Probing ion-specific effects on aqueous acetate solutions: Ion pairing versus water structure modifications

From the Free Ligand to the Transition Metal Complex: FeEDTA- Formation Seen at Ligand K-Edges

Chelating agents are an integral part of transition metal complex chemistry with broad biological and industrial relevance. The hexadentate chelating agent ethylenediaminetetraacetic acid (EDTA) has the capability to bind to metal ions at its two nitrogen and four of its carboxylate oxygen sites. We use resonant inelastic X-ray scattering at the 1s absorption edge of the aforementioned elements in EDTA and the iron(III)-EDTA complex to investigate the impact of the metal–ligand bond formation on the electronic structure of EDTA. Frontier orbital distortions, occupation changes, and energy shifts through metal–ligand bond formation are probed through distinct spectroscopic signatures.

Metal-ligand interactions between the hexadentate chelating agent EDTA and iron(III) ions in the high spin iron(III)-EDTA complex are probed using ligand K-edge X-ray absorption and resonant inelastic X-ray scattering. Distinct spectral signatures of metal–ligand bond formation e.g. frontier orbital distortions, occupation changes, and energy shifts are detected at the coordinating sites. The absence of strong spin orbit coupling effects allows for direct interpretation based on TD-DFT based spectrum simulations.

Substituent effects in aqueous solutions of carboxylate salts studied by x-ray absorption spectroscopy at the oxygen K-edge

The present study aims at probing the influence of different substituents of sodium carboxylate salts R-COO^-:Na⁺ in aqueous solutions, with R = H, CH₃, C₂H₅, CH₂Cl, CF₃, and C₆H₅. X-ray absorption spectroscopy was used in the oxygen K-edge region to highlight the effect of R on the energy position of the O1s-to-π_COO* resonance of the carboxylate ion. Ab initio static exchange and ΔSCF calculations are performed and confirm the experimental observations. We qualitatively discuss the results on the basis of the polar properties of these groups as well as on the basis of the π_COO* orbital energy in the ground states, the oxygen 1s orbital ionization energy, and the O1s-to-π_COO* resonance energy.

Synthesis, Complexation Properties, and Evaluation of New Aminodiphosphonic Acids as Vector Molecules for 68Ga Radiopharmaceuticals

Two new aminodiphosphonic acids derived from salicylic acid and its phosphonic analogue were prepared through a simple and efficient synthesis. 2-[(2-Amino-2,2-diphosphono)ethyloxy]-benzoic acid 8 and 2-[(2-amino-2,2-diphosphono)ethyloxy]-5-ethyl-phenylphosphonic acid 9 were evaluated for their applicability as ⁶⁸Ga binding bone-seeking agents. Protonation constants of 8 and 9 and stability constants of the Ga³⁺ complexes with 8 and 9 in water were determined. The stability constant of Ga³⁺ complex with fully phosphorylated acid 9 (logK_GaL = 31.92 ± 0.32) significantly exceeds stability constant of Ga³⁺ complex with 8 (logK_GaL = 26.63 ± 0.24). Ligands 8 and 9 are as effective for Ga³⁺ cation binding as ethylenediamine-N,N’-diacetic-N,N’-bis(methy1enephosphonic) acid and ethylenediamine-N,N,N’,N’-tetrakis(methylenephosphonic) acid, respectively. The labelling process and stability of [⁶⁸Ga]Ga-8 and [⁶⁸Ga]Ga-9 were studied. Both 8 and 9 readily form ⁶⁸Ga-complexes stable to ten-fold dilution with saline. However, in fetal bovine serum, only [⁶⁸Ga]Ga-9 was stable enough to be subject to biological evaluation. It was injected into rats with bone pathology and aseptic inflammation of soft tissues. For [⁶⁸Ga]Ga-9 in animals with a bone pathology model in 60 and 120 min after injection, a slight accumulation in the pathology site, stable blood percentage level, and moderate accumulation in the liver were observed. For animals with an aseptic inflammation, the accumulation of [⁶⁸Ga]Ga-9 in the pathology site was higher than that in animals with bone pathology. Moreover, the accumulation of [⁶⁸Ga]Ga-9 in inflammation sites was more stable than that for [⁶⁸Ga]Ga-citrate. The percentage of [⁶⁸Ga]Ga-9 in the blood decreased from 3.1% ID/g (60 min) to 1.5% ID/g (120 min). Accumulation in the liver was comparable to that obtained for [⁶⁸Ga]Ga-citrate.

Restructuring of Hydration Shell Water due to Solvent-Shared Ion Pairing (SSIP): A Case Study of Aqueous MgCl2 and LaCl3 Solutions

Hydration of ions plays a crucial role in interionic interactions and associated processes in aqueous media, but selective probing of the hydration shell water is nontrivial. Here, we introduce Raman difference with simultaneous curve fitting (RD-SCF) analysis to extract the OH-stretch spectrum of hydration shell water, not only for the fully hydrated ions (Mg²⁺, La³⁺, and Cl^-) but also for the ion pairs. RD-SCF analyses of diluted MgCl₂ (0.18 M) and LaCl₃ (0.12 M) solutions relative to aqueous NaCl of equivalent Cl^- concentrations provide the OH-stretch spectra of water in the hydration shells of fully hydrated Mg²⁺ and La³⁺ cations relative to that of Na⁺. Integrated intensities of the hydration shell spectra of Mg²⁺ and La³⁺ ions increase linearly with the salt concentration (up to 2.0 M MgCl₂ and 1.3 M LaCl₃), which suggests no contact ion pair (CIP) formation in the MgCl₂ and LaCl₃ solutions. Nevertheless, the band shapes of the cation hydration shell spectra show a growing signature of Cl^--associated water with the rising salt concentration, which is a manifestation of the formation of a solvent-shared ion pair (SSIP). The OH-stretch spectrum of the shared/intervening water in the SSIP, retrieved by second-round RD-SCF analysis (2RD-SCF), shows that the average H-bonding of the shared water is weaker than that of the hydration water of the fully hydrated cation (Mg²⁺ or La³⁺) but stronger than that of the anion (Cl^-). The shared water displays an overall second-order dependence on the concentration of the interacting ions, unveiling 1:1 stoichiometry of the SSIP formed between Mg²⁺ and Cl^- as well as La³⁺ and Cl^-.

Expansion of Ion Effects on Water Induced by a High Hydrophilic Surface of a Polymer Network

The spatial extent and anion-cation cooperativity of the ion effect on the structure and dynamics of water have long been debated but are still controversial. Previously, we experimentally demonstrated the extensive and cooperative effect of ions on water in a polyamide network by measuring the reflection wavelength (λ) on the ion sensor of poly(N-isopropylacrylamide) (PNIPAAm) hydrogel-immobilized photonic crystals. In the present study, we investigated the influence of the polymer surface on the ion effect by adopting a highly hydrophilic poly(N-isopropylacrylamide-co-N-acryloylaza-18-crown-6) hydrogel as a sensor matrix. In alkaline earth metal salt solutions, the copolymer hydrogel membrane sensor showed the redshift of λ for the specific combination of cations and anions, that is, Ca²⁺/Cl^- and Sr²⁺/NO₃^-, which resulted from the concerted binding of ion pairs to the copolymer receptor. In alkali metal salt solutions, the ion sensor showed the blueshift of λ originating from the osmotic dehydration suppressed by the salts. The strength of the ion effect was evaluated by the average osmotic pressure (Π_A) required for the salt-inhibited dehydration in the early stage of hydrogel contraction. From the calculation results of Π_A for the copolymer and PNIPAAm hydrogels, it was found that the high hydrophilic copolymer surface more significantly enhanced the ion effect of structure-making cations (i.e., Li⁺) compared with borderline (Na⁺) and structure-breaking (K⁺ and Cs⁺) cations. Furthermore, the ion effect exhibited the higher ion cooperativity in combination with chloride anions than with nitrate anions. The enhancement of the long-range cooperative ion effect is derived from the expansion of the interactions between ions, water molecules, and the hydrophilic polymer network.

Quantitative Evaluation of Long-Range and Cooperative Ion Effect on Water in Polyamide Network

Despite long-standing research efforts to elucidate the specific ion effect on the structure and dynamics of water, the spatial extent affected by ions and the cooperativity of ions and counterions are still controversial. Here, we demonstrate an undoubtable evidence of long-range and cooperative ion effect on water molecules in a polyamide network by using a precision ion sensor of photonic crystal hydrogel membrane. The ion effect was quantitatively evaluated by means of the osmotic work per unit cell volume change of photonic crystal, W_unit, required for the ion-inhibited dehydration, which means a suppressed migration of water molecules by the extensive effect of ions beyond their immediate hydration shells. It was found that  W_unit required for 14 vol % contraction of the membrane sensor in LiCl aqueous solutions was 7.7 times larger than that in Sr(NO₃)₂ solutions. The combination of structure-making Ca²⁺ and Sr²⁺ with nitrate anions lowered the ion effect than the chloride salts of borderline Na⁺ and Ba²⁺. Furthermore, the nitrate salt of Sr²⁺ exhibited a lower ion effect than the chloride salts of structure-breaking K⁺ and Cs⁺. These results have revealed that the ion effect acts to water extensively, which is modulated by cooperative interactions of ions and counterions.

Cationic and Anionic Impact on the Electronic Structure of Liquid Water

Hydration shells around ions are crucial for many fundamental biological and chemical processes. Their local physicochemical properties are quite different from those of bulk water and hard to probe experimentally. We address this problem by combining soft X-ray spectroscopy using a liquid jet and molecular dynamics (MD) simulations together with ab initio electronic structure calculations to elucidate the water-ion interaction in a MgCl₂ solution at the molecular level. Our results reveal that salt ions mainly affect the electronic properties of water molecules in close vicinity and that the oxygen K-edge X-ray emission spectrum of water molecules in the first solvation shell differs significantly from that of bulk water. Ion-specific effects are identified by fingerprint features in the water X-ray emission spectra. While Mg²⁺ ions cause a bathochromic shift of the water lone pair orbital, the 3p orbital of the Cl^- ions causes an additional peak in the water emission spectrum at around 528 eV.

Investigation of the Ionic Hydration in Aqueous Salt Solutions by Soft X-ray Emission Spectroscopy

Understanding the molecular structure of the hydration shells and their impact on the hydrogen bond (HB) network of water in aqueous salt solutions is a fundamentally important and technically relevant question. In the present work, such hydration effects were studied for a series of representative salt solutions (NaCl, KCl, CaCl2, MgCl2, and KBr) by soft X-ray emission spectroscopy (XES) and resonant inelastic soft X-ray scattering (RIXS). The oxygen K-edge XES spectra could be described with three components, attributed to initial state HB configurations in pure water, water molecules that have undergone an ultrafast dissociation initiated by the X-ray excitation, and water molecules in contact with salt ions. The behavior of the individual components, as well as the spectral shape of the latter component, has been analyzed in detail. In view of the role of ions in such effects as protein denaturation (i.e., the Hofmeister series), we discuss the ion-specific nature of the hydration shells and find that the results point to a predominant role of anions as compared to cations. Furthermore, we observe a concentration-dependent suppression of ultrafast dissociation in all salt solutions, associated with a significant distortion of intact HB configurations of water molecules facilitating such a dissociation.

X-ray and Electron Spectroscopy of Water

Here we present an overview of recent developments of X-ray and electron spectroscopy to probe water at different temperatures. Photon-induced ionization followed by detection of electrons from either the O 1s level or the valence band is the basis of photoelectron spectroscopy. Excitation between the O 1s and the unoccupied states or occupied states is utilized in X-ray absorption and X-ray emission spectroscopies. These techniques probe the electronic structure of the liquid phase and show sensitivity to the local hydrogen-bonding structure. Both experimental aspects related to the measurements and theoretical simulations to assist in the interpretation are discussed in detail. Different model systems are presented such as the different bulk phases of ice and various adsorbed monolayer structures on metal surfaces.

A liquid flatjet system for solution phase soft-x-ray spectroscopy

We present a liquid flatjet system for solution phase soft-x-ray spectroscopy. The flatjet set-up utilises the phenomenon of formation of stable liquid sheets upon collision of two identical laminar jets. Colliding the two single water jets, coming out of the nozzles with 50 μm orifices, under an impact angle of 48° leads to double sheet formation, of which the first sheet is 4.6 mm long and 1.0 mm wide. The liquid flatjet operates fully functional under vacuum conditions (<10(-3) mbar), allowing soft-x-ray spectroscopy of aqueous solutions in transmission mode. We analyse the liquid water flatjet thickness under atmospheric pressure using interferomeric or mid-infrared transmission measurements and under vacuum conditions by measuring the absorbance of the O K-edge of water in transmission, and comparing our results with previously published data obtained with standing cells with Si3N4 membrane windows. The thickness of the first liquid sheet is found to vary between 1.4-3 μm, depending on the transverse and longitudinal position in the liquid sheet. We observe that the derived thickness is of similar magnitude under 1 bar and under vacuum conditions. A catcher unit facilitates the recycling of the solutions, allowing measurements on small sample volumes (∼10 ml). We demonstrate the applicability of this approach by presenting measurements on the N K-edge of aqueous NH4 (+). Our results suggest the high potential of using liquid flatjets in steady-state and time-resolved studies in the soft-x-ray regime.

Probing Interfacial Water on Nanodiamonds in Colloidal Dispersion

The structure of interfacial water layers around nanoparticles dispersed in an aqueous environment may have a significant impact on their reactivity and on their interaction with biological species. Using transmission soft X-ray absorption spectroscopy in liquid, we demonstrate that the unoccupied electronic states of oxygen atoms from water molecules in aqueous colloidal dispersions of nanodiamonds have a different signature than bulk water. X-ray absorption spectroscopy can thus probe interfacial water molecules in colloidal dispersions. The impacts of nanodiamond surface chemistry and concentration on interfacial water electronic signature are discussed.

On the Origin of the Improvement of Electrodeposited MnOx Films in Water Oxidation Catalysis Induced by Heat Treatment

Manganese oxides (MnOx ) are considered to be promising catalysts for water oxidation. Building on our previous studies showing that the catalytic activity of MnOx films electrodeposited from aqueous electrolytes is improved by a simple heat treatment, we have explored the origin of the catalytic enhancement at an electronic level by X-ray absorption spectroscopy (XAS). The Mn L-edge XA spectra measured at various heating stages were fitted by linear combinations of the spectra of the well-defined manganese oxides-MnO, Mn3 O4 , Mn2 O3 , MnO2 and birnessite. This analysis identified two major manganese oxides, Mn3 O4 and birnessite, that constitute 97 % of the MnOx films. Moreover, the catalytic improvement on heat treatment at 90 °C is related to the conversion of a small amount of birnessite to the Mn3 O4 phase, accompanied by an irreversible dehydration process. Further dehydration at higher temperature (120 °C), however, leads to a poorer catalytic performance.

Valence holes observed in nanodiamonds dispersed in water

Colloidal dispersion is essential for most nanodiamond applications, but its influence on nanodiamond electronic properties remains unknown. Here we have probed the electronic structure of oxidized detonation nanodiamonds dispersed in water by using soft X-ray absorption and emission spectroscopies at the carbon and oxygen K edges. Upon dispersion in water, the π* transitions from sp(2)-hybridized carbon disappear, and holes in the valence band are observed.

Probing hydrogen bonding orbitals: resonant inelastic soft X-ray scattering of aqueous NH3

Resonant inelastic soft X-ray scattering was used to probe the hydrogen bonding orbitals in aqueous ammonia.

Ion-Solvation-Induced Molecular Reorganization in Liquid Water Probed by Resonant Inelastic Soft X-ray Scattering

The molecular structure of liquid water is susceptible to changes upon admixture of salts due to ionic solvation, which provides the basis of many chemical and biochemical processes. Here we demonstrate how the local electronic structure of aqueous potassium chloride (KCl) solutions can be studied by resonant inelastic soft X-ray scattering (RIXS) to monitor the effects of the ion solvation on the hydrogen-bond (HB) network of liquid water. Significant changes in the oxygen K-edge emission spectra are observed with increasing KCl concentration. These changes can be attributed to modifications in the proton dynamics, caused by a specific coordination structure around the salt ions. Analysis of the spectator decay spectra reveals a spectral signature that could be characteristic of this structure.

Incisive probing of intermolecular interactions in molecular crystals: core level spectroscopy combined with density functional theory

The α-form of crystalline para-aminobenzoic acid (PABA) has been examined as a model system for demonstrating how the core level spectroscopies X-ray photoelectron spectroscopy (XPS) and near-edge X-ray absorption fine-structure (NEXAFS) can be combined with CASTEP density functional theory (DFT) to provide reliable modeling of intermolecular bonding in organic molecular crystals. Through its dependence on unoccupied valence states NEXAFS is an extremely sensitive probe of variations in intermolecular bonding. Prediction of NEXAFS spectra by CASTEP, in combination with core level shifts predicted by WIEN2K, reproduced experimentally observed data very well when all significant intermolecular interactions were correctly taken into account. CASTEP-predicted NEXAFS spectra for the crystalline state were compared with those for an isolated PABA monomer to examine the impact of intermolecular interactions and local environment in the solid state. The effects of the loss of hydrogen-bonding in carboxylic acid dimers and intermolecular hydrogen bonding between amino and carboxylic acid moieties are evident, with energy shifts and intensity variations of NEXAFS features arising from the associated differences in electronic structure and bonding.

Probing ion-specific effects on aqueous acetate solutions: Ion pairing versus water structure modifications

The effect of monovalent cations (Li(+), K(+), NH4 (+), Na(+)) on the water structure in aqueous chloride and acetate solutions was characterized by oxygen K-edge X-ray absorption spectroscopy (XAS), X-ray emission spectroscopy, and resonant inelastic X-ray scattering (RIXS) of a liquid microjet. We show ion- and counterion dependent effects on the emission spectra of the oxygen K-edge, which we attribute to modifications of the hydrogen bond network of water. For acetates, ion pairing with carboxylates was also probed selectively by XAS and RIXS. We correlate our experimental results to speciation data and to the salting-out properties of the cations.