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Activity coefficients in aqueous solutions of alkali metal nitrates at 298 K have been calculated by the generalized Debye–Huckel theory using experimental values of the static dielectric constant of solutions. It has been shown that calculation without optimization of model parameters in a number of systems reproduces the dependence of activity coefficients on cation radius and the levelling of potassium, rubidium, and cesium cations influence on activity coefficients in the presence of nitrate ions. The effect of hydration and ion association on the thermodynamic properties in the series of studied systems has been considered.

During lithium-ion battery charging and discharging, carbonate electrolytes degrade from redox side reactions to produce electrode surface films. The composition of these films depends on the compo...

A study is performed of the electrical conductivity of a number of ionic liquids based on 1-alkyl-3-methylimidazolium quaternary salts with inorganic anions in acetonitrile. The Lee–Wheaton approach is used to calculate the ion association constants Ka, limiting molar electrical conductivity (λ0), and the Gibbs energy of association (ΔG) in solutions. It is shown that the nature and size of the anion has a decisive influence on the association of the studied ionic liquids. It is found that the limiting molar electrical conductivity of bromides is lower than that of hexafluorophosphates and tetrafluoroborates.

Sodium citrate (Na3Cit) has a crucial role in many biological and industrial processes. Yet, quantitative information on its hydration and the ion association between Na+ and Cit3– ions in a broad ...

The review generalizes and analyzes published data on different methods of studying structural characteristics of the solvation environment of alkali metal ions (except for Li+) in various electron-donor oxygen-containing solvents. Coordination numbers of alkali metal ions, interspecies distances, and types of ion association are discussed. The solvation structures of alkali metal cations in water and some nonaqueous systems are compared. The number of solvent molecules and the distance between the cation and oxygen atom of solvent molecule in the first coordination sphere are independent of physicochemical properties of the solvent.

The NaCl–H2O binary is a first approximation to fluids in the deep crust and upper mantle. Such fluids are fundamental to understanding geophysical properties, metal transport, and ore genesis within the Earth. Consequently, recent experimental studies have sought to determine the equation of state of the NaCl–H2O binary as a function of composition. However, experimental characterization is not straightforward at extreme conditions and there is so far little understanding of the molecular nature of associated fluids. Variations in the structure of water, ion solvation, and ion association affect properties such as ionic strength and electrical conductivity. These properties in turn influence geophysical signatures and metal solubilities at high pressure and temperature (P,T). We performed a series of NPT ab initio molecular dynamics simulations as a function of pressure (0.5–4.5 GPa), temperature (20–800 °C), and composition (1–14 m NaCl) to develop a molecular-level understanding of high P,T NaCl brines. Comparison of simulation results with experimental and theoretical densities provides a foundation for testing current levels of theory. The simulations have allowed us to determine the nature of ion solvation, ion association, and the effect of solutes on the solvent at high P,T. The PBE functional used here is known to over-structure water and overestimate the density of aqueous fluids at ambient conditions. This is due to an inadequate treatment of hydrogen bonding and dispersion. However, we find excellent agreement between theory and experimental densities at elevated P,T. Accordingly, we have extrapolated the existing equation of state to regions that have not been measured experimentally. From the O–O pair distribution functions, we interpret the improved agreement as resulting from the breakdown of hydrogen bonding at high P,T. At ambient conditions, the presence of Na and Cl has a strong effect on the structure of water. However, at high P,T, even 14 m NaCl has no effect on water structure. Nevertheless, Na and Cl are highly associated. We propose that NaCl brines at high P,T are best described as hydrous melts rather than as aqueous electrolytes.

Solid polymer electrolytes are key components in many electrochemical devices. For an in-depth study of the basic parameters of such electrolytes, we developed a previously proposed method in order to determine the charge carrier density (n), mobility (μ), and diffusion coefficient (D) of ionic conductors using electrochemical impedance analysis. This reinforced method was tested with a composite solid electrolyte based on polyethylene oxide, ethylene carbonate, LiCF3SO3, and alumina filler by analyzing DC conductivity, frequency dependence of AC conductivity, and the complex dielectric function. The results show a clear picture of the temperature dependence of the parameters n, μ, and D; for example, at 20 °C, more than 15% of the total number of ions available in the electrolyte are mobile, and this value decreases with increasing temperature, most probably due to increased ion association in agreement with measurements using other techniques. The increase in ionic conductivity with increasing temperature is thus due to an increased mobility of the ionic species.

The use of dielectric relaxation spectroscopy (DRS) for studying electrolyte solutions is reviewed, focussing on the authors’ investigations over the last three decades. It is shown that this often-overlooked technique provides powerful insights into the nature of ion-ion and ion-solvent interactions. DRS is revealed to be particularly useful for detection of weak ion association and, due to its unique ability to detect solvent-separated species, the quantitation of ion pairing. It is demonstrated that DRS correctly determines chemical speciation for ion-paired systems where major spectroscopic techniques (NMR, Raman, UV-vis) fail. DRS also provides important insights into ion solvation. In aqueous solutions, it has been used to build up a coherent set of ‘effective’ hydration numbers for ions based on the dynamics of proximate water molecules, and has a unique ability to detect ‘slow’ water resulting from hydrophilic and hydrophobic hydration of solutes. DRS has been especially useful for characterising the behaviour of ionic liquids (ILs), e.g. showing they possess rather low dielectric constants and, surprisingly, contain no significant concentrations of ion pairs. Neat ILs and their mixtures with molecular solvents are shown by ultra-broadband DRS to exhibit extremely complicated behaviour especially at frequencies in the THz region.

ChemPhysChem 10, (2021). doi:10.1002/cphc.202100602, Published by Wiley-VCH Verl., Weinheim

Superconcentrated electrolytes, being highly thermodynamically nonideal, provide a stringent proving ground for continuum transport theories. Herein, we test an ostensibly complete model of LiPF6 in ethyl-methyl carbonate (EMC) based on the Onsager-Stefan-Maxwell theory from irreversible thermodynamics. We perform synchronous magnetic resonance imaging (MRI) and chronopotentiometry to examine how superconcentrated LiPF6:EMC responds to galvanostatic polarization and open-circuit relaxation. We simulate this experiment using an independently parametrized model with six composition-dependent electrolyte properties, quantified up to saturation. Spectroscopy reveals increasing ion association and solvent coordination with salt concentration. The potentiometric MRI data agree closely with the predicted ion distributions and overpotentials, providing a completely independent validation of the theory. Superconcentrated electrolytes exhibit strong cation-anion interactions and extreme solute-volume effects that mimic elevated lithium transference. Our simulations allow surface overpotentials to be extracted from cell-voltage data to track lithium interfaces. Potentiometric MRI is a powerful tool to illuminate electrolytic transport phenomena.

Ionic liquid (IL) materials are promising electrolytes with striking physicochemical properties for energy and environmental applications. Heterogeneous structures and transport quantities of monomeric and polymeric ILs are intrinsically intercorrelated and span multiple spatiotemporal scales, which is more feasible for coarse-grained (CG) simulations than atomistic modelling. Herein we constructed a novel CG model for ethyl-imidazolium tetrafluoroborate ILs with varied cation alkyl chains ranging from C2 to C20, and the interaction parameters were validated against representative static and dynamic properties that were obtained from atomistic reference simulations and experimental characterizations at relevant thermodynamic states. This CG model was extended to study thermotropic phase behaviors of monomeric ILs and to explore ion association structures and ion transport quantities in polymeric ILs with different architectures. A systematic analysis of structural and dynamical quantities identifies an evolution of liquid morphology from homogeneous to nanosegregated structures and then a smectic mesomorphism via a gradual lengthening of cation alkyl chains, and thereafter a distinct structural transition characterized by a monotonic decrease in orientational and translational order parameters in a sequential heating cascade. Backbone and pendant polymeric ILs exhibit evident anion association structures with cation monomers and polymer chains, and striking intra- and interchain coordinations between cation monomers owing to an intrinsic polymer architecture effect. Such a peculiar ion pairing association leads to a progressive increase in anion intrachain hopping probabilities, and a concomitant decrease in anion interchain hopping events with a gradual lengthening of polymeric ILs. The anion diffusivities in polymeric ILs are intrinsically correlated with ion pairing association lifetimes and ion structural relaxation times via a universal power law correlation D ∼ τ−1, irrespective of polymer architectures.

In this study, the ion association characteristics and the hydration dynamics of Mg2+ ions in MgSO4 aqueous solution at various concentrations were systematically investigated using density functional theory and classical molecular dynamics simulations. The results show that contact associated (CA) and solvent separated (SS) structures of Mg2+ and SO42− ions are the dominant species and play different roles in the crystallization and gelation processes of MgSO4 aqueous solution due to their distinctive association trend and the distinctive dynamics of their second hydration shells. The CA structures are more stable and the further aggregation of these clusters is sluggish in MgSO4 aqueous solution. The supersaturation or the formation of gel state can be partially attributed to the abundance of CA structures in supersaturated MgSO4 aqueous solution. In contrast, although the ion exchange in SS structures occurs frequently, the aggregation of SS structures is active and can be greatly promoted by increasing of the concentration. The dynamics of the second hydration shells around Mg2+ ions of SS structures also becomes more active as the concentration increases. The dynamic evolution characteristics of SS structures demonstrate their aggregation tendency for nucleation in the oversaturated MgSO4 aqueous solution, which helps to understand the nucleation mechanism in MgSO4 aqueous solution and the role of ion hydration dynamics in the nucleation process.

Structure of alkaline aqueous NaBH4 solutions at NaBH4 concentrations of 1.0 to 9.1 mol·dm−3 (almost saturated) as well as a 1.3 mol·dm−3 NaOH solution was studied by X-ray diffraction and Empirical Potential Structure Refinement (EPSR) modelling. In the 1.0 mol dm−3 alkaline NaBH4 solution, Na+ is surrounded by around six water molecules with octahedral geometry. With an increase in NaBH4 concentration, the hydration number of Na+ decreases from 4.8 ± 1.1 at 2.25 mol·dm−3 solution to 2.9 ± 1.3 at 9.1 mol·dm−3 with an almost constant Na+-O (W: H2O) distance of 2.34 A. The decrease in hydration number for Na+ is compensated by the formation of ion pairs between Na+ and BH4− to ensure an octahedral hydrated geometry for Na+. About 6.0 ± 1.6 water molecules are likely to bond to BH4− via tetrahedral edges or tetrahedral corners without a very specific hydration geometry, giving rise to each hydrogen atom of BH4− bound to 2.3 ± 1.0 water molecules through dihydrogen bonds. The hydration number around BH4− decreases with increasing concentration. BH4− tends to form contact ion pairs with Na+ at the Na-B distances of 3.24 A and 2.82 A in tetrahedral-corner-shared and tetrahedral-edge-shared fashions, respectively; the number of Na-B interactions increases from 0.3 ± 0.5 in 1.0 mol·dm−3 (almost negligible) to 2.1 ± 0.9 in 9.0 mol·dm−3. Tetrahedral-edge-shared bidentate ion pair is likely to be the preferred and dominant ion cluster over the wide concentration range. Ion association between the Na+ and BH4− was crosschecked by density function theory (DFT) calculations on [NaBH4(H2O)6] clusters.

The choline ion (Ch+) is ubiquitous in nature and also its synthetic homologue, chlorocholine (ClCh+), is widely used. Nevertheless, surprisingly little information on the hydration and counter-ion binding of these cations can be found in the literature. In this contribution we report effective hydration numbers, determined by dielectric relaxation spectroscopy, and ion-pair association constants with Cl-, determined by dilute-solution conductivity measurements. In combination with RISM calculations the obtained data suggest that for Ch+ water is bound to the hydroxy group via hydrogen bonds whereas for ClCh+ a rather stiff clathrate-like shell around the chlorine atom seems to be formed. With Cl- both cations form contact ion pairs with association constants of only ∼2 to 3 M-1.

The hydrolysis of Ba2+and Ra2+was studied at 25 °C in aqueous mixtures of NaOH and NaClO4using an ion exchange method and radiotracer and batch techniques. The distribution of133Ba and226Ra between solid (ion exchange resin) and aqueous (mixture of NaOH and NaClO4) phases was measured via gamma spectrometry and liquid scintillation counting. The total ionic strength was kept constant and the concentration of NaOH in the aqueous phase was varied from 0 (pure NaClO4) to pure NaOH from sample to sample. It was shown that an increase of the Ba2+or Ra2+concentration in the aqueous phase with an increase of the NaOH concentration cannot be explained solely by Ba2+or Ra2+activity coefficient differences in the NaOH and NaClO4media (salting out) and that weak BaOH+and RaOH+ion pairing occurs in the systems studied. A model for weak ion association was developed and apparent BaOH+and RaOH+stability constants were derived assuming the formation of weak aqueous NaOH(aq) ion pairs via non-linear curve fitting. It was demonstrated that systematic uncertainties have a much greater contribution to the NaOH(aq), BaOH+and RaOH+stability constant uncertainty budget compared to stochastic uncertainties and a method for estimation of the systematic uncertainties was proposed. The method combines fitting, restricted primitive model computations with surveyed literature data that resulted in a stability constant for NaOH(aq) that ranged from 0 to 1 at ionic strengths below 5 mol·kg−1(i.e. KNaOH= 0.5 ± 0.5, where the uncertainty is a systematic 95% confidence interval). The variation of KNaOHallowed the estimation of the systematic 95% confidence interval in the apparent stability constants of BaOH+and RaOH+. The specific ion interaction theory was used to extrapolate the derived logarithms of the BaOH+and RaOH+apparent stability constants to zero ionic strength (log10K = 0.7 ± 0.2 for both ion pairs) and obtain the relevant ion interaction parameters. It was shown that both the Ba2+and Ra2+ions have similar activity coefficients and undergo similar short-range interactions in aqueous NaOH-NaClO4media.

In this study, molecular dynamics simulations were used to investigate the role of hydration shells on the stabilization of ion clusters. The stabilization of [CaClx]2−x (x = 4–6) species can be enhanced by their hydration shells. The shrinkage of the third hydration shell can be observed for the [CaClx]2−x complex while not for the [CuCl]+ or [CaCl2]0 species in an aqueous solution. The shrinking third shell together with the second shell of Ca2+ form a compact inner hydration shell around those clusters promoting their existence in an aqueous solution. Their inner hydration shells, forming a distinctive hydrogen bond network around the ion cluster, act as a “cage” structure, confining multi-Cl− ions around Ca2+, and promote the occurrence of [CaClx]2−x species in the aqueous solution. Our results demonstrate that the structural characteristics of some ion clusters combined with their distinctive hydration characteristics have a huge impact on ion association in an aqueous phase.

In this paper, we investigate the consequences of ion association, coupled with the considerations of finite size effects and orientational ordering of Bjerrum pairs as well as ions and water molecules, on electric double layer near charged surfaces. Based on the lattice statistical mechanics accounting for finite sizes and dipole moments of ions, Bjerrum pairs and solvent molecules, we consider the formation of Bjerrum pairs and derive the mathematical expressions for Bjerrum pair number density as well as cation/anion number density and water molecule number density. We reveal the several significant phenomena. Firstly, it is shown that our approach naturally yields the equilibrium constant for dissociation-association equilibrium between Bjerrum pairs and ions. Secondly, at low surface charge densities, an increase in the bulk concentration of Bjerrum pairs enhances the permittivity and decreases the differential capacitance. Next, for cases where Bjerrum pairs in an alcohol electrolyte solution have a high value of dipole moment, Bjerrum pair number density increases with decreasing distance from the charged surface, and differential capacitance and permittivity is high compared to ones for the cases with lower values of Bjerrum-pair dipole moments. Finally, we show that the difference in concentration and dipole moment of Bjerrum pairs can lead to some variation in osmotic pressure between two similarly charged surfaces., Comment: 28pages, 13 figures

Mechanistic pathways relevant to mineralization are not well‐understood fundamentally, let alone in the context of their biological and geological environments. Through quantitative analysis of ion association at near‐neutral pH, we identify the involvement of HCO3 − ions in CaCO3 nucleation. Incorporation of HCO3 − ions into the structure of amorphous intermediates is corroborated by solid‐state nuclear magnetic resonance spectroscopy, complemented by quantum mechanical calculations and molecular dynamics simulations. We identify the roles of HCO3 − ions as being through (i) competition for ion association during the formation of ion pairs and ion clusters prior to nucleation and (ii) incorporation as a significant structural component of amorphous mineral particles. The roles of HCO3 − ions as active soluble species and structural constituents in CaCO3 formation are of fundamental importance and provide a basis for a better understanding of physiological and geological mineralization., An important role of bicarbonate at near‐neutral pH during CaCO3 formation was found. Its structural incorporation in amorphous intermediates, revealed by a combination of potentiometric titrations, NMR, computer simulations, and other techniques, can be quantitatively explained by a model based on the binding of bicarbonate ions to pre‐nucleation clusters. The findings seem to be crucial for a better understanding of, e.g., CaCO3 biomineralization.

Polymerized ionic liquids (polyILs) combine the unique properties of ionic liquids (ILs) with macromolecular polymers. But anion diffusivities in polyILs can be three orders of magnitude lower than that in ILs. Endeavors to improve ion transport in polyILs urgently need in-depth insights of ion transport in polyILs. As such in the work we compared ion transport in poly (1-butyl-3-vinylimidazolium-tetrafluoroborate) (poly ([BVIM]-[BF4])) polyIL and 1-butyl-3-methylimidazolium tetrafluoroborate ([BMIM]-[BF4]) IL. The diffusivities of ions in the polyIL and IL were measured and computed. According to the results of the molecular dynamics simulations performed, in the IL the coupling motion between an anion and the ions around determines the ion diffusivities, and the ion association lifetime gives the time scale of ion transport. But in the polyIL, the hopping of an anion among cages composed of cationic branch chains determines the diffusivity, and the associated anion transport time scale is the trap time, which is the time when an anion is caught inside a cage, not the ion association lifetime, as Mogurampelly et al. regarded. The calculation results of average displacements (ADs) of the polyIL chains show that, besides free volume fraction, average amplitudes of the oscillation of chains and chain translation speed lead to various diffusivities at various temperatures.

In concentrated electrolytes with asymmetric or irregular ions, such as ionic liquids and solvent-in-salt electrolytes, ion association is more complicated than simple ion-pairing. Large branched aggregates can form at significant concentrations at even moderate salt concentrations. When the extent of ion association reaches a certain threshold, a percolating ionic gel network can form spontaneously. Gelation is a phenomenon that is well known in polymer physics, but it is practically unstudied in concentrated electrolytes. However, despite this fact, the ion-pairing description is often applied to these systems for the sake of simplicity. In this work, drawing strongly from established theories in polymer physics, we develop a simple thermodynamic model of reversible ionic aggregation and gelation in concentrated electrolytes accounting for the competition between ion solvation and ion association. Our model describes, with the use of several phenomenological parameters, the populations of ionic clusters of different sizes as a function of salt concentration; it captures the onset of ionic gelation and also the post-gel partitioning of ions into the gel. We discuss the applicability of our model, as well as the implications of its predictions on thermodynamic, transport, and rheological properties.

Detailed speciation of electrolytes as a function of chemical system and concentration provides the foundation for understanding bulk transport as well as possible decomposition mechanisms. In particular, multivalent electrolytes have shown a strong coupling between anodic stability and solvation structure. Furthermore, solvents that are found to exhibit reasonable stability against alkaline-earth metals generally exhibit low permittivity, which typically increases the complexity of the electrolyte species. To improve our understanding of ionic population and associated transport in these important classes of electrolytes, the speciation of Mg(TFSI)2 in monoglyme and diglyme systems is studied via a multiscale thermodynamic model using first-principles calculations for ion association and molecular dynamics simulations for dielectric properties. The results are then compared to Raman and dielectric relaxation spectroscopies, which independently confirm the modeling insights. We find that the significant presence of free ions in the low-permittivity glymes in the concentration range from 0.02 to 0.6 M is well-explained by the low-permittivity redissociation hypothesis. Here, salt speciation is largely dictated by long-range electrostatics, which includes permittivity increases due to polar contact ion pairs. The present results suggest that other low-permittivity multivalent electrolytes may also reach high conductivities as a result of redissociation.

The micro-structure of hydration shell of solute in water is significant for understanding the properties of aqueous solutions. Raman spectroscopy has been employed for studying the hydration shell structure of the solute for decades, however, Raman imaging data is still seriously overlapped, making it challenging to obtain information on the spectrum of hydrated water molecules. In this paper, Raman spectroscopy was employed to study the O-H vibration peaks of LiCl aqueous solution and LiCl-MgCl2-H2O mixed aqueous solution. The changes of stretching vibration peak of 2800∼3800 cm-1 O-H and hydrogen bond network structure in aqueous solution were analyzed at room temperature and ion association. With the increase of magnesium salt ratio, the damage of solute to the bulk water gradually decreases in the mixed solution, which indicated that LiCl has a more significant influence on the bulk water molecules. It is mainly due to the intense hydration of Li+, which can not only affect the water molecules in the first hydration shell but also affect the water molecules in the second hydration shell. The number of water molecules in the first hydration shell were obtained by extracting the spectra of different solute first hydration shells from the solution spectra. Those spectra of the hydration shell were employed to study the micro-structures of the first hydration shells of anions, and the aggregation behavior of ions in the the mixed solution.

The conductivity of polymerized ionic liquid graft copolymers (PILGCs) is usually three orders of magnitude lower than ionic liquids (ILs); researches are being performed actively to improve ion transport in PILGCs. In the research, the composite membrane of poly(1-butyl-3-vinylimidazolium-acetate) (poly([BVIM]-[OAc])) PILGC and NPs filled with 1-butyl-3-methylimidazolium acetate ([BMIM]-[OAc]) IL was developed. Ion transport in the composite is a combination of that in PILGC and IL and mechanisms underlying anion transport in the PILGC and the IL should both be explored in order to obtain profound knowledge of ion transport (mechanism) in the composite. Researchers still have discrepancy in the mechanisms of ion transport in PILGCs. We explored the mechanisms and time scales of ion transport in the PILGC and compared the differences in ion transport phenomena and mechanisms in the PILGC and the IL. 2D-IR experiments were performed and the results show that the peak-shape of the spectra of SCN− in the PILGC changes on a much slower time scale than that in the IL, indicating that the IL completes structure decay in a way significantly faster than the PILGC. Anion diffusivities in the composite, PILGC and IL at various temperatures were measured and calculated. Subdiffusion phenomena are caused by hindrance influence of surrounding units on anion motion. Therefore exploring subdiffusive regime gives mechanisms underlying ion transport: the subdiffusion phenomena of anion transport in the IL are caused by the coupling between anion motion and the motion of ions around, while that in the PILGC are caused by the trap of anions in cages composed of the PILGC chains before they get the opportunities to hop to cages nearby. According to our analysis, van der Waals (VDW) interactions play a more significant role than electrostatic interactions, including ion association interactions, in influencing the magnitude of anion diffusivities in the PILGC. It is wrong to explore ion transport mechanism via only analyzing electrostatic interactions between ions; anion transport is along PILGC chains most probably in successive steps and between PILGC chains most probably in separate steps, depending on jumping between neighboring cages. The trap time of an anion inside a cage, instead of ion association time, gives the time scale that associates with anion transport. The anion transport mechanism in the composite is a combination of fast structural relaxation in the IL and slow hopping between cages in the PILGC.

Conductance data for Et4NBr, Et4NBF4, Bu4NBr, Bu4NBF4 in acetonitrile for the molar concentration range of 2·10-4–1·10-2 mol·dm-3 over the temperature range from 5 to 55 °C are reported. Limiting molar conductivities and ion association constants were determined by using the Lee-Wheaton equation for the symmetrical electrolytes. On the basis of the preliminary conductometric data analysis it was established that the closest approach parameter is almost independent from the temperature for all studied acetonitrile solutions. Therefore, the closest approach parameter was adopted as a sum of cation and anion radii for further conductometric data treatment. The limiting conductivities of Br-, BF4-, Et4N+ and Bu4N+ ions and the structure-dynamic parameter of ion-molecular interaction obtained from the experimental data on limiting molar conductivities were evaluated in the framework of the approach proposed by authors [Kalugin O. N., Vjunnik I. N. Limiting ion conductance and dynamic structure of the solvent in electrolyte solution. Zh. Khim. Fiz. (Rus.) 1991, 10 708-714]. Elongation of the alkyl radical of the tetraalkylammonium cation from Et4N+ to Bu4N+ leads to a significant increase in the structure-dynamic parameter, which indicates the dynamic structuring of the solvent near the tetrabutylammonium ion and increased solvophobic solvation of the Bu4N+ compared to Et4N+. Ion association constants are discussed in terms of competition between Coulomb and non-Coulomb forces in terms of short-range square-mound potential. An increase in the ion association constants in the sequence Bu4NBF4

The three-phase electrochemistry of 2,2-diphenyl-1-picrylhydrazyl (DPPH) has been studied by attaching a droplet of nitrobenzene (NB) containing DPPH to a graphite electrode in an aqueous electrolyte solution. Since DPPH can be reduced to DPPH− and oxidized to DPPH+, the accompanying ion transfer to NB and ion pair formation in NB are accessible. The anion transfer from water to nitrobenzene is accompanied by the formation of ion pairs [DPPH+An−] with nitrate, hexafluorophosphate, perchlorate and trichloroacetate. The ion pair formation of DPPH− with tetrabutylammonium cations is very weak. When the DPPH is dissolved in molten paraffin together with the salt tetrabutylammonium tetrafluoroborate (TBATFB), and composite electrodes are produced by mixing the paraffin with graphite powder, DPPH exhibits a typically surface electrochemical response providing a rather stable system DPPH/DPPH−.

The structural and thermodynamic properties of Na+ - Cl- ion-pair association in water-CO2 binary mixtures in supercritical conditions for infinitely dilute solutions are studied using constrained molecular dynamics simulations over a wide range of compositions. It is found that solvation structure varies dramatically with the solvent composition. Contact ion pairs (CIPs) are found to be more stable than all other configurations as seen from the potentials of mean force (PMFs). PMFs of the NaCl ion pair in pure CO2 look almost like the pair potential between the ion pair. Stabilities of CIPs increase with increase in the mole fraction of CO2. An increment in the average number of hydrogen bonds with an increase in the mole fraction of H2O in the bulk as well as in the solvation shell of the ions is observed. Ion-pair association in aqueous CO2 mixtures in supercritical conditions is found to be endothermic and driven by entropy. Preferential solvation analysis shows that both Na+ and Cl- ions are preferentially solvated by water and even a small percentage of water in the mixture prevents CO2 molecules from entering the first solvation shell of ions due to the strong hydrophilicity of the ions. (C) 2018 Elsevier B.V. All rights reserved.

金沢大学理工研究域物質化学系, Molecular association between biocompatible dendritic polymers, dendrigraft poly-l-lysines (DGLs), and an anionic fluorescent probe, 8-anilino-1-naphthalenesulfonate (ANS-), was studied at the polarized water|1,2-dichloroethane (DCE) interface. The fluorescence intensity of ANS measured in aqueous solution was enhanced by the coexistence of DGLs over a wide pH range (2 < pH < 10), where ANS and DGL exist as a monoanionic form and a polycation, respectively. The voltammetric responses indicated that the positively charged DGLs were adsorbed at the water|DCE interface, whereas ANS- was transferred across the interface accompanied by the adsorption process. The interfacial behavior of the DGL-ANS associates was analyzed by potential-modulated fluorescence (PMF) spectroscopy. The PMF results demonstrated that the ion association between DGLs and ANS at the water|DCE interface is strongly affected by the applied potential and the dendritic generation of DGL. By applying appropriate potentials, the ANS anion was dissociated from its ion associate with DGLs at the interface and transferred into the organic phase, whereas DGLs remained in the aqueous phase. The Gibbs free energy of ion association (ΔGD···ANS) was estimated for the second-fourth generation DGLs (DGL-G2-G4) and the G4 polyamidoamine (PAMAM) dendrimer as a control. The highest stability of the DGL-G4-ANS associate manifested itself through ΔGD···ANS: DGL-G4-ANS (>G4 PAMAM dendrimer-ANS) > DGL-G3-ANS > DGL-G2-ANS. The results elucidated the efficient anion-binding ability of higher generation DGLs and its potential dependence at the liquid|liquid interface. © 2018 American Chemical Society., Embargo Periods 12 Months

We studied ion concentration profiles and the charge density gradient caused by electrode reactions in weak electrolytes by using the Poisson--Nernst--Planck equations without assuming charge neutrality. In weak electrolytes, only a small fraction of molecules is ionized in bulk. Ion concentration profiles depend on not only ion transport but also the ionization of molecules. We considered the ionization of molecules and ion association in weak electrolytes and obtained analytical expressions for ion densities, electrostatic potential profiles, and ion currents. We found the case that the total ion density gradient was given by the Kuramoto length which characterized the distance over which an ion diffuses before association. The charge density gradient is characterized by the Debye length for 1:1 weak electrolytes. We discuss the role of these length scales for efficient water splitting reactions using photo-electrocatalytic electrodes., 5 figures

Association constants and single-ion conductivities of the bromide salts of five isomeric tetrabutylammonium ions were determined with dilute-solution conductivity measurements for aqueous solutions at 298.15 K. In addition, for two of the isomers dielectric relaxation experiments were performed. It was found that, despite similar size of all cations, the hydrodynamic radii of the branched isomers were significantly smaller than that of the un-branched Bu4N + species. It appears that the more compact branched cations exert only a minor perturbation on the H-bond network of the surrounding solvent and easily strip off their hydration shell. This effect, and the —on average— closer approach between anions and cations in ion pairs of Bu2 isoBu2NBr and isoBu4NBr probably explain the rather large association constants of the latter salts.

This work describes designing and using a simple automated preconcentration system for estimation of tungsten in soil spectrophotometrically. The preconcentration of tungsten is based upon the retention of ion associate of [RB+. H W O 4 - ] inside a minicolumn packed with polyurethane foam. The retained ion associate is then automatically and quantitatively eluted by acetone. The tungsten concentration was estimated by measuring the absorbance of elution at 584 nm. The absorbance of elution follows Beer's law in concentration range of 0.5–100 µgL−1 with a detection limit of 0.09 µgL−1. The enhancement factor and relative standard deviation of proposed methodology were 50 and 1.7%, respectively. The system developed in this research is characterized by its low cost and high efficiency, as it uses low-cost PUF as a stationary phase and a few number of pumps and valves. On the other hand, the designed system can be used, in the future, with many detection techniques easily. The composition of the ion association formed between W(VI) ions and RB was calculated by continuous variation method (Job plot) to be 1:1 (W6+:RB). On the other hand, the ion associate in its solid state was synthesized and characterized using elemental analysis and ATR–FTIR technique. The retention of tungsten (VI) ions in the form of [RB+. H W O 4 - ] ion association on the surface of PUFs was confirmed by SEM images and XRF spectra. Moreover, the kinetic and thermodynamic behaviors of the retention of [RB+. H W O 4 - ] on the polyurethane foams were studied in detail. The kinetic models revealed that the retention of [RB+. H W O 4 - ] on PUF surface obeys the pseudo-second-order reaction. The thermodynamic investigations confirmed that the adsorption of [RB+. H W O 4 - ] on PUF surface was spontaneous at 298 K and the interaction between [RB+. H W O 4 - ] and active sites available on PUF surface was physical and electrostatic in nature.

We study the solvation properties of standard lithium salts in various aprotic apolar solvents using conceptual density functional theory. The electronegativity and the chemical hardness of the species are introduced as reliable criteria for identifying suitable solvent-ion combinations. All salt solvation energies reveal positive values as a consequence of the ion association reaction which implies a tendecy for increased ion pair formation. Our findings reveal that frequently used solvents like propylene carbonate, ethylene carbonate, γ -butyrolactone and dimethoxyethane mainly favor the solvation of the lithium ion due to their nucleophilic and thus Lewis-basic properties. The solvation of standard anions like BF 4 − , PF 6 − and BOB − is of minor importance in terms of their contribution to the total solvation energy. Based on the values for the electronegativity and the chemical hardness, we show that the increased tendency towards ion pair formation can be avoided by using more acidic and softer anions in combination with more basic and softer solvents. A variation of the cation does not significantly lower the amount of ion pairs. Our results are in qualitative agreement with recent empirical assumptions and shed more light on the underlying interaction patterns.

A model that considers the contributions from hydration, ion association, and electrostatic interactions to the nonideality of 2‒1 electrolyte solutions is substantiated. The parameters of the model’s equations are the mean ion hydration number, the spread of the distribution of hydrated ion stoichiometric coefficients in the standard state, and the number of association. The model is successfully used to describe literature experimental data on the concentration dependence of osmotic coefficients of aqueous CaCl2 solutions at temperatures ranging from 0 to 100°C. The modeling of the above systems shows that as the temperature rises, the hydration number falls slightly, the distribution of the hydration number broadens, and the ion paring of the salt rises by the first degree.

The thermodynamics has been studied of Pb forms in soil solution for the case of calcareous ordinary chernozem of the Southern Russia, northern zone of the Krasnodar krai for conditions of phosphogypsum utilization into the soil layer of 20–40 cm at doses of 0, 10, 20, and 40 t ha− 1. The application of phosphogypsum increased the contents of total Pb by 4.2; the content of water-soluble forms – by 17.7. As a result of ion association in soil solution are presented electrically neutral ion pairs СаСО30; CaSO40, MgCO30, MgSO40, and charged ion pairs CaHCO3+, MgHCO3+, NaCO3−, NaSO4−, CaOH+, MgOH+. The contents of soluble Pb2 + forms in water extracts were calculated by mathematical chemical-thermodynamic model of ion pairs association in soil solution using algorithm and computer program ION-2. The coefficient of heavy metal ion association kas in soil solution is proposed for calculation of the heavy metal equilibrium concentration in soil solution. The model shows that calculated association coefficient of Pb2 + is up to 25.0, Pb2 + is presented mostly in the form of PbOH+ and Pb(OH)20. The quantity of PbCO30 + Pb(CO3)22 − and PbHCO3+ is lower than content of hydroxide-complexes by 10–15 times. At the phosphogypsum dose of 40 t ha− 1, the association coefficient of Pb2 + ion decreases by 2.0 times comparing to initial soil, and its activity increases by 60.0%. The molar fraction of hydroxo complexes decrease for 4.0, carbonate for 5.0, and sulfate associates – increase for 4.5%. The dangerous increase of activity of Pb2 + ion in soil solution after phosphogypsum apply up to 90.0% of its concentration is revealed.

The alternative models of ionic equilibria in acetonitrile solutions of alkaline earth percholorates in the presence of 3-hydroxyflavone (HL) were studied by conductometry at 288.15, 298.15, 308.15, 318.15, and 328.15 K. Using a procedure specially developed for this task it was shown that in the solutions containing Ca(ClO 4 ) 2 and Sr(ClO 4 ) 2 the complexation of the cation (M 2 + ) by the HL molecule leads to the formation of the doubly-charged [M(HL)] 2 + complex species. In contrast to them, in the presence of Ba(ClO 4 ) 2 HL interacts with the ion pair BaClO 4 + forming the singly-charged [BaClO 4 (HL)] + complex. The limiting equivalent conductivities as well as the constants of complexation were estimated. The latter were calculated by taking into account the ion association between cation and anion of initial salts and activity coefficients, that is the ‘true’ thermodynamic constants were found. The reliability of the proposed approach was additionally checked by analyzing artificially noised model experimental data. It was shown that the fitted parameters can be satisfactorily reproduced even at the noise level equal to 2%. The limiting equivalent conductivities of the [Ca(HL)] 2 + , [Sr(HL)] 2 + and [BaClO 4 (HL)] + complex species were interpreted in terms of the Stokes radii whose values indicate very weak solvation of the formed complexes. The variation of the constants of complexation among cations was found to be in agreement with the values of primary association constants.

It is demonstrated theoretically how the ionic association of the metal cation and/or protonated form of the neutral indicator (chromoionophore) with the anionic sites in the polymeric optode phase alters the dynamic range and the sensitivity of the optode response. Explicit response equations are derived for several limiting cases where the metal ion or the protonated indicator form ion pairs with the anionic sites. The optode response is also simulated numerically with no assumptions on the association degree of any type of the species present in the membrane phase. Simulations show that association in the optode phase may result in significant errors when optode is used as an analytical tool. The results are confirmed experimentally using pH/pM optodes based on chromoionophore ETH2439 as a model system. It is demonstrated that by selection of a proper ionic additive one can optimize the optode response range for the desired concentration range.

In this paper, five novel cholinium-based organic magnetic ionic liquids (MILs) were synthesized. The carbon chain links to the nitrogen atom from cationic nucleus. The MILs were comprehensively identified by FT-IR, HPLC, UV–Vis, 1H NMR and ESI-MS. The MILs have interesting performance to form aqueous two-phase system and can respond to the external magnetic field after the formation of aqueous two-phase system, which can assists magnetically phase separation. The magnetic susceptibility of the MILs was measured at 298 K. Other physical properties such as conductivities, acidity and density were measured in range of 278.15 K to 323.15 K for further application investigation. Satisfactory results were obtained after the data of physical properties were correlated. Based on the measurements and correlations above, some discussions were carried out, including the ion association and ion diffusion, the influences of MILs carbon chain and temperature on the molar conductivity, the influences of the cation size on the molar volume, acidity variation of the MILs.

The density functional theory B3LYP/6-31+G(d,p) method in the framework of the discrete-continuum model has been applied to study the tetramer structure of a lithium thiocyanate complex in triethylamine (Et3N) with the use of experimental and calculated manifestations of ion association in the vibrational spectrum of the anion. Analysis of topological characteristics of the electron density distribution at critical point (3,–1) of the [Li+NCS−]4 and [Li+NCS−]4Et3N complexes has been performed.

Structurally unique ionic liquids phosphonium-phosphate and phosphonium-thiophosphate, having both phosphorus based counter ions, in which the anionic part is represented by di-aryl phosphate or di-aryl thiophosphate and cations been tetraalkylphosphonium groups, behave differently in terms of their NMR behaviour. While phosphonium-phosphates show significant changes in its 1H, 13C and 31P NMR chemical shifts vis. a vis. corresponding chemical shifts for a physical mixture of tetraalkylphosphonium bromide and di-aryl phosphate, phosphonium-thiophosphates behave almost similarly in terms of NMR with their synthetic precursors, hence indicating phosphate-phosphonium interaction has a significant covalent component resembling more to a liquid ion pair while thiophosphate-phosphonium interaction is principally ionic in nature. Translational diffusion behavior studied by PFGSE-NMR experiments and ionic conductivities of these ionic liquids in chloroform solution corroborated the hypothesis. The effect of variable alkyl chain length in phosphonium cation is effectively observed in the extent of ion association. Results of this study may provide insight into the solution state behavior of these ionic liquids, would help to classify those in terms of their strength of ion association and thus potential application thereof.

Manifestations of ion association of lithium cation with BrO3 - ion in the vibrational spectrum have been studied by IR spectroscopy. The microstructure of the bromate ion with lithium cation in a contact ion pair in an aprotic solvent has been determined by the density functional theory method in the B3LYP/cc-pVTZ approximation, and the vibrational spectrum of coordinated bromate ion has been calculated.

Ion association, leading to the formation of clusters in ionic liquids, is investigated within the framework of classical density functional theory. Clusters are incorporated into a generic coarse-grained model for imidazolium-based ionic liquids confined by planar charged surfaces. We find that the short-ranged structure adjacent to surfaces is remarkably unaffected by the degree of ion association. The physical implications of ion clustering only become apparent in equilibrium properties that depend upon the long-range screening of charge, such as the asymptotic behavior of forces between charged surfaces and the differential capacitance around low surface potentials. Surface forces show a long-range exponential decay, which depends primarily on the concentration of nonassociated ions, while the differential capacitance seems to be a sensitive function of the internal structure of clusters. Furthermore, the size of the ion clusters only slightly influences surface forces, but has a significant effect on...

The phenomenon of spinodal decomposition in weakly charged polyelectrolyte solutions is studied theoretically within the random phase approximation. A novel feature of the theoretical approach is that it accounts for the effects of ionic association, i.e. ion pair and multiplet formation between counterions and ions in polymer chains, as well as the dependence of local dielectric permittivity on the polymer volume fraction Φ. The main focus is on the spinodal instability of polyelectrolyte solutions towards microscopic phase separation. It has been shown that increasing the binding energy of ions decreases the classical microphase separation region (possible at low polymer concentrations) due to the effective neutralization of the chains. A qualitatively new type of microphase separation is found in the presence of a dielectric mismatch between polymer and solvent. This new branch of microphase separation is realized at high polymer concentrations where ion association processes are the most pronounced. Typical microstructures are shown to have a period of a few nanometers like in ionomers. The driving force for the microphase formation of a new type is more favourable ion association in polymer-rich domains where ionomer-type behavior takes place. Effective attraction due to ion association promotes microscopic as well as macroscopic phase separation, even under good solvent conditions for uncharged monomer units of polymer chains. Polyelectrolyte-type behavior at low Φ and ionomer-type behavior at high Φ result in the presence of two critical points on the phase diagrams of polyelectrolyte solutions as well as two separate regions of possible microscopic structuring. Our predictions on the new type of microphase separation are supported by experimental data on polymer solutions, membranes and gels.

We discuss here carboxylate graphene nanopore adsorbed with bivalent cations for Mg2+/Li+ separation based on molecular dynamic simulations. The results show that these graphene nanopores exhibit higher permeating preference to Li+ than to Mg2+, with the Mg2+/Li+ ratio down to 0.2–0.5, even when the pristine hole diameter is larger than 2 nm. The Li+ selectivity can be attributed to the block of ion hydration layers and ion association. The distribution analyses of ion in the nanopore indicate that Li+ can be closer to Ca2+ of nanopores, and the looser hydration layers of Li+ make it be less blocked by the hydration layers of Ca2+, resulting in the larger available area in the nanopore for Li+ to transport. The ion association of Mg2+ or Li+ with Cl− can be enhanced near nanopores, the stronger ion association of Mg2+ with Cl− leads to a stronger permeating resistance for Mg2+. This work displays the possibility of high lithium ion selectivity in large graphene nanopores, which can lower the cost of graphene perforation, and may provide inspiration for further efficient extraction of Li+ from salt-lake brines.

In this paper, Hg(2+) ions are demonstrated to form anionic [HgI4](2-) complexes after interacting with massive amount of I(-) ions. Subsequently, the addition of tetradecyl pyridyl bromide (TPB) can make [HgI4](2-) anionic complexes react with univalent tetradecyl pyridyl cationic ions (TP(+)), forming dispersed ion-association complexes (TP)2(HgI4). Due to the extrusion action of water and Van der Waals force, the hydrophobic ion-association complexes aggregate together, forming dispersed nanoparticles with an average size of about 8.5nm. Meanwhile, resonance Rayleigh scattering (RRS) intensity is apparently enhanced due to the formation of (TP)2(HgI4) ion-association nanoparticles, contributing to a novel technique for Hg(2+) detection. The wavelength of 365nm is chosen as a detection wavelength and several conditions affecting the RRS responses of Hg(2+) are optimized. Under the optimum condition, the developed method is used for the determination of Hg(2+) in aqueous solution and the detection limit is estimated to be 0.8ngmL(-1). Finally, the practical application of the developed method can be confirmed through the detections of Hg(2+) in waste and river water samples with satisfactory results.

Ion association in solutions of lithium salts in mixtures of alkyl carbonates carries significant impact on the performance of lithium ion batteries. Focusing on lithium bis(oxalato)borate, LiBOB, in binary solvents based on ethylene carbonate, EC, we show that neither continuum nor discrete solvation approaches are capable of predicting physically meaningful results. So-called mixed or the discrete–continuum solvation approach, based on explicit consideration of an ion solvatocomplex combined with estimation of the medium polarization effect, is required in order to characterize the ion association at the quantitative level. The calculated changes of the Gibbs free energy are overestimated by nearly an order of magnitude by the purely continuum and purely discrete approaches, with the values having the opposite signs. The physically balanced discrete–continuum description predicts weak ion association. The numerical data obtained with density functional theory are validated using coupled-cluster calculat...

Hofmeister's “water absorbing effect” of ions is quantified. A clear definition is provided for the term “hydration number” as the average number of water molecules bound to the solute more strongly (by at least 13.3 kcal mol− 1) than are bound to other waters. Such hydration numbers are reported for various ions of electrolytes along with the extent of ion pair formation of each salt. Experimental measurements of various colligative properties demonstrate that the hydration numbers and extents of ion pair formation remain constant over large ranges of concentration and that solutions behave “ideally” up to the point where about 50% of the total water is strongly bound to the solute and is not available to act as solvent for additional solute.

The electrical conductances of dilute solutions of the ionic liquids 1-ethyl-3-methylimidazolium tetrafluoroborate [emim][BF4] and 1-butyl-3-methylimidazolium tetrafluoroborate [bmim][BF4] in 1-butanol have been measured in the temperature range from T/K = (283.15 to 318.15) at 5 K intervals. The ionic association constants, KA, the limiting molar conductances, Λo, and distance parameters, R, were obtained using the low concentration Chemical Model (lcCM). The examined ionic liquids are strongly associated in 1-butanol over the whole temperature range. From the temperature dependence of the limiting molar conductivities the Eyring’s activation enthalpy of charge transport was determined. The thermodynamic functions such as Gibbs energy, Δ G A o , entropy, Δ S A o , and enthalpy of the process of ion pair formation, Δ H A o , were calculated from the temperature dependence of the association constants. Negative values of Δ G A o indicate that the formation of ion pairs is a spontaneous process. The increase of temperature leads to more negative Δ G A o values, which means shifting the equilibrium toward the formation of ion pairs. The entropy values of association are positive, which means that as a result of association, the arrangement of the system decreases in comparison with the arrangement associated with the solvation of free ions.

© 2015 Elsevier B.V. The properties of aqueous ionic solutions are determined by strong electrostatic interactions, which are influenced by polarizability and charge transfer interactions. Potential models which include polarizability and charge transfer have been developed for water and single ion properties. Here, the ion-ion interactions are optimized so that the osmotic pressure as a function of concentration is reproduced. Using the optimized potentials, the amount of ion pairing and larger cluster formation is calculated. For NaCl, NaI, and KCl, there is a small amount of pairing, with larger clusters present as well. For KI, there is much more pairing and much larger clusters are observed. The amount of pairing is consistent with the law of matching affinities, with the pairs that show the least pairing also being the most mis-matched in terms of size or solvation free energy. The charge transfered from the anions to water is more than from is transferred from the water to cations, so the water molecules acquire a negative charge, which increases with ion concentration.

A micro-volume spectrofluorimetric coupled ultrasound-assisted ion association dispersive liquid–liquid microextraction (USA-IA-DLLME) procedure for the total determination and speciation of chromium(III & VI) species has been established. This demonstration was based upon the formation of a complex ion associate between halochromate (CrO3Cl−) and rhodamine 6G hydrochloride dye (RG+). At optimal conditions, the method exhibited a three order of magnitude wide linear concentration range (1.0–1000 μg L−1), with detection and quantification limits of 0.57 μg L−1 and 1.9 μg L−1, respectively. Competent cations, anions and oxoanions did not interfere with the chromium(VI) determination. Chromium(III) was also evaluated after its conversion to chromate with H2O2 in alkaline media. The nature of the extractant and the disperser, their volumes and the extraction time were optimized. The fluorescence quenching mechanism of the complex ion associate was discussed. The method was validated by the determination of chromium(VI) in real water (sea and tap water) samples. The results were also compared successfully with those from inductively coupled plasma-optical emission spectrometry (ICP-OES) in terms of the student’s t- and F test data at 95% confidence.

Herein, we propose an evanescent-wave fiber optic sensing technique for the anionic dye uranine based on ion association extraction. The sensor was prepared by removing a section of the cladding from a multimode fiber and hydrophobization of the exposed core surface. Uranine was extracted in association along with hexadecyltrimethylammonium (CTA) ion onto the fiber surface and detected via absorption of the evanescent wave generated on the surface of the exposed fiber core. The effect of CTA+ concentration added for ion association was investigated, revealing that the absorbance of uranine increased with increasing CTA+ concentration. A change in the sensor response as a function of the added uranine concentration was clearly observed. The extraction data were analyzed using a distribution equilibrium model and a Freundlich isotherm. The uranine concentration in the evanescent field of the fiber optic was up to 54 times higher than that in the bulk solution, and the limit of detection (3&sigma, ) for uranine was found to be 1.3 nM.

The predictive electrolyte model COSMO-RS-ES is refined to improve the description of systems at 25{\deg}C in which strong ion pairing is expected due to a low static permittivity of the liquid phase. Furthermore, the short-range ion energy interaction equations have been modified to better describe the misfit and energy interaction terms between ions and solvent molecules. In addition, the salt solubility database is extended with additional non-aqueous systems containing solvents that have a low (\epsilon_s