Specific Salt Effects Research Articles

Comparison of the impact of anion and cation selection onto cation exchange chromatography of model proteins

For a further improvement of ion exchange chromatography for protein purification, the optimization of mobile phase composition is an important factor. The effects of salt type selection on cation exchange chromatography (CEC) have been investigated in this work. Hereby, the significance of co-ion and displacing ion selection were compared. Therefore, the suitability of a constant cationic or anionic strength for multivalent salts for the determination of salt impact were evaluated. The retention and separation behavior of the three model proteins bovine serum albumin, lysozyme and α-chymotrypsin were investigated with varying salts in the elution buffer. For this, five different cations and five different anions were used. Hereby, the effects of anion and cation selection were analyzed independently. It was shown that a constant cationic strength is a suitable setup for the determination of salt effects on protein retention in CEC, as the resulting retention factors did not show a clear trend regarding ion valence. It was observed that for the used process conditions the anion selection could change the separation factor by up to 30% and the cation selection by up to 16%. These changes were attributed to protein specific salt effects of citrate on bovine serum albumin and magnesium on α-chymotrypsin. It was demonstrated that at constant cationic strength the co-ion selection can be of the same or even of higher importance for protein separation as the selection of the displacing ion in CEC.

Enhanced interfacial salt effect on extraction and separation of Er(III) from Mg(II), Al(III), Fe(III) sulfate aqueous solutions using bubble-supported organic liquid membrane

Understanding the essence of specific salt effect on mass transfer of metal ions acrossing liquid–liquid interface is of crucial importance to intensify extraction and separation of target metal ions. In present work, the effect from various typical coexisting electrolyte salt cations, Mg(II), Al(III), Fe(III) cations, in the in-situ leaching solutions of ion-adsorption type rare earth ores on extraction and separation of heavy rare earths (Er(III) ions as a representative) by our previously suggested new method, bubble-supported organic liquid membrane (BSOLM) extraction, was investigated. It is found that extraction and separation of low concentration Er(III) ions can be significantly enhanced, even when the concentrations of Mg(II), Al(III) and Fe(III) cations are low enough. It cannot be explained by the traditional salt effect theory. Experiments revealed that BSOLM extraction results in an enhanced interfacial salt effect due to its essence of mass transfer only occurring on the surface of thin-layer organic extractant liquid membrane. It is benefit for intensifying the interfacial competitive adsorption of coexisting salt cations. The competitive hydration of those salt cations at interface promotes the extraction of low concentration Er(III) ions, whereas their competitive adsorption is unfavorable of the separation of Er(III). The present work highlights the microscopic nature of specific interfacial effect of various coexisting electrolyte salt cations on extraction of target rare-earth ions. It is benefit for development of new methods to intensify extraction and selective separation of low concentration rare-earth ions from the electrolyte salt in-situ leaching solutions of ion-adsorption type rare earth ores.

Validation of specific cation partitioning to molecular surfaces using fluorescent carbon quantum dots

Specific salt effects are significantly involved in the aqueous colloidal solutions of proteins, nucleic acids, surfactants and nanomaterials. The mechanism of such effects has been systematically investigated here with a simple nanomaterial-based model system. Water-soluble fluorescent carbon quantum dots were synthesized for this purpose. This research work highlights the unique advantage offered by fluorescent carbon quantum dots to be utilized as a sensing probe for the detection of selective ion interactions based on their fluorescence turn-on response. Synthesized carbon quantum dots exhibited surface potential dependent strong fluorescence emission. It was further found that the functional group chemistry on the carbon quantum dot surface plays a crucial role in the ion partitioning. A remarkable enhancement of emission intensity observed in the presence of metal cations can be attributed to their relative tendency to form ion-pairs with the nanomaterial surface functionalities. Carbon quantum dots with deprotonated carboxylic groups on their surface showed a stronger affinity for strongly hydrated cations as compared to weakly hydrated cations. Uncharged yet polar nanomaterial surface, however, showed no selectivity towards different ions.

Specific Salt Effect on the Interaction between Rare Earth Ions and Trioctylphosphine Oxide Molecules at the Organic–Aqueous Two-Phase Interface: Experiments and Molecular Dynamics Simulations

Understanding the essence of the specific salt effect on the enhancement of the transport of metal ions across the liquid/liquid interface during the process of solvent extraction is of crucial importance for the development of a new approach to extract and selectively separate various valuable metals from complex aqueous solutions. However, some abnormal experimental phenomena involved in the salt effect on the liquid/liquid solvent extraction could not be understood only from the conventional interpretation based on the salting-out ability of salt ions. The knowledge into the microscopic mechanism behind the specific salt effect was urgent. Herein, as an example, the effect of adding various salts on the extraction performance of rare earth ion Pr3+ using trioctylphosphine oxide (TOPO) as the organic extractant was investigated. It was revealed that the difference in the interface propensity of different salt anions enriched at the organic-aqueous two-phase interface played a crucial role in promoting the interaction of TOPO molecules with Pr3+ions, despite the occurrence of the salting-out effect of those salt anions with the increase of their concentrations in aqueous solutions. The interfacial interaction mechanism obtained by molecular dynamics simulations revealed that both the interface propensity and the salting-out ability of the coexisting salt anions contributed to the enhancement in the extraction of Pr3+ into the TOPO organic phase. However, when the concentrations of coexisting salts in aqueous solutions were low enough, the extraction of Pr3+ was mainly dominated by the interface propensity of those added salt anions but not their salting-out ability. With the increase in the concentration of salts, the salting-out effect gradually became significant and, therefore, began to join with the interface propensity of salt anions to co-dominate the transport of Pr3+ ions across the liquid/liquid interface. The present study highlights the microscopic nature of the salt effect on promoting the extraction of rare earth ions and suggests that the interaction of organic extractant molecules with rare earth ions at liquid/liquid interface was dependent not only upon the salting-out ability of the coexisting salt ions but also their interface propensity.

Halide-Dependent Dissolution of Dicalcium Phosphate Dihydrate and Its Modulation by an Organic Ligand

In situ atomic force microscopy (AFM) combined with X-ray photoelectron spectroscopy (XPS) and ζ potential were used to investigate how citrate (50 μM) modified the nanoscale dissolution of brushite (dicalcium phosphate dihydrate, CaHPO4·2H2O) by NaX (X = Cl, Br, or I) at a constant pH of 7.0. Results showed that on the brushite (010) surface, halide ions with large sizes (such as I–) enhanced adsorption of Na+ that cannot be desorbed by water flow, inhibiting the retreat rates of [1̅00]Cc steps. The introduction of 50 μM citrate resulted in desorption of Na+ ions and caused the disappearance of specific salt effects on the dissolution of the brushite (010) face. With further increasing salt concentrations up to 100 mM, pit deepening was initiated, and it could possibly be attributed to hydrated Na+ ions at the negatively charged surface that orient the interfacial water so as to increase the entropy of the interfacial system, and hence the dissolution rate. Following the addition of 50 μM citrate to 100 mM salt solutions, deep pits immediately disappeared, suggesting that citrate covers the brushite (010) surface to form a protective film to decrease the number of active sites of dissolution reactions on the brushite surface. The findings improve fundamental understanding of biomineral interfacial dissolution, with clinical implications of bone demineralization and resorption as well as prevention of osteoporosis.

Viscometry of polyelectrolyte solutions: Star-like versus linear poly[[2-(methacryloyloxy)ethyl] trimethylammonium iodide] and specific salt effects

The intrinsic viscosities, [η], of the 3-arm star polyelectrolyte in pure water are for a given molar mass considerably lower than for the linear product because of the higher monomer concentration and charge density in isolated coils. These effects are much more pronounced than in the case of uncharged macromolecules. Extra salt (NaCl, NaI, CaCl2) reduces the solution viscosities of the 3-arm star polymer less than of the linear product. The transition of [η] from the value in pure water to the minimum saturation value at high salt concentrations follows a Boltzmann sigmoid. In saline solvents the changes of the viscosities with rising polymer concentration depend strongly on the chemical nature of the salt and on the molecular architecture of the solute. The present findings demonstrate the necessity to account for thermodynamic interactions between all components of the mixture, in addition to the usual electrostatic considerations. These considerations should turn out helpful for a better understanding of salt induced topological transitions of charged biopolymers.

Tuning Polymer-Protein Interaction with Salt

Biological nanopores are known to interact with synthetic and biological polymers, enabling their use in label-free single-molecule analytical tasks such as sequencing and/or mass discrimination. The latter, called nanopore-based single molecule mass spectrometry (Np-SMMS) has, to date, only been shown for one synthetic polymer, poly(ethyleneglycol) (PEG). It is based on the fact that binding of PEG inside the pore gives rise to blocks of ionic current, with the degree of block being detectably different for PEG molecules differing in size by only one monomer1,2.In order to extend the range of application of Np-SMMS, and to obtain information on the mechanism underying mass sensitivity of polymer-induced pore block, we have begun to test the interaction of other neutral polymers with biological nanoores. Here, we report on poly(dimethylacrylamide) (PDMA), a water-soluble neutral polymer. Under conditions used for Np-SMMS of PEG with alpha-hemolysine (4M KCl, +40 mV), we obtained current blocks with polydisperse PDMA (Mn 1500 g/mol by MALDI) that were low in frequency (<0.1 Hz/ µM) short (tau <100 µs) and noisy, resulting in little mass resolution compared to PEG. We reasoned that we might take advantage of a specific salt effect of fluoride anion reported for the aHL pore3 in order to increase blocking frequency and dwell time. Using electrolyte solutions consiting of 4M K+ as cation and various proportions of Cl- and F- as anions (1:1, 2:1, 3:1, 4:1) we were able to obtain longer events (tau up to 400 µs) and higher frequencies (up to 0.15 Hz/µM), allowing significantly better mass resolution for PDMA than in 4 M KCl. However, the large noise component in the blocked current levels for PDMA as opposed to PEG remained, still compromising the peak-to-valley ratio of histograms. These findings suggest that the specific salt effect of F- on polymer-protein interaction is independent of the polymer and may be useful in tuning polymer pore interaction for a range of analytes.(1) Robertson et al. Single-Molecule Mass Spectrometry in Solution Using a Solitary Nanopore. Proc. Natl. Acad. Sci. U S A 2007, 104, 8207-8211.(2) Baaken et al. High-Resolution Size-Discrimination of Single Nonionic Synthetic Polymers with a Highly Charged Biological Nanopore. ACS Nano 2015, 9, 6443-6449.(3) Rodrigues et al. Hofmeister Effect in Confined Spaces: Halogen Ions and Single Molecule Detection. Biophys. J. 2011, 100, 2929-2935.

Specific salt effects on the hydrolysis reaction rate of tropolone tosylate in binary MeCN–H2O media containing n-Bu4NOH

The hydrolysis reaction rates of tropolone tosylate have been investigated in 20–90% (v/v) MeCN–H2O binary media containing n-Bu4NOH at 50°C. Increasing concentration of n-Bu4NOH causes an increase in the hydrolysis rate. Two diverse effects have been observed with increasing content of MeCN, i.e., a decrease in the rate and the successive increase for up to the 60% (v/v) MeCN and further increasing MeCN proportions. At a constant n-Bu4NOH concentration, the added salts (0.0–1.0moldm−3) have caused various influences on the “pseudo” first order rate constant [log (k/s−1)] of the substrate. Among alkali metal and alkaline earth metal perchlorates, NaClO4 and Ba(ClO4)2 cause the obvious rate acceleration especially in the media of higher MeCN proportions. However, LiClO4 and Ca(ClO4)2 cause the rate deceleration, whereas Mg(ClO4)2 has resulted in a total suppression of the hydrolysis reaction. All the salts of NaN3, Et4NCl, and Et4NBr appear to increase the log (k/s−1) value in the order of Et4NCl–Et4NBr<NaN3. The Arrhenius plots of the hydrolysis in the 50% (v/v) MeCN–H2O binary media containing 2.0mmoldm−3n-Bu4NOH in the presence of various kinds of salts have been evaluated. All the salt systems have given a good linearity from 35 to 60°C with the activation energy (Ea) of 83.7, 79.3, 81.9, 79.1, 87.1, and 79.1kJmol−1 for no salt, 0.50moldm−3 LiClO4, NaClO4, Ba(ClO4)2, Et4NBr, and NaN3, respectively. These large values suggest that the hydrolysis reactions in the presence of various salts are controlled by the normal temperature dependent and non-catalytic mechanism with the ions from the added salt. The emerged scenarios have been discussed in terms of changes in the structure of bulk water and/or activities of H2O and OH− in the presence of both the added organic solvent and salts, and also in terms of the nucleophilicities of anions or the coordination abilities of the metal ions in the “modified” media.

Vapor-Pressure Osmometry and Conductivity Determination of Salting-Out Effects in Aqueous Surface-Active Ionic Liquid 1-Dodecyl-3-methylimidazolium Bromide Solutions

Systematic studies on the vapor-liquid equilibria (VLE) and conductometric properties of aqueous solutions of model surface-active ionic liquid 1-dodecyl-3-methylimidazolium bromide ([C12mim]Br) are performed in the absence and presence of a large series of electrolytes in order to achieve a deeper understanding about the molecular mechanism behind the specific salt effect on the aggregation behavior of [C12mim]Br in aqueous solution. For this purpose, 6 chloride electrolytes (NaCl, KCl, NH4Cl, (CH3)4NCl, MgCl2 and FeCl3) and 5 sodium electrolytes (NaCl, NaNO3, Na2CO3, Na2SO4, and Na3Cit.) were used in order to individualize the effect of the anion and the cation. The values of the critical aggregation concentration (CAC) were obtained and it was found that all the investigated electrolytes have salting-out effect on the aggregation of [C12mim]Br in aqueous solutions, leading to significant downward shifts of the CAC. The magnitude of the shifts depends on the water-structuring nature of the electrolyte and follows the Hofmeister series. Furthermore the effect of electrolyte on the degree of anionic binding and thermodynamic parameters of aggregation for [C12mim]Br in aqueous solutions were determined.

Salt-effects in aqueous surface-active ionic liquid 1-dodecyl-3-methylimidazolium bromide solutions: Volumetric and compressibility property changes and critical aggregation concentration shifts

Systematic studies on the volumetric and compressibility properties of aqueous solutions of model surface-active ionic liquid 1-dodecyl-3-methylimidazolium bromide ([C12mim]Br) are performed in the absence and presence of a large series of electrolytes in order to achieve a deeper understanding about the molecular mechanism behind the specific salt effect on the aggregation behavior of [C12mim]Br in aqueous solution. For this purpose, 6 chloride electrolytes (NaCl, KCl, NH4Cl, (CH3)4NCl, MgCl2 and FeCl3) and 5 sodium electrolytes (NaCl, NaNO3, Na2CO3, Na2SO4, and Na3Cit.) were used in order to individualize the effect of the anion and the cation. The values of the critical aggregation concentration (CAC) were obtained and it was found that all the investigated electrolytes have salting-out effect on the aggregation of [C12mim]Br in aqueous solutions, leading to significant downward shifts of the CAC. The magnitude of the shifts depends on the water-structuring nature of the electrolyte and follows the Hofmeister series. Changes in the apparent molar volumes and isentropic compressibilities upon micellization were derived using a pseudophase-transition approach and the infinite dilution apparent molar properties of the monomer and micellar form of [C12mim]Br were determined.

Intrinsic viscosities of polyelectrolytes: specific salt effects and viscometric master curves

Dilute solutions of the sodium salt of polystyrene sulfonic acid (PSS-Na) were measured viscometrically as a function of composition in aqueous solvents of different salinity, where the extra salt was either NaCl or CaCl2. Such experiments yield {η}, the generalized intrinsic viscosities (hydrodynamic specific volume) of the polyelectrolyte for arbitrary polymer concentrations, c. In the limit of infinite dilution {η} becomes identical to the intrinsic viscosity [η]. For NaCl {η} decreases monotonously with rising c, whereas maxima are passed in the case of CaCl2. Condensing c and the concentration of extra salt in the mixed solvent into a single variable enables the establishment of predictive master curves. The viscometrically observed changes in the spatial extension of the individual polymer coils are discussed in light of the corresponding thermodynamic information.

Specific salt effects on thermophoresis of charged colloids

We study the Soret effect of charged polystyrene particles as a function of temperature and electrolyte composition. As a main result we find that the Soret coefficient is determined by charge effects, and that non-ionic contributions are small. In view of the well-known electric-double layer interactions, our thermal field-flow fractionation data lead us to the conclusion that the Soret effect originates to a large extent from diffusiophoresis in the salt gradient and from the electrolyte Seebeck effect, both of which show strong specific-ion effects. Moreover, we find that thermophoresis of polystyrene beads is fundamentally different from proteins and aqueous polymer solutions, which show a strong non-ionic contribution.

Helix Formation by Alanine-Based Peptides in Pure Water and Electrolyte Solutions: Insights from Molecular Dynamics Simulations

Specific ion effects on oligopeptide conformations in solution are attracting strong research attention, because of their impact on the protein-folding problem and on several important biological-biotechnological applications. In this work, we have addressed specific effects of electrolytes on the tendency of oligopeptides toward formation and propagation of helical segments. We have used replica-exchange molecular dynamics (REMD) simulations to study the conformations of two short hydrophobic peptides [Ace-(AAQAA)3-Nme (AQ), and Ace-A8-Nme (A8)] in pure water and in aqueous solutions of sodium chloride (NaCl) and sodium iodide (NaI) with concentrations of 1 and 3 M. The average helicities of the AQ peptide have been analyzed to yield Lifson-Roig (LR) parameters for helix nucleation and helix propagation. The salt dependence of these parameters suggests that electrolytes tend to stabilize the helical conformations of short peptides by enhancing the helix nucleation parameter. The helical conformations of longer oligopeptides are destabilized in the presence of salts, however, because the helix propagation parameters are reduced by electrolytes. On top of this general trend, we observe a significant specific salt effect in these simulations. The hydrophobic iodide ion in NaI solutions has a high affinity for the peptide backbone, which reflects itself in an enhanced helix nucleation and a reduced helix propagation parameter with respect to pure water or NaCl solutions. The present work thus explains the computational evidence that electrolytes tend to stabilize the compact conformations of short peptides and destabilize them for longer peptides, and it also sheds additional light on the specific salt effects on compact peptide conformations.

Specific Salt and pH Effects on Foam Film of a pH Sensitive Surfactant

Steady state foams made of a pH sensitive surfactant, nonaoxyethylene oleylether carboxylic acid, with ion complexing properties was studied using small angle neutron scattering (SANS). The effect of pH variation and salt addition on the foam film thickness was investigated and discussed in terms of the influent parameters stabilizing the foam such as surface properties and electrostatic effects determined by tensiometry and zeta potential measurements. The decrease in the film thickness by adding mono (Na(+)) and divalent (Ca(2+)) salts is classically explained by screening of the double layer in foam films (transverse interactions). On the contrary, addition of acid or complexing ion (Nd(3+)) results in an increase in the film thickness and can be analyzed in terms of cohesive forces between surfactants at the liquid/gas interface (lateral interactions). pH and specific salt effects revealed that foams produced by nonaoxyethylene oleylether carboxylic acid are of interest in the potential use of this surfactant in ion separation process.

Interpreting Ion-Specific Effects on the Reduction of an Arenediazonium Ion by t-Butylhydroquinone (TBHQ) Using the Pseudophase Kinetic Model in Emulsions Prepared with a Zwitterionic Sulfobetaine Surfactant

Specific salt effects on the reduction of an amphiphilic arenediazonium ion, 16-ArN2(+), by TBHQ in opaque, stirred, and kinetically stable emulsions prepared with a zwitterionic sulfobetaine surfactant are consistent with the chameleon effect: selective anion binding/induced cation binding in the interfacial region of the emulsions. Added NaX salts with different anions decrease the observed first-order rate constant, k(obs), for the reduction in the order X(-) = ClO4(-) > Br(-) ≈ CCl3CO2(-) > Cl(-) > MeSO3(-). Added MCln salts of increasing cation valence at constant total Cl(-) concentration increase kobs in the order M(n+) = Cs(+) < Ca(2+) < Al(3+) in the same emulsions. These results, combined with recent results for nonionic and ionic emulsions, demonstrate that pseudophase kinetic models provide general, coherent explanations of chemical reactivity in homogeneous micelles, microemulsions, vesicles, and now biphasic emulsions and with all types of basic surfactant structures: nonionic, cationic, anionic, and now zwitterionic.

Partitioning water potential and specific salt effects on seed germination of Kochia scoparia

Partitioning water potential and specific salt effects on seed germination of Kochia scoparia

Partitioning Water Potential and Specific Salt Effects on Seed Germination of Kochia scoparia

Salinity is one of the environmental factors that have a critical influence on the germination of halophyte seeds and plant establishment. Salinity affects imbibitions, germination and root elongation. However, the way in which NaCl exerts its influence on these vital processes (seed germination), whether it is through an osmotic effect or specific ion toxicity, is still not resolved. Seeds of the halophyte Kochia scoparia were treated with various iso-osmotic solutions of NaCl and polyethylene glycol 6000 (PEG) over the water potential range of 0 to -1.9 MPa. After 10d of treatment, the ungerminated seeds were transferred to distilled water for 3 d. The germination results revealed that both NaCl and PEG inhibited germination and seedling growth, but the effects of NaCl compared to PEG were less on final germination. The seeds of kochia differ in their response to salinity and failure of seeds to recover from high salinity when transferred to deionized water, revealed the toxicity of NaCl. In contrast, the increase in germination and during the recovery period after exposure of PEG suggested that PEG was not toxic. It was concluded that at low levels of salinity the main effect of NaCl was to reduce the rate of germination due to the reduce water potential and at a higher salinity NaCl had a toxic effect since many seeds failed to germinate even salinity stress was removed.

Specific Salt Effects on Poly(ethylene oxide) Electrolyte Solutions

Exploring the mechanism of specific salt effects in electrolyte solutions is an old and attractive subject. It has been gradually realized that the competition at the molecular level plays an important role. Aiming to include molecular details as many as possible, we combine molecular dynamics (MD) simulations with a molecular theory to study specific salt effects on poly(ethylene oxide) (PEO) solutions with the addition of monovalent salt. Radial distribution functions obtained from MD simulations provide microscopic structures of different components as well as interactions between various species. On the basis of these interactions, we construct the molecular theory with four assumptions: (1) an ion along with bound water in the first shell works as a single entity; (2) short-ranged interactions among various species are modeled as hydrogen-bonding interactions; (3) the ability of a hydrated ion to provide donors/acceptors for hydrogen bonding is governed by the charge density; (4) contact ion pairs are included, especially in the cases of small cations. The molecular theory is generalized with the explicit inclusion of ion−PEO, ion−water, ion−ion, water−water, and water−PEO hydrogen bonds. This means the molecular-scale structure and interaction are included within the frame of the theory. Theoretically calculated cloud points verify that the salting-out ability for alkali metal ions follows the series of K+ > Rb+ > Cs+ > Na+ > Li+, which is in agreement with the experimental observations. Here, the competition among ion−PEO, ion−water, and water−PEO interactions and the impact of steric repulsions induced by the introduction of ions are two essential factors determining the phase behavior of PEO solutions. The combined methods bridge the microscopic interactions and structures to the macroscopic behavior.

Liquid Interface Functionalized by an Ion Extractant: The Case of Winsor III Microemulsions

The present work shows for the first time that tributylphosphate (TBP), the major ion extractant used in the reprocessing of spent nuclear fuel, acts efficiently as a cosurfactant in the formation of three-phase microemulsions. The system is composed of water, dodecane, TBP, and an extremely hydrophilic sugar surfactant, n-octyl-β-glucoside. The investigation of the three-phase region (Winsor III), the so-called "fish-cut" diagrams, revealed that TBP exhibits cosurfactant behavior comparable to that of classical cosurfactants n-pentanol and n-hexanol. Upon increasing the cosurfactant/surfactant molar ratio, TBP appears to be more efficient than single-chain alcohols in raising the spontaneous curvature of the adsorbed surfactant film toward oil. This is a direct consequence of the different lateral packing of TBP and n-pentanol or n-hexanol in the mixed surfactant film, with TBP having three alkyl chains and so a higher hydrophobic volume than those n-alcohols. This property is underlined by the interfacial film composition, which is determined by the chemical analysis of the excess phases. It gives a surfactant to cosurfactant molar ratio of 1:1 for TBP and 1:3 for n-hexanol. Moreover, the local microstructure of the microemulsion becomes dependent on the addition of salt when n-alcohol is replaced by TBP. A specific salt effect is also observed and rationalized in terms of the complexing property of TBP and Hofmeister's effects. Treatment of the small-angle neutron scattering (SANS) data gives access to (i) the length scales characterizing the microemulsions (i.e., the persistence length, ξ, and aqueous or organic domain sizes, D*) and (ii) the specific surface, Σ. It results that a subtle change is highlighted in the TBP microemulsion structure, in terms of connectivity, according to the type of salt added.

Salt Effect on the Aggregation Behavior of 1-Decyl-3-methylimidazolium Bromide in Aqueous Solutions

Understanding of the specific salt effect on the aggregation behavior of ionic liquids (ILs) is relevant to multiple applications. In this work, the influence of a series of 15 salts on the aggregation behavior of [C(10)mim]Br in aqueous solutions has been investigated by conductivity, fluorescence, and dynamic light scattering. It was shown that NaCl, NaBr, NaI, CH(3)CO(2)Na, NaSCN, NaNO(3), NaBrO(3), NaClO(3), C(6)H(5)COONa, Na(2)CO(3), Na(2)SO(4), Na(2)C(4)H(4)O(6), and Na(3)CH(5)O(7) have salting-out effect, whereas FeBr(3) and AlBr(3) have salting-in effect on the aggregation of [C(10)mim]Br in aqueous solutions. The effect of anions of the added sodium salts on the critical aggregation concentration (CAC), degree of anionic binding (beta), and aggregation number (N(agg)) of the IL basically follows the Hofmeister series, and the CAC values decrease but the beta and N(agg) values increase with increasing concentration of the salts. Hydrophobicity of the anions is suggested to play important roles in the salt effect on the aggregation of [C(10)mim]Br in aqueous solutions. Furthermore, the IL aggregates were found to grow slowly as the increase of the salt concentrations under studied static conditions, and resulting in the increased aggregation number of the IL. These results are expected to be useful in the applications of ionic liquids.