Structural Features of Ion Hydration in Sodium Nitrate and Thiosulfate

The features of formation of hydration spheres around electrolyte ions in aqueous solutions of sodium nitrate and thiosulfate in a wide concentration range (from 2 to 42 wt %) at temperatures from 278.15 to 318.15 K were determined from the isoentropy compressibility data. The structural characteristics of the solute hydration complexes were determined. The hydration numbers decrease with increasing concentration and are independent of temperature. Na2S2O3 has the highest hydration number at infinite dilution (h ⁰) and is characterized by the lowest molar isoentropy compressibility of water in the hydration spheres of the ions (S,1hV1h). Sodium thiosulfate, compared to sodium nitrate, interacts with water stronger, and its aqueous solutions show a greater degree of ordering.     

The influence of pressure on the structure of aqueous solutions of NaCl over the pressure range 0.1–1000 MPa according to the integral equation method

The influence of pressure (0.1–1000 MPa) on the structure of aqueous solutions of NaCl (1.91–3.08 m) at constant temperatures of 298 and 623 K was studied by the integral equation method. The most substantial structural rearrangement was found to occur at pressures exceeding 150 MPa. Solution structure formation at 298 K was characterized by a substantial decrease in interparticle distances and a baric distortion of the tetrahedral network of water, which resulted in an increase in the hydration of ions and a decrease in the fraction of ion pairs. Structure changes under compression conditions at 623 K were similar to those observed at 298 K, but the network of water H-bonds was already destroyed in solutions at the higher temperature, and hydration-separated ion pairs did not form over the whole pressure range studied. Ions partially dehydrated at 623 K virtually fully restored the hydration spheres they had at 298 K as the pressure increased to 1000 MPa.     

Electron Transport Properties of Ions in Aqueous Solutions of Sodium Selenite

Ion diffusion kinetics has been studied using the data of conductivity measurements for aqueous solutions of sodium selenite with different concentrations and at different temperatures. Molecular and ionic self-diffusion coefficients have been determined for infinitely dilute solutions in the temperature range 288 K-313 K. The limiting values of ion mobility and changes in the energies of translation of water molecules from ions’ hydration shell have been found. At elevated temperatures, ΔE _tr ⁰ increases for both ions in direct proportion to the crystallographic radius of the latter. Ion hydration numbers at 298 K have been calculated. The results of this study are interpreted in the light of Samoilov’s theory on positive and negative hydration of ions.Original Russian Text Copyright © 2004 by L. T. Vlaev and S. D. Genieva__________Translated from Zhurnal Strukturnoi Khimii, Vol. 45, No. 5, pp. 870–876, September–October, 2004.     

Acoustic Study of Solvent Coordination in the Hydration Shells of Potassium Iodide

The isentropic compressibilities of aqueous solutions of potassium iodide, from dilute to almost saturated, were determined at 288 to 308 K based on precise measurements of the speed of ultrasound. Using proper correlations, the hydration numbers (h) were calculated as well as the molar volume and compressibility parameters of the hydrated complexes (V _h , β _h V _h ) of water in the hydration shell (V _1h , β_1h V _1h), and of the cavity containing stochiometric mixtures of K⁺ and I⁻ ions (V _2h, β_2h V _2h). It is revealed that under the studied conditions, the obtained values of h and β _h V _h are independent of temperature whereas the molar compressibility of the hydration shell β _h V _h) is independent of concentration. The electrostatic field of the ions is shown to influence the temperature dependence of the molar volume of water in the hydration shell more substantially than a change of pressure alone influences the temperature dependence of the molar volume of pure water.     

Temperature and Concentration Dependence of the Electrical Conductance,Diffusion, and Kinetics Parameters of the Ions in Aqueous Solutions of Sulfuric Acid,Selenic Acid,and Potassium Tellurate

The temperature and concentration dependences of the electrical conductance of aqueous solutions of sulfuric acid, selenic acid, and potassium tellurate were studied. The coefficients of the corresponding empirical equations were determined, and the values of equivalent conductances of the anions were evaluated at infinite dilution at the experimental temperatures. The values of the coefficients in the Fuoss and Onsager equation were evaluated for the three electrolytes at 298 K. The values of the molecular and ionic coefficients of self-diffusion at infinite dilution were calculated in the temperature range 288–318 K. The change of the translational energy Δ E∘_tr. of water molecules in the ionic hydration sphere was determined. The number of water molecules participating in the ionic hydration sphere at 298 K and the changes of Gibbs free energy, enthalpy, and entropy of activation of ionic conductance were calculated. The results obtained were interpreted according to the Samoylov’s theory of positive and negative hydration of ions. The differences observed in the temperature dependences of the mentioned parameters were explained in terms of the different radii and hydration numbers of the ions.     

Bulk properties and hydration numbers of ammonium nitrate and ammonium chloride in solution: A structural and thermodynamic analysis

The apparent volumes of the salts in the systems H₂O-NH₄Cl (298 K) and H₂O-NH₄NO₃ (273 K, 298 K, and 323 K) are reproduced with an accuracy of 0.03–0.01 cm³/mol by the equation ? = ?⁰ + Aw ₂ ^0.5 + Bw ₂, where w ₂ is the salt content (mass fractions). The study shows that there is a correspondence between the critical (for determining the hydration number) structural parameters-the intrinsic volume of the electrolyte and the volume of water in ion hydration shells-and the limiting (at w ₂ = 1) partial molar volumes of the components. The hydration numbers at infinite dilution are 6.9 for NH₄Cl at 298 K and 9.1, 6.7, and 6.4 for NH₄NO₃ at 273 K, 298 K, and 323 K. The water volume in ion hydration shells decreases in the sequence: No ₃ ^? , Cl^?, and NH ₄ ⁺ . The hydration numbers decrease with increasing salt concentration. The study shows that within a simpler model ? = ?⁰ + aw ₂ ^0.5 , the hydration numbers are temperature independent.     

Structures of the nearest surroundings of the K<Superscript>+</Superscript>, Rb<Superscript>+</Superscript>, and Cs<Superscript>+</Superscript> ions in aqueous solutions of their salts

Published data on structural characteristics of hydration of K⁺, Rb⁺, and Cs⁺ ions in aqueous solutions of their salts under standard conditions, including authors’ X-ray diffraction data, are summarized and correlated. The structural parameters of the nearest surrounding of the K⁺, Rb⁺, and Cs⁺ ions, such as the coordination numbers, interparticle distances, and types of ionic association, are discussed. It is noted that, because of weak tendency of these cations to hydration, the parameters of their coordination spheres strongly depend on the concentration and chemical nature of counterions.     

Hydration heat capacities of hydrophobic solutes at 248–373 K

A new equation is suggested to define the temperature dependence of the Gibbs energy of hydration of hydrophobic substances: ΔG ⁰ = b ₀ + b ₁ T + b ₂lnT. According to this equation, the hydration heat capacity is in inverse proportion to temperature. Consistent values of hydration heat capacity of nonpolar solutes have been obtained for different temperatures using data on solubility and dissolution enthalpy. The contributions of the hydrocarbon radicals and OH group to the heat capacity of hydration of the compounds were found for the temperature range 248–373 K. The hydration heat capacity of the hydroxyl group has a weak dependence on temperature and increases by only 12 J/(mol·K) in the specified temperature interval. Changes in the hydration entropy of hydrophobic and OH groups are calculated for the temperature increasing from 248 K to 373 K.     

Volume dependence of thermal conductivity and isothermal bulk modulus up to 1 GPa for poly(vinyl acetate)

The thermal conductivity λ and heat capacity per unit volume of poly(vinyl acetate) (260 kg mol⁻¹ in weight average molecular weight) have been measured in the temperature range 150–450 K at pressures up to 1 GPa using the transient hot-wire method, which yielded λ = 0.19 W m⁻¹ K⁻¹ at atmospheric pressure and room temperature. The bulk modulus K has been measured in the temperature range 150–353 K up to 1 GPa. At atmospheric pressure and room temperature, K = 4.0 GPa and (∂K/∂p)_T = 8.3. The volume data were used to calculate the volume dependence of λ, $g = - \left( {\frac{{\partial \lambda /\lambda }}{{\partial V/V}}} \right)_T .$ The values for g of the liquid and glassy states were 3.0 and 2.7, respectively, and g of the latter was almost independent of volume and temperature. Theoretical models can predict the value for g of the glassy state to within 25%. © 1998 John Wiley & Sons, Inc. J Polym Sci B: Polym Phys 36: 1451–1463, 1998     

Solvation of magnesium chloride and sulfate in aqueous solutions at 278.15–323.15 K

Isoentropy compressibilities of aqueous magnesium chloride and sulfate were determined based on precision measurements of ultrasound velocity, density, and isobaric heat capacity at low to high concentrations at 278.15–323.15 K. The hydration numbers h and the molar parameters of volume and compressibility were calculated based on thermodynamically correct equations for hydration complexes (V _h , β _h V _h ), water in the hydration shell (V _1h , β_1h V _1h ), and the void containing a stoichiometric mixture of ions (V _2h , β_2h V _2h ). The h and β _h V _h values were found to be independent of temperature; the molar compressibility of the hydration sphere (β_1h V _1h ) and the stoichiometric mixture of ions without a hydration shell (β_2h V _2h ) were independent of the concentration under the stated conditions. The effect of the electrostatic field of ions on the temperature dependence of the molar volume of water in the hydration sphere was more significant than the effect of pressure on the temperature dependence of the molar volume of bulk water. This is attributed to changes in the dielectric constant of water in the vicinity of the electrolyte ions.     

Pressure and temperature dependence of hydrophobic hydration: volumetric, compressibility, and thermodynamic signatures

The combined effect of pressure and temperature on hydrophobic hydration of a nonpolar methanelike solute is investigated by extensive simulations in the TIP4P model of water. Using test-particle insertion techniques, free energies of hydration under a range of pressures from 1 to 3000 atm are computed at eight temperatures ranging from 278.15 to 368.15 K. Corresponding enthalpy, entropy, and heat capacity accompanying the hydration process are estimated from the temperature dependence of the free energies. Partial molar and excess volumes calculated using pressure derivatives of the simulated free energies are consistent with those determined by direct volume simulations; but direct volume determination offers more reliable estimates for compressibility. At 298.15 K, partial molar and excess isothermal compressibilities of methane are negative at 1 atm. Partial molar and excess adiabatic (isentropic) compressibilities are estimated to be also negative under the same conditions. But partial molar and excess isothermal compressibilities are positive at high pressures, with a crossover from negative to positive compressibility at approximately 100-1000 atm. This trend is consistent with experiments on aliphatic amino acids and pressure-unfolded states of proteins. For the range of pressures simulated, hydration heat capacity exhibits little pressure dependence, also in apparent agreement with experiment. When pressure is raised at constant room temperature, hydration free energy increases while its entropic component remains essentially constant. Thus, the increasing unfavorability of hydration under raised pressure is seen as largely an enthalpic effect. Ramifications of the findings of the authors for biopolymer conformational transitions are discussed.     

Partial oxidation of ethane in a fuel cell-like electrochemical reactor in the gas phase under mild conditions

Partial oxidation of ethane with dioxygen in a gas-phase, fuel cell-like electrochemical reactor under mild conditions is reported. Ethane is shown to be selectively oxidized to ethanol and acetaldehyde on the carbon gas-diffusion cathode in the presence of transition metal ions (Fe²⁺, Cu²⁺). The reaction proceeds at ambient pressure and temperature in the range from 318 to 353 K.     

Pressure Dependence of the Solubilities of Anthracene and Phenanthrene in Water at 25°C

Solubilities of anthracene and phenanthrene in water were measured at 298.15K at pressures to 200 MPa and were found to decrease with increasing pressure.From the pressure coefficients of the solubilities, the volume changesaccompanying the dissolution were estimated to be 15.1±0.6 cm³-mol–1 for anthraceneand 12.4±0.3 cm³-mol–1 for phenanthrene. The partial molar volumes of thesesolutes in water are presumed to decrease with increasing pressure, contrary to thenegative compressibility of alkylbenzenes previously observed in water. Volumechanges accompanying hydrophobic hydration are also estimated to be 1.4cm³-mol–1 for anthracene and 4.1 cm³-mol–1 for phenanthrene, respectively. Thesepositive values are opposite to the negative ones usually observed for hydrophobichydration. The hydration structure of these hydrocarbons is discussed.     

Stability of Anionic Complexes in Halide Melts of Bivalent Metals

The effect of volume variation at the possible dissociation equilibria of (MX₄)^2– anionic complexes in halide melts of bivalent metals are analyzed in terms of the mean-sphere approximation (MSA) of the statistical theory. Within the framework of the simplified model of charged hard spheres of different diameters and valences, the complete system of equilibrium equations is obtained, i.e., equations of the law of mass action and equations of state. This system makes possible self-correlated calculations of both the equilibrium concentration of autocomplexes and the melt density. It is shown that the simplest approximation of the complex diameter as the treble diameter of simple ions overestimates the effects of volume variations when considering dissociation. Taking into account the superposition of spheres makes it possible to describe the smoother volume variations with the temperature.     

Self‐Assembled Poly(N‐methylaniline)–Lignosulfonate Spheres: From Silver‐Ion Adsorbent to Antimicrobial Material

Self‐assembled poly(N‐methylaniline)–lignosulfonate (PNMA–LS) composite spheres with reactive silver‐ion adsorbability were prepared from N‐methylaniline by using lignosulfonate (LS) as a dispersant. The results show that the PNMA–LS composite consisted of spheres with good size distribution and an average diameter of 1.03–1.27 μm, and the spheres were assembled by their final nanofibers with an average diameter of 19–34 nm. The PNMA–LS composite spheres exhibit excellent silver‐ion adsorption; the maximum adsorption capacity of silver ions is up to 2.16 g g^?1 at an adsorption temperature of 308 K. TEM and wide‐angle X‐ray results of the PNMA–LS composite spheres after absorption of silver ions show that silver ions are reduced to silver nanoparticles with a mean diameter of about 11.2 nm through a redox reaction between the PNMA–LS composite and the silver ions. The main adsorption mechanism between the PNMA–LS composite and the silver ions is chelation and redox adsorption. In particular, a ternary PNMA–LS–Ag composite achieved by using the reducing reaction between PNMA–LS composite spheres and silver ions can be used as an antibacterial material with high bactericidal rate of 99.95 and 99.99 % for Escherichia coli and Staphylococcus aureus cells, respectively.     

Heats of immersion in relation to water losses of chromium exchanged mordenite

Chromium-exchanged mordenite samples were thermally dehydrated in vacuo over the temperature range 100–480°C. The effects of thermal treatment on water losses and heats of immersion in water, methanol, and cyclohexane are discussed.From the results obtained, it is concluded that the chromium ions reflect smaller heats of ion hydration compared with the sodium ions in sodium mordenite¹. The displaying parameters are, the hydration of metal cations, filling of the vacated pore structure, and the structural collapse at the high temperature studied. It was found that the heats of immersion decrease with increasing molecular size of the wetting liquid.     

Volumetric properties of water–poly(acrylic acid) salt systems from the pure solid to highly concentrated solutions

Volumetric properties of poly(acrylic acid) alkali-metal salts (Li, Na, K, Rb, and Cs) with different degrees of neutralization and water contents were studied in the range from pure solid to highly concentrated solutions. The apparent partial molar volume ?₂ of the polymer and the partial molar volume V₁ of water were calculated from density data. The value of ?₂ decreased with decreasing polymer concentration and eventually leveled off. Values of V₁, which at low water contents were much smaller than that of free water, increased with increasing water content and eventually reached a constant value equivalent to that of free water, thus indicating the appearance of free water. Water contents corresponding to the appearance of free water increased in the order of Li < Na < K < Rb < Cs, differing from the usual trend of hydration numbers observed in dilute solutions. The change of the slope of the plots of V₁ versus composition suggested a change in the hydration mechanism. For Li, Na, and K salts, the limiting values of V₁ at very low water content is considerably smaller than the 18 cm³/mol of free water. In contrast, for Rb and Cs salts, these values were relatively large, indicating the relatively weak electrostriction effects of these ions.     

Calculation of the transport properties of aqueous species at pressures to 5 KB and temperatures to 1000°C

The limiting equivalent conductances at temperatures from 0° to 1000°C and pressures from 1 to 5000 bars of a large number of aqueous ions have been calculated from limiting equivalent conductances of electrolytes reported in the literature. The limiting equivalent conductances of individual ions typically increase by a factor of about 15 with increasing temperatures from 0° to 1000°C and decrease about 30 percent with increasing pressure from 1 to 5 kb. The equivalent conductance of H₂O approximated by the sum of the limiting equivalent conductances of H⁺ and OH^– is essentially independent of pressure, but increases from about 350 to a maximum of approximately 1800 S-cm²-equiv^–1 in response to an increase in temperature from 0° to 500°C at 1kb. Stokes' law radii and Walden products generated from the computed limiting equivalent conductances of ions exhibit changes over the temperature and pressure range of interest by as much as 100 percent for all of the ions except H⁺ and OH^–, which vary by an order of magnitude. Apparent solvation numbers calculated as a function of pressure and temperature from the Stokes' law radii using the volume and dielectric constant of H₂O and Born coefficients of the individual ions approach infinity at the critical point of H₂O. Residual friction coefficients as a general rule approach zero as temperatures increases to 1000°C. The excess limiting equivalent conductances of the hydrogen and hydroxyl ions computed from the differences between the limiting equivalent conductances of HCl and KCl, and NaOH and NaCl, respectively, increases with increasing pressure, and maximize at 250°C.