Specific Cation Effects Research Articles

Effect of seawater and dissolved cations on scorodite synthesis via iron coprecipitation

In recent years, one of the more effective developments in the fixation of arsenic from nonferrous waste has been the synthesis of scorodite via the co-precipitation of arsenic with iron during Fe(II) oxidation. This work examines the impact of cation impurities on the process with the aim of widening the application of this approach to incorporate the use of seawater and the treatment of ferrous metallurgy waste. Scorodite formation was promoted by the presence of cations in the order of Cu2+ >> Zn2+ > Al3+ > Ca2+ ∼ Mg2+, with differences in precipitation behaviour, yield and kinetics due to specific cation effects on iron oxidation. A grain size increase was observed with Cu, while the opposite was found with Al, attributed to crystallisation from solution over epitaxial growth. The use of seawater decreased As precipitation by <4 % without affecting grain size, showing the potential to be utilised as reaction medium. While the presence of Na+ hinders As precipitation, the impact appears less pronounced with seawater due to competing effects from other cations. Since scorodite precipitation is slowed down by high Fe(III) in the feed, a pre-reduction approach with Al, Cu and Zn base metals was examined for conversion of Fe(III) in metallurgical waste to Fe(II) prior to co-precipitation with As. While this produced stable scorodite precipitates, the yield and presence of unstable by-products were impacted by the method of reductant addition.

Influence of Cations on Direct CO2 Capture and Mineral Film Formation: The Role of KCl and MgCl2 at the Air/Electrolyte/Iron Interface.

Uncovering the mechanisms associated with CO2 capture through mineralization is vital for addressing rising CO2 levels. Iron in planetary soils, the mineral cycle, and atmospheric dust react with CO2 through complex surface chemistry. Here, the effect of cations on the growth of carbonate films on iron surfaces was investigated. In situ polarized modulated infrared reflection absorption spectroscopy was used to measure CO2 adsorption and oxidation of iron in MgCl2(aq) and KCl(aq), compared to FeCl2(aq) at the air/electrolyte/iron interface. The cation was found to influence the film composition and growth rates, as corroborated by infrared and photoelectron spectroscopy. In MgCl2(aq), a mixture of hydromagnesite, magnesite, and a Mg hydroxy carbonate film was grown on iron, while in KCl(aq), a potassium-rich bicarbonate film was grown. The cations were found to affect the rates of hydroxylation and carbonation, confirming a specific cation effect on carbonate film growth. In the submerged region, a heterogeneous mixture of lepidocrocite and iron hydroxy carbonate was produced, suggesting that Fe2+ dominates the surface products. Surface roughness measurements from in situ atomic force microscopy indicate iron initially corrodes faster in MgCl2(aq) than KCl(aq), due to the Cl- ions that initiate pitting and corrosion. In this region, cations were not found to affect the morphologies. This study shows surface corrosion is necessary to provide nucleation sites for film growth and that the cations influence the carbonate film, relevant for CO2 capture and planetary processes.

The Unexpected Role of Cations in the Self-Assembly of Positively Charged Amphiphiles at Liquid/Liquid Interfaces.

Conventional wisdom suggests that cations play a minimal role in the assembly of cationic amphiphiles. Here, we show that at liquid/liquid (L/L) interfaces, specific cation effects can modulate the assemblies of hydrophobic tails in an oil phase despite being attached to cationic headgroups in the aqueous phase. We used oligo-dimethylsiloxane (ODMS) methyl imidazolium amphiphiles to identify these specific interactions at hexadecane/aqueous interfaces. Small cations, such as Li+, bind to the O atoms in the ODMS tail and pin it to the interface, thereby imposing a kinked conformation─as evidenced by vibrational sum frequency generation spectroscopy and molecular dynamics simulations. While larger Cs+ ions more readily partition to the interface, they do not form analogous complexes. Our data not only point to ways for controlling amphiphile structure at L/L interfaces but also suggest a means for the separation of Li+, or related applications, in soft-matter electronics.

Tailoring the humidity response of cellulose nanocrystal-based films by specific ion effects

HypothesisThe optical properties and humidity response of iridescent films made of cellulose nanocrystal (CNC) and polyethylene glycol (PEG) can be tailored by the incorporation of electrolytes chosen based on specific ion effects (SIE). ExperimentsA series of inorganic salts comprising five different cations and five anions based on the Hofmeister series were mixed with CNC/PEG suspensions, followed by an air-dried process into iridescent solid films. These films were tested in changing relative humidity (RH) environments from 30% to 90% and their photonic properties and mass change monitored. The underlying structures and the mechanism of their formation were quantified in terms of interparticle distance derived from small angle X-ray scattering experiment and pitch size quantified by scanning electron microscope (SEM). FindingsThe specific color and color range of CNC/PEG based films are controlled by a specific anion effect achieved by selection of the salt while the specific cation effect is negligible. The salting-in type anions with the same valency result in a red-shift color when films are in the dried state. The salting-in type leads to a greater color changing range during RH changes than the salting-out type. The resultant mass gain/loss trend is consistent with the color change. In contrast, cations do not show any relationships between salting-in effect and the measured properties as observed for anions. The observed SIE can be used to engineer CNC/polymer-based humidity and bio-diagnostic colorimetric indicator devices.

Specific Cation Effect on the Flotation of Graphite

Inorganic electrolytes are assumed to significantly impact the flotation performance of graphite resources; however, the underlying mechanism is still unclear. In this work, the effect of three inorganic salts, including KCl, MgCl2, and AlCl3, was studied on the flotation of graphite. Flotation results indicated significantly increased graphite recovery with the addition of KCl regardless of pulp pH. MgCl2 improved the flotation performance under acidic and slightly alkaline conditions, while AlCl3 had a better activation on graphite flotation under strong acidic and alkaline conditions. Contact angle and Zeta potential results confirmed that electrolytes substantially reduced the absolute value of surface charge and the hydration of graphite surface, thus improving the hydrophobicity. Froth stability studies indicated that multivalent Al3+ and Mg2+ ions exhibit superior foaming performance than K+ ions and contribute to more stable and abundant foam. Additionally, these salt ions increased the surface tension and prevented bubble coalescence, contributing to nanobubble formation. Therefore, adding MgCl2 and KCl is of great significance for improved graphite flotation under neutral conditions.

Design principles of PI(4,5)P2 clustering under protein-free conditions: Specific cation effects and calcium-potassium synergy

Phosphatidylinositol 4,5-bisphosphate (PIP2) clustering is a key component in cell signaling, yet little is known about the atomic-level features of this phenomenon. Network-theoretic analysis of multimicrosecond atomistic simulations of PIP2 containing asymmetric bilayers under protein-free conditions, presented here, reveals how design principles of PIP2 clustering are determined by the specific cation effects. Ca2+ generates large clusters (6% are pentamer or larger) by adding existing PIP2 dimers formed by strong O‒Ca2+‒O bridging interactions of unprotonated P4/P5 phosphates. In contrast, monovalent cations (Na+ and K+) form smaller and less-stable clusters by preferentially adding PIP2 monomers. Despite having the same net charge, the affinity to P4/P5 is higher for Na+, while affinity toward glycerol P1 is higher for K+. Consequently, a mixture of K+ and Ca2+ (as would be produced by Ca2+ influx) synergistically yields larger and more stable clusters than Ca2+ alone due to the different binding preferences of these cations.

Specific cation effects on surface reactions of HgCl2 in clay-water systems

The significant complexity of the chemical forms and reactions of HgCl2 in clay-water systems make elucidating the corresponding interaction mechanisms and activities challenging. The specific ion effect and its mechanisms for HgCl2 adsorption at charged clay surfaces were investigated in this study. Experimental results showed that (1) at a fixed background concentration, the adsorption capacity and adsorption energy of HgCl2 on montmorillonite surface decreased in the order Li+> > Na+ > K+, and HgCl2 adsorption exhibited a strong specific cation effect. (2) For the same background cation (Li+, Na+ or K+), the adsorption capacity and energy of HgCl2 decreased as the cation concentration increased. An analysis of the origin of the adsorption energy for HgCl2 showed that this energy linearly increased with the surface electric field strength of montmorillonite. On the basis of orbital asymmetric hybridization theory, we speculated that specific adsorption of the HgCl2 molecule occurs via “polarization-induced covalent bonding” adsorption between O (clay surface) and Hg (HgCl2) atoms. The adsorption energy linearly increased with the surface electric field strength in the clay-water system. These findings provide useful information on modifying the physical, chemical and biological activities of HgCl2 in clay-water systems.

Comparing interfacial cation hydration at catalytic active sites and spectator sites on gold electrodes: understanding structure sensitive CO2 reduction kinetics.

Hydrated cations present in the electrochemical double layer (EDL) are known to play a crucial role in electrocatalytic CO2 reduction (CO2R), and numerous studies have attempted to explain how the cation effect contributes to the complex CO2R mechanism. CO2R is a structure sensitive reaction, indicating that a small fraction of total surface sites may account for the majority of catalytic turnover. Despite intense interest in specific cation effects, probing site-specific, cation-dependent solvation structures remains a significant challenge. In this work, CO adsorbed on Au is used as a vibrational Stark reporter to indirectly probe solvation structure using vibrational sum frequency generation (VSFG) spectroscopy. Two modes corresponding to atop adsorption of CO are observed with unique frequency shifts and potential-dependent intensity profiles, corresponding to direct adsorption of CO to inactive surface sites, and in situ generated CO produced at catalytic active sites. Analysis of the cation-dependent Stark tuning slopes for each of these species provides estimates of the hydrated cation radius upon adsorption to active and inactive sites on the Au electrode. While cations are found to retain their bulk hydration shell upon adsorption at inactive sites, catalytic active sites are characterized by a single layer of water between the Au surface and the electrolyte cation. We propose that the drastic increase in catalytic performance at active sites stems from this unique solvation structure at the Au/electrolyte interface. Building on this evidence of a site-specific EDL structure will be critical to understand the connection between cation-dependent interfacial solvation and CO2R performance.

The Influence of Spectator Cations on Solvent Reorganization Energy Is a Short-Range Effect.

In this manuscript, we use classical molecular dynamics simulation to explore the origin of specific cation effects on the rates of bulk-phase aqueous electron transfer (ET) reactions. We consider 0.6 M solutions of Cl- and a series of different cations: Li+, Na+, K+, Rb+, and Cs+. We evaluate the collective electrostatic fluctuations that drive Marcus-like ET and find that they are essentially unaffected by changes in the cationic species. This finding implies that the structure making/breaking properties of various cations do not exert a significant influence on bulk-phase ET reactions. We evaluate the role of ion pairing in these specific cation effects and find, unsurprisingly, that model redox anions that are more highly charged tend to pair more effectively with spectator cations than their monovalent counterparts. We demonstrate that this ion pairing significantly affects local electrostatic fluctuations for the anionic redox species and thus conclude that ion pairing is one of the likely sources of rate-dependent cation effects in aqueous ET reactions.

Bulk Phase Behavior vs Interface Adsorption: Specific Multivalent Cation and Anion Effects on BSA Interactions.

Proteins are ubiquitous and play a critical role in many areas from living organisms to protein microchips. In humans, serum albumin has a prominent role in the foreign body response since it is the first protein which will interact with, e.g., an implant or stent. In this study, we focused on the influence of salts (i.e., different cations (Y3+, La3+) and anions (Cl-, I-) on bovine serum albumin (BSA) in terms of its bulk behavior as well as the role of charges for protein adsorption at the solid-liquid interface in order to understand and control the underlying molecular mechanisms and interactions. This is part of our group's effort to gain a deeper understanding of protein-protein and protein-surface interactions in the presence of multivalent ions. In the bulk, we established two new phase diagrams and found not only multivalent cation-triggered phase transitions, but also a dependence of the protein behavior on the type of anion. The attractive interactions between proteins were observed to increase from Cl- < NO3- < I-, resulting in iodide preventing re-entrant condensation and promoting liquid-liquid phase separation in bulk. Using ellipsometry and a quartz-crystal microbalance with dissipation (QCM-D), we obtained insight into the growth of the protein adsorption layer. Importantly, we found that phase transitions at the substrate can be triggered by certain interface properties, whether they exist in the bulk solution or not. Through the use of a hydrophilic, negatively charged surface (native silica), the direct binding of anions to the interface was prevented. Interestingly, this led to re-entrant adsorption even in the absence of re-entrant condensation in bulk. However, the overall amount of adsorbed protein was enhanced through stronger attractive protein-protein interactions in the presence of iodide salts. These findings illustrate how carefully chosen surface properties and salts can directly steer the binding of anions and cations, which guide protein behavior, thus paving the way for specific/triggered protein-protein, protein-salt, and protein-surface interactions.

Mechanisms of Al3+ Dimerization in Alkaline Solutions.

The molecular speciation of aluminum (Al3+) in alkaline solutions is fundamental to its precipitation chemistry within a number of industrial applications that include ore refinement and industrial processing of Al wastes. Under these conditions, Al3+ is predominantly Al(OH)4-, while at high [Al3+] dimeric species are also known to form. To date, the mechanism of dimer formation remains unclear and is likely influenced by complex ion···ion interactions. In the present work, we investigate a suite of potential dimerization pathways and the role of ion pairing on energetics using static DFT calculations and DFT and density functional tight binding molecular dynamics. Specific cation effects imparted by the background electrolyte cations Na+, Li+, and K+ have been examined. Our simulations predict that, when the Al species are ion-paired with either cation, the formation of the oxo-bridged Al2O(OH)62- is favored with respect to the dihydroxo-bridged Al2(OH)82-, in agreement with previous spectroscopic work. The formation of both dimers first proceeds by bridging of two monomeric units via one hydroxo ligand, leading to a labile Al2(OH)82- isomer. The effect of contact ion pairing of Li+ and K+ on the dimerization energetics is distinctly more favorable than that of Na+, which may have an effect on further oligomerization.

Cation Specific Effects on the Domain-Domain Interaction of Heterogeneous Dimeric Protein Revealed by FRET Analysis.

Specific monovalent cation effects on the domain-domain interaction of heterogeneous dimeric protein were investigated using green fluorescent protein (GFP)-glutathione-s-transferase (GST) fusion protein as a model protein. Conjugating N-terminal of GST domain with a fluorescence probe Cyanine3, complementary increase and decrease of fluorescence intensities of Cyanine3 and GFP were recognized on the exclusive excitation of GFP and further the fluorescence decay of GFP was remarkably accelerated to show that an excellent Förster type of resonance excitation energy transfer (FRET) pair was constructed between GFP- and GST-domain. The spectral overlap integral and critical distance of the FRET pair were estimated to be 5.96×1013 M-1cm3 and 62.5 Å, respectively. The FRET rate and efficiency evaluated by fluorescence lifetime of the energy donor, GFP, were influenced by the monovalent cations included in the buffer solution to suggest that the domain-domain interactions of GFP-GST fusion protein would be susceptible to cation species and their concentrations. The order affecting the domain-domain interaction was estimated to be Li+>NH4+ >Na+>K+>Cs+, almost corresponding to the reverse Hofmeister series.

Specific monovalent cation effect on protein-protein interactions revealed by protein rotational diffusion analysis

The protein-protein interactions induced by cation specific effects were characterized by the rotational diffusion analysis. The rotational diffusion coefficient, Drot, estimated by time-resolved fluorescence anisotropy of fluorescent-labeled lysozyme was reduced responding to monovalent cation chlorides and was able to be approximated by quadratic functions of lysozyme concentration. The resultant first and second terms of lysozyme concentration were observed to be negative and positive according to the species and concentrations of monovalent cations, respectively. These two hydrodynamic interaction parameters demonstrated that the attractive and also repulsive interactions were induced between lysozymes in the order of inverse Hofmeister effect.

Specific ion effects of divalent cations on the aggregation of positively charged goethite nanoparticles in aqueous suspension

Nanoparticles, especially natural nanoparticles, are closely related to many environmental processes. In this study, the aggregation kinetics of goethite nanoparticles (GNPs) in the presence of Mg(NO3)2, Ca(NO3)2 and Cu(NO3)2 were investigated by dynamic light scattering. The specific cation effects of Cu2+, Ca2+ and Mg2+ on the aggregation of the positively charged GNPs were observed. The results showed that: (1) the mean aggregation rate followed the order of Cu2+ < Ca2+ < Mg2+, e.g. under an electrolyte concentration of 50 mM, the mean aggregation rate in the presence of Cu2+, Ca2+ and Mg2+ were 0, 38.3 and 49.5 nm min−1, respectively; (2) the critical coagulation concentration (CCC) for the presence of Cu2+, Ca2+ and Mg2+ of GNPs were respectively 117, 65.3 and 37.9 mM, which was consistent with the orders of aggregation rate; and (3) correspondingly, the activation energy was the highest for the presence of Cu2+, followed by Ca2+ and then by Mg2+, e.g. under an electrolyte concentration of 5 mM, activation energy for the presence of Cu2+, Ca2+ and Mg2+ approached infinity, 1.1 RT and 0.88 RT, respectively. We conclude that, the difference in surface reactions of the three cations with the surface functional group of −OH resulted in the difference in particle aggregation: Cu2+ produced the strongest covalent bonding, and followed by Ca2+ and Mg2+; the covalent bonding in forming −OCu+ could be explained by the classic theory, but the covalent bonding in forming −OCa+ or –OMg+ might be a polarization induced one.

Effect of the combined use of γ-cyclodextrin and a chiral ionic liquid on the enantiomeric separation of homocysteine by capillary electrophoresis

The enantioseparation of the non-protein amino acid homocysteine by CE was investigated in this article using eleven neutral CDs and five chiral ionic liquids as chiral selectors. Using a previous derivatization step with FMOC and the subsequent separation under neutral conditions, homocysteine enantiomers were only separated when γ-CD or (R)-N,N,N-trimethyl-2-aminobutanol-bis(trifluoromethane-sulfon)imidate (EtCholNTf2) were employed as sole chiral selectors in the separation buffer. On the one hand, γ-CD gave rise to the enantiomeric separation in 10min with a resolution value of 1.9, whereas EtCholNTf2 let to obtain a resolution value of 1.4 in more than 50min. Then, the evaluation of the combined use of both selectors was also carried out, resulting in a considerable increase in the Rs. The best enantioseparation for homocysteine was obtained when 10mM EtCholNTf2 was added to 50mM phosphate buffer (pH 7.0) containing 2mM γ-CD. In an attempt to discriminate specific chiral cation effect from the salt effect, the influence of adding LiNTf2 to the separation medium was also evaluated, resulting in lower resolution values for homocysteine when compared to those achieved with the addition of EtCholNTf2, indicating a synergistic effect between EtCholNTf2 and γ-CD. Interestingly, the enantiomer migration order changed depending on the use of a single chiral selector or dual systems. When EtCholNTf2 or γ-CD were employed as sole chiral selectors, D-enantiomer was the first-migrating enantiomer. However, an inversion in the migration order was observed when both selectors were combined in a dual system being the L-enantiomer the first-migrating one.

Specific Cation Effects on SCN- in Bulk Solution and at the Air-Water Interface.

The large and sparsely hydrated thiocyanate anion, SCN-, plays a prominent role in the study of specific ion effects in biological, colloid, and atmospheric chemistry due to its extreme position in the Hofmeister series. Using atomistic modeling of aqueous SCN- solutions, we provide novel insight at the molecular scale into the experimentally observed differences in ion pairing, clustering, reorientation dynamics, mutual diffusion, and solubility between the sodium, Na+, and the potassium, K+, salt. Compared to KSCN, NaSCN has a less pronounced tendency to ion pairing; nevertheless, at high salt concentrations, we observe a strong attraction between Na+ cations and the nitrogen end of SCN-, resulting in larger and more closely packed ion clusters. To accurately model aqueous SCN- solutions in computer simulations, we develop a thermodynamically consistent force field rooted in quantum-chemical calculations and refined using the Kirkwood-Buff theory. The force field is compatible with the extended simple point charge and three-point optimal point charge classical water models and reproduces experimental activity derivatives and air-water surface tension for a wide range of salt concentrations.

Thermodynamic Properties of l-Aspartates of Alkali and Alkali-Earth Metals in Aqueous Solutions at 298.15 and 310.15 K and Specific Cation Effects on Biomolecule Solvation

Vapor pressure osmometry was applied to the systems calcium l-aspartate ((S)-aminobutanedioic acid calcium salt) + water for varying molalities of Ca–l-Asp (mCa–l-Asp = 0.01–1.02 mol·kg−1) and guanidinium hydrochloride (methanamidine hydrochloride) + sodium L–aspartate ((S)–aminobutanedioic acid sodium salt) + water, varying the molalities of GndmCl and Na–l-Asp (mNa–l-Asp = 0.1, 0.25, 0.4, 0.57 mol·kg−1 and mGndmCl = 0.1–1.1 mol·kg−1) at T = 298.15 K and 310.15 K. From vapor pressure osmometry, activities of water, and the corresponding osmotic coefficients of the mixtures Ca–l-Asp + water and Na–l-Asp + GndmCl + water have been calculated, both being directly related to the chemical potentials of the different species and therefore to their Gibbs energy. Mean molal ion activity coefficients were obtained from experimental data fits with the Pitzer equations and the corresponding dual and triple interaction parameters were derived for the Ca–l-Asp + water binary system. β(2) Pitzer parameters different from zero are required for Ca–l-Asp in water to reproduce the osmotic coefficient decrease with increasing concentration. Mean Spherical Approximation parameters accounting for Coulomb and short range interactions that describe the calcium and magnesium aspartates and glutamates are given. The decrease in the chemical potential of the aspartates corresponds to the Hofmeister series: NaAsp > Mg(Asp)2 > CaAsp. A strong interaction between amino acid and salt due to specific dispersion interactions in amino acid salt systems containing guanidinium based salt has been revealed that is in agreement with MD and half-empirical quantum-chemical calculations.

Molecular dynamics study of structure and vibrational spectra at zwitterionoic lipid/aqueous KCl, NaCl, and CaCl2 solution interfaces.

Molecular dynamics (MD) simulations of KCl, NaCl, and CaCl2 solution/dipalmytoylphosphatidylcholine lipid interfaces were performed to analyze heterodyne-detected vibrational sum frequency generation (HD-VSFG) spectra in relation to the interfacial water structure. The present MD simulation well reproduces the experimental spectra and elucidates a specific cation effect on the interfacial structure. The K+, Na+, and Ca2+ cation species penetrate in the lipid layer more than the anions in this order, due to the electrostatic interaction with negative polar groups of lipid, and the electric double layer between the cations and anions cancels the intrinsic orientation of water at the water/lipid interface. These mechanisms explain the HD-VSFG spectrum of the water/lipid interface and its spectral perturbation by adding the ions. The lipid monolayer reverses the order of surface preference of the cations at the solution/lipid interface from that at the solution/air interface.

Specific cation effects at aqueous solution−vapor interfaces: Surfactant-like behavior of Li+ revealed by experiments and simulations

It is now well established by numerous experimental and computational studies that the adsorption propensities of inorganic anions conform to the Hofmeister series. The adsorption propensities of inorganic cations, such as the alkali metal cations, have received relatively little attention. Here we use a combination of liquid-jet X-ray photoelectron experiments and molecular dynamics simulations to investigate the behavior of K+ and Li+ ions near the interfaces of their aqueous solutions with halide ions. Both the experiments and the simulations show that Li+ adsorbs to the aqueous solution-vapor interface, while K+ does not. Thus, we provide experimental validation of the "surfactant-like" behavior of Li+ predicted by previous simulation studies. Furthermore, we use our simulations to trace the difference in the adsorption of K+ and Li+ ions to a difference in the resilience of their hydration shells.

A Colloidal Description of Intermolecular Interactions Driving Fibril-Fibril Aggregation of a Model Amphiphilic Peptide.

We apply a kinetic analysis platform to study the intermolecular interactions underlying the colloidal stability of dispersions of charged amyloid fibrils consisting of a model amphiphilic peptide (RADA 16-I). In contrast to the aggregation mechanisms observed in the large majority of proteins and peptides, where several elementary reactions involving both monomers and fibrils are present simultaneously, the system selected in this work allows the specific investigation of the fibril-fibril aggregation process. We examine the intermolecular interactions driving the aggregation reaction at pH 2.0 by changing the buffer composition in terms of salt concentration, type of ion as well as type and concentration of organic solvent. The aggregation kinetics are followed by dynamic light scattering, and the experimental data are simulated by Smoluchowski population balance equations, which allow to estimate the energy barrier between two colliding fibrils in terms of the Fuchs stability ratio (W). When normalized on a dimensionless time weighted on the Fuchs stability ratio, the aggregation profiles under a broad range of conditions collapse on a single master curve, indicating that the buffer composition modifies the aggregation kinetics without affecting the aggregation mechanism. Our results show that the aggregation process does not occur under diffusion-limited conditions. Rather, the reaction rate is limited by the presence of an activation energy barrier that is largely dominated by electrostatic repulsive interactions. Such interactions could be reduced by increasing the concentration of salt, which induces charge screening, or the concentration of organic solvent, which affects the dielectric constant. It is remarkable that the dependence of the activation energy on the ionic strength can be described quantitatively in terms of charge screening effects in the frame of the DLVO theory, although specific anion and cation effects are also observed. While anion effects are mainly related to the binding to the positive groups of the fibril surface and to the resulting decrease of the surface charge, cation effects are more complex and involve additional solvation forces.