Viscosity B-coefficients of the aqueous cyanide and azide ions

A number of factors have led us to re-examine the literature assignments of the viscosity B-coefficients of the individual cyanide and azide ions in aqueous solutions at 25°C. The revised values, obtained from mutually confirmatory correlations, are: B(CN^–)=–0.020±0.007 and B(N ₃ ^– )=–0.014#±#0.006>dm³-mol^–1     

Thermal diffusion of hydrogen chloride in aqueous solutions of N,N-dimethylformamide and acetone

The initial and stationary temperature coefficients of voltage were measured at 298.15 K for symmetrical thermogalvanic cells with silver-chloride and quinhydrone electrodes in the HCl-H₂O-S systems (S is N,N-dimethylformamide or acetone) to determine Soret coefficients, the standard entropies of thermal diffusion transfer of hydrogen chloride, and the standard entropies of H⁺ and Cl^? ions moving in water mixtures with N,N-dimethylformamide and acetone. The Agar model was used to calculate the standard entropies of thermal diffusion transfer of ions and the standard partial molar entropies of the H⁺ and Cl^? ions.     

Homogeneous nucleation temperatures in aqueous mixed salt solutions

This is the first report on the measurement of homogeneous nucleation temperature, TH, in the presence of aqueous mixed salt systems of varying compositions and ionic strengths. The TH,m (TH value in aqueous mixed salt system) data for these systems have been analyzed in terms of a simple empirical equation. The TH,m values in simple aqueous mixed salts like NaCl-KCl can be approximated by linear summation of the products of ionic strength fraction and the TH values of pure salt solutions at the same ionic strength as that of the mixture. The empirical parameter, q0, indicating ionic interaction is related to the viscosity B-coefficients. The TH,m data, though correlated on the basis of the B-coefficients also depends upon the mixing of two ions of like charges. Further, a linear correlation exists between the q0 parameter and self-diffusion coefficient, D0, of the ionic solute. The q0 parameter is also well correlated with the rotational correlation time, tauch/tauc0 of the ionic species involved in the mixtures. It is possible to compute TH,m for the salt mixtures with no common ions from the knowledge of the TH,m values of the salt mixtures with common ions.     

The thermodynamic characteristics of adsorption of hydrogen,sodium, and potassium chlorides at the aqueous solution-gas interface

Thermodynamic models of the adsorption of ions at the interphase boundary between a solution of a 1,1-electrolyte and a gas are suggested. The experimental surface tension isotherms and the isotherms of excess adsorption of hydrogen, sodium, and potassium chlorides from aqueous solutions were used to show that the formation of the surface layer followed both the mechanism of coadsorption of the anion and cation and the mechanism of predominant adsorption of one of the ions. The calculated total adsorption isotherms were used to obtain the dependences of the heats and entropies of adsorption on the amount of the ion adsorbed. The results are discussed in terms of the solvation and desolvation of electrolyte ions in bulk solution and at liquid-vapor interfaces.     

Entropy characteristic of solvation and thermal diffusion of hydrogen chloride in water-1-propanol solutions: A thermoelectrochemical determination

Thermogalvanic cells with silver chloride and quinhydrone electrodes in the HCl-H₂O-1-PrOH system are studied experimentally. The results are used to determine standard entropies of thermal diffusion transport of hydrogen chloride, entropies of mobile H⁺ and Cl^? ions, and Soret coefficients of the electrolyte at 298 K. Thermal diffusion entropies and partial molar entropies of said ions in water-1-propanol solutions are calculated within the Agar model. The results are interpreted with the application of basic concepts of the De Bethune theory concerning thermal diffusion transport in electrolytes.     

Ion hydration near air/water interfaces and the structure of liquid surfaces

Negative adsorption of ions, commonly observed at air-water interfaces, is examined in terms of models of restricted polarization of the solvent by ions at the interface and the structure of the liquid interface. The Born and other models of ionic hydration are applied to evaluate the self-energy of the ion arising in the region of solvent near its interface and in the vacuum or vapour beyond. The adsorption energy of an ion varies substantially with distance from the liquid interface so that a distribution of ions arises as a function of distance from the interface. Integration of this distribution gives an expression, and results, for the ionic surface excess. The diffuse-layer potential, which an unequal distribution of cations and anions give rise to, gives a contribution to the surface potential of the electrolyte solution at finite concentrations.Structural aspects of the liquid interface at which ions are negatively adsorbed are discussed in terms of Stefan's ratio and the superficial excess entropies of various liquid surfaces. These entropies are related to the cohesive energy densities of the bulk liquids. Ion solvent-structure co-sphere interactions with structured interfaces will lead to specificity of negative adsorption of ions.     

A statistical thermodynamic supermolecule-continuum study of ion hydration: Cell and shell methods

A theoretical study of ion hydration using the statistical thermodynamic supermolecule-continuum method is described. The cell and shell methods are used for configurational averaging. Enthalpies, free energies and entropies are calculated for Li⁺, Na⁺, K⁺, F^– and Cl^– each four coordinated with water. The results are in reasonable accord with experiment. A comparison of the site method, cell method and shell method results is presented. The supermolecule-continuum approach to solvent effects seems to be capable of accommodating essential features for the calculation of solvation energy and solvent effects on structure and properties.     

Standard characteristics of thermodiffusion of hydrogen chloride in the water-2-propanol system at 298.15 K

The initial and stationary temperature voltage coefficients for symmetric thermogalvanic cells with silver/silver chloride and quinhydrone electrodes in the HCl-H₂O-2-PrOH system are measured at 298.15 K. From the experimental data obtained, the standard entropies of thermodiffusional transfer of the electrolyte, the standard entropies of moving hydrogen and chlorine ions, and the Soret coefficients of HCl in aqueous solutions of 2-propanol are calculated. The influence of the composition of the mixed solvent and the nature of ions on the entropy characteristics is discussed.     

Improvements in estimated entropies and related thermodynamic data for aqueous metal ions

New estimated standard entropies for some aqueous metal ions are obtained by taking account of magnetic and symmetry contributions. By combining them with an analysis of literature data, improved experimental and estimated values are derived for the standard enthalpies and Gibbs energies of formation of the aqueous ions of titanium, vanadium, chromium, manganese, cerium, and praseodymium. Separate entropy correlations are used for each primary coordination number, and the size dependence is represented by the reciprocal of the metal-oxygen distance in that coordination. The new scheme is consistent with recent work on the coordination of Hg(2+)(aq), Pb(2+)(aq), and tripositive rare earth ions. It differs from its predecessors in indicating a larger variation of the standard molar entropies of aqueous ions with coordination number. The value of S(Θ)(Be(2+), aq) is discussed in this context.     

Approaches to hydration, old and new: Insights through Hofmeister effects

Hydration effects in colloidal interactions or problems involving electrolytes are usually taken care of by effective electrostatic potentials that subsume notions like hydrated ion size, Gurney potentials, soft and hard, chaotropic and cosmotropic ions, and inner and outer Helmholtz planes. Quantum fluctuation (dispersion) forces between ions and between ions and surfaces are missing from classical theories, at least not explicit in standard approaches to hydration. This paper outlines an evolving back-to-basics approach that allows these ion specific forces to be included in theories quantitatively. In this approach ab initio quantum mechanics is used to calculate dynamic polarisabilities of ions and to quantify bare ion radii. The ionic dispersion potentials between ions, and between ions and surfaces in water can then be given explicit analytic form from an extension of Lifshitz theory. They are included in the theory along with electrostatic potentials. In a first stage the primitive (continuum solvent) model provides a skeletal theory on which to build in hydration. Extension of the ab initio calculations to include “dressed” ions, i.e. water hydration shells for cosmotropic ions, quadrupolar and octupolar polarisability contributions and; for colloids, allowance for a surface hydration layer, permits quantification of Hofmeister effects and Gurney potentials. With these extensions, primary hydration forces (short range repulsion) arise due to an interplay between surface hydration layers and specific ion interactions. Apparent longer range “secondary hydration forces” are shown to be a consequence of ion-surface dispersion interactions and are not true “hydration forces”.     

The changes in free energies and entropies from analytical radial distribution functions

The changes in Helmholtz free energies and entropies in dense fluids have been evaluated using three known analytical expressions for radial distribution functions (RDFs) of Lennard–Jones (L-J) fluid. This method provides a simpler and a more expeditious way for the calculation of free energy and entropy in L-J dense fluids through statistical mechanics. Previously, integral equations or perturbation theories were used for this purpose. Such approach not only tests the power of analytical distribution functions in predicting the changes in Helmholtz free energies and entropies, but also specifies better expressions in determining these properties. The results are compared with experimental data and an accurate analytic equation of state for the L-J fluid. It is shown if an expression properly presents RDFs as a function of interparticle distance, density and temperature, it is possible to calculate the changes in Helmholtz free energies and entropies from analytical distribution functions.     

The Normal-Mode Entropy in the MM/GBSA Method: Effect of System Truncation, Buffer Region, and Dielectric Constant

We have performed a systematic study of the entropy term in the MM/GBSA (molecular mechanics combined with generalized Born and surface-area solvation) approach to calculate ligand-binding affinities. The entropies are calculated by a normal-mode analysis of harmonic frequencies from minimized snapshots of molecular dynamics simulations. For computational reasons, these calculations have normally been performed on truncated systems. We have studied the binding of eight inhibitors of blood clotting factor Xa, nine ligands of ferritin, and two ligands of HIV-1 protease and show that removing protein residues with distances larger than 8-16 ? to the ligand, including a 4 ? shell of fixed protein residues and water molecules, change the absolute entropies by 1-5 kJ/mol on average. However, the change is systematic, so relative entropies for different ligands change by only 0.7-1.6 kJ/mol on average. Consequently, entropies from truncated systems give relative binding affinities that are identical to those obtained for the whole protein within statistical uncertainty (1-2 kJ/mol). We have also tested to use a distance-dependent dielectric constant in the minimization and frequency calculation (ε = 4r), but it typically gives slightly different entropies and poorer binding affinities. Therefore, we recommend entropies calculated with the smallest truncation radius (8 ?) and ε =1. Such an approach also gives an improved precision for the calculated binding free energies.     

Effects of sulphate,calcium and aluminum ions upon the hydration of sulphoaluminate belite cement

In the model sulphoaluminate belite cement, the process of hydration is governed by the diffusion and transport phenomena of the main ionic species. The sulphate components and combined sulphate and aluminium ions, exert an accelerating effect upon the kinetics of sulphoaluminate belite cement hydration. Aluminium and calcium ions delay the hydration by creating a retarding layer which can be considered a co-precipitate of aluminium and calcium hydroxides. This is revealed in the calorimetric curves by the duration of induction period and also by the intensities of the main peaks. The appearance of small additional peaks characterizes the formation of primary ettringite, due to the presence of sulphate ions in aqueous solution. The intensities of these peaks depend on the ion concentration too.     

The thermal diffusion characteristics of hydrogen and alkali metal chlorides in aqueous-methanolic solutions at 298.15 K

The paper presents the results of experimental studies of thermoelectrochemical systems comprising silver chloride and quinhydrone electrodes in aqueous-methanolic solutions of hydrogen, lithium, potassium, and cesium chlorides. These results and literature data are used to calculate the standard entropies of transfer of electrolytes, Soret thermal diffusion coefficients, and entropies of moving ions at 298.15 K. The influence of the nature of electrolytes and mixed solvent composition on the characteristics of thermal diffusion transfer in the systems studied is discussed.     

An empirical potential function for the interaction between univalent ions in water

A hydration-shell model has been developed for calculating the interaction energy between ions in water. The hydration shell around each ion contains a few tightly bound water molecules and a larger number of less tightly bound molecules. The energies of their interaction with the ion and the size of the hydration shell have been derived from published experimental data for ion-water clusters in the gas phase. An expression derived for the interaction energy of two univalent ions in water incorporates the following effects: a Lennard-Jones 6–12 interaction, a Coulomb interaction between the charges, the polarization of the hydration shells by a neighboring ion, and an energy term for the removal of water from the hydration shells when the hydration shells of two ions overlap. The effective dielectric constant at small ion-ion distances is the only adjustable parameter. Computed interaction energies for aqueous solutions of twelve alkali halides match experimental values, derived from conductimetric measurements, with an average error of ±14%.     

Thermal diffusion of alkali bromides

The molar entropies (or heats) of transport of dilute aqueous alkali bromide solutions have been measured by the potentiometric method using the silver, silver bromide thermocell at a mean temperature of 25°C and over a concentration range of 5×10^–4 to 0.1M. Experimental results were extrapolated to infinite dilution to obtain the standard molar entropies of transport. The limiting behavior observed is compared with theory based on the electrostatic model.The standard transported entropy of Br^– was also derived by extrapolating the steady-state thermoelectric powers to infinite dilution. The absolute ionic entropies of transport of alkali metal ions have been estimated based on Gurney's scale. The results obtained are compared with that derived f¹/om the previous work on alkali chlorides.     

SN2 reactions in the gas phase. Structure of the transition state

The alkylhalide-halide association ions, [RX₂]^? that are observed in the negative chemical ionization mass spectra of alkyl halides appear to be directly related to the corresponding S_N2 transition states in solution. ‘Frontside’ association of halide ions with bridgehead alkyl halides does not occur in our system. The Change in heats and entropies of association for the chloromethane series is consistent with delocalization in the [RX]₂^? ions.     

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.     

Estimating the Gibbs Hydration Energies of Actinium and Trans-Plutonium Actinides

The use of actinides for medical, scientific and technological purposes has gained momentum in the recent years. This creates a need to understand their interactions with biomolecules, both at the interface and as they become complexed. Calculation of the Gibbs binding energies of the ions to biomolecules, i. e., the Gibbs energy change associated with a transfer of an ion from the water phase to its binding site, could help to understand the actinides’ toxicities and to design agents that bind them with high affinities. To this end, there is a need to obtain accurate reference values for actinide hydration, that for most actinides are not available from experiment. In this study, a set of ionic radii is developed that enables future calculations of binding energies for Pu³⁺ and five actinides with renewed scientific and technological interest: Ac³⁺, Am³⁺, Cm³⁺, Bk³⁺ and Cf³⁺. Reference hydration energies were calculated using quantum chemistry and ion solvation theory and agree well for all ions except Ac³⁺, where ion solvation theory seems to underestimate the magnitude of the Gibbs hydration energy. The set of radii and reference energies that are presented here provide means to calculate binding energies for actinides and biomolecules.     

Solubilization of methane,ethane, propane andn-butane in aqueous solutions of sodium dodecylsulfate

The solubilities of methane, ethane, propane, and n-butane were measured in aqueous solutions of sodium dodecylsulfate (NaDS) (0–0.1M) from 15 to 27°C. From these measurements the standard Gibbs energies, entropies, and enthalpies for the process of transferring the solute molecules from the gaseous phase into the solutions were calculated. An approximate relationship was found relating the volume fraction of the micelles to NaDS concentration.