Structure Of Aqueous Electrolyte Solutions Research Articles

Ionization Difference between Weak and Strong Electrolytes as Perturbed by Conductivity Spectra Analysis

While the static structure of aqueous electrolytes has been studied for decades, their dynamic microscopic structure remains unresolved yet critical in many areas. We report a comparative study of dc and ac (1 Hz to 20 GHz) conductivity data of weak and strong electrolytes, highlighting previously missing differences and similarities. Based on these results, we introduce into consideration the intrinsic short-lived ions of water, namely, excess protons (H3O+) and proton holes (OH-). We show that the model accounting for the neutralization of these ions by the species of electrolyte explains the conductivity of aqueous solutions in the concentration range 10-7-10 M. Based on independent experimental data, we hypothesize that the aggregation of the species in weak electrolytes may determine the main difference between the conductivity of weak and strong electrolytes. Our results push forward the understanding of the dynamic structure of aqueous electrolyte solutions and are important to nanofluidic, biological, and electrochemical systems.

Specific counter-cation effect on the molecular orientation of thiocyanate anions at the aqueous solution interface.

Understanding the interfacial structure of aqueous electrolyte solutions is important and relevant to a wide range of systems, ranging from atmospheric aerosols to electrochemistry, and biological environments. Though significant efforts have been made to unravel the interfacial structure of water molecules, the structure and dynamics of ions at the interface have not yet been fully elucidated. Here, the interfacial structure of the aqueous solution was investigated directly by monitoring the thiocyanate (SCN-) anions using surface-specific sum frequency generation (SFG) vibrational spectroscopy. The molecular orientation of the SCN- anions and their adsorption behavior at the air/water interface were systematically determined by quantitative polarization analysis. The transition dipole of the CN stretching of the SCN- anion is oriented around 44° from the surface normal of the NaSCN aqueous solution surface and remained unchanged with the bulk concentration varying from 1 mol kg-1 to 13 mol kg-1. The free energy of adsorption of SCN- anions at the air/water interface was determined to be -1.53 ± 0.04 kcal mol-1. Furthermore, a new SFG peak positioned at 2080 cm-1 in the ppp polarization combination was observed at the air/15.0 mol kg-1 NaSCN aqueous solution interface for the first time. Concentration-dependent SFG analysis and density functional theory (DFT) calculation further revealed that the SCN- anions form an ion clustering structure at the air/water interface. The subtle and specific Na+ and K+ counter-cation effects on the interfacial structure of the SCN- anions at the aqueous solution interface were also observed, which showed that ion cooperativity plays an important role in affecting the interfacial structure of ions at the air/water interface. The results are expected to yield significant insights into the understanding of the structure of aqueous solution surfaces and the molecular level mechanism of the cationic Hofmeister effect.

Structural Inhomogeneity in Electrolyte Solutions: The Calcium Perchlorate–Water System

A new approach has been proposed to study the structure of aqueous electrolyte solutions. NIR, Raman and attenuated total reflectance, Fourier transform infra-red (ATR FTIR) spectra have been measured for aqueous calcium perchlorate solutions in the 0.22–4.3 mol·L−1 (0.22–7.46 mol·kg−1) concentration range at 25 °C. By the methods of principal component analysis (PCA) and multivariate curve resolution—alternating least squares (MCR-ALS) the number, spectra and concentration profiles have been determined for spectroscopically distinguishable forms of water and ClO 4 − ion in solutions. The results have been analyzed using a phenomenological model, establishing thereby: concentration ranges for structural rearrangements of the solution, the nature of structural microirregularities and different states of the ClO 4 − ion in the areas of domination of the natural water structure, and of cybotactic groups of calcium perchlorate hexa and tetra hydrates.

Thermodynamics and structure of aqueous electrolyte solutions of glycine at different temperatures

The enthalpies of dissolution of glycine in aqueous electrolyte solutions are measured by calorimetry at temperatures of 288 K, 298 K, and 313 K in the molality range from 0.25 mol/kg to 6 mol/kg NaCl and to 4.5 mol/kg KCl. The enthalpy and heat capacity parameters are determined for the pair and triple interactions of glycine with electrolyte in water. Special (singular) points are discovered on the concentration dependences of the heats of dissolution of glycine in aqueous electrolyte solutions and in urea solutions at different temperatures. At these points, such a structurally sensitive property as the entropy of dissolution is temperature independent and the partial heat capacity of glycine is equal to that of the crystalline amino acid. The position of the singular point on the concentration axis is identical for NaCl and KCl solutions; in the case of urea solutions, the singular point is achieved at a much higher concentration of urea.

Structure of aqueous electrolyte solutions estimated by near infrared spectroscopy and chemometric analysis of spectral data

The IR spectra of aqueous solutions of MClO4, MNO3, MCl (M = Li, Na), CsCl, and Na2SO4 were measured at frequencies of 5400–7500 cm−1 over a wide concentration range from 0.12 M up to saturation concentrations at 25°C. Chemometric analysis of the experimental data matrix was performed. Regularity was detected in distribution of the IR spectra of the studied aqueous solutions on the the plot of analysis scores by the principal component method depending on the nature of the anion-water interaction. The influence of the nature of the salt anion and cation on the water structure in aqueous electrolyte solutions was demonstrated. The existence and interconversion of the spectral forms of water with changing electrolyte concentration in solution were revealed.

Detection of Hydrate Forms of Lithium and Sodium Perchlorates in Aqueous Solutions Using near Infrared Spectroscopy

A new approach to the study of concentrated electrolyte water solutions is proposed. NIR spectra of aqueous solutions of LiClO4 and NaClO4 were studied. The pure spectra and concentration profiles of spectral forms in the solutions were found by deconvolution of an experimentally measured data matrix. The chemometric analysis and the generalised phenomenological model of the structure of aqueous electrolyte solutions were used to establish that the variation in spectra of LiClO4 solutions with concentration is due to antibatic changes in the fractions of water that is unbonded with ions and of water in the structure of the cybotactic group of lithium perchlorate trihydrate. Corresponding changes in NaClO4 solutions are also associated with interconversion of sodium perchlorate forms of di- and monohydrates. Hydrate forms of lithium and sodium perchlorates in aqueous solutions are described, and a cybotactic group in a saturated solution is found to be equivalent to a solid-phase structure fragment. A strategy for selecting the nature of a background electrolyte is recommended.

Significant Hydration Shell Formation Instead of Hydrogen Bonds in Nanoconfined Aqueous Electrolyte Solutions

Nanoscale confined electrolyte solutions are frequently observed, specifically in electrochemistry and biochemistry. However, the mechanism and structure of such electrolyte solutions are not well understood. We investigated the structure of aqueous electrolyte solutions in the internal nanospaces of single-walled carbon nanotubes, using synchrotron X-ray diffraction. The intermolecular distance between the water molecules in the electrolyte solution was increased because of anomalously strong hydration shell formation. Water correlation was further weakened at second-neighbor or longer distances. The anomalous hydrogen-bonding structure improves our understanding of electrolyte solutions in nanoenvironments.

Structure of aqueous electrolyte solutions near a hydrophobic surface

The structure of aqueous solutions of 1:1 salts (KCl, NaCl, KF, and CsI) near a hydrophobic surface is analysed using the angle-dependent integral equation theory. Water molecules are taken to be hard spheres imbedded with multipolar moments including terms up to octupole order, and hard spherical ions are immersed in this model water. The many-body interactions associated with molecular polarizability are treated at the self-consistent mean field level. The effects of catio nic and anionic sizes and salt concentration in the bulk are discussed in detail. As the salt concentration increases, the layer of water molecules next to the surface becomes denser but its orientational order remains almost unchanged. The concentration of each ion at the surface can be drastically different from that in the bulk. As a striking example, at sufficiently low salt concentrations, the concentration of I is about 500 times higher than that of F at the surface.

Structure of Aqueous Electrolyte Solutions by X-ray Diffraction

Cadmium nitrate and cesium chloride solutions were studied in a wide concentration range by the X-ray diffraction method. The structure of aqueous solutions of the electrolytes is similar to the structure of solid phases that crystallize from them.

Analysis of excess Gibbs energy of electrolyte solutions: a new model for aqueous solutions

This paper presents an analysis of the excess Gibbs free energy of aqueous electrolytes. The analysis of experimental data leads to the conclusion that the equilibrium state for dilute univalent electrolytes in water involves an intercalation of water and ionic liquid crystal domains. Excess free energy of the solution is determined by the Madelung energy of hydrated ion-pair liquid crystals, and the energy associated with a shift in the structural equilibrium of water. The data that point to such a model include: molecular orbital–molecular dynamics applied to electrolyte water systems; Raman spectra; infrared spectra; magnetic resonance spectra of ions; the apparent density of water; and the excess free energy of electrolytes in aqueous solutions. Molecular orbital–molecular dynamics calculations of relatively large water clusters containing a molecule of sodium iodide show that the solvent separated ion pair exists in a substantial potential well compared to other possible structures. Raman spectra of univalent electrolyte solutions as a function of concentration can be quantitatively modeled using only the spectra of pure water and electrolyte solution at the concentration of the solvent separated ion pair. The other observations are consistent with the structures proposed from the Raman spectral study. The new model provides a satisfactory account of the fact that the excess free energy of dilute (<0.2 mol/l) solutions is generally more negative than anticipated on the basis of Debye–Hückel theory, and that the equilibrium evidence points to the same functional behavior at very low concentrations as is seen at 0.05 mol/l. We present a testable hypothesis that the excess free energy, and other thermodynamic properties of the solutions do not follow the Debye–Hückel limiting law. The tests of this hypothesis must involve only equilibrium measurements at concentrations between 0.05 and 0.0005 mol/l. This hypothesis concerning the structure of aqueous electrolyte solutions is not in conflict in any way with the Debye–Hückel–Onsager theory of electrical conductivity.

Molecular-dynamics simulations of ion size effects on the fluid structure of aqueous electrolyte systems between charged model electrodes

The effect of ion size on the structure of aqueous electrolyte solutions between charged nonpolarizable surfaces or electrodes is investigated using molecular-dynamics simulations of discrete water molecules and ions confined to a slab geometry. Long-range intermolecular Coulombic interactions are calculated using the particle–particle–particle–mesh method with a modification to account for the slab geometry. Density distribution and potential profiles are reported for 1 M aqueous electrolyte solutions with ±0.1 C/m2 electrode surface charge at the electrode surfaces. Five different models for the ions are studied. The models can be characterized as (1) ions of equal size, (2) smaller cations, (3) larger anions, (4) smaller cations and larger anions, and (5) ions representing aqueous NaCl. Compared to the equal-size ion reference case, smaller cation size decreases the contact adsorption at the cathode, but interestingly anion size tends to moderate this effect somewhat. Whereas there is no contact adsorption of anions when the ions are the same size, an increase in anion size causes significant replacement of adsorbed water molecules at the anode with anions. In the case studied here, the larger anion size produced a 20 fold increase in localized density at the electrode surface. The electrostatic potential profile tends to be relatively insensitive to the changes in ion size.

Application of luminescence methods in the study of the structure of aqueous electrolyte solutions

Abstract Luminescence methods have been applied to the study of solutions of concentrated aqueous electrolytes of LiCl, CaCl 2 , LiBr, CsCl with addition to those of LnCl 3 , NiCl 2 , MnCl 2 . The rate constants of energy transfer from Tb(III) ions to Er(III) ions to Er(III), Ni(II), Mn(II) were measured in solutions of different electrolytes. So as the energy transfer rate depends on the distance between the energy donor and acceptor, it can be shown from this study, that the labile nanostructures of lanthanide ions are formed in concentrated aqueous solutions of LiCl and to a lesser degree in solutions of CaCl 2 . The concentration of lanthanide ions in labile nanostructures is close to that of a saturated aqueous solution of LnCl 3 . The labile nanostructures consist of no less than several tens of lanthanide ions which exchange with ions of the solution during the time of less than one μs. The mixed labile nanostructures of lanthanide and Ni(II) or Mn(II) ions are also formed in concentrated LiCl solutions, but they are less stable. The dependence of labile nanostructure stability on temperature and on the addition to the electrolyte solution of other multicharge cations was also studied. The nature of the phenomenon of the labile nanostructure formation is discussed.

An Investigation of the Structure of Aqueous Electrolyte Solutions by Statistical Geometry

Abstract Statistical geometry is employed for a quantitative description of deviations from regular octahedrality of water around monovalent ions and from regular tetrahedrality of solvent water around water molecules. For these deviations a new criterion is presented by taking into account octahedral and tetrahedral spatial symmetry groups. A geometrical definition of structure making and structure breaking is proposed and illustrated on the basis of molecular dynamics simulations of 2.2 molal aqueous KCl and LiI solutions with the flexible BJH model of water.

Interatomic structure of aqueous ionic solutions

Abstract

The Structure of Aqueous Electrolyte Solutions: Comparis on of Computer Simulation and Experiment

Abstract This review compares results of neutron and X-ray diffraction experiments with computer simulation and theoretical calculation for aqueous electrolyte solutions at the atomic level in terms of the partial radial distribution functions of several ionic solutions, and includes results for the ion-water and ion-ion structure of systems containing alkali ions, alkaline earth ions, transition metal cations and a few anions

The structure of aqueous electrolyte solutions as derived from MD (molecular dynamics) simulations

Abstract

Structure of aqueous electrolyte solutions. Ionic internal pressures and proton chemical shifts

The total ionic pressure changes indicate that most ions break the structure of water. The polyvalent cations and the SO2–4(and possibly the CO2–3) ion are exceptions since they enhance the structure. This agrees with the total proton chemical shift effects. On the other hand, the hydrogen-bond pressures of cations enhance structure whilst the corresponding pressures of anions break the structure (decrease hydrogen bonding).The ionic pressures ascribed to the first and second cospheres of the cation are proportional to the electric fields at the H2O centre in the first and second cospheres, respectively. The corresponding components of the proton chemical shift, δ+, are proportional to these pressures. A third component of δ+ is proportional to the combined ion–ion and ion–water-orientation pressures. The overall results for the cations are compatible with a distorted interstitial-type arrangement of the cations which bond to an average of 3.5 water molecules, have 6 H2O molecules in the first cosphere and 11 in the second.The anions behave quite differently. The effects due to the electric deformation of the water molecules are offset by those due to intermolecular effects. The electrostatic (no charge transfer) hydrogen-bond pressures which are associated with decreases in hydrogen-bonding for most anions (except F–, SO2–4 and CO2–3) effectively dominate. Thus the observed shift for anions appears to derive from this type of effect alone. The hydration numbers are 6 (or 8 for ions such as ClO–4) for the first cosphere. Unlike the halides the influence of the oxyanions extends beyond the first cosphere so that altogether up to 21 water molecules are affected by the ions. The foregoing applies for solutions at 1 mol (kg H2O)–1 at 25 °C and 1 bar pressure. The equations derived for δ+ and δ– indicate that the components of the shifts will all be functions of temperature, pressure and concentration.

Structure of aqueous electrolyte solutions. Thermodynamic internal pressure

The thermodynamic internal pressures of aqueous electrolyte solutions have been analysed using three models (the Tammann–Tait–Gibson, TTG, the orientation defect and the non-interacting homomorph, NIH) together with the infrared (i.r.) and dielectric structural temperatures. The effect of ionic volumes is to change water–water interactions. This leads to a change (increase or decrease) in the internal pressure of the water of pww bar. The ionic-charge effects cause an increase in pressure of (pe–plr) bar, where pe is the TTG effective pressure and plr is the long-range pressure (deriving from forces proportional to z2/r and to z2). pel(a component of pe and arising from the electrical deformation of the water molecules) plus pww are mainly responsible for the changes in pressure associated with the hydrogen-bond energy. The pressure changes associated with second cosphere events can be ascribed to intermolecular forces and small changes in the free OH.

Thermal studies on the dehydration of polystyrene—divnylbenzene-type ion exchangers

Thermal studies on fully swollen polystyrene—divinylbenzene-type resins variously cross-linked Dowex 50W (Li +) resisn adn Dowex 1-X8 (Cl −, Br −, I − and NO − 3 resin forms have been carried out. Thermogarvimetric analysis of the resisn showed that apparently all of the water was lost in one step between 300 and 383°K. Isothermal kinetic data for the resins best fitted the contracting envelope equations. Nearly 70% of the total water in the resisn, corresponding to the osmotic water in the resin phase, was lost at a constant rate. The activation energies for this part of the dehydration for Li + and NO − 3 resins were greater and those for Cl −, Br − and I − resins were smaller than the enthalphy of evaporatin of normal water. This indicates a more ordered state of water in Li + and NO − 3 resins adn more disordered state in Cl −, Br − and I − resins than in normal water. These results are discussed in terms of a generally accepted model for the structure of aqueous electrolyte solutions.

Internal pressure and its application for the analysis of the structure of aqueous electrolyte solutions at high temperatures and pressures

Internal pressure and its application for the analysis of the structure of aqueous electrolyte solutions at high temperatures and pressures