Microscopic Study of Ionic Liquid−H2O Systems: Alkyl-Group Dependence of 1-Alkyl-3-Methylimidazolium Cation

Water binding and hygroscopicity in π-conjugated polyelectrolytes

The presence of water strongly influences structure, dynamics and properties of ion-containing soft matter. Yet, the hydration of such matter is not well understood. Here, we show through a large study of monovalent π-conjugated polyelectrolytes that their reversible hydration, up to several water molecules per ion pair, occurs chiefly at the interface between the ion clusters and the hydrophobic matrix without disrupting ion packing. This establishes the appropriate model to be surface hydration, not the often-assumed internal hydration of the ion clusters. Through detailed analysis of desorption energies and O-H vibrational frequencies, together with OPLS4 and DFT calculations, we have elucidated key binding motifs of the sorbed water. Type-I water, which desorbs below 50 °C, corresponds to hydrogen-bonded water clusters constituting secondary hydration. Type-II water, which typically desorbs over 50-150 °C, corresponds to water bound to the anion under the influence of a proximal cation, or to a cation‒anion pair, at the cluster surface. This constitutes primary hydration. Type-III water, which irreversibly desorbs beyond 150 °C, corresponds to water kinetically trapped between ions. Its amount varies strongly with processing and heat treatment. As a consequence, hygroscopicity-which is the water sorption capacity per ion pair-depends not only on the ions, but also their cluster morphology.

Anomalous Dependence of Translational Diffusion on the Water Mole Fraction for Solute Molecules Dissolved in a 1-Butyl-3-methylimidazolium Tetrafluoroborate/Water Mixture

Translational diffusion coefficients of carbon monoxide (CO), diphenylacetylene (DPA), and diphenylcyclopropenone (DPCP) were determined in mixtures of 1-butyl-3-methylimidazolium tetrafluoroborate ([C₄mim]BF₄) and water using transient grating spectroscopy at different mole fractions of water (x_w). While DPA exhibited a larger diffusion coefficient than DPCP at low water mole fractions (x_w < 0.7), as observed for conventional liquids and ionic liquids (ILs), it was smaller at high mole fractions (x_w > 0.9). The apparent molecular radius of DPA determined using the Stokes-Einstein equation at x_w > 0.9 is close to the radius of an IL cluster in a water pool as determined from small-angle neutron scattering experiments (J. Bowers et al., Langmuir, 2004, 20, 2192-2198), suggesting that the DPA molecules are trapped in IL clusters in the water pool and move together. The solvation state of DPCP in the mixture was studied using Raman spectroscopy. Dramatically strong water/DPCP hydrogen bonding was observed at higher water mole fractions, suggesting that DPCP is located near the cluster interfaces. The large diffusion coefficient of DPCP suggests that hopping of DPCP between IL clusters occurs through hydrogen bonding with water.

Exploring the Microscopic Aspects of 1-Methyl-3-octylimidazolium Tetrafluoroborate Mixtures with Formamide, N-Methylformamide, and N,N-Dimethylformamide by Multiple Spectroscopic Techniques and Computations

The microscopic aspects of 1-methyl-3-octylimidazolium tetrafluoroborate ([MOIm][BF₄]) mixtures with formamide (FA), N-methylformamide (NMF), and N,N-dimethylformamide (DMF) were investigated using spectroscopic techniques of femtosecond Raman-induced Kerr effect spectroscopy (fs-RIKES), FT-IR, and NMR. Molecular dynamics simulations and quantum chemistry calculations were also performed. According to fs-RIKES, the first moment of the low-frequency spectrum bands mainly originating from the intermolecular vibrations in the [MOIm][BF₄]/FA and [MOIm][BF₄]/DMF systems changed gradually with the molecular liquid mole fraction X_ML but that in the [MOIm][BF₄]/NMF system was constant up to X_NMF = 0.7 and then gradually increased in the range of X_NMF ≥ 0.7. Excluding the contribution of the 2D hydrogen-bonding network due to the presence of FA in the low-frequency spectrum band, the X_ML dependence of the normalized first moment of the low-frequency band in the [MOIm][BF₄]/FA and [MOIm][BF₄]/NMF systems revealed that the normalized first moment did not remarkably change in the range of X_ML < 0.7 but drastically increased in X_ML ≥ 0.7. FT-IR results indicated that the amide C═O band shifted to the low-frequency side with increasing X_ML for the three mixtures due to the hydrogen bonds. The imidazolium ring C-H band also showed a similar tendency to the amide C═O band. ¹⁹F NMR probed the microenvironment of [BF₄]^- in the mixtures. The [MOIm][BF₄]/NMF and [MOIm][BF₄]/DMF systems showed an up-field shift of the F atoms of the anion with increasing X_ML, and the [MOIm][BF₄]/FA system exhibited a down-field shift. Steep changes in the chemical shifts were confirmed in the region of X_ML > 0.8. On the basis of the quantum chemistry calculations, the observed chemical shifts with increasing X_ML were mainly attributed to the many-body interactions of ions and amides for the [MOIm][BF₄]/FA and [MOIm][BF₄]/DMF systems. Meanwhile, the long distance between the cation and the anion was due to the high dielectric medium for the [MOIm][BF₄]/NMF system, which led to an up-field shift.

Synthesis of 4-Amino-N-[2 (diethylamino)Ethyl]Benzamide Tetraphenylborate Ion-Associate Complex: Characterization, Antibacterial and Computational Study

The 4-amino-N-[2 (diethylamino) ethyl] benzamide (procainamide)-tetraphenylborate complex was synthesized by reacting sodium tetraphenyl borate with 4-amino-N-[2 (diethylamino) ethyl] benzamide, chloride salt, and procainamide in deionized water at room temperature through an ion-associate reaction (green chemistry) at room temperature, and characterized by several physicochemical methods. The formation of ion-associate complex between bio-active molecules and/or organic molecules is crucial to comprehending the relationships between bioactive molecules and receptor interactions. The solid complex was characterized by infrared spectra, NMR, elemental analysis, and mass spectrometry, indicating the formation of ion-associate or ion-pair complex. The complex under study was examined for antibacterial activity. The ground state electronic characteristics of the S1 and S2 complex configurations were computed using the density functional theory (DFT) approach, using B3LYP level 6-311 G(d,p) basis sets. R² = 0.9765 and 0.9556, respectively, indicate a strong correlation between the observed and theoretical ¹H-NMR, and the relative error of vibrational frequencies for both configurations was acceptable, as well. HOMO and LUMO frontier molecular orbitals and molecular electrostatics using the optimized were used to obtain a potential map of the chemical. The n → π* UV absorption peak of the UV cutoff edge was detected for both configurations of the complex. Spectroscopic methods were structures used to characterize the structure (FT-IR and 1HNMR). In the ground state, DFT/B3LYP/6-311G(d,p) basis sets were used to determine the electrical and geometric properties of the S1 and S2 configurations of the title complex. Comparing the observed and calculated values for the S1 and S2 forms, the HOMO-LUMO energy gap of compounds was 3182 and 3231 eV, respectively. The small energy gap between HOMO and LUMO indicated that the compound was stable. In addition, the MEP reveals that positive potential sites were around the PR molecule, whereas negative potential sites were surrounding the TPB site of atoms. The UV absorption of both arrangements is comparable to the experimental UV spectrum.

Interfacial Water and Microheterogeneity in Aqueous Solutions of Ionic Liquids

In this work, aqueous solutions of two prototypical ionic liquids (ILs), [BMIM][BF4] and [BMIM][TfO], were investigated by UV Raman spectroscopy and small-angle neutron scattering (SANS) in the water-rich domain, where strong heterogeneities at mesoscopic length scales (microheterogeneity) were expected. Analyzing Raman data by a differential method, the solute-correlated (SC) spectrum was extracted from the OH stretching profiles, emphasizing specific hydration features of the anions. SC-UV Raman spectra pointed out the molecular structuring of the interfacial water in these microheterogeneous IL/water mixtures, in which IL aggregates coexist with bulk water domains. The organization of the interfacial water differs for the [BMIM][BF4] and [BMIM][TfO] solutions, being affected by specific anion-water interactions. In particular, in the case of [BMIM][BF4], which forms weaker H-bonds with water, the aggregation properties clearly depend on concentration, as reflected by local changes in the interfacial water. On the other hand, stronger water-anion hydrogen bonds and more persistent hydration layers were observed for [BMIM][TfO], which likely prevent changes in IL aggregates. The modeling of SANS profiles, extended to [BPy][BF4] and [BPy][TfO], evidences the occurrence of significant concentration fluctuations for all of the systems: this appears as a rather general phenomenon that can be ascribed to the presence of IL aggregation, mainly induced by (cation-driven) hydrophobic interactions. Nevertheless, larger concentration fluctuations were observed for [BMIM][BF4], suggesting that anion-water interactions are relevant in modulating the microheterogeneity of the mixture.

A New Compartmentalized Scale (PN) for Measuring Polarity Applied to Novel Ether-Functionalized Amino Acid Ionic Liquids

Functionalized and environmentally friendly ionic liquids are required in many fields, but convenient methods for measuring their polarity are lacking. Two novel ether-functionalized amino acid ionic liquids, 1-(2-methoxyethyl)-3-methylimidazolium alanine ([C₁OC₂mim][Ala]) and 1-(2-ethoxyethyl)-3-methylimidazolium alanine ([C₂OC₂mim][Ala]), were synthesized by a neutralization method and their structures confirmed by NMR spectroscopy. Density, surface tension, and refractive index were determined using the standard addition method. The strength of intermolecular interactions within these ionic liquids was examined in terms of standard entropy, lattice energy, and association enthalpy. A new polarity scale, P_N, is now proposed, which divides polarity into two compartments: the surface and the body of the liquid. Surface tension is predicted via an improved Lorentz-Lorenz equation, and molar surface entropy is used to determine the polarity of the surface. This new P_N scale is based on easily measured physicochemical parameters, is validated against alternative polarity scales, and is applicable to both ionic and molecular liquids.

Influence of water on the microscopic dynamics of 1-butyl-3-methylimidazolium tetrafluoroborate studied by means of quasielastic neutron scattering

We present a systematic study on the effect of water on the microscopic dynamics of 1-butyl-3-methylimidazolium tetrafluoroborate by means of quasielastic neutron scattering. By mixing the ionic liquid with either heavy or light water, the different contributions to the quasielastic broadening could be identified and treated separately. This study was performed at room temperature, which is more than 15 °C above the demixing line. Our results show that even small amounts of water accelerate the diffusion mechanisms considerably. While samples with small water percentage reveal a diffusion process confined within ionic liquid nanodomains, an admixture of more than 15 wt. % water relieves the confinement. Furthermore, the presence of two water species was identified: one behaving as free water, whereas the other was interpreted as a component bound to the ionic liquid motion. Based on the fact that water preferentially binds to the BF4 anion, which itself has a negligible contribution to the scattered intensity, our experiments reveal unprecedented information about the microscopic anion dynamics.

Anion Effects on the Mixing States of 1-Methyl-3-octylimidazolium Tetrafluoroborate and Bis(trifluoromethylsulfonyl)amide with Methanol, Acetonitrile, and Dimethyl Sulfoxide on the Meso- and Microscopic Scales

The mixing states of two imidazolium-based ionic liquids (ILs) with different anions, 1-methyl-3-octylimidazolium tetrafluoroborate (C₈mimBF₄) and bis(trifluoromethylsulfonyl)amide (C₈mimTFSA), with three molecular liquids (MLs), methanol (MeOH), acetonitrile (AN), and dimethyl sulfoxide (DMSO), have been investigated on both mesoscopic and microscopic scales using small-angle neutron scattering (SANS), infrared (IR), and ¹H and ¹³C nuclear magnetic resonance (NMR) spectroscopy. Additionally, molecular dynamics (MD) simulations have been conducted on the six combinations of ILs and MLs to observe the states of their mixtures on the atomic level. The SANS profiles of the IL-ML mixtures suggested that MeOH molecules only form clusters in both C₈mimBF₄ and C₈mimTFSA, whereas AN and DMSO were homogeneously mixed with ILs on the SANS scale. MeOH clusters are more enhanced in BF₄^--IL than TFSA^--IL. The microscopic interactions among IL cations, anions, and MLs should contribute to the mesoscopic mixing states of the IL-ML mixtures. In fact, the IL cation-anion, cation-ML, anion-ML, and ML-ML interactions observed by IR, NMR, and MD simulations clarified the reasons for the mixing states of the IL-ML binary solutions observed by the SANS experiments. In neat ILs, the imidazolium ring of the IL cation more strongly interacts with BF₄^- than TFSA^- due to the higher charge density of the former. The interaction of anions with the imidazolium ring is more easily loosened on adding MLs to ILs in the order of DMSO > MeOH > AN. It does not significantly depend on the anions. However, the replacement of the anion on the imidazolium ring by an ML depends on the anions; the replacement is more proceeded in the order of MeOH > DMSO > AN in BF₄^--IL, while DMSO > MeOH > AN in TFSA^--IL. On the other hand, the solvation of both anions by MLs is stronger in the order of MeOH > DMSO ≈ AN. Despite the stronger interactions of MeOH with both cations and anions, MeOH molecules are heterogeneously mixed with both ILs to form clusters in the mixtures. Therefore, the self-hydrogen bonding among MeOH molecules most markedly governs the mixing state of the binary solutions among the abovementioned interactions.

Concerns and breakthroughs of combining ionic liquids with microwave irradiation for the synthesis of Ru nanoparticles via decarbonylation

HYPOTHESIS

Combination of microwave irradiation (MWI) and ionic liquids (IL) is widely used for the synthesis of nanoparticles (NP) via decarbonylation of zero-valent metal carbonyl precursors. However, we carefully raise a question as to whether this combination is always beneficial. Upon MWI, highly-absorbing materials such as ILs would be subject to local intense heating, likely resulting in the occurrence of localized chemical decomposition. The decomposition is expected to influence the growth mechanism of NPs due to changes in the electrostatic and steric effects. If the assumption is valid, it should be possible to decompose IL and destabilize the NPs by modifying the amplitude of the incident microwaves. In other words, it should also be possible to control the particle aggregation by circumventing the decomposition of the IL.

EXPERIMENTS

A series of comparative studies were conducted using a model system (i.e. [BMIm][BF₄] and Ru₃(CO)₁₂). Variables were systematically controlled. After MWI, the decrease in colloidal stability of NPs was identified.

FINDINGS

In the formation of Ru NPs via decarbonylation, the association between incident microwave intensity, chemical decomposition of IL, and initiation of particle aggregation has been demonstrated. Conditions that can accelerate or alleviate the decomposition and the aggregation are also corroborated.

Structure, Molecular Interactions, and Dynamics of Aqueous [BMIM][BF4] Mixtures: A Molecular Dynamics Study

Molecular dynamics simulations with many-body polarizable force fields were carried out to investigate the thermodynamic, structural, and dynamic properties of aqueous solutions of 1-butyl-3-methylimidazolium tetrafluoroborate ([bmim][BF₄]). The radial distribution functions exhibit well-defined features, revealing favored structural correlations between [bmim]⁺, [BF₄]^-, and H₂O. The addition of water is shown to alter ionic liquid structural organizations by replacing counterions in the coordination shells and disrupt the cation-anion network. At low water concentration, the majority of water molecules are isolated from each other and have lower average dipole moment than that in pure water. With increasing hydration level, while [bmim][BF₄] ionic network breaks up and becomes isolated ion pairs or free ions in the dilute limit, water begins to form clusters of increasing sizes and eventually forms a percolating network. As a result, the average water dipole moment increases and approaches its bulk value. Water is also observed to have a substantial influence on the dynamics of ionic liquids. At low water content, the cation and anion have similar diffusion coefficients due to the correlated ionic motion of long-lived ion pairs. As the water concentration increases, both ions exhibit greater mobility and faster rotations from the breakup of ionic network. Consequently, the ionic conductivity of [bmim][BF₄] aqueous solutions rises with increasing water composition.

Current Status of AMOEBA-IL: A Multipolar/Polarizable Force Field for Ionic Liquids

Computational simulations of ionic liquid solutions have become a useful tool to investigate various physical, chemical and catalytic properties of systems involving these solvents. Classical molecular dynamics and hybrid quantum mechanical/molecular mechanical (QM/MM) calculations of IL systems have provided significant insights at the atomic level. Here, we present a review of the development and application of the multipolar and polarizable force field AMOEBA for ionic liquid systems, termed AMOEBA–IL. The parametrization approach for AMOEBA–IL relies on the reproduction of total quantum mechanical (QM) intermolecular interaction energies and QM energy decomposition analysis. This approach has been used to develop parameters for imidazolium– and pyrrolidinium–based ILs coupled with various inorganic anions. AMOEBA–IL has been used to investigate and predict the properties of a variety of systems including neat ILs and IL mixtures, water exchange reactions on lanthanide ions in IL mixtures, IL–based liquid–liquid extraction, and effects of ILs on an aniline protection reaction.

Spectroscopic and computational insights into the ion-solvent interactions in hydrated aprotic and protic ionic liquids

Ionic liquids (ILs) and their aqueous solutions are emerging media for solving and manipulating biochemical molecules such as proteins. Unleashing the full potential however requires a detailed mechanistic understanding of how suitable protic and aprotic ILs behave in the presence of water in the first place. The present work aims at making an important step by performing a combined experimental and computational study of two selected ILs and their mixtures with water: the aprotic cholinium propionate ([Chl][Pro]) and the protic N-methyl-2-pyrrolidonium propionate ([NMP][Pro]). IR and Raman spectroscopy reveal stronger ion-solvent interactions in [Chl][Pro]-H₂O systems compared to [NMP][Pro]-H₂O mixtures. This can be explained by the tightly packed ion-pair associations in [NMP][Pro] comprising the protic -N⁺-H counterpart, which allows the establishment of highly directional and strong interionic hydrogen bonds. The spectral decomposition of the O-D stretching band into three sub-peaks showed that the protic [NMP][Pro] favors the self-association of water molecules. On the other hand, the predominant fraction of water-anion/cation aggregates exists in aprotic [Chl][Pro]. These hydrated systems can be envisaged using quantum-chemical calculations in the following way: H₂O[Chl]⁺H₂O[Pro]^-H₂O and H₂O[NMP]⁺[Pro]^-H₂O, which implied preferable solvent-shared ion-pair (SIP) configurations for [Chl][Pro]-H₂O systems, whereas the contact ion-pair (CIP) state prevails for the [NMP][Pro]-H₂O systems. The latter holds even in the water-rich regime. In future work, these findings will be the basis for an understanding of the underlying principles that govern the interactions of ions with bio-molecules in aqueous solutions.

Coordination of Nd(iii) and Eu(iii) with monodentate organophosphorus ligands in ionic liquids: spectroscopy and thermodynamics

The thermodynamics and coordination nature of the complexes of Ln(iii) with three monodentate organophosphorus ligands in ionic liquids have been elucidated and illustrated by spectroscopic and calorimetric techniques.

The role of viscosity in various dynamical processes of different fluorophores in ionic liquid–cosolvent mixtures: a femtosecond fluorescence upconversion study

Literature reports provide ample evidence of the dynamical studies of various fluorophores in different room-temperature ionic liquid (RTIL)–cosolvent mixtures.

Excited-state prototropism of 7-hydroxy-4-methylcoumarin in [Cnmim][BF4] series of ionic liquid–water mixtures: insights on reverse micelle-like water nanocluster formation

This study explores the excited-state prototropic behavior of 7-hydroxy-4-methylcoumarin dye in ionic liquid–water media, to reveal the intriguing reverse micelle formation in these solvent systems.

Initial dissolution of D2O at the gas-liquid interface of the ionic liquid [C4min][NTf2] associated with hydrogen-bond network formation

We have studied the initial dissolution of D₂O at the interfacial surface of the flowing jet sheet beam of the ionic liquid (IL) [C₄min][NTf2] using the King and Wells method as a function of both the temperature and collision energy of the IL. The initial dissolution probability of D₂O into the IL [C₄min][NTf2] was found to follow the general propensity that the solubility of gases into a liquid decreases with temperature. However, a large partial molar enthalpy and entropy for the initial dissolution of D₂O in the IL [C₄min][NTf2] were observed from the temperature dependence of the initial dissolution probability: ΔH_l = -53 ± 8 kJ mol^-1, ΔS_l = -210 ± 30 J mol^-1 K^-1. In addition, it was found that the collision energy significantly reduced the initial dissolution probability. We propose that the associated D₂O molecules at the interface of the IL [C₄min][NTf2] make a hydrogen-bond network around the [NTf2]^- anion before dissolution into the deeper portion of the interface layer.

Water Nanocluster Formation in the Ionic Liquid 1-Butyl-3-methylimidazolium Tetrafluoroborate ([C4mim][BF4])-D2O Mixtures

The microstructure of mixtures of deuterated water in the ionic liquid [C4mim][BF4] is investigated by small-angle neutron scattering (SANS) measurement. In the salt-rich region, water dissolves in the ionic liquid up to 0.7 mole fraction, whereupon distinct, nanometer-sized water clusters are observed. These water nanoclusters increase in size with increasing water addition while the mole ratio of water dissolved into the ionic liquid nanostructure increases from 2 to 4. These results provide direct confirmation for recent simulations as well insight into the source of nonidealities in some thermophysical and transport properties (e.g., density and viscosity) of salt-rich aqueous mixtures reported in the literature.

Investigation of the influence of alkyl side chain length on the fluorescence response of C153 in a series of room temperature ionic liquids

Variation of average solvation time with the product of temperature averaged viscosity and the radius of the cation of different room temperature ionic liquids (RTILs) with varying cationic chain length.

Measurement of vaporization enthalpy by isothermogravimetrical method and prediction of the polarity for 1-alkyl-3-methylimidazolium acetate {[Cnmim][OAc] (n = 4, 6)} ionic liquids

The ΔglH°m for [C_nmim][OAc] (n = 4, 6) were measured using isothermogravimetrical approach and explored by molecular dynamics simulations. The δ_μ can be estimated easily, and good agreement with experiences.

Molecular dynamics investigation of the vibrational spectroscopy of isolated water in an ionic liquid

Experimental studies examining the structure and dynamics of water in ionic liquids (ILs) have revealed local ion rearrangements that occur an order of magnitude faster than complete randomization of the liquid structure. Simulations of an isolated water molecule embedded in 1-butyl-3-methyl imidazolium hexafluorophosphate, [bmim][PF6], were performed to shed insight into the nature of these coupled water-ion dynamics. The theoretical calculations of the spectral diffusion dynamics and the infrared absorption spectra of the OD stretch of isolated HOD in [bmim][PF6] agree well with experiment. The infrared absorption line shape of the OD stretch is narrower and blue-shifted in the IL compared to those in aqueous solution. Decomposition of the OD frequency time correlation function revealed that translational motions of the anions dominate the spectral diffusion dynamics.

Ionic liquid-induced formation of the α-helical structure of β-lactoglobulin

Structural modification of bovine milk β-lactoglobulin (β-LG) in aqueous 1-butyl-3-methylimidazolium nitrate ([bmim][NO3]) and ethylammonium nitrate ([EAN][NO3]) solutions has been investigated by Fourier transform infrared and circular dichroism spectroscopy. Remarkably, high ionic liquid (IL) concentrations (>15 mol %IL) caused formation of a non-native α-helical structure of β-LG and disruption of its tertiary structure. Furthermore, while [bmim][NO3] promoted protein aggregation, [EAN][NO3] inhibited it probably owing to differences in the unique solution structure (nanoheterogeneity) of the ILs by the different cationic species. The IL-induced α-helical formation of β-LG shows a behavior similar to the alcohol denaturation, but a disordered structure-rich state was observed in the β-α transition process by adding IL, in contrast to the case of an aqueous alcohol solution of protein. We propose that the molten salt-like property of aqueous IL solutions strongly support α-helical formation of proteins.

Theoretical studies of the 1-ethyl-3-methylimidazolium glycine, [EMIM][Gly], ionic liquid – water mixture — I. Prediction of the solvation and structural properties

Gaussian-based HF/MP2 and DFT/B3LYP methods have been explored to study the microsolvation of glycine anion with water, [Gly]–(W)n, and 1-ethyl-3-methylimidazolium glycine ionic liquid (IL) with water, [EMIM][Gly](W)n, n = 1–6 and 12. The water molecules are either isolated or aggregated around [Gly]– and [EMIM][Gly]. Their electronic structures have been calculated clearly to verify the molecular state of H2O and the results were used to compare with experiments. The water effect on the interaction energies and local packing of [EMIM][Gly] is considered. We identified an important factor: the variation of [EMIM][Gly](W)n polarity with the different numbers of H2O. As the amount of H2O increases, the polar network is continuously broken up. The dipole moments are changed to be lowest when the H-bonding ability of [Gly]– is almost saturated. Meanwhile, the nonpolar groups of the cation form an enhanced aggregation. Such observation can provide initial insights for the experimental nanostructural evolution in the IL–water mixtures.