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ABSTRACT: Aqueous solutions of sodium propanoate (NaOPr) and n-butanoate (NaOBu) have been studied at concentrations, c ≤ 3 M, by broadband dielectric relaxation spectroscopy over the frequency range 0.2 ≤ ν/GHz ≤ 89 at 25 °C. Three relaxation modes were resolved, centred at (approximately) 1, 8 and 18 GHz, for both sets of solutions. The two faster modes were assigned to the cooperative relaxation of "slow" and bulk water molecules. Detailed analysis of the spectra indicated that both OPr(-) and OBu(-) were strongly hydrated, with ~23 and ~33 slow water molecules per anion respectively at infinite dilution. These effective hydration numbers include ~6 water molecules hydrophilically-bound to the carboxylate moiety, with the remainder arising from the hydrophobic hydration of the nonpolar alkyl chains. The latter shows a characteristic rapid decrease with increasing solute concentration, which facilitated the separation of the hydrophobic and hydrophilic contributions. The lowest frequency mode was a composite with contributions from ion-cloud, ion-pair and anion relaxations. While this low intensity mode provided specific evidence of weak ion pairing between Na(+)(aq) and the carboxylate anions reliable estimates of the association constant could not be made because of its composite nature.
The Journal of Physical Chemistry B 01/2013; · 3.70 Impact Factor
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ABSTRACT: A detailed investigation of the binary mixtures of the ionic liquids (ILs) 1-N-R-3-N-methylimidazolium tetrafluoroborate (R = ethyl, n-butyl, n-hexyl) with the important molecular solvent acetonitrile (AN) over the entire composition range has been made at 25 °C using broadband dielectric spectroscopy. All spectra showed two modes: a Cole-Cole (CC) mode centered at ~2 GHz and a Debye mode centered at ~50 GHz. However, detailed analysis indicated both relaxations were composites. The Debye mode arises from the rotational diffusion of free AN molecules with contributions from ultrafast vibrations and librations of the ILs. The CC mode corresponds to the jump rotation of the imidazolium cations and the hindered rotational diffusion of "slow" AN molecules solvating them. At very low IL concentrations 1:1 contact ion pairs are dominant. Overall, these IL + AN mixtures can be divided into two broad regions: at IL mole fraction (x(IL)) ≲ 0.2 the IL behaves as a rather weakly associated conventional electrolyte while at x(IL) ≳ 0.2 it takes on its IL characteristics, "lubricated" by the AN.
The Journal of Physical Chemistry B 06/2012; 116(25):7509-21. · 3.70 Impact Factor
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ABSTRACT: Isopiestic measurements of mixed electrolyte solutions involving sodium hydroxide and other components of Bayer liquors were
performed at 50 °C and 100 °C. Most of the systems studied obey Zdanovskii’s rule exactly or very closely. However, those
with sodium chloride as one of their components show deviations from Zdanovskii’s rule, which are well predicted by our thermodynamic
modelling software based on Pitzer’s equations.
Journal of Solution Chemistry 04/2012; 36(11):1619-1634. · 1.41 Impact Factor
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ABSTRACT: The structure and dynamics of ionic liquids (ILs) are unusual due to the strong interactions between the ions and counter ions. These microscopic properties determine the bulk transport properties critical to applications of ILs such as advanced fuel cells. The terahertz dynamics and slower relaxations of simple alkylammonium nitrate protic ionic liquids (PILs) are here studied using femtosecond optical Kerr-effect spectroscopy, dielectric relaxation spectroscopy, and terahertz time-domain spectroscopy. The observed dynamics give insight into more general liquid behaviour while comparison with glass-forming liquids reveals an underlying power-law decay and relaxation rates suggest supramolecular structure and nanoscale segregation.
Faraday Discussions 01/2012; 154:145-53; discussion 189-220, 465-71. · 5.00 Impact Factor
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ABSTRACT: Dielectric relaxation (DR) spectra have been measured for aqueous solutions of sodium formate (NaOFm) and sodium acetate (NaOAc) over a wide range of frequencies (0.2 ≤ ν/GHz ≤ 89) up to solute concentrations c is less or approximately equal to 3.2 M and is less or approximately equal to 3.7 M, respectively, at 25 °C. Measurements were also made on NaOAc(aq) at 15 ≤ T/°C ≤ 35. In addition to the usual dominant bulk-water relaxation process at ~20 GHz, one or two further relaxation modes were detected. One process, centered at ~8 GHz and observed for both NaOFm(aq) and NaOAc(aq), was attributed to the presence of slow water in the hydration shells of the anions. A lower-frequency process at ~0.6 GHz, observed only for NaOAc(aq) at c is less or approximately equal to 1 M, was thought to be due to the presence of very small concentrations of ion pairs. Detailed analysis of the spectra indicated that very few (<2 per anion) water molecules were irrotationally bound (frozen) on the DR time scale. Nevertheless, both anions are strongly hydrated, as evidenced by the significant amounts of slow water detected. Such H(2)O molecules with reduced dynamics result from two distinct effects. The first is the relatively strong hydrophilic interaction of water with the -COO(-) moiety, which is similar for the two anions and little affected by increasing solute concentration. The second (for OAc(-) only) is the hydrophobic hydration of the -CH(3) group, which is fragile, decreasing markedly with increasing solute concentration and temperature. The activation parameters for bulk-water relaxation in NaOAc(aq) indicated a breakdown of the bulk water structure at high solute concentrations.
The Journal of Physical Chemistry B 11/2011; 116(1):314-23. · 3.70 Impact Factor
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ABSTRACT: The thermodynamic properties of the binary aqueous solutions of 183 electrolytes at 25 °C and 1 bar have been fitted using a standard form of the Pitzer equations. Where possible, all thermodynamic properties have been treated simultaneously, in contrast to previous compilations of Pitzer parameters. Prior to fitting, a critical assessment of the available information for each system was made using the JESS database and software. Employing linear regression with singular value decomposition and using an appropriate objective function criterion, more than two-thirds of the systems could be satisfactorily fitted to the upper concentration limit of the available data. Only six electrolytes proved to be completely intractable using the present Pitzer model. All of these systems (which included HF, H2SO4, and H3PO4) are known to exhibit significant changes in chemical speciation at low concentrations (even though ion association per se does not preclude a satisfactory fit). The present Pitzer ion-interaction parameters provide a coherent, up-to-date set of empirical coefficients that can be combined in a self-consistent manner to produce multicomponent electrolyte solution models having a minimum of computational uncertainty in bulk solution properties such as density, heat capacity, and water activity.
11/2011;
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ABSTRACT: Ion association of the ionic liquid [bmim][Cl] in acetonitrile and in water was studied by dielectric spectroscopy for salt concentrations c ≤ 1.3 M at 298.15 K and by measurement of molar electrical conductivities, Λ, of dilute solutions (c ≤ 0.006 M) in the temperature range 273.15 ≲ T/K ≤ 313.15. Whilst acetonitrile solutions of [bmim][Cl] exhibit moderate ion pairing, with an association constant of K°(A) ≈ 60 M(-1) and increasing with temperature, [bmim][Cl] is only weakly associated in water (K°(A) ≈ 6 M(-1)) and ion pairing decreases with rising temperature. Only contact ion pairs were detected in both solvents. Standard-state enthalpy, entropy and heat capacity changes of ion association were derived, as well as the activation enthalpy of charge transport and the limiting conductivity of the cation, λ(∞) ([bmim](+)). These data, in conjunction with effective solvation numbers obtained from the dielectric spectra, suggest that the solvation of [bmim](+) is much weaker in water than in acetonitrile.
Physical Chemistry Chemical Physics 09/2011; 13(39):17588-98. · 3.57 Impact Factor
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ABSTRACT: Solutions of sodium trifluoromethanesulfonate, magnesium trifluoromethanesulfonate, and barium perchlorate in N,N-dimethylformamide (DMF) have been investigated using broadband dielectric relaxation spectroscopy at 25 °C. All spectra were dominated by a solvent relaxation process centered at ∼15 GHz but also exhibited one (for NaCF(3)SO(3)) or two (for the 2:1 salts) low-amplitude processes, centered at frequencies below 2 GHz, that could be attributed to the presence of ion pairs. Effective solvation numbers calculated from the solvent relaxation amplitudes indicated strong solvation of all three cations, with evidence for the formation of a second solvation sheath for Mg(2+) and possibly Ba(2+). Detailed analysis of the solute-related processes showed that solvent-shared ion pairs (SIPs) were formed in NaCF(3)SO(3) solutions in DMF. The data for Mg(CF(3)SO(3))(2) and Ba(ClO(4))(2) solutions were not definitive but, consistent with the solvation evidence, favored the presence of double solvent-separated ion pairs and SIPs. Overall association constants, K(A), were small for all three salts in DMF and increased in the order: NaCF(3)SO(3)<Ba(ClO(4))(2)<Mg(CF(3)SO(3))(2).
The Journal of Physical Chemistry B 02/2011; 115(10):2234-42. · 3.70 Impact Factor
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ABSTRACT: A detailed investigation using broadband dielectric relaxation spectroscopy (DRS) has been made of the aqueous solutions of guanidinium chloride and carbonate, GdmCl(aq) and Gdm₂CO₃(aq), at 25 °C. The spectra indicate that Gdm(+) ions, C(NH₂)₃(+), do not bind strongly to water nor are they hydrophobically hydrated; rather they appear to have a most unusual ability to dissolve in water without altering its dynamics. Although DRS is particularly sensitive to the presence of ion pairs, only weak ion pairing was detected in Gdm₂CO₃(aq) solutions and none at all in GdmCl(aq). Surprisingly, no evidence was found for the existence of the higher order homo- and heteroionic nanoscale aggregates that have been identified in recent years by Mason and co-workers using molecular dynamics simulations and neutron diffraction. Possible reasons for this discrepancy are discussed. The present DR spectra and other solution properties of GdmCl(aq) and Gdm₂CO₃(aq), such as apparent molar volumes and electrical conductivities, are shown to have strong similarities to those of the corresponding Na+ salts. However, such solutions also differ remarkably from their Na(+) analogues (and all other simple electrolytes in aqueous solution) in that their average water relaxation times correlate strongly with their bulk viscosities. The biological implications of the present results are briefly discussed.
The Journal of Physical Chemistry B 10/2010; 114(43):13617-27. · 3.70 Impact Factor
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ABSTRACT: The JESS software package, which is a widely-used tool for modelling chemical speciation in complex aqueous environments, has been extended to allow comprehensive predictions of physicochemical properties for strong electrolytes in aqueous solution. Another large database, this time of physicochemical property data, has been added to the JESS suite, along with the computational methods which automatically turn these diverse literature data into a thermodynamically-consistent calculation for water activities, densities, heat capacities, etc. Given the recent emphasis on the role of water activity in predicting electrolyte mixing behaviour, we expect that this capability will lead to major changes in the way aquatic chemistry is modelled in future.
Talanta 04/2010; 81(1-2):142-8. · 3.79 Impact Factor
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ABSTRACT: 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.
Physical Chemistry Chemical Physics 10/2009; 11(40):8984-99. · 3.57 Impact Factor
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ABSTRACT: 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.
Journal of the American Chemical Society 08/2009; 131(31):11140-6. · 9.91 Impact Factor
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ABSTRACT: Dielectric spectra over the frequency range of 0.2 less, similar nu/GHz < or = 89 have been measured for the room-temperature ionic liquid 1-N-ethyl-3-N-methylimidazolium ethylsulfate ([emim][EtSO(4)], IL) and its mixtures with dichloromethane (DCM) at temperatures of 5 < or = vartheta/ degrees C < or = 65 and 25 degrees C respectively. The spectra of the neat IL at all temperatures and those of the mixtures could be satisfactorily fitted by assuming three relaxation modes, a Cole-Cole process at lower frequencies and two Debye processes at higher frequencies. Consistent with previous studies, detailed analysis of the first (lowest-frequency) process, centered at 0.2-2 GHz depending on temperature and composition, indicated that it is mainly due to the reorientation of the dipolar [emim](+) cations. At high dilutions in the mixtures (x(IL) less, similar 0.2), contact ion pairs also contribute to this mode. The second mode at approximately 8 GHz, which is absent from the dielectric spectra of previously studied imidazolium salts and their mixtures with DCM, is assigned to reorientation of the dipolar [EtSO(4)](-) anions. The highest-frequency mode (located at approximately 80 GHz) in the mixtures is a composite of low-energy intermolecular vibrations originating from the IL and the rotational diffusion of DCM molecules. Detailed analysis of the spectra reveals marked orientational correlations of the IL components, with the cation dipoles showing a strong preference for parallel and the anions showing preference for antiparallel arrangements. These effects are the probable cause of the unusually high dielectric constant of [emim][EtSO(4)]. The structure of the IL appears to be maintained up to quite high dilutions (x(IL) > or = 0.2) in DCM.
The Journal of Physical Chemistry B 06/2009; 113(28):9527-37. · 3.70 Impact Factor
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ABSTRACT: 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.
ChemPhysChem 02/2009; 10(4):723-33. · 3.41 Impact Factor
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ABSTRACT: Densities and heat capacities of various 1:1 and higher-charged electrolytes have been measured in N,N-dimethylformamide (DMF) at 25 degrees C using a series-connected flow densimeter and Picker calorimeter. Standard molar volumes V (o) and isobaric heat capacities C p (o) derived from these data were split into their ionic contributions using the tetraphenylphosphonium tetraphenylborate (TPTB) reference electrolyte assumption. The values so obtained have enabled a meaningful separation of the effects of cationic size and charge for the first time in a nonaqueous solvent. As in water, V (o)(M (n+) ) values in DMF are markedly more negative for higher-charged cations due to increasing electrostriction of the solvent. In contrast, ionic charge has a much smaller effect on C p (o)(M (n+) ) in DMF than in water. Ionic volumes in DMF show the expected dependence on size but those of small monatomic monovalent cations and anions imply a significant difference in accessibility of the electron donor and acceptor sites on the DMF molecule. Ionic heat capacities in DMF show a relatively weak dependence on ionic size that, when corrected for charge, is opposite to that in water. Both V (o)(R 4N (+)) and C p (o)(R 4N (+)) in DMF show the usual linear dependence on carbon number but differ from their values in aqueous solution due to the absence of hydrophobic interactions in DMF.
The Journal of Physical Chemistry B 10/2008; 112(39):12366-73. · 3.70 Impact Factor
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ABSTRACT: 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.
The Journal of Physical Chemistry B 10/2008; 112(41):12913-9. · 3.70 Impact Factor
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ABSTRACT: Ion association in aqueous solutions of scandium sulfate has been investigated at 25 degrees C and at concentrations from 0.01 to 0.8 M by broadband dielectric spectroscopy over the frequency range 0.2 <or= nu/GHz <or= 89. Detailed analysis of the spectra reveals the presence of both inner- and outer-sphere 1:1 [ScSO 4] (+)(aq) complexes, similar to solutions of other high-valent metal sulfates. Outer-outer-sphere 1:1 complexes are probably also formed, but their contribution is swamped by the presence of higher-order inner-sphere complexes. The latter predominate in the more concentrated solutions, causing major changes to the low-frequency end of the spectrum. The data, while not definitive, are consistent with fac-[Sc(SO 4) 3(OH 2) 3] (3-) as the major species present. The speciation is strikingly different from that recently reported for aluminum sulfate solutions and indicates that the often-postulated similarity between the aqueous chemistry of Al(III) and Sc(III) has to be treated with caution.
Inorganic Chemistry 09/2008; 47(19):8619-28. · 4.60 Impact Factor
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ABSTRACT: When salts are added to water, generally the viscosity increases, suggesting that the ions increase the strength of the water's hydrogen-bond network. However, infrared pump-probe measurements on electrolyte solutions have found that ions have no influence on the rotational dynamics of water molecules, implying no enhancement or breakdown of the hydrogen-bond network. Here, we report optical Kerr effect and dielectric relaxation spectroscopic measurements, which have enabled us to separate the effects of rotational and transitional motions of the water molecules. These data show that electrolyte solutions behave like a supercooled liquid approaching a glass transition in which rotational and translational molecular motions are decoupled. It is now possible to understand previously conflicting viscosity data, nuclear magnetic resonance relaxation, and ultrafast infrared spectroscopy in a single unified picture.
The Journal of Chemical Physics 05/2008; 128(16):161102. · 3.33 Impact Factor
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ABSTRACT: 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.
The Journal of Physical Chemistry B 04/2008; 112(16):4854-8. · 3.70 Impact Factor
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ABSTRACT: Densities of sodium hydroxide solutions at concentrations of 0.1 m/mol·kg-1 8 have been measured by vibrating-tube densimetry over the temperature range 323 ≤ T/K ≤ 573 at a pressure of 10 MPa. The purpose-built densimeter, which is capable of measuring densities of corrosive fluids at temperatures up to 573 K and pressures up to 30 MPa, has a platinum−rhodium vibrating tube with inlet/outlet tubes of platinum and an optical detection system. Calibrations were carried out with water and aqueous solutions of sodium chloride at temperatures up to 523 K. At higher temperatures, nitrogen was substituted as the auxiliary density reference because of uncertainties in the NaCl(aq) densities. The apparent molar volumes of NaOH(aq) calculated from the measured densities were fitted with an extended Redlich−Meyer equation. The present results are in good agreement with literature data but are more precise. The current global accuracy in the densities of NaOH(aq) appears to be about 0.1 %, being significantly better at lower concentrations and temperatures but slightly worse at higher concentrations and temperatures. More accurate knowledge of the densities of the reference materials (other than water) used for calibrating vibrating-tube densimeters under extreme conditions is probably required to improve the accuracy of such data.
09/2007;
