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Glenn Hefter

Publications and source records attributed to Glenn Hefter.

14 recordsLinked to original sources
PubMed2006

Temperature effects on ion association and hydration in MgSO4 by dielectric spectroscopy.

A detailed investigation of aqueous solutions of magnesium sulfate has been made by dielectric relaxation spectroscopy (DRS) over a wide range of frequencies (0.2 MgSO(4) (0)(aq) is in good agreement with literature data at lower temperatures but is overestimated at higher temperatures due to processing difficulties. Despite the limited precision of the spectra, analysis of the individual steps in the ion-association process is possible for the first time. The 2SIPs are formed with little disturbance to their hydration shells, the (partial) destruction of which appears to occur mostly during the formation of SIPs. Effective hydration numbers derived from the DRS spectra indicate that both Mg(2+) and SO(4) (2-) influence solvent water molecules beyond their first hydration spheres but that MgSO(4)(aq) is less strongly hydrated than the previously studied CuSO(4)(aq).

Journal Article↗
PubMed2006

Ion association and hydration in aqueous solutions of copper(II) sulfate from 5 to 65 degrees C by dielectric spectroscopy.

Aqueous solutions of copper(II) sulfate have been studied by dielectric relaxation spectroscopy (DRS) over a wide range of frequencies (0.2 less, similar nu/GHz < or = 89), concentrations (0.02 < or = m/mol kg(-1) less, similar 1.4), and temperatures (5 < or = t/ degrees C < or = 65). The spectra show clear evidence for the simultaneous existence of double-solvent-separated, solvent-shared, and contact ion pairs at all temperatures, with increasing formation especially of contact ion pairs with increasing temperature. The overall ion association constant corresponding to the equilibrium: Cu2+(aq) + SO4(2-)(aq) right harpoon over left harpoon CuSO4(0)(aq) was found to be in excellent agreement with literature data over the investigated temperature range. However, the precision of the spectra and other difficulties did not allow a thermodynamic analysis of the formation of the individual ion-pair types. Effective hydration numbers derived from the DRS spectra were high but consistent with simulation and diffraction data from the literature. They indicate that both ions influence solvent water molecules beyond the first hydration sphere. The implications of the present findings for previous observations on copper sulfate solutions are briefly discussed.

Anions↗
PubMed2006

Hydration of tetraphenylphosphonium and tetraphenylborate ions by dielectric relaxation spectroscopy.

A systematic study of the dielectric relaxation spectra of aqueous solutions of NaBPh4 and Ph4PCl has been made at solute concentrations of 0.02 < or = c/M < or = 0.82 and 0.20, respectively, and over a wide range of frequencies (0.2 < or = nu/GHz < or = 89) at 25 degrees C. The spectra were best described by a superposition of four Debye processes, consisting of a very small ion-pair contribution with an average relaxation time of about 300 ps, a "slow"-water relaxation at 17 ps, and two bulk-water relaxations at 8 ps and 0.25 ps, respectively. The slow-water process has been assigned to the presence of a sheath of water molecules surrounding BPh4- and Ph4P+, whose structure has been enhanced by its proximity to the bulky hydrophobic phenyl rings. A structure-making effect on the remaining solvent water can also be observed at low concentrations. More importantly, BPh4- and Ph4P+ show almost identical hydration characteristics, which provides indirect support for the use of the tetraphenylphosphonium tetraphenylborate reference electrolyte assumption in deriving single-ion thermodynamic properties.

Journal Article↗
PubMed2006

Effects of nonionic surfactant C12E5 on the cooperative dynamics of water.

A dielectric relaxation study of binary mixtures of nonionic surfactant C12E5 + water has been made as a function of temperature in the isotropic micellar, lamellar, and hexagonal regions of the phase diagram. Two dielectric dispersion steps were found and could be assigned to the intermolecular cooperative dynamics of water at the micellar interface and in the bulk water domains. A quantitative analysis is given. The relaxation amplitudes were used to determine effective hydration numbers. The activation energies of water relaxation were calculated from the relaxation times. The data indicate weaker surfactant-water and water-water interactions near the micellar interface compared to those of bulk liquid water. Further analysis revealed the presence of water clusters large enough to show a cooperative relaxation mode even at high surfactant concentrations. However, the relaxation time of this mode is larger compared to that of pure water. This points out the importance of confinement effects on water dynamics.

Journal Article↗
PubMed2006

Chemical speciation in concentrated alkaline aluminate solutions in sodium, potassium and caesium media. Interpretation of the unusual variations of the observed hydroxide activity.

A detailed electrochemical investigation using H2/Pt electrode potentiometry as well as Raman and NMR spectroscopy was carried out to develop a comprehensive chemical explanation for the unusual patterns of hydroxide concentrations observed in strongly alkaline, highly concentrated aluminate solutions (Bayer-liquors). For this, aluminate solutions with various alkaline metal background cations were investigated. The effect of the temperature on the observed patterns was also studied, and for comparison with solutions of similar concentrations, the chemical speciation of borate solutions was also studied. The formation of the NaOH 0 ion-pair has been proven with the formation constant (defined in terms of activities) beta 0 = 0.78 +/- 0.08. The formation of analogous KOH 0 or CsOH 0 ion-pairs under the experimental conditions applied is negligible. Assuming the formation of the NaAl(OH)4 (0) ion-pair is not necessary for modeling the experimental findings, as its formation causes only secondary effects on the potentiometric patterns. It has also been shown that all experimental data can be interpreted quantitatively if the formation of the doubly charged dimeric aluminate species is included in the calculation of the changes in the mean activity coefficients. The formation constant of the aluminate dimer could not be estimated purely from the H2/Pt potentiometric data but a lower limit for its formation constant (defined in terms of activities) has been derived. These conclusions are in full congruency with those derived from the Raman spectra of solutions with similar concentrations, so the two independent experimental methods lead to the same set of chemical species in highly concentrated alkaline aluminate solutions.

Journal Article↗
PubMed2006

Ion pairing.

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Journal Article↗
PubMed2005

Ultrasonic velocities, densities, viscosities, electrical conductivities, Raman spectra, and molecular dynamics simulations of aqueous solutions of Mg(OAc)2 and Mg(NO3)2: Hofmeister effects and ion pair formation.

The ultrasonic velocities, densities, viscosities, and electrical conductivities of aqueous solutions of magnesium nitrate and magnesium acetate have been measured from dilute to saturation concentrations at 0 < or = t/degrees C < or = 50. The temperature derivative of the isentropic compressibility, kappa(s), became zero at 2.28 and 2.90 mol kg(-1) for Mg(OAc)2 and Mg(NO3)2 solutions, respectively, at 25 degrees C. The total hydration numbers of the dissolved ions were estimated to be, respectively, 24.3 and 19.2 at these concentrations. Differences in kappa(s) for various M2+ salts, using the present and literature data, correlated with reported M2+-OH2 bond lengths and to a lesser extent with cationic charge densities (ionic radii). The influence of anions on kappa(s) appears to follow the Hofmeister series and also correlates approximately with the anionic charge density. Substantial differences between Mg(OAc)2(aq) and Mg(NO3)2(aq) occur with respect to their structural relaxation times (derived from compressibility and viscosity data) and their electrical conductivities. These differences were attributed to a much greater ion association in Mg(OAc)2 solutions. Raman spectra recorded at 28 degrees C confirmed the presence of various types of contact ion pairs including mono- and bidentate complexes in Mg(OAc)2(aq). In Mg(NO3)2(aq), only noncontact ion pairs appear to be formed even at high concentrations. The experimental results are supported by molecular dynamics simulations, which also reveal the much stronger tendency of OAc- compared to NO3- to associate with Mg2+ in aqueous solutions. The simulations also allow an evaluation of the ion-ion and ion-water radial distribution functions and cumulative sums and provide a molecular picture of ion hydration in Mg(OAc)2(aq) and Mg(NO3)2(aq) at varying concentrations.

Journal Article↗
PubMed2005

Quantitative determination of an aluminate dimer in concentrated alkaline aluminate solutions by Raman spectroscopy.

Raman spectra of concentrated alkaline aluminate solutions in various M'OH media (M'(+) = Na(+), K(+), Cs(+) and (CH(3))(4)N(+)) have been investigated systematically as a function of concentration and water activity, a(w). All spectra at [Al(III)](T) < or = 1 M and at 0.1 < or = [M'OH](T)/M < or = 5 (where the square brackets denote concentrations and the subscript T totals) exhibit one significant Raman mode in the low frequency region, at ca. 620 cm(-1), due to the symmetric Al(OH)(4)(-) stretch. At higher [Al(III)](T) and [M'OH](T) new modes appear at 530-550 and 700-720 cm(-1). The intensities of these new bands depend on [Al(III)](T) and a(w) but are independent of [OH(-)](T) and are only slightly cation-dependent. All three bands shift towards higher wavenumbers at [M'OH](T) > 10 M, probably due to ion-pairing. Spectra at [M'OH](T) < 10 M have been interpreted quantitatively by assuming that the integrated peak area of the 620 cm(-1) mode is linearly proportional to [Al(OH)(4)(-)] at constant a(w) and that the only significant equilibrium in these systems is the formation of a dimer that can be represented as (Al(OH)(4))(2)(2-)(aq), although it may exist in an oxo-bridged form such as [(HO)(3)Al-O-Al(OH)(3)](2). The (aquated) species Na(+), OH(-), Al(OH)(4)(-), the dimer, and their ion-pairs, were sufficient to interpret all the Raman observations. No evidence was found for various other species that have been claimed to exist in concentrated alkaline aluminate solutions.

Alkalies↗
PubMed2005

Is there an anionic Hofmeister effect on water dynamics? Dielectric spectroscopy of aqueous solutions of NaBr, NaI, NaNO3, NaClO4, and NaSCN.

A systematic study of the dielectric relaxation spectra of aqueous solutions of NaBr, NaI, NaNO(3), NaClO(4), and NaSCN has been made over a wide range of frequencies (0.2 < or = nu/GHz < or = 89) and solute concentrations (0.05 < or = c/M < or = 1.5) at 25 degrees C. The spectra could be adequately described by a single Cole-Cole (CC) process, symmetrically broadened relative to that of pure water. However, similar quality fits were also obtained with a three-Debye-process (3D) model consisting of a small ion-pair contribution at lower frequencies and two solvent relaxations at higher frequencies. Assuming the ions to be solvent separated, the 3D model provided estimates of their association constants and their rate constants for formation and dissociation. The bulk water relaxation times obtained from both models showed almost no dependence on the nature of the anion. Nevertheless, there were subtle differences in the concentration dependences of the relaxation times which correlated with some, but not all, of the anion properties that are believed to be relevant for explaining the anionic Hofmeister series.

Journal Article↗
PubMed2003

Mononuclear cyano- and hydroxo-complexes of iron(III).

A detailed investigation of the iron(III)-cyanide and iron(III)-hydroxide systems has been made in NaClO(4) media at 25 degrees C, using combined UV-vis spectrophotometric and pH-potentiometric titrations. For the Fe(III)/OH- system, use of low total Fe(III) concentrations (< or =10 microM) and a wide pH range (0 < or = pH < or = 12.7) enabled detection of six mononuclear complexes, corresponding to the following equilibria: Fe3+(aq)+rH2O<=>Fe(OH)r(3-r)+(aq) + rH(+)(aq), where r = 1-6 with stability constants (log *beta 1r) of -2.66, -7.0, -12.5, -20.7, -30.8, and -43.4, respectively, at I = 1 M (NaClO(4)). It was also found to be possible to measure, for the first time, stability constants for most of the following equilibria: Fe3+(aq)+qCN-(aq)<=>Fe(CN)q(3-q)+(aq), despite a plethora of complicating factors. Values of log beta(1q) = 8.5, 15.8, 23.1, and 38.8 were obtained at I = 1.0 M (NaClO(4)) for q = 1-3 and 6, respectively. No reliable evidence could be obtained for the intermediate (q = 4 or 5) complexes. Similar results were obtained for both systems at I = 0.5 M(NaClO(4)). Spectra for the individual mononuclear complexes detected for Fe(III) with OH- and CN- are reported. Attempted measurements on the Fe(II)/CN- system were unsuccessful, but values of log beta(16)(Fe(CN)(6)(4-)) = 31.8 and log beta(15)(Fe(CN)(5)(3-) approximately 24 were estimated from well established electrode potential and other data.

Journal Article↗
PubMed2003

Hydration and ion pairing in aqueous sodium oxalate solutions.

Dielectric spectra have been measured for aqueous sodium oxalate solutions up to the saturation concentration (0.04 < or = c[mmol L-1] < or = 0.25) at 25 degrees C over the approximate frequency range 0.2 < or = v [GHz] < or = 20. The spectra exhibit a process at about 1 GHz associated with the presence of ion pairs, in addition to the dominant solvent relaxation process at about 18 GHz. Detailed analysis of the solvent dispersion amplitude indicates that the oxalate ion is highly hydrated but that its solvation sheath is "fragile", decreasing quickly with increasing solute concentration. The NaOx(aq)- ion pair is shown to be of the double-solvent-separated (2SIP) type, with an infinite dilution association constant KA = 1.04 +/- 0.02. Analysis of the ion-pair relaxation time as a function of solute concentration gave rate constants for the formation (k12 = (7.3 +/- 0.4) 10(9) L mol-1 s-1) and dissociation (k21 = (6.7 +/- 0.5) 10(8) s-1) of the ion pair. These values are reasonably close to the diffusion-controlled values predicted by the Eigen theory, consistent with a 2SIP structure for the ion pair.

Journal Article↗