G.T. Hefter

Dynamics of Imidazolium Ionic Liquids from a Combined Dielectric Relaxation and Optical Kerr Effect Study: Evidence for Mesoscopic Aggregation

We have measured the intermolecular dynamics of the 1,3-dialkylimidazolium-based room-temperature ionic liquids (RTILs) [emim][BF(4)], [emim][DCA], and [bmim][DCA] at 25 degrees C from below 1 GHz to 10 THz by ultrafast optical Kerr effect (OKE) spectroscopy and dielectric relaxation spectroscopy (DRS) augmented by time-domain terahertz and far-infrared FTIR spectroscopy. This concerted approach allows a more detailed analysis to be made of the relatively featureless terahertz region, where the higher frequency diffusional modes are strongly overlapped with librations and intermolecular vibrations. Of greatest interest though, is an intense low frequency (sub-alpha) relaxation that we show is in accordance with recent simulations that have reported mesoscopic structure arising from aggregates or clusters--structure that explains the anomalous and inconveniently high viscosities of these liquids.

Interactions and dynamics in electrolyte solutions by dielectric spectroscopy

Despite its immense abilities to quantify many aspects of ion-ion and ion-solvent interactions, dielectric relaxation spectroscopy (DRS) has long been neglected as a tool for the investigation of the structure and dynamics of electrolyte solutions. The reasons for this are briefly discussed and it is shown that many of the difficulties associated with this technique have been overcome in recent years by technological developments. Representative applications of DRS to the investigation of ion solvation and ion association in electrolyte solutions of chemical, industrial, geochemical and biological interest, including room temperature ionic liquids and polyelectrolyte systems, are discussed. The advantages of linking DRS measurements to information obtained from other experimental techniques and from computer simulations are highlighted.

Temperature Dependence of the Dielectric Properties and Dynamics of Ionic Liquids

Dielectric spectra were measured for eight, mostly imidazolium-based, room temperature ionic liquids (RTILs) over a wide range of frequencies (0.2 < or = nu/GHz < or = 89) and temperatures (5 < or = theta/degrees C < or = 65). Detailed analysis of the spectra shows that the dominant low frequency process centred at ca. 0.06 to 10 GHz (depending on the salt and the temperature) is better described using a symmetrically broadened Cole-Cole model rather than the asymmetric Cole-Davidson models used previously. Evaluation of the temperature dependence of the static permittivities, effective dipole moments, volumes of rotation, activation energies, and relaxation times derived from the dielectric data indicates that the low frequency process cannot be solely due to rotational diffusion of the dipolar imidazolium cations, as has been thought, but must also include other contributions, probably from cooperative motions. Analysis of the Debye process observed at higher frequencies for these RTILs is not undertaken because it overlaps with even faster processes that lie outside the range of the present instrumentation.

Interactions and Dynamics in Ionic Liquids

Precise dielectric spectra have been determined at 25 degrees C over the exceptionally broad frequency range of 0.1 <or= nu/GHz <or= 3000 for the imidazolium-based room-temperature ionic liquids (RTILs) [bmim][BF4], [bmim][PF6], [bmim][DCA], and [hmim][BF4]. The spectra are dominated by a low-frequency process at approximately 1 GHz with a broad relaxation time distribution of the Cole-Davidson or Cole-Cole type, which is thought to correspond to the rotational diffusion of the dipolar cations. In addition, these RTILs possess two Debye relaxations at approximately 5 GHz and approximately 0.6 THz and a damped harmonic oscillation at approximately 2.5 THz. The two higher-frequency modes are almost certainly due to cation librations, but the origin of the approximately 5 GHz mode remains obscure.

Chemical speciation of environmentally significant heavy metals with inorganic ligands. Part 1: The Hg2+– Cl–, OH–, CO32–, SO42–, and PO43– aqueous systems (IUPAC Technical Report)

This document presents a critical evaluation of the equilibrium constants and reaction enthalpies for the complex formation reactions between aqueous Hg(II) and the common environmental inorganic ligands Cl–, OH–, CO32–, SO42–, and PO43–. The analysis used data from the IUPAC Stability Constants database, SC-Database, focusing particularly on values for 25 °C and perchlorate media. Specific ion interaction theory (SIT) was applied to reliable data available for the ionic strength range Ic < 3.0 mol dm–3. Recommended values of log10βp,q,r° and the associated reaction enthalpies, ∆rHm°, valid at Im = 0 mol kg–1 and 25 °C, were obtained by weighted linear regression using the SIT equations. Also reported are the equations and specific ion interaction coefficients required to calculate log10βp,q,r° values at higher ionic strengths and other temperatures. A similar analysis is reported for the reactions of H+ with CO32– and PO43–. Diagrams are presented to show the calculated distribution of Hg(II) amongst these inorganic ligands in model natural waters. Under typical environmental conditions, Hg(II) speciation is dominated by the formation of HgCl2(aq), Hg(OH)Cl(aq), and Hg(OH)2(aq).

Complexation of iron (III) and iron (II) by citrate. Implications for iron speciation in blood plasma

Estimates of the concentrations and identity of the predominant complexes of iron with the low-molecular-mass ligands in vivo are important to improve current understanding of the metabolism of this trace element. These estimates require a knowledge of the stability of the iron-citrate complexes. Previous studies on the equilibrium properties of the Fe(III)-citrate and Fe(II)-citrate are in disagreement. Accordingly, in this work, glass electrode potentiometric titrations have been used to re-determine the formation constants of both the Fe(III)- and Fe(II)-citrate systems at 25 degrees C in 1.00 M (Na)Cl and the reliability of these constants has been evaluated by comparing the measured and predicted redox potentials of the ternary Fe(III)-Fe(II)-citrate system. The formation constants obtained in this way were used in computer simulation models of the low-molecular-mass iron fraction in blood plasma. Redox equilibria of iron are thus included in large models of blood plasma for the first time. The results of these calculations show the predominance of Fe(II)-carbonate complexes and a significant amount of aquated Fe(II) in human blood plasma.

Raman spectroscopic investigation of speciation in MgSO4(aq)

Careful measurements have been made of the Raman spectra of aqueous solutions of Mg(ClO4)2, MgCl2, (NH4)2SO4 and MgSO4 down to 50 cm−1 and, in some cases, to extremely low concentrations (≥0.06 mmol kg−1) and high temperatures (≤200 °C). In MgSO4(aq), the well known asymmetry in the ν1-SO42− mode at ∼980 cm−1 that develops with increasing concentration has been assigned to a mode at 993 cm−1 associated with the formation of an MgOSO3 contact ion pair (CIP). Confirmation of this assignment is provided by the simultaneous and quantitative appearance of stretching modes for the Mg–OSO3 bond of the ligated SO42− at 245 cm−1 and for the (H2O)5MgOSO3 unit at 328 cm−1. The CIP becomes the dominant species at higher temperatures. Alternative explanations of the broadening of the ν1-SO42− mode are shown to be inconsistent with this and other Raman spectral evidence such as the similarity of the ν1-SO42− mode for MgSO4 in H2O and D2O. After subtraction of the CIP component at 993 cm−1, the ν1-SO42− band in MgSO4(aq) showed systematic differences from that in (NH4)2SO4(aq). This is consistent with a previously undetected ν1-SO42− mode at 982.2 cm−1 that can be assigned to the presence of solvent-shared ion pairs (SIPs). In solutions with high Mg2+/SO42− concentration ratios, a further ν1-SO42− mode was observed at 1005 cm−1, which has been tentatively assigned to a Mg2SO42+(aq) triple ion. All of these observations are shown to be in excellent agreement with recent dielectric relaxation spectroscopy measurements. In addition, the correct relationship between the Mg2+/SO42− association constant determined by Raman spectroscopic measurements and those obtained by other techniques is derived. It is shown that thermodynamic data measured by Raman spectroscopy for systems involving other (Raman-undetected) ion-pair types in addition to CIPs, cannot and should not be compared directly with those obtained by traditional techniques.

Chemical speciation of environmentally significant metals with inorganic ligands Part 2: The Cu2+-OH-, Cl-, CO32-, SO42-, and PO43- systems (IUPAC Technical Report)

The numerical modeling of CdII speciation amongst the environmental inorganic ligands Cl–, OH–, CO32–, SO42–, and PO43– requires reliable values for the relevant stability (formation) constants. This paper compiles and provides a critical review of these constants and related thermodynamic data. It recommends values of log10βp,q,r° valid at Im = 0 mol kg–1 and 25 °C (298.15 K), along with the equations and empirical reaction ion interaction coefficients, ∆ε , required to calculate log10βp,q,r values at higher ionic strengths using the Brønsted–Guggenheim–Scatchard specific ion interaction theory (SIT). Values for the corresponding reaction enthalpies, ∆rH, are reported where available. Unfortunately, with the exception of the CdII-chlorido system and (at low ionic strengths) the CdII-sulfato system, the equilibrium reactions for the title systems are relatively poorly characterized. In weakly acidic fresh water systems (–log10 {[H+]/c°} < 6), in the absence of organic ligands (e.g., humic substances), CdII speciation is dominated by Cd2+(aq), with CdSO4(aq) as a minor species. In this respect, CdII is similar to CuII [2007PBa] and PbII [2009PBa]. However, in weakly alkaline fresh water solutions, 7.5 < –log10 {[H+]/c°} < 8.6, the speciation of CdII is still dominated by Cd2+(aq), whereas for CuII [2007PBa] and PbII [2009PBa] the carbonato- species MCO3(aq) dominates. In weakly acidic saline systems (–log10 {[H+]/cϒ} < 6; –log10 {[Cl–]/c°} < 2.0) the speciation is dominated by CdCln(2–n)+ complexes, (n = 1–3), with Cd2+(aq) as a minor species. This is qualitatively similar to the situation for CuII and PbII. However, in weakly alkaline saline solutions, including seawater, the chlorido- complexes still dominate the speciation of CdII because of the relatively low stability of CdCO3(aq). In contrast, the speciation of CuII [2007PBa] and PbII [2009PBa] in seawater is dominated by the respective species MCO3(aq). There is scope for additional high-quality measurements in the Cd2+ + H+ + CO32– system as the large uncertainties in the stability constants for the Cd2+-carbonato complexes significantly affect the speciation calculations.

From ionic liquid to electrolyte solution: dynamics of 1-N-butyl-3-N-methylimidazolium tetrafluoroborate/dichloromethane mixtures.

Dielectric spectra have been measured at 25 degrees C for mixtures of the room temperature ionic liquid 1- N-butyl-3- N-methylimidazolium tetrafluoroborate (IL) with dichloromethane (DCM) over the entire composition range at frequencies 0.2 less than or approximately nu/GHz < or = 89. The spectra could be satisfactorily fitted by assuming only two relaxation modes: a Cole-Cole process at lower frequencies and a Debye process at higher frequencies. However, detailed analysis indicated that both spectral features contain additional modes, which could not be resolved due to overlaps. The spectra indicate that the IL appears to retain its chemical character to extraordinarily high levels of dilution ( x IL greater than or approximately 0.5) in DCM. At even higher dilutions ( x IL less than or approximately 0.3), the IL behaves as a conventional but strongly associated electrolyte.

Iron chelators of the pyridoxal isonicotinoyl hydrazone class Part II. Formation constants with iron(III) and iron(II)

Formation constants for the iron(III) complexes of the orally effective iron chelator pyridoxal isonicotinoyl hydrazone (PIH) and three analogues: pyridoxal benzoyl hydrazone (PBH), 3-hydroxy- isonicotinaldehyde isonicotinoyl hydrazone (IIH) and salicylaldehyde isonicotinoyl hydrazone (SIH), have been determined by a combination of spectrophotometry and potentiometry. All four ligands bind iron(III) strongly giving, at physiological pH 7.4, values of pM (−log[uncomplexed metal]) between 27.7 and 50, comparable to or greater than those for transferrin (25.6) and desferrioxamine B (28.6). The complexation of Fe(II) by PIH has also been studied and has been found to be appreciable but very much weaker than that for Fe(III).