Lattice polarization effects in electrochromic switching in WO3−x films studied by pulse-nanogravimetric technique

The interactions of various types of cations with the tungsten trioxide lattice have been investigated to evaluate possible intercalation of these cations and the occurrence of lattice polarization leading to the near-surface structural lattice damage. The interactions of cations, such as the large monovalent cations (K⁺, Et₄N⁺, CtMe₃N⁺ cations), transition metal dications (Ni²⁺), heavy metal ions (Cd²⁺), and representative lanthanides (La³⁺) and actinides (Th⁴⁺), in competition with intercalation of H⁺ ions have been investigated using pulse-nanogravimetric technique. The effects of these cations in electrochromic processes of WO₃ proceeding during cathodic reduction have been assessed. For all of the metal ions studied, a large increase in the apparent mass uptake (up to eightfold) in comparison to pure H⁺ ion ingress was observed upon the film coloration induced by a cathodic potential pulse. The experiments indicate that the apparent mass gains, although large, are insufficient to confirm predominant contribution of metal ions in the ion transport along the channels in WO₃ lattice. The lower decoloration rate in the case of Ni²⁺ ions, in comparison to H⁺ and other transition metal cations (Cd²⁺), has been attributed to a slow dissociation of Ni²⁺ ions from lattice-bound oxygen atoms. For et₄N⁺ cation, which is too large to enter channels in WO₃, a dissociative reduction of the WO₃ and severe lattice damage was observed. Among the metal ions investigated, only K⁺ ions have been found to cause a dissociative reduction of WO₃ and near-surface lattice damage. Strong lattice polarization effects and irreversible binding have been found for La³⁺ and Th⁴⁺ cations.     

Sensitivity and Selectivity of Polypyrrole Based AC‐Amperometric Sensors for Electroinactive Ions – Frequency and Applied Potential Influence

Oxidation/reduction of polypyrrole films coupled with ion exchange on the polymer/solution interface can be utilized for amperometric sensing of electroinactive ions. Anion or cation exchanging films (polypyrrole doped by chloride or poly(4‐styrenesulfonate) ions, respectively) can be used to determine common anions (as Cl^?, NO , SO etc) or cations (K⁺, Na⁺, Li⁺, Ca²⁺, Mg²⁺) under conditions of alternating current (AC) amperometry in the range 10^?4–1 M. A sensitivity can be tuned by choosing appropriate electrode potential, corresponding to polypyrrole oxidation (anion‐exchanging films) or reduction (cation‐exchangers). Electrochemical impedance spectroscopy and AC‐voltammetry studies have shown that applied frequency and potential could also affect the observed dependence of the signal (admittance or AC‐current) on ion concentration. For high frequency the sensitivity is higher but selectivity lower, due to influence of solution conductivity on the response. For low frequencies the sensitivity is lower; however, a selectivity increase was observed due to diverse mobility of ions in the polymer film. Selectivity of AC‐amperometric responses was studied both in separate and mixed solutions.     

Bifunctional Ion‐Selective Electrode Based on C60‐Cryptand 22 for Silver and Iodine Ions

Fullerence C60‐cryptand 22 was prepared and successfully applied as the electric carrier in the PVC electrode membrane of a bifunctional ion‐selective electrode for cations, e.g., Ag⁺ ions as well as anions, e.g., I^? ions. The bifunctional ion‐selective electrode based on C60‐cryptand 22 can be applied as a Silver (Ag⁺) ion selective electrode with an internal electrode solution of 10^?3 M AgNO₃ in water (pH = 6.3), or as an Iodide (I^?) ion selective electrode with an acidic internal electrode solution of 10^?4 M KI_(aq) (pH = 2) in which the cryptand 22 is protonated, and the C60‐cryptand 22 is changed to C60‐Cryptand22–H⁺ and becomes an anionic electro‐carrier to absorb the I^? ion. The Ag⁺ ion selective electrode based on C60‐cryptand 22 gave a linear response with a near‐Nernstian slope (59.5 mV decade^?1) within the concentration range 10^?1‐10^?3 M Ag⁺_(aq). The Ag⁺ ion electrode exhibited comparatively good selectivity for silver ions, over other transition‐metal ions, alkali and alkaline earth metal ions. The Ag⁺ ion selective electrode with good stability and reproducibility was successfully used for the titration of Ag⁺_(aq) with Cl^? ions. The Iodide (I^?) Ion selective electrode based on protonated C60–cryptand22‐H⁺ also showed a linear response with a nearly Nernstian slope (58.5 mV decade^?1) within 10^?1 ‐ 10^?3 M I^? _(aq) and exhibited good selectivity for I^? ions and had small selectivity coefficients (10^?2–10^?3) for most of other anions, e.g., F^? , OH^?, CH₃COO^?, SO₄^2?, CO₃^2?, CrO₄^2?, Cr₂O₇^2? and PO₄^3? ions.     

On‐site sampling of inorganic contamination on the metal surface and analysis with capillary electrophoresis

Pipes are the primary structural elements used for transporting fluid in various industries. The most common damage mechanism is corrosion, which occurs in pipes surface of turbine. The corrosive compounds for pipes are inorganic ion (Na⁺, Cl^?, NH₄⁺, NO₃^?, etc.) and grinding oil. For rapid and quantitative detection of inorganic ions on site, more reliable and reproducible analytical methods are demanded. A highly efficient solid–liquid sampling collection system is introduced in this work. Papering on the sample surface, inorganic cations and anions were simultaneously collected and analyzed by capillary electrophoresis with indirect ultraviolet detection. As a result, five cations (Na⁺, K⁺, NH₄⁺, Ca²⁺, Mg²⁺) and three anions (Cl^?, NO₃^?, SO₄^2?) were completely separated. The efficiency of the sampling and ability of capillary electrophoresis analysis were presented by the determination of trace‐level (mg/m²) contaminants. The recoveries of cations and anions on the paper from metal surface were between 86.6 and 107.2%, and the relative standard deviations were less than 12.85%.     

Cationic and anionic environments in LiTFSI-doped di-ureasils with application in solid-state electrochromic devices

Fourier Transform mid-infrared and Raman spectroscopies were used to investigate the cation/polymer, cation/urea bridge, cation/anion and hydrogen bonding interactions in poly(oxyethylene) (POE)/siloxane di-ureasil networks prepared by the sol–gel route and doped with lithium bis(trifluoromethanesulfonyl)imide (LiTFSI). Materials with compositions 200 ?n ? 5 (where n expresses the molar ratio OCH₂CH₂/Li⁺) were studied. The Li⁺ ions coordinate to the urea carbonyl oxygen atoms over the whole range of salt concentration considered. Bonding to the ether oxygen atoms of the POE chains occurs at n ? 40, although a significant fraction of the POE chains remain non-coordinated. In these high salt content samples, the cations interact with the anions forming contact ion pairs. “Free” ions are probably the main charge carriers at the room temperature conductivity maximum of these ormolytes.     

Photo Decoloration of Methylene Blue with Ribose under Optimum Conditions by Visible Radiation

Photo decoloration of the methylene blue (MB) with reducing sugar, ribose (RH), was investigated on an especially designed optical processor using monochromatic radiation of 661 nm through a red filter. The dye molecule gets excited into triplet transient species (MBT) during flushing with lifetime of 10.1 ms into acetate buffered aqueous alcoholic medium, which later on reduces to protonated leuco dye (MBH). Photolysis of the aqueous alcholic medium generated highly reactive oxygen radical (O－•) with the production of 2e－, which led to probable oxidation of the ribose into respective acid while hydrogen abstraction and 2e－ reduced the dye (MB) into MBH by following reaction     

Study of the Photodegradation Kinetics and Pathways of Hexaflumuron in Liquid Media

Hexaflumuron, one of the benzoylphenylurea insect growth regulators, can be leached into surface water and thus having a potential impact on aquatic organisms. In this study, the photodegradation processes of hexaflumuron under high‐pressure mercury lamp irradiation were assessed. The photodegradation kinetics were studied, as were the effects of pH, different light sources, organic solvents and environmental substances, including nitrate ions (NO₃^?), nitrite ions (NO₂^?), ferrous ions (Fe²⁺), ferric ions (Fe³⁺), humic acid, sodium dodecyl sulfate (SDS) and hydrogen peroxide (H₂O₂). Three photodegradation products in methanol were identified by gas chromatography‐mass spectrometry (GC‐MS). In general, the degradation of hexaflumuron followed first‐order kinetics. In the four media studied, the photodegradation rate order was n‐hexane > methanol > ultrapure water > acetone. Faster degradation was observed under high‐pressure mercury lamp irradiation than under xenon lamp irradiation. The pH had a considerable effect, with the most rapid degradation occurring at pH 5.0. The photodegradation rate of hexaflumuron was promoted in the presence of NO₃^?, NO₂^?, Fe²⁺, humic acid, SDS and H₂O₂, but inhibited by Fe³⁺. Moreover, the presumed photodegradation pathway was proposed to be the cleavage of the urea linkage.     

The Application of Cryptand 22 as a Bifunctional Stationary Phase for Ion Chromatography

Poly (styrene/divinyl benzene) with cryptand 22 as an anchoring group was synthesized and applied as a bifunctional packing material for the separation of both cations and anions. At pH < 2, the resin can be protonated and applied as an anion exchanger for the separation of anions; with water as eluent, inorganic anions such as F^?, Cl^?, Br^?, NO₃^?”, I^? were well separated. After deprotonation at pH> 10, the resin became a cation exchanger and successfully separated alkali metal ions such as Li⁺, K⁺ and Cs⁺ with methanol as eluent. The effects of solvents, flow rate and temperature on the separation of various ions were also investigated.     

Heterogeneous surface saponification of suspension‐polymerized monodisperse poly(vinyl acetate) microspheres using various ions

Poly(vinyl alcohol) (PVA)/poly(vinyl acetate) (PVAc) microspheres with a skin/core structure were prepared through the heterogeneous surface saponification of PVAc microspheres suspension‐polymerized. The PVA skin formed through the heterogeneous saponification was hydrogel swellable in water. In addition, to obtain monodisperse PVA/PVAc microspheres having various skin/core ratios and morphologies, the ion‐specificities to the heterogeneous saponification were investigated using SO, Cl^?, NO, Br^?, and I^? for anions and Li⁺, Na⁺, and K⁺ for cations, respectively. The ions were not specific significantly to the rate of the heterogeneous saponification, while were related to the degree of saponification (DS). DSs had different values between by weight loss (DS_w) and by proton nuclear magnetic resonance spectroscopy (DS_NMR) measurements. The order of DS_ws was SO < Cl^? < NO < Br^? < I^? for anions and K⁺ < Na⁺ < Li⁺ for cations, and that of DS_NMRs, I^? < Br^? < NO < Cl^? < SO for anions and Li⁺ < Na⁺ < K⁺ for cations. The differences in values between DS_ws and DS_NMRs were caused by the dissolution of PVA skin and were significantly decreased for SO. The peaks at melting temperature of PVA were sharp and their areas were large for ions deswelling PVA skins.     

Double collision experiments

A variety of double collision experiments, whereby fast species undergo collisional interactions in two distinct regions of a mass spectrometer, are described. These include two-stage charge reversal of negative ions, two-stage double electron transfer from targets to cations, neutralization-reionization experiments as well as delayed analysis of organic cations formed in a one-step charge reversal of anions. Experiments have been performed on a number of systems of current interest in gas-phase ion chemistry. It is concluded that autoelectron detachment of benzyl anions leads to benzyl radicals, whereas the collisionally induced electron detachment produces a mixture of benzyl and tropyl radicals. By contrast, electron detachment from [H₃CNH]^? is not a metastable process and occurs only after excitation to produce H₃CNH˙ radicals, which do not rearrange into the thermodynamically more stable H₂CNH₂˙. It is shown that in the double electron transfer reactions H⁺ + Xe→H˙ + Xe⁺˙ and H˙ + Xe→H^? + Xe⁺˙, excited states are produced. From double collision experiments on methyl formate ions, it is concluded that the non-decomposing ions have undergone rearrangement on the time-scale of 10 μs into the distonic isomer, \documentclass{article}\pagestyle{empty}\begin{document}$ {\rm H} - \mathop {\rm C}\limits^ + ({\rm OH}){\rm O}\mathop {\rm C}\limits^. {\rm H}_2 $\end{document}. Finally, it is shown that short-lived (<0.2 μs) [H₂, C, N]⁺ ions generated by charge-reversal of [H₂CN]^? have the [H₂CN]⁺ structure, whereas most of the long-lived (10 μs) ions have the [HCNH]⁺ structure.     

Zeta Potential of Pyrophyllite in Aqueous Solutions of Alkaline and Alkaline Earth Metal Cations and Low‐Molecular‐Weight Organic Anions

Electrokinetic behavior of pyrophyllite in aqueous solutions of K⁺, Na⁺, Mg²⁺, and Ca²⁺ ions and effect of low‐molecular‐weight organic anions like acetate (C₂H₃O₂ ^?), oxalate (C₂O₄ ^2?), and citrate (C₆H₅O₇ ^3?) on the zeta potential was investigated. Pyrophyllite has higher negative zeta potential values in the presence of alkaline metal cations than in water, and it comes close to positive values in the presence of alkaline earth metal cations at low pH values. Results revealed that presence of low‐molecular‐weight organic anions led to a decrease in the zeta potential in the order of acetate>citrate>oxalate.     

Evaluation of the stability of dye heteroassociates formed in aqueous solutions

The additive tetraphenylarsonium-tetraphenylborate model of interactions was found to be applicable to the problem of “preexperimental” evaluation of the stability of associates formed by dye cations (Ct⁺) and anions (An^?) in aqueous solutions. The possibility of predicting equilibrium association constants K _as from preliminarily calculated ΔG(Ct⁺) and ΔG(An^?) and of solving the inverse problem was analyzed. The invariability of the ΔG(Ct⁺) and ΔG(An^?) values and the problem of bringing calculation results in consistency with the experimental K _as values are discussed.     

Interface physicochemical processes controlling sulphate anion incorporation in porous anodic alumina and their dependence on the thermodynamic and transport properties of cations

Aluminium was anodised in H₂SO₄, LiHSO₄, NaHSO₄, KHSO₄, Mg(HSO₄)₂ and Al(HSO₄)₃ electrolytes. The kinetics of growth of porous anodic alumina films and of the pore wall oxide dissolution during anodisation was studied. Based on the derived kinetic parameters, suitable physicochemical processes in the barrier layer electrolyte interface controlling the anion incorporation in the barrier layer were suggested and relevant models were formulated. According to these processes Al³⁺ and H⁺ ions are rejected from the pore base surface in the attached double layer, where Al³⁺ ions are solvated, and are transferred to the pore filling solution. The strongly different mobilities of Al³⁺ and H⁺ and the necessary space negative charge density distribution in the double layer result in similar concentration distributions of Al³⁺ and anions inside it, which differ strongly from that of H⁺. These Al³⁺ and anion concentrations increase with decreasing mobility of the main cations in the solution which depends on their hydration enthalpy and transport mechanism. The concentration of incorporated anions inside both a thin surface layer of the barrier layer and the double layer vary similarly. For identical surface density and base diameter of pores the decrease of the above mobility reinforces anion incorporation.     

Tunable separation of anions and cations by column switching in ion chromatography

A convenient ion chromatography method has been proposed for the routine and simple determination of anions (Cl⁻, SO₄²⁻ and NO₃⁻) and/or cations (Na⁺, NH₄⁺, K⁺, Mg²⁺ and Ca²⁺) using a single pump, a single eluent and a single detector. The present system used cation-exchange and anion-exchange columns connected in series via two 6-port switching valves or a single 10-port valve. The connection order of the ion-exchange columns could be varied by switching the valve(s). The present system therefore allowed the separation of either cations or anions in a single chromatographic run. While one ion-exchange column is being operated, the other ion-exchange column is being conditioned, i.e., the columns are always ready for analysis at any time. When 2.4 mM 5-sulfosalicylic acid was used as the eluent, the three anions and the five cations could be separated on the anion-exchange column and cation-exchange column, respectively. In order to obtain the separations of the target ions, the injection valve was placed between the two columns. Complete separations of the above anions or cations were demonstrated within 10 min each. The detection limits at S/N = 3 were 19-50 ppb (μg/l) for cations and 10-14 ppb for anions. The relative standard deviations of the analyte ions were less than 1.1, 2.9 and 2.8% for retention time, peak area and peak height, respectively. This proposed technique was applied to the determination of common anions and cations in river water samples.     

Interfering influence of silver on the voltammetric behaviour of Cd2+ and Zn2+ ions on hmde in supporting electrolytes containing nitrates

A new kind of secondarily formed peaks was found in cyclic and stripping voltammetry in neutral sulphate, perchlorate and nitrate supporting electrolytes containing some divalent cations and a substance (for example O₂), the reduction of which gives as a by-product OH^? ions. The hydroxides deposited in the vicinity of the mercury electrode, in the course of a cathodic scan, react during the anodic scan according to the reaction Hg+Me(OH)₂=Hg(OH)₂+Me²⁺+2e forming a new, separate anodic peak.It was found that silver exerts a catalytic effect on the reduction of NO₃^? ions on the mercury electrode. In neutral nitrate supporting electrolyte containing Ag⁺ ions the hydroxides of some cations (Cd²⁺, Zn²⁺, Mn²⁺, Co²⁺ and Ni²⁺) were deposited during the cathodic scan or during the preelectrolysis. Afterwards, in the course of the anodic scan, a new peak, of the kind described above, was observed. The same effect was formerly interpreted, for Zn²⁺ and Cd²⁺, as evidence for the formation of intermetallic compounds, AgZn and AgCd.     

Cu(II) Hexacyanoferrate(III) Modified Carbon Paste Electrode; Application for Electrocatalytic Detection of Nitrite

Copper hexacyanoferrate (CuHCF) film‐modified carbon paste electrode (CPE) has been prepared from various electrolytic aqueous solutions using consecutive cyclic voltammetry. The cyclic voltammograms showed the direct deposition of CuHCF films from the mixing of Cu²⁺ and Fe(CN)₆^3? ions and each time with one of the six cations: H⁺, Na⁺, K⁺, NH₄⁺, Mg²⁺, and Al³⁺. The CuHCF film showed a single redox couple that exhibited a cation effect (Na⁺, K⁺, Mg²⁺, and NH₄⁺) and anion effect (Cl^?, NO₃^?, SO₄^2?, ClO₄^?, and BrO₃^?) in the cyclic voltammograms. Voltammetric studies have indicated that in presence of nitrite, the cathodic peak current of CuHCF increases, followed by a decrease in the corresponding anodic current. This indicated that nitrite was reduced by the redox mediator immobilized on the electrode surface via an electrocatalytic mechanism. The process of reduction and its kinetics were investigated by using cyclic voltammetry, differential pulse voltammetry, chronoamperometry and chronocoulometry techniques. The electrocatalytic ability about 800 mV can be seen. The rate constant of the catalytic reduction of nitrite was found to be 7.9×10⁵ cm³ mol^?1 s^?1. Linearity range obtained was 5×10^?5?8.4×10^?3 by cyclic voltammetry and 8×10^?6?1.3×10^?3 and 4×10^?3?2×10^?2 by differential pulse voltammetry.     

Effect of acid and alkali treatments of a forest waste,Pinus brutia cones,on adsorption efficiency of methyl green

The removal of methyl green (MG) dye from aqueous solutions using acid- or alkali-treated Pinus brutia cones (PBH and PBN) waste was investigated in this work. Adsorption removal of MG was conducted at natural pH, namely, 4.5 ± 0.10 for PBH and near 4.8 ± 0.10 for PBN. The pseudo-second-order model appeared to be the most appropriate to describe the adsorption process of MG on both PBN and PBH with a correlation coefficient R² > 0.999. Among the tested isotherm models, the Langmuir isotherm was found to be the most relevant to describe MG sorption onto modified P. brutia cones with a correlation factor R² > 0.999. The ionic strength (presence of other ions: Cl^?, Na⁺, and SO₄^2?) also influences the adsorption due to the change in the surface properties; it had a negative impact on the adsorption of MG on these two supports. A reduction of 68.5% of the adsorption capacity for an equilibrium dye concentration Ce of 30 mg/L was found for the PBH; while with PBN no significant influence of the ionic strength on adsorption was observed, especially in the presence of NaCl for dye concentrations superior to 120 mg L^?1.     

“Best Match” Model and Effect of Na+/H+ Exchange on Anion Attachment to Peptides and Stability of Formed Adducts in Negative Ion Electrospray Mass Spectrometry

The “Best Match” model has been extended to account for the role that Na⁺/H⁺ exchange plays on anion attachment in negative ion electrospray. Without any Na⁺/H⁺ exchange on (Glu) fibrinopeptide B, the higher basicity anions F^? and CH₃COO^? can hardly form observable adducts; however, after multiple Na⁺/H⁺ exchanges, adduct formation is enabled. Moreover, dissociation pathways of CF₃COO^? adducts with singly deprotonated peptides that have undergone 0 to 3 Na⁺/H⁺ exchanges exhibit a shift in CID product ions from losing predominately CF₃COOH (case of 0 Na⁺/H⁺ exchanges) to losing predominately CF₃COO^? (case of 3 Na⁺/H⁺ exchanges). These phenomena can be rationalized by considering that Na⁺ cations exchange at, and serve to “block”, the most acidic sites, thereby forcing implicated anions to attach to lower acidity protons. In addition to forming ion pairs with carboxylate groups, Na⁺ also participates in formation of tri-atomic ions of the form ANaA^? during adduct dissociation. The fact that low gas-phase basicity (GB) anions preferentially form ANaA^? species, even though high GB anions form more stable tri-atomic species, indicates that the monatomic ions were not in close contact in the initial adduct. The propensity for formation of stable anionic adducts is dependent on the degree of matching between anion GBs and GB_app of deprotonated sites on the peptide. The GB_app is raised dramatically as the charge state of the peptide increases via a through-space effect. The presence of Na⁺ on carboxylate sites substantially decreases the GB_app by neutralizing these sites, while slightly increasing the intrinsic GBs by an inductive effect.

Quantification of Ion‐Pairing Effects on the Nucleophilic Reactivities of Benzoyl‐ and Phenyl‐Substituted Carbanions in Dimethylsulfoxide

Second‐order rate constants for the reactions of acceptor‐substituted phenacyl (PhCO?CH^??Acc) and benzyl anions (Ph?CH^??Acc) with diarylcarbenium ions and quinone methides (reference electrophiles) have been determined in dimethylsulfoxide (DMSO) solution at 20 °C. By studying the kinetics in the presence of variable concentrations of potassium, sodium and lithium salts (up to 10^?2 mol L^?1), the influence of ion‐pairing on the reaction rates was examined. As the concentration of K⁺ did not have any influence on the rate constants at carbanion concentrations in the range of 10^?4–10^?3 mol L^?1, the acquired rate constants could be assigned to the reactivities of the free carbanions. The counter ion effects increase, however, in the series K⁺<Na⁺<Li⁺, and the sensitivity of the carbanion reactivities toward variation of the counter ion strongly depends on the structure of the carbanions. The reactivity parameters N and s_N of the free carbanions were derived from the linear plots of log k₂ against the electrophilicity parameters E of the reference electrophiles, according to the linear‐free energy relationship log k₂(20 °C)=s_N(N+E). These reactivity parameters can be used to predict absolute rate constants for the reactions of these carbanions with other electrophiles of known E parameters.     

Effects of Ions on the OH Stretching Band of Water as Revealed by ATR-IR Spectroscopy

The effects of various cations (Li⁺, Na⁺, K⁺, Rb⁺, Cs⁺, Mg²⁺, Ca²⁺, Sr²⁺, Ba²⁺, Mn²⁺, Co²⁺, and Ni²⁺) and anions (Cl^?, Br^?, I^?, \( {\text{NO}}_{3}^{ - } \) , \( {\text{ClO}}_{4}^{ - } \) , \( {\text{HCO}}_{3}^{ - } \) , and \( {\text{CO}}_{3}^{2 - } \) ) on the molar absorptivity of water in the OH stretching band region (2,600–3,800 cm^?1) were ascertained from attenuated total reflection infrared spectra of aqueous electrolyte solutions (22 in all). The OH stretching band mainly changes linearly with ion concentrations up to 2 mol·L^?1, but several specific combinations of cations and anions (Cs₂SO₄, Li₂SO₄, and MgSO₄) present different trends. That deviation is attributed to ion pair formation and cooperativity in ion hydration, which indicates that the extent of the ion–water interaction reflected by the OH stretching band of water is beyond the first solvation shell of water molecules directly surrounding the ion. The obtained dataset was then correlated with several quantitative parameters representing structural and dynamic properties of water molecules around ions: ΔG _HB, the structural entropy (S _str), the viscosity B-coefficient (B _η ), and the ionic B-coefficient of NMR relaxation (B _NMR). Results show that modification of the OH stretching band of water caused by ions has quasi-linear relations with all of these parameters. Vibrational spectroscopy can be a useful means for evaluating ion–water interaction in aqueous solutions.