Aqueous solutions of divalent chlorides: ions hydration shell and water structure

By combining neutron diffraction and Monte Carlo simulations, we have determined the microscopic structure of the hydration ions shell in aqueous solutions of MgCl(2) and CaCl(2), along with the radial distribution functions of the solvent. In particular the hydration shell of the cations, show cation specific symmetry, due to the strong and directional interaction of ions and water oxygens. The ions and their hydration shells likely form molecular moieties and bring clear signatures in the water-water radial distribution functions. Apart from these signatures, the influence of divalent salts on the microscopic structure of water is similar to that of previously investigated monovalent solutes, and it is visible as a shift of the second peak of the oxygen-oxygen radial distribution function, caused by distortion of the hydrogen bond network of water.     

SCF MO LCGO studies on the hydration of ions: The systems H+H2O,Li+H2O,and Na+H2O

The energy surfaces of the systems LiOH ₂ ⁺ and NaOH ₂ ⁺ are studied for a number of different geometries within the SCF MO LCAO framework, using a gaussian basis set to approximate the wavefunction. In the minimum energy geometry of both systems the positive ion is bound to the oxygen atom of the water molecule. The computed binding energies and bond distances are: B ^SCF(LiOH ₂ ⁺ ) = 36.0 kcal/mole, d(LiO) = 3.57 a.u., and B ^SCF(NaOH ₂ ⁺ ) = 25.2 kcal/mole, d(NaO) = 4.23 a.u., resp. The results are compared with those of H₃O⁺ and discussed in view of ion-solvent interaction in aquous solutions.It is a pleasure to thank our technical staff for the careful preparation of the input for the programs and for its enthusiastic and skilful assistance in running the computer.     

Effect of solute size and solute-water attractive interactions on hydration water structure around hydrophobic solutes.

Using Monte Carlo simulations, we investigated the influence of solute size and solute-water attractive interactions on hydration water structure around spherical clusters of 1, 13, 57, 135, and 305 hexagonally close-packed methanes and the single hard-sphere (HS) solute analogues of these clusters. We obtain quantitative results on the density of water molecules in contact with the HS solutes as a function of solute size for HS radii between 3.25 and 16.45 A. Analysis of these results based on scaled-particle theory yields a hydration free energy/surface area coefficient equal to 139 cal/(mol A2), independent of solute size, when this coefficient is defined with respect to the van der Waals surface of the solute. The same coefficient defined with respect to the solvent-accessible surface decreases with decreasing solute size for HS radii less than approximately 10 A. We also find that solute-water attractive interactions play an important role in the hydration of the methane clusters. Water densities in the first hydration shell of the three largest clusters are greater than bulk water density and are insensitive to the cluster size. In contrast, contact water densities for the HS analogues of these clusters decrease with solute size, falling below the bulk density of water for the two largest solutes. Thus, the large HS solutes dewet, while methane clusters of the same size do not.     

Influence of concentration and temperature on the dynamics of water in the hydrophobic hydration shell of tetramethylurea

We study the influence of the amphipilic compound tetramethylurea (TMU) on the dynamical properties of water, using dielectric relaxation spectroscopy in the regime between 0.2 GHz and 2 THz. This technique is capable of resolving different water species, their relative fractions, and their corresponding reorientation dynamics. We find that the reorientation dynamics of water molecules in the hydration shell of the hydrophobic groups of TMU is between 3 (at low concentrations) and 10 (at higher concentrations) times slower than the dynamics of bulk water. The data indicate that the effect of hydrophobic groups on water is strong but relatively short-ranged. With increasing temperature, the fraction of water contained in the hydrophobic hydration shell decreases, which implies that the overall effect of hydrophobic groups on water becomes smaller.     

Effect of the Zn2+ and Hg2+ ions on the structure of liquid water

The effect of ions on the structure of liquid water is still not completely understood, despite extensive experimental and theoretical studies. A combined XANES and molecular dynamics investigation on diluted Zn(2+) and Hg(2+) aqueous solutions reveals that the influence of a single ion on the bonding pattern of water molecules is strongly dependent on the nature of the ion. While the structure of water is not altered by the presence of the Zn(2+) ion, the Hg(2+) cation has a strong impact on the hydrogen-bond network of water that extends beyond the first coordination shell.     

Quantized hydration energies of ions and structure of hydration shell from the experimental gas-phase data

With previous data on alkali metal and halide ions included [Rais, J.; Okada, T. Anal. Sci. 2006, 22, 533], we analyzed rather broad data on ionic hydration from the point of view of gaseous cluster energetics. We have now added alkaline earth cations, Zn(2+), H(+), OH(-), Cu(+), Ag(+), Bi(+), Pb(+), and alkylammonium cations. The present analysis revealed the octa-coordinated nature of alkaline earth cations, which is not fully pronounced for Be(2+) and Zn(2+), existence of Eigen protonium complex, which is trigonally hydrated, and particular property of the first OH-, H(2)O cluster. Whereas these findings are generally in accordance with theoretical model calculation studies, we have foreseen in addition tetrahedral hydration for halide anions and Rb(+) and Cs(+), as well as for alkylammonium ions. The obtained picture of the quantized solvation of ions is mirrored in the ionization potentials of outer electrons of pertinent atoms. This is a second independent phenomenon, and together, they invoked a common pattern formation ("Aufbau") obeying tetra- and octa-coordinated principles.     

SCF MO LCGO studies on the hydration of ions: The system Li+·2H2O

The energy of the dihydrated lithium cation Li⁺·2H₂O is studied in several different points within the SCF MO LCAO framework, using a gaussian basis set to approximate the wavefunction. The computed binding energies (hydration energies) and bond distances are compared to the values found for the monohydrate. The results are discussed in view of ion-solvent interaction, and especially of the effect of ions on adjacent hydrogen bonds, in aqueous solutions.It is a pleasure to thank our technical staff for the careful preparation of the input for the programs and for its enthusiastic and skilful assistance in running the computer.     

Hydroxy double salt anion exchange kinetics: effects of precursor structure and anion size

(1)H NMR spectroscopy and powder X-ray diffraction have been used to explore the details of anion exchange reactions of two layered hydroxy double salts (HDSs), zinc copper hydroxy acetate (ZCA), nickel zinc hydroxy acetate (NZA), and a related layered material, zinc hydroxy acetate (ZHA), at room temperature (21-22 degrees C). Reactions that followed Avrami-Erofe'ev kinetics with respect to temporal profiles for acetate release, ZCA with butyrate (k = 1.7 x 10(-3) s(-1)), and octanoate (k = 0.79 x 10(-3) s(-1)) anions, as well as ZHA with octanoate (k = 2.6 x 10(-3) s(-1)), demonstrate that rate constants for acetate release are influenced by the exchange anion relative size as well as by the solid precursor structure/composition. The reaction of NZA with octanoate deviated from expected Avrami-Erofe'ev behavior, with evidence for an intermediate species in the solid phase that may influence the rate of acetate release into solution. The reaction of ZCA with formate anions exhibited a unique zeroth-order kinetics release of acetate, providing the possibility of developing tunable nanostructured anion release sources by use of variations in the size of the exchange species.     

Influence of H+ and calcium ions on surface functional groups of Synechococcus PCC 7942 cells

The chemistry of the surface functional groups of picocyanobacteria Synechococcus PCC 7942 cells was examined as a function of H+ and calcium concentrations. Titration experiments, infrared spectroscopy, biosorption experiments, and chemical modeling were used to gain insight into the mechanisms of biosorption. The pKa and concentration of active sites on the cell wall were clarified with the aid of potentiometric titration. Modeling calculations and infrared spectra are consistent with pKa's values of 4.3, 5.2, 6.9, 9.1, and 10.0 and a total concentration of 7.8 x 10(-4) mol g(-1). Spectral analysis of an aqueous cellular suspension revealed a presence of carboxyl, amide, phosphate, hydroxyl, and carbohydrate moieties. Correspondence between spectral data and potentiometric titration curves supported the hypothesis that carboxylate groups and phosphodiester groups mediate calcium adsorption to bacterial cells. This process is strongly pH dependent. In the second part of the experimental work, Synechococcus cells were suspended in the presence of different calcium concentrations. Mechanistic modeling demonstrated that the calcium adsorption phenomenon can be described taking into account only two mechanisms: ion exchange and complexation.     

Effect of surface polarity on water contact angle and interfacial hydration structure

We perform molecular dynamics simulations of water in the presence of hydrophobic/hydrophilic walls at T = 300 K and P = 0 GPa. For the hydrophilic walls, we use a hydroxylated silica model introduced in previous simulations [Lee, S. H.; Rossky, P. J. J. Chem. Phys. 1994, 100, 3334. Giovambattista, N.; Rossky, P. J.; Debenedetti, P. G.; Phys. Rev. E 2006, 73, 041604.]. By rescaling the physical partial atomic charges by a parameter 0 相似文献   

Electronic structure,energy of hydration,and stability of metal aquo ions

The stability of metal aquo ions with respect to redox reactions is determined by the ionization energies of the atoms and the Gibbs energies of hydration for the ions(–_hG⁰). We present critically selected values of –_hG⁰ for 55 metal ions, determined from electrochemical, thermochemical, and spectra data. We consider the factors determining the values of –_hG⁰ (charges, ionic radii, electronic structure, and relativistic effects). For isoelectronic ions, we observe correlations between the ratios of the Gibbs energies of hydration for these ions with different charges and the ratios of their ionic radii. Based on the use of these correlations, we find –_hG⁰ for a number of aquo ions not observed experimentally and we estimate the unknown oxidation-reduction potentials for the pairs of ions M³⁺/M²⁺. We formulate the principles for stabilization of unstable oxidation states of the metals by including the corresponding ions in complexes with certain classes of ligands.Translated from Teoreticheskaya i Éksperimental'naya Khimiya, Vol. 30, No. 1, pp. 1–11, January–February, 1994.     

Influence of pyrogenic silica hydration on water diffusion in surface layers

The surface hydration of pyrogenic silica (aerosil) has been studied by slow neutron scattering. It was shown that the rate of diffusion changes with the thickness of the layer monotonically. The mean square displacement of water molecules from the equilibrium position in aerosil hydration shell is smaller than in bulk water, but twice larger than in ice.     

The influence of ions on the structure of water

Journal of Structural Chemistry - The main features of the changes produced by ions in water may be summarized as follows. The influence of ions on the structure of the “free” water...     

Ab initio rigid water: effect on water structure, ion hydration, and thermodynamics

We investigate the liquid structure, ion hydration, and some thermodynamic properties associated with the rigid geometry approximation to water by applying ab initio molecular dynamics simulations (AIMD) with the Perdew-Burke-Ernzerhof (PBE) exchange-correlation functional at T = 320 K. We vary the rigid water geometry in order to locate a class of practical water models that yield reasonable liquid structure and dynamics, and to examine the progression of AIMD-predicted water behavior as the OH bond length varies. Water constrained at the optimal PBE gas phase geometry yields reasonable pair correlation functions. The predicted liquid phase pressure, however, is large ( approximately 8.0 kbar). Although the O-H bond in water should elongate when transferred from gas to the condensed phase, when it is constrained to 0.02, or even just 0.01 A longer than the optimal gas phase value, liquid water is predicted to be substantially overstructured compared to experiments. Zero temperature calculations of the thermodynamic properties of cubic ice underscore the sensitivity toward small variations in the O-H bond length. We examine the hydration structures of potassium, chloride, and formate ions in one rigid PBE water model. The results are in reasonable agreement with unconstrained AIMD simulations.     

Influence of tetraalkylammonium ions on structure of nonaqueous solvents

Summary The half-wave potentials of the redox system [Co(en)₃]³⁺/[Co(en)₃]²⁺ have been influenced by the tetraalkylamonium cation of the supporting electrolyte used. This was explained with the specific interactions (solvation) of the tetraalkylamonium ion with the nonaqueous solvent.

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Einfluß von Tetraalkylammoniumionen auf die Struktur nichtwäßriger Lösungsmittel (Kurze Mitt.)

Zusammenfassung Das Halbwellenpotential des Redoxsystems [Co(en)₃]³⁺/[Co(en)₃]²⁺ wird von Tetraalkylammonium-Ionen des Stützelektrolyten beeinflußt. Dies wird durch die spezifischen Wechselwirkungen (Solvatation) der Ionen mit dem nichtwäßrigen Lösungsmittel erklärt.