.0?mol?L^?1. Following Hofmeister's series, the dispersions of PMMA/Tween-20 destabilized faster in the presence of NaF than with NaSCN. After the phase separation, the systems were homogenized and except for the dispersions in NaF, re-dispersed particles took longer to destabilize, indicating that anions adsorbed on the particles, creating a new surface. Except for F^? ions, the adsorption of anions on the polar outmost shell was evidenced by means of tensiometry and small-angle X-ray scattering measurements. Fluoride ions induced the dehydration of the polar shell, without affecting the polar shell electron density, and the formation of very large aggregates. A model was proposed to explain the colloidal behavior in the presence of Hofmeister ions.     

Calibration-free concentration determination of charged colloidal nanoparticles and determination of effective charges by capillary isotachophoresis

Although colloidal nanoparticles show an electrophoretic heterogeneity under the conditions of capillary electrophoresis, which can be either due to the particle-size distribution and/or the particle shape distribution and/or the zeta-potential distribution, they can form correct isotachophoretic zones with sharp-moving boundaries. Therefore, the technique of isotachophoresis permits to generate plugs in which the co-ions and counter ions of the original colloidal solution are removed and replaced by the buffering counter ions of the leading electrolyte. It is shown that analytical isotachophoresis can be used to measure directly, without calibration, the molar (particle) concentration of dispersed ionic colloids provided that the transference number and the mean effective charge number of the particles (within the isotachophoretic zone) can be determined with adequate accuracy. The method can also be used to measure directly the effective charge number of biomacromolecules or colloidal particles, if solutions with known molar (particle) concentration can be prepared. The validity of the approach was confirmed for a model solution containing a known molar concentration of bovine serum albumin.