Halide Ions In Water Research Articles

Hydrophilic Versus Hydrophobic Coupling in the Pressure Dependence of the Chemical Potential of Alkali Metal and Halide Ions in Water.

We computed the chemical potential for some alkali metal ions (K+, Rb+, and Cs+) and two halide ions (Br- and I-) in aqueous solution at ambient T and various pressures in the range 1-8000 atm. Results were obtained from classic Monte Carlo simulations in the NPT ensemble by means of the free energy perturbation method. Here, the chemical potential is computed as the sum of a term relative to a Lennard-Jones solute and a term relative to the process in which this solute is transformed into the ion. Hydrophobic and hydrophilic features of these two components of the chemical potential show opposite behaviors under isothermal compression. The increase in pressure determines an increase in the hydrophobic component, which becomes more positive with a stronger effect for larger ions. Correspondingly, the values of the hydrophilic component become more negative for alkali ions, whereas they are only slightly affected by compression for halide ions. Hydrophobic-hydrophilic quasi-compensation in the slopes is observed for Rb+. For a smaller ion, such as K+, the dependence on pressure of the hydrophilic component is slightly dominant. For a larger ion, as observed in the cases of Cs+, Br-, and I-, the hydrophobic component assumes the determinant role. Pressure dependence of the chemical potential is little affected by corrections introduced for molecular potential truncation. This view can change for possible boundary artifacts that could have affected the static electrostatic potential. Some inference is obtained from comparison with experimental data at 1 atm on the free energy of hydration. Discrepancies show the characteristic asymmetry between cations and anions. The further addition of a correction based on the static potential significantly reduces these discrepancies with important error cancellation on the sum of chemical potentials of ions of opposite charge. The correction is applied also at higher pressures, and results are compared with those obtained by adding an alternative correction that is based on the water number density. Regardless of the ion, changes of the chemical potential induced by an increase in pressure appear to be dominated by the hydrophobic component, in particular when using the alternative correction. For bromide and iodide electrolytes, the two corrections give chemical potentials in good agreement.

A Self-Assembled Cage Binding Iodide Anions over Other Halide Ions in Water.

A triangular-prism shaped cage was self-assembled by the formation of a dynamic covalent bond, namely a hydrazone, in acidic aqueous solution. The hexacationic host bears a number of relatively acidic protons pointing inside the cage cavity, which is able to accommodate an iodide anion selectively in water over other halide anions such as F- , Cl- , and Br- , which are more hydrated. As a comparison, a macrocycle analogue bearing fewer positive charges and fewer hydrogen bond donors shows no anion binding ability. As inferred from the single-crystal structure and theoretical calculations, the ability of the cage to selectively recognize iodide anions results from the combination of electrostatic forces, C-H⋅⋅⋅I- hydrogen bonds, and the hydrophobic effect.

A Self‐Assembled Cage Binding Iodide Anions over Other Halide Ions in Water

AbstractA triangular‐prism shaped cage was self‐assembled by the formation of a dynamic covalent bond, namely a hydrazone, in acidic aqueous solution. The hexacationic host bears a number of relatively acidic protons pointing inside the cage cavity, which is able to accommodate an iodide anion selectively in water over other halide anions such as F−, Cl−, and Br−, which are more hydrated. As a comparison, a macrocycle analogue bearing fewer positive charges and fewer hydrogen bond donors shows no anion binding ability. As inferred from the single‐crystal structure and theoretical calculations, the ability of the cage to selectively recognize iodide anions results from the combination of electrostatic forces, C−H⋅⋅⋅I− hydrogen bonds, and the hydrophobic effect.

Aqueous solvation of the chloride ion revisited with density functional theory: impact of correlation and exchange approximations.

The specificity of aqueous halide solvation is fundamental to a wide range of bulk and interfacial phenomena spanning from biology to materials science. Halide polarizability is thought to drive the ion specificity, and if so, it is essential to have an accurate description of the electronic properties of halide ions in water. To this end, the solvation of the chloride anion, Cl- has been reinvestigated with state-of-the-art density functional theory. Specifically, the PBE-D3, PBE0-D3, and SCAN functionals have been employed to probe the impact of correlation and exchange approximations. Anticipating the findings, adding exact exchange improves the electronic structure, but simultaneously significantly reduces the Cl- polarizability, resulting in an over-structured Cl-O radial distribution function (RDF) and longer water H-bond lifetimes to Cl-. SCAN does not yield as much improvement in the energetics of Cl- relative to bulk water, but does result in a smaller reduction of the polarizability and thus a less structured Cl-O RDF, which agrees better with experiment. Special consideration is therefore warranted in assessing the impact of exchange on the energy, charge density, and the charge density response when designing and testing hybrid functionals for aqueous halide solvation.

Extreme surface propensity of halide ions in water.

Water possesses an extremely high polarity, making it a unique solvent for salts. Indeed, aqueous electrolyte solutions are ubiquitous in the atmosphere, biology, energy applications and industrial processes. For many processes, chemical reactions at the water surface are rate determining, and the nature and concentration of the surface-bound electrolytes are of paramount importance, as they determine the water structure and thereby surface reactivity. Here we investigate the dynamics of water molecules at the surface of sodium chloride and sodium iodide solutions, using surface-specific femtosecond vibrational spectroscopy. We quantify the interfacial ion density through the reduced energy transfer rates between water molecules resulting from the lowered effective interfacial density of water molecules, as water is displaced by surface active ions. Our results reveal remarkably high surface propensities for halogenic anions, higher for iodide than for chloride ions, corresponding to surface ion concentrations several times that of the bulk.

The structure of ionic aqueous solutions at interfaces: An intrinsic structure analysis

We investigate the interfacial structure of ionic solutions consisting of alkali halide ions in water at concentrations in the range 0.2-1.0 molal and at 300 K. Combining molecular dynamics simulations of point charge ion models and a recently introduced computational approach that removes the averaging effect of interfacial capillary waves, we compute the intrinsic structure of the aqueous interface. The interfacial structure is more complex than previously inferred from the analysis of mean profiles. We find a strong alternating double layer structure near the interface, which depends on the cation and anion size. Relatively small changes in the ion diameter disrupt the double layer structure, promoting the adsorption of anions or inducing the density enhancement of small cations with diameters used in simulation studies of lithium solutions. The density enhancement of the small cations is mediated by their strong water solvation shell, with one or more water molecules "anchoring" the ion to the outermost water layer. We find that the intrinsic interfacial electrostatic potential features very strong oscillations with a minimum at the liquid surface that is ∼4 times stronger than the electrostatic potential in the bulk. For the water model employed in this work, SPC/E, the electrostatic potential at the water surface is ∼-2 V, equivalent to ∼80 k(B)T (for T = 300 K), much stronger than previously considered. Furthermore, we show that the utilization of the intrinsic surface technique provides a route to extract ionic potentials of mean force that are not affected by the thermal fluctuations, which limits the accuracy of most past approaches including the popular umbrella sampling technique.

Ion-Pair Formation of Sodium Salts of Several Oxo Anions and Cadmium Halides in Water and the Distribution of Monovalent Sulfate Ion Pairs in Nitrobenzene

Ion-pair formation constants (Kn0, n = 1, 2) of Na2SO4, Na2S2O3, Na2CrO4, CaCl2, CdCl2, CdBr2, and CdI2 in water were determined at 298 K and an ionic strength of 0 mol·dm−3 by potentiometry with Na+-, Cl−-, Br−-, and I−-selective electrodes. Simultaneously, equations for the Kn0 determination based on the DeFord−Hume method were derived. The thus-obtained K10 values for Na2SO4, Na2S2O3, CdCl2, and CaCl2 were compared with those reported before, and the validity of their values was confirmed. Interactions of Na+ with oxo anions (X2−) and Cd2+ with the halide ions in water were characterized by considering hydration to and Coulombic forces acting between these ions. Then, a difference in K10 between CdI+ and NaSO4− was interpreted as that in the Coulombic force between the pairing ions. Also, the K10 difference between Na+BPh4− and Ph4As+Cl− or CaCl+ and NaSO4− was examined by using a thermodynamic cycle of these salts in a water/nitrobenzene (NB) distribution system. Furthermore, the distribution constant...

The Ionic Work Function and its Role in Estimating Absolute Electrode Potentials

The experimental determination of the ionic work function is briefly described. Data for the proton, alkali metal ions, and halide ions in water, originally published by Randles (Randles, J. E. B. Trans Faraday Soc. 1956, 52, 1573) are recalculated on the basis of up-to-date thermodynamic tables. These calculations are extended to data for the same ions in four nonaqueous solvents, namely, methanol, ethanol, acetonitrile, and dimethyl sulfoxide. The ionic work function data are compared with estimates of the absolute Gibbs energy of solvation obtained by an extrathermodynamic route for the same ions. The work function data for the proton are used to estimate the absolute potential of the standard hydrogen electrode in each solvent. The results obtained here are compared with those published earlier by Trasatti (Trasatti, S. Electrochim. Acta 1987, 32, 843) and more recently by Kelly et al. (Kelly, C. P.; Cramer, C. J.; Truhlar, D. G. J. Phys. Chem. B 2006, 110, 16066. Kelly, C. P.; Cramer, C. J.; Truhlar, D. G. J. Phys. Chem. B 2007, 111, 408). A comparison of the ionic work function with the absolute Gibbs solvation energy permits an estimation of the surface potential of the solvent. The results show that the surface potential of water is small and positive whereas the surface potential of the nonaqueous solvents considered is negative. The sign of the surface potential is consistent with the known structure of each solvent.

Photophysics and Photochemistry of a Water-Soluble C60 Dendrimer: Fluorescence Quenching by Halides and Photoinduced Oxidation of I-

Using electronic absorption spectroscopy, and steady-state and time-resolved fluorescence, we studied the singlet state properties of dendro[60]fullerene, a C60 methanoderivative whose addend is a water-soluble dendrimer. Fluorescence quenching, especially by halide ions in water, was characterized at several temperatures. From a new relation for the activation energy of a diffusion-influenced reaction and from a comparison with the quenching in organic solvents, it is concluded that the quenching occurs by the external heavy-atom effect. An efficient photoinduced oxidation of iodide in aerated solutions and in the presence of a fullerene is also reported. The photoinduced oxidation mechanism exclusively involves ground-state oxygen.

Computer simulation studies of the structure and dynamics of ions and non–polar solutes in water

The mobility of simple ions such as alkali–metal and halide ions at room temperature shows two anomalies. Firstly, there are maxima in mobilities as a function of ion size for both positive and negative ions and, secondly, the maximum for negative ions occurs at a larger ionic radius than the maximum for positive ions. Theoretical treatments of this problem are reviewed and it is concluded that a molecular treatment of the system is needed to understand the results. Computer simulation using the simple point charge model (SPC/E) for water reproduced the observations and is used to discuss the application of theories. In particular, the nature of the first solvation shell is correlated with ion mobility. Simulation reveals a further anomaly, namely that if the charge is removed from a large ion, then it moves more slowly. This is interpreted as the result of formation of a solvent cage around the hydrophobic solute. The changes in local structure resulting from changes in charge and size also affect the solvation thermodynamics. Simulations show that the solvation entropy has a double maximum when viewed as a function of charge. The local minimum near zero charge is interpreted as being due to hydrophobic order, and the maxima as the result of structure breaking. This double maximum in the entropy of solvation is a signature of the hydrophobic cage effect. Comparisons are made between ion mobilities in liquid water at ambient and supercritical conditions.

Solvation thermodynamic functions in the mean spherical approximation: Behavior near the solvent critical region

Explicit expressions of the mean spherical approximation (MSA) solvation thermodynamic functions, the Gibbs free energy, energy, enthalpy, entropy, apparent molar heat capacities, and partial molar volume are derived starting from the Helmholtz free energy. The thermodynamic consistency of the MSA solvation thermodynamic functions are discussed. The limiting behavior of the MSA solvation thermodynamic functions is compared to the Born theory. The effect of the solvent structure on the Gibbs and the Helmholtz free energy in the MSA is discussed. The results for alkali and halide ions in water are compared with experimental data. The apparent molar heat capacities at constant volume and pressure, and the partial molar volume are calculated along an isobaric line to the critical region of a dipolar liquid. Some recent experiments on solvation thermodynamics in the critical region are discussed.

A SIMPLE MODEL FOR ION SOLVATION WITH NON ADDITIVE CORES

Abstract : The solvation of monoatomic ions at infinite dilution in polar solvents is considered within the context of the mean spherical approximation (MSA). The role of both ion-dipole and dipole-dipole interactions in determining the Gibbs energy and entropy of solvation is considered in detail for the alkali metal and halide ions in water. The significance of the MSA parameters with respect to local interactions of water molecules with the ions and each other is discussed.

Determination of Individual Partial Molar Volumes of Ions in Water from Measurements of Ultrasonic Vibration Potentials

Abstract A new apparatus for measurements of ionic vibration potentials generated by the propagation of ultrasonic waves through electrolyte solutions was constructed, and used for the determination of partial molar volumes of ions in aqueous solutions of uni-univalent electrolytes. The apparatus, based on the method of ultrasonic vibration potentials (Zana and Yeager), was improved in two points through the introduction of the Doppler effect: a small measuring cell (about 50 cm3) and very low noises (50 nV). Then, measurements of temperature variation of ultrasonic vibration potentials have become possible. Partial molar volumes of hydrogen, alkali metal, and halide ions in water were determined in the range of temperature, 15–45 °C.

Synthesis, Characterization, and Isomerization of trans- and cis-[CoCl2(1,2- or 1,3-diphosphine)2]+ and Related Complexes

Abstract A number of new diphosphine complexes of cobalt(III), trans- and cis-[CoCl2(dmpe, dmpp, dbpe, or dpee)2]+, trans- and cis-[CoBr2(dmpe)2]+, trans- and cis-[CoCl(H2O)(dmpe)2]2+, trans-[Co(I2 or BrI)(dmpe)2]+, and [Co(CO3)(dmpp or dbpe)]+ have been prepared and characterized, where dmpe, dmpp, dbpe, and dpee denote 1,2-bis(dimethylphosphino)ethane, 1,3-bis(dimethylphosphino)propane, 1,2-bis(dibutylphosphino)ethane, and cis-1,2-bis(diphenylphosphino)ethylene, respectively. The trans isomers of the dihalogeno complexes are stable to aquation, while the cis isomers release one of the halide ions in water fairly rapidly to yield cis-[CoX(H2O)(diphosphine)2]+ which isomerizes to the trans isomer on exposure of ultraviolet light. The cis-[CoCl2(diphosphine)2]+ complexes in methanol or ethanol solution isomerize to the trans isomer, and the rates were followed spectrophotometrically in the temperature range of 39.0–65.0 °C. The rates have a first-order dependence on the complex concentration. Both the ΔH\eweq and ΔS\eweq values for the isomerization increase in the order diphosphine=dmpe<dmpp<dbpe<dpee. This order coincides with the increasing order in cone angle of phosphine ligands, and suggests that the difference in rates among the complexes comes from a steric factor. Absorption spectra of all the complexes have been recorded and the results compared with those of the corresponding ethylenediamine complexes.

Solvated positron chemistry competitive positron reactions with halide ions in water

It is shown by means of the angular correlation technique that the binding of positrons to halides is strongly influenced by solvation effects. For aqueous solutions we find increasing values for the binding energies between the halide and the positron with increasing mass of the halide. This is contrary to the calculations of Cade and Farazdel for the vacuum case.

Solvation of halide ions in water and dipolar aprotic solvents studied by halogen nucleus nuclear magnetic resonance

Solvation of halide ions in water and dipolar aprotic solvents studied by halogen nucleus nuclear magnetic resonance