Solvation Sphere Research Articles

Solvation structure and thermodynamics for Ln(III), (Ln = Pr, Nd, Tb, and Dy) complexes in phosphonium-based ionic liquids evaluated by Raman spectroscopy and DFT calculation

The coordination states of trivalent praseodymium, neodymium, terbium, and dysprosium complexes in triethyl-n-octylphosphonium bis(trifluoromethylsulfonyl)amide ([P2228][TFSA]) ionic liquid were examined using Raman spectroscopy. In [P2228][TFSA], the concentration dependence of lanthanide (Ln) ions on the Raman spectrum was investigated in the range of 0.24–0.48 mol kg−1 of Ln(III), (Ln = Pr, Nd, Tb, and Dy) in [P2228][TFSA]. Based on classical analysis, solvation numbers n of Ln(III) (Ln = Pr, Nd, Tb, and Dy) in [P2228][TFSA] were determined as 4.80 ± 0.04, 5.08 ± 0.03, 5.15 ± 0.04, and 4.92 ± 0.03, respectively, at 373 K.Based on the temperature dependence of the Raman spectrum measured at temperatures between 298 and 398 K, thermodynamic parameters with ΔisoG, ΔisoH, and ΔisoS for the isomerism of [TFSA]− from the trans- to the cis-coordinated isomer in the bulk and first solvation sphere of the centered Ln3+ (Ln = Pr, Nd, Tb, and Dy) cation in [P2228][TFSA] were evaluated in this study. As indicated by the positive value of ΔisoH(bulk) = 6.86 kJ mol−1, the trans-[TFSA]− was found to be the dominant contributor to enthalpy. In [P2228][TFSA], the value of TΔisoS(bulk) = 7.92 kJ mol−1 was slightly larger than that of ΔisoH(bulk), indicating that cis-[TFSA]− was entropy controlled. Conversely, in the first solvation sphere of the Ln3+ cation, ΔisoH(Ln) became significantly negative, indicating stabilization of cis-[TFSA]− isomers via enthalpic contributions. This result clearly indicates that the cis-[TFSA]− conformer bound to Ln3+ cation is the preferred coordination state of [Ln(III)(cis–TFSA)5]2− in [P2228][TFSA], because ΔisoH(Ln) contributes to a significant decrease in ΔisoG(Ln).Furthermore, DFT calculations using the Amsterdam Density Functional package were used to investigate the optimized geometries and binding energies of [Ln(III)(cis-TFSA)5]2−, (Ln = Pr, Nd, Tb, and Dy) clusters. The bonding energy, ΔEb, was calculated as ΔEb = Etot(cluster) − Etot(Ln3+) − nEtot([TFSA]−), and ΔEb ([Ln(III)(cis-TFSA)5]2−), (Ln = Pr, Nd, Tb, and Dy) were calculated as −4349.2 ± 7.2, −4354.6 ± 7.6, −4278.6 ± 7.2, and −4296.4 ± 7.8 kJ mol−1, respectively. The thermodynamic properties were consistent with the average atomic charges and bond distances of these clusters. Additionally, the highest occupied molecular orbital and lowest unoccupied molecular orbital estimated from DFT methods were consistent with the coordination states of [Ln(III)(cis-TFSA)5]2−, (Ln = Pr, Nd, Tb, and Dy) analyzed by Raman spectroscopy.

A Variational Model of Charged Drops in Dielectrically Matched Binary Fluids: The Effect of Charge Discreteness

This paper addresses the ill-posedness of the classical Rayleigh variational model of conducting charged liquid drops by incorporating the discreteness of the elementary charges. Introducing the model that describes two immiscible fluids with the same dielectric constant, with a drop of one fluid containing a fixed number of elementary charges together with their solvation spheres, we interpret the equilibrium shape of the drop as a global minimizer of the sum of its surface energy and the electrostatic repulsive energy between the charges under fixed drop volume. For all model parameters, we establish the existence of generalized minimizers that consist of at most a finite number of components “at infinity”. We also give several existence and non-existence results for classical minimizers consisting of only a single component. In particular, we identify an asymptotically sharp threshold for the number of charges to yield existence of minimizers in a regime corresponding to macroscopically large drops containing a large number of charges. The obtained non-trivial threshold is significantly below the corresponding threshold for the Rayleigh model, consistently with the ill-posedness of the latter and demonstrating a particular regularizing effect of the charge discreteness. However, when a minimizer does exist in this regime, it approaches a ball with the charge uniformly distributed on the surface as the number of charges goes to infinity, just as in the Rayleigh model. Finally, we provide an explicit solution for the problem with two charges and a macroscopically large drop.

Druglike Molecules Binding to Large Membrane Proteins: Absolute Binding Free Energy Computation.

In this research, we employed the alchemical double-decoupling method alongside restraining potentials, coupled with the FEPMD method, to ascertain the standard binding free energy of a drug-like molecule termed BHQ and three analogous compounds engineered with progressive addition of bulky para-alkyl groups binding to SERCA (Ca2+-ATPase of skeletal muscle sarcoplasmic reticulum). Integral transmembrane proteins represent crucial drug targets in numerous therapeutic interventions, presenting computational challenges due to their considerable system sizes. Our approach integrated the generalized born potential method and the spherical solvent boundary potential method, allowing us to explicitly focus on the active binding site while treating the remainder of the system implicitly. We evaluated contributions to the standard binding free energy from distinct interaction potentials: electrostatic, repulsive, dispersive, and restraining potentials, computed separately. The resulting absolute binding free energy of BHQ (11.63 kcal/mol) closely aligns with experimental measurements (10.56 kcal/mol). Notably, an accurate estimation of the absolute binding free energy was achieved for the simplest analog, created with the addition of a single para-methyl group. However, the analog with two para-methyl groups exhibited the highest binding free energy, which disagreed with experimental results. Determining the binding free energy of the BHQ analog engineered with three para-methyl groups presented challenges in convergence and resulted in the lowest free energy among the three.

Microsolvation of cobalt, nickel, and copper atoms with ammonia: a theoretical study of the solvated electron precursors.

The s-block metals dissolved in ammonia form metal-ammonia complexes with diffuse electrons which could be used for redox catalysis. In this theoretical paper, we investigated the possibility of the d-bloc transition metals (Mn, Fe, Co, Ni, and Cu) solvated by ammonia. It has been demonstrated that both Mn and Fe atoms undergo into an oxidative reaction with NH3 forming an inserted species, HMNH2. On the contrary, the Co, Ni, and Cu atoms can accommodate four NH3, via the coordination bond, to form the first solvation sphere within C2v, D2d, and Td point groups, respectively. Addition of a fifth NH3 constitute the second solvation shell by forming hydrogen bond with the other NH3s. Interestingly, M(NH3)4 (M = Co, Ni, and Cu) is a so-called solvated electron precursor and should be considered as a monocation M(NH3)4+ kernel in tight contact with one electron distributed over its periphery. This nearly free electron could be used to capture a CO2 molecule and engages in a reduction reaction. Geometry optimization of the stationary points on the potential energy surface was performed using density functional theory - CAM-B3LYP functional including the GD3BJ dispersion contribution - in combination with the 6-311 + + G(2d, 2p) basis set for all the atoms. All first-principles calculations were performed using the Gaussian 09 quantum chemical packages. The natural electron configuration of transition atom engaged in the compounds has been found using the natural bond orbital (NBO) method. We used the EDR (electron delocalization range) approach to analyze the structure of solvated electrons in real space. We also used the electron localization function (ELF) to measure the degree of electronic localization within a chemical compound. The EDR and ELF analyses are done using the TopMod and Multiwfn packages, respectively.

Coordination of dissolved transition metals in pristine battery electrolyte solutions determined by NMR and EPR spectroscopy.

The solvation of dissolved transition metal ions in lithium-ion battery electrolytes is not well-characterised experimentally, although it is important for battery degradation mechanisms governed by metal dissolution, deposition, and reactivity in solution. This work identifies the coordinating species in the Mn2+ and Ni2+ solvation spheres in LiPF6/LiTFSI-carbonate electrolyte solutions by examining the electron-nuclear spin interactions, which are probed by pulsed EPR and paramagnetic NMR spectroscopy. These techniques investigate solvation in frozen electrolytes and in the liquid state at ambient temperature, respectively, also probing the bound states and dynamics of the complexes involving the ions. Mn2+ and Ni2+ are shown to primarily coordinate to ethylene carbonate (EC) in the first coordination sphere, while PF6- is found primarily in the second coordination sphere, although a degree of contact ion pairing does appear to occur, particularly in electrolytes with low EC concentrations. NMR results suggest that Mn2+ coordinates more strongly to PF6- than to TFSI-, while the opposite is true for Ni2+. This work provides a framework to experimentally determine the coordination spheres of paramagnetic metals in battery electrolyte solutions.

On the formation of inner-sphere solvation complexes in dilute acetonitrile solutions of the Hg(NO3)2 and Hg(TfO)2 salts: a Molecular Dynamics study

The solvation properties of dilute acetonitrile solutions of the Hg(NO3)2 and Hg(TfO)2 salts have been investigated by using Molecular Dynamics simulations. The Hg2+ ion in acetonitrile was found to coordinate both counterions and acetonitrile molecules in its first solvation sphere by forming inner-sphere complexes. However, different geometries of the Hg2+ solvation polyhedra are formed depending on the counterion: in the Hg(NO3)2 salt solution, Hg2+ forms a 8-fold cluster of acetonitrile and nitrate ligands with a square antiprism symmetry, while in the system containing the Hg(TfO)2 salts, the Hg2+ solvation shell is composed of eight ligands arranged in a C2 symmetry.

On The Nature of FeIV =Oaq in Aqueous Media: A DFT analysis.

FeIV =Oaq is a key intermediate in many advanced oxidation processes and probably in biological systems. It is usually referred to as FeIV =O2+ . The pKa's of FeIV =Oaq as derived by DFT are: pKa1=2.37 M06 L/6-311++G(d,p) (SDD for Fe) and pKa2=7.79 M06 L/6-311++G(d,p) (SDD for Fe). This means that in neutral solutions, FeIV =Oaq is a mixture of (H2 O)4 (OH)FeIV =O+ and (H2 O)2 (OH)2 FeIV =O. The oxidation potential of FeIV =Oaq in an acidic solution, E0 {(H2 O)5 FeIV =O2+ /FeIII (H2 O)6 3+ , pH 0.0} is calculated with and without a second solvation sphere and the recommended value is between 2.86 V (B3LYP/Def2-TZVP, with a second solvation sphere) and 2.23 V (M06 L/Def2-TZVP without a second solvation sphere). This means that FeIV =Oaq is the strongest oxidizing agent formed in systems involving FeVI O4 2- even in neutral media.

Unraveling the Ag+ ion coordination and solvation thermodynamics in the 1-butyl-3-methylimidazolium tetrafluoroborate ionic liquid

The solvation of the Ag+ ion in the 1-butyl-3-methylimidazolium tetrafluoroborate ([C4mim][BF4]) ionic liquid (IL) has been studied by means of experimental and theoretical methods with the aim of elucidating the cation coordination structure and thermodynamic properties. Car-Parrinello molecular dynamics (CPMD) simulations showed that the Ag+ ion is coordinated by an average number of four [BF4]− anions in a pseudo-tetrahedral geometry. A high configurational disorder of the first solvation sphere is found, where the anions can be found both in mono- and bidentate coordination mode around the Ag+ ion. Also, a solvational equilibrium is observed as due to [BF4]− anion dissociation along the trajectory. The analysis of X-ray absorption spectroscopy data confirmed the picture provided by the CPMD simulation. Classical molecular dynamics simulations were carried out to obtain the single-ion solvation thermodynamic parameters. The negative water → IL free energy of transfer suggests that the Ag+ ion is more favorably solvated in the [C4mim][BF4] IL than in water. This behavior is due to a balance between the enthalpic and entropic contributions, which allows to find a rationale to the strong solvation capabilities of BF4-based ILs towards Ag+.

Solvation–Reduction Coupling in Ca2+ Electroactivity in Glyme‐Based Electrolytes: A First Principles Study

AbstractThe electrochemical activity of solvated Ca2+ in glyme‐based electrolytes is investigated using grand canonical density functional theory approach and Fukui functions. The obtained results reveal that the length of glyme molecules has little effect on the reduction potentials, but has significant impacts on the effective electron transfer process. In short chain glymes, the transferred electron is located on a Ca2+ center and the organic part of the solvation sphere, leading to a direct Ca2+ reduction and a partial degradation of the glyme molecules. As the glyme's length increases, the reduction process turns into the formation of solvated electrons rather than Ca2+ reduction, unless a partial desolvation occurs. Consequently, an effective Ca2+ reduction in long chain glyme‐based electrolytes is controlled by a (partial) desolvation of the solvation sphere. These results can be used as guiding information to design new electrolytes having the Ca2+ reduction potential in an accessible voltage range together with an effective Ca2+ reduction process. The methodology developed in this study can be universally applied to investigate the thermodynamic and kinetic properties of other battery systems using metal anodes, which might lead to a paradigm shift in the design of prospective electrolytes for future battery technologies.

Composition of the Solvation Shell of the Selected Cyclic Ethers (1,4-Dioxane, 12-Crown-4, 15-Crown-5 and 18-Crown-6) in a Mixture of Formamide with Water at Four Temperatures

The solution enthalpy of 15-crown-5 and 18-crown-6 ethers in the mixture of formamide (F) and water (W) was measured at four temperatures: 293.15 K, 298.15 K, 303.15 K, 308.15 K. The standard molar enthalpy of solution, ΔsolHo, depends on the size of cyclic ethers molecules and the temperature. With increasing temperature, the values of ΔsolHo become less negative. The values of the standard partial molar heat capacity Cp,2o of cyclic ethers at 298.15 K have been calculated. The Cp,2o=f(xW) curve shape indicates the hydrophobic hydration process of cyclic ethers in the range of a high-water content in the mixture with formamide. The enthalpic effect of preferential solvation of cyclic ethers was calculated and the effect of temperature on the preferential solvation process was discussed. The process of complex formation between 18C6 molecules and formamide molecules is observed. The cyclic ethers molecules are preferentially solvated by formamide molecules. The mole fraction of formamide in the solvation sphere of cyclic ethers has been calculated.

Conceptual and Practical Aspects of Metal-Organic Frameworks for Solid-Gas Reactions.

The presence of site-isolated and well-defined metal sites has enabled the use of metal-organic frameworks (MOFs) as catalysts that can be rationally modulated. Because MOFs can be addressed and manipulated through molecular synthetic pathways, they are chemically similar to molecular catalysts. They are, nevertheless, solid-state materials and therefore can be thought of as privileged solid molecular catalysts that excel in applications involving gas-phase reactions. This contrasts with homogeneous catalysts, which are overwhelmingly used in the solution phase. Herein, we review theories dictating gas phase reactivity within porous solids and discuss key catalytic gas-solid reactions. We further treat theoretical aspects of diffusion within confined pores, the enrichment of adsorbates, the types of solvation spheres that a MOF might impart on adsorbates, definitions of acidity/basicity in the absence of solvent, the stabilization of reactive intermediates, and the generation and characterization of defect sites. The key catalytic reactions we discuss broadly include reductive reactions (olefin hydrogenation, semihydrogenation, and selective catalytic reduction), oxidative reactions (oxygenation of hydrocarbons, oxidative dehydrogenation, and carbon monoxide oxidation), and C-C bond forming reactions (olefin dimerization/polymerization, isomerization, and carbonylation reactions).

Hydroxide Chemoselectivity Changes with Water Microsolvation.

Solvent molecules are known to affect chemical reactions, especially if they interact with one or more of the reactants or catalysts. In ion microsolvation, i.e., solvent molecules in the first solvation sphere, strong electronic interactions are created, leading to significant changes in charge distribution and consequently on their nucleophilicity/electrophilicity and acidity/basicity. Despite a long history of research in the field, fundamental issues regarding the effects of ion microsolvation are still open, especially in the condensed phase. Using reactions between hydroxide and relatively stable quaternary ammonium salts as an example, we show that water microsolvation can change hydroxide's chemoselectivity by differently affecting its basicity and nucleophilicity. In this example, the hydroxide reactivity as a nucleophile is less affected by water microsolvation than its reactivity as a base. These disparities are discussed by calculating and comparing oxidation potentials and polarizabilities of the different water-hydroxide clusters.

Direct Observation of Contact Ion-Pair Formation in La3+ Methanol Solution.

An approach combining molecular dynamics (MD) simulations and X-ray absorption spectroscopy (XAS) has been used to carry out a comparative study about the solvation properties of dilute La(NO3)3 solutions in water and methanol, with the aim of elucidating the still elusive coordination of the La3+ ion in the latter medium. The comparison between these two systems enlightened a different behavior of the nitrate counterions in the two environments: while in water the La(NO3)3 salt is fully dissociated and the La3+ ion is coordinated by water molecules only, the nitrate anions are able to enter the metal first solvation shell to form inner-sphere complexes in methanol solution. The speciation of the formed complexes showed that the 10-fold coordination is preferential in methanol solution, where the nitrate anions coordinate the La3+ cations in a monodentate fashion and the methanol molecules complete the solvation shell to form an overall bicapped square antiprism geometry. This is at variance with the aqueous solution where a more balanced situation is observed between the 9- and 10-fold coordination. An experimental confirmation of the MD results was obtained by La K-edge XAS measurements carried out on 0.1 M La(NO3)3 solutions in the two solvents, showing the distinct presence of the nitrate counterions in the La3+ ion first solvation sphere of the methanol solution. The analysis of the extended X-ray absorption fine structure (EXAFS) part of the absorption spectrum collected on the methanol solution was carried out starting from the MD results and confirmed the structural arrangement observed by the simulations.

(Digital Presentation) Tailoring Solvation and Counterion Complexation in the Electrolyte for Enhanced Titanium Redox Flow Battery Performance

The use of titanium (Ti) in redox flow batteries (RFBs) is of increasing interest as Ti is 100x as abundant[1] worldwide as vanadium (V) with a 10x lower cost of production. The half-cell potential of the Ti4+/Ti3+ redox couple is 0.1 V (vs. SHE) as compared to -0.26 V (vs. SHE) for V3+/V2+, which makes the Ti4+/Ti3+ redox couple less prone to parasitic hydrogen evolution reactions. Aiming to optimize electrolyte formulation for Ti-based RFBs, we measured the transport and kinetic parameters for the Ti3+/Ti4+ redox couple in various acidic supporting electrolytes [2, 3, 4] such as H2SO4, HCl, HNO3 and CH3SO3H (methanesulfonic acid). The solubility and electrochemical reversibility of the Ti3+/Ti4+ redox couple was examined with a view toward increasing the energy density and energy efficiency of Ti-based RFBs. The solvation sphere around the redox active species was manipulated by varying the counterion and by careful control of the redox active species to counterion ratio. The solvation and complexation behavior were correlated to the transport and kinetic properties through the impact of the solvent reorganization energy () on the electrochemical Thiele Modulus[5]. The kinetics of the Ti3+/Ti4+ redox couple diverged by >2x between the forward and reverse reactions and the diffusion coefficients were found to vary by an order of magnitude with varying complexation and solvation. Finally, we also demonstrate the predictive ability of the electrochemical Thiele Modulus by correlating the predictions based on the effectiveness factor to polarization performance of Ti symmetric cells.

How solvent determines the molecular reactive conformation and the selectivity: Solvation spheres and energy

In solution, the solvent determines the molecular conformation and the chemical reaction viability and selectivity. When solvent-solute and solvent-solvent interactions present similar strengths, explicit salvation is the best way to describe a system. The problem to solve is how big the explicit shell should be. In this paper, we want to answer one of the fundamental questions in the implementation of explicit solvation, exactly how many solvent molecules should be added and where they should be placed. Here we determine the first solvent sphere around a molecule and describe how it controls the conformation and selectivity of a selected reaction. NMR experiments were carried out to identify the number of solvent molecules around the solute that constitutes the first solvent sphere, and the interaction between this solvent sphere and the solute was detected using DFT and QTAIM calculations. A new approach to the solvation energy is presented. Finally, we established the role of solvent molecules in the conformation of the solute and in the transition states that produce the two possible products of the reaction.

Beryllium ion coordination in Ammonia, methanol and water solvents

• Be is a key component of materials indispensable in today’s industries and its solvation behavior is a topic of interest. • The study highlight the coordination behavior of beryllium ion with water, ammonia and methanol solvents. • Fitting procedure to obtain Binding energy convergence. • Temperature effects on the isomeric distribution for each cluster size. • MP2 method as reliable method to describe the solvation process around cations. The solvation of B e 2 + [ M g N H 3 n = 1 - 10 ] 2 + cation in ammonia, methanol and water has been studied at Ab initio MP2 level using cluster models in which the solvent molecules have been distributed between the first and second solvation shells. In all cases, the first solvation sphere results to be saturated with four solvent molecules in a tetrahedral arrangement. The second shell contains instead up to 8,12 and 16 ligands in the case of methanol, water and ammonia, respectively. Solvent molecules in the second solvation shell serve to stabilizes the structures through the formation of a number of hydrogen bonds and improves the agreement with the available experimental data. From obtained distances values it can be deduced that beryllium ion establishes with the solvent molecules, inside the first coordination shell, bonds of covalent nature (less than1.8Å) which are longer than those with the magnesium ion. Using a fitting procedure, binding energy values of 29.2, 46.9 and 68.6 kcal/mol, for ammonia, water and methanol clusters, have been obtained, respectively. The clustering enthalpies and free energies have been also evaluated. Temperature effects on the isomeric distribution for each cluster and for all considered solvents, have been calculated in the range of 25–40 K and results show that only the tetracoordinated systems are populate.

Solvation structure of the Hg(NO3)2 and Hg(TfO)2 salts in dilute aqueous and methanol solutions: An insight into the Hg2+ coordination chemistry

A synergic approach combining X-ray absorption spectroscopy (XAS) and Molecular Dynamics (MD) has been used to investigate the solvation properties of dilute aqueous and methanol solutions of the Hg(NO3)2 and Hg(TfO)2 (where TfO−=trifluoromethanesulfonate or triflate) salts. A new Lennard-Jones potential has been developed to be used in classical MD simulations of Hg2+-containing systems by refining the force-field parameters to reproduce the Hg2+ ion coordination in water in agreement with the XAS data. A different behavior of the Hg2+ ion in water and methanol has been highlighted by the analysis of the MD simulations carried out with the newly developed interaction potential: in aqueous solutions of the Hg(NO3)2 and Hg(TfO)2 salts, the Hg2+ first coordination shell is composed only of water molecules, while in methanol solutions of the same salts the Hg2+ cation coordinates both counterions and methanol molecules in its first solvation sphere. The MD results have been confirmed by comparison with the Hg L3-edge XAS experimental data. Independently of the formation of ion pairs with the counterions, the Hg2+ ion in solution tends to form heptacoordinated first shell complexes, with an arrangement of the ligands depending on the nature of the solvent: in water a 7-fold cluster of water molecules in a C2 symmetry is found, while in methanol the Hg2+ solvation shell is composed of both counterions and methanol molecules, with oxygen atoms coordinating Hg2+ arranged in a distorted pentagonal bipyramid geometry. These findings represent a significant step forward in the understanding of the solvation chemistry of the Hg2+ ion, which is fundamental to improve the efficiency of mercury removal procedures that are crucial to the safety of water resources.

Voltammetric Analysis of the Ferrocenium/Ferrocene Redox Couple in Litfsi-Acetonitrile Highly Concentrated Electrolyte

Research on electrolytes have a defining role for the progression of current energy storage device and electrocatalysis. For instance, the electrolyte is often a limiting factor in the life span of Li-ions batteries due to slow decomposition at electrodes. Intensive research is therefore ongoing to increase the stability of the electrolyte. One example of such research is on highly concentrated electrolyte (HCE) which are obtained when the number of ions of a salt nears or equals that of the solvent molecules, while maintaining a liquid state. These highly concentrated electrolytes (HCE) or "solvent-in-salt" solutions differ in properties from conventional ones because all solvent molecules are occupying the ion solvation sphere where they are strongly coordinated to the cation. While on the application side, the attention is directed towards their use in batteries or supercapacitors, current research on HCE aims at increasing the fundamental understanding of their properties mostly the transport properties of the Li+ or the formation of the solid electrolyte interface (SEI). Interestingly, there are no reports on the study of heterogeneous electron transfer (ET) in HCE, yet the absence of "free" solvent molecule suggests that ET in HCE might be different from conventional electrolytes because of the importance of solvent reorganization during ET. We therefore investigate the mass transport and ET of dissolved ferrocene (Fc) and with ferrocene terminated self-assemble monolayer, (Fc-SAM). Ferrocene was selected as the redox center for its "ideal" outer shell electron transfer mechanism and well-defined electrochemical properties. To establish the effect of salt concentration, the electrochemical measurements were done in a series of electrolytes composed of Lithium bis(trifluoromethanesulfonyl)imide in acetonitrile with concentrations spanning from diluted (0.3 M) to highly concentrated (4.1 M) solutions. The diffusivity of Fc (Shoup-Szabo) is found to follow the trend with viscosity (η) expected from the Stokes-Einstein relation over the entire concentration range. The ET rate constant k0 variation with η on the other hand diverges from what is expected from a purely adiabatic ET. Possible explanations for deviation involve different dielectric properties, strong coordination of the solvent and particular double-layer structure of HCE, properties that are not fully established. This lack of knowledge highlights the importance of increasing fundamental research on the topic of electrochemistry in HCE.

On the Role of Water in the Formation of a Deep Eutectic Solvent Based on NiCl2·6H2O and Urea.

The metal-based deep eutectic solvent (MDES) formed by NiCl2·6H2O and urea in 1:3.5 molar ratio has been prepared for the first time and characterized from a structural point of view. Particular accent has been put on the role of water in the MDES formation, since the eutectic could not be obtained with the anhydrous form of the metal salt. To this end, mixtures at different water/MDES molar ratios (W) have been studied with a combined approach exploiting molecular dynamics and ab initio simulations, UV–vis and near-infra-red spectroscopies, small- and wide-angle X-ray scattering, and X-ray absorption spectroscopy measurements. In the pure MDES, a close packing of Ni2+ ion clusters forming oligomeric agglomerates is present thanks to the mediation of bridging chloride anions and water molecules. Conversely, urea poorly coordinates the metal ion and is mostly found in the interstitial regions among the Ni2+ ion oligomers. This nanostructure is disrupted upon the introduction of additional water, which enlarges the Ni–Ni distances and dilutes the system up to an aqueous solution of the MDES constituents. In the NiCl2·6H2O 1:3.5 MDES, the Ni2+ ion is coordinated on average by one chloride anion and five water molecules, while water easily saturates the metal solvation sphere to provide a hexa-aquo coordination for increasing W values. This multidisciplinary study allowed us to reconstruct the structural arrangement of the MDES and its aqueous mixtures on both short- and intermediate-scale levels, clarifying the fundamental role of water in the eutectic formation and challenging the definition at the base of these complex systems.

Inclusion of solvent polarizability in the modeling of ion-dipole mixtures within the mean spherical approximation (MSA)

The fluctuating polarization model of Høye and Stell [J. Chem. Phys. 72, 1597 (1980); J. Chem. Phys. 73, 461 (1980)] is introduced in the non-primitive mean spherical approximation (MSA) framework for an arbitrary mixture of hard spherical ions and polar polarizable solvent molecules. The thermodynamic properties are derived for this more realistic representation of an aqueous electrolyte. The obtained explicit formulas constitute a thermodynamically consistent set.