Richard Buchner

The effect of alkyl-chain branching on the hydration and ion association of tetrabutylammonium cations

Co-reporter:Nicolás Moreno-Gómez, Edgar F. Vargas, Richard Buchner

Journal of Molecular Liquids 2017 Volume 248(Volume 248) pp:

Publication Date(Web):1 December 2017

DOI:10.1016/j.molliq.2017.09.123

•The mobility of isomeric tetrabutylammonium ions strongly depends on their shape.•With retardation factors of ∼1.5 to 2.5 rotation of hydrating H2O is only weakly impeded.•Branched isomers easily strip off their hydrophobic hydration shell.•Compared to the other isomers the compact Bu2isoBu2N + and isoBu4N + cations show notable ion association with Br −.Association constants and single-ion conductivities of the bromide salts of five isomeric tetrabutylammonium ions were determined with dilute-solution conductivity measurements for aqueous solutions at 298.15 K. In addition, for two of the isomers dielectric relaxation experiments were performed. It was found that, despite similar size of all cations, the hydrodynamic radii of the branched isomers were significantly smaller than that of the un-branched Bu4N + species. It appears that the more compact branched cations exert only a minor perturbation on the H-bond network of the surrounding solvent and easily strip off their hydration shell. This effect, and the —on average— closer approach between anions and cations in ion pairs of Bu2 isoBu2NBr and isoBu4NBr probably explain the rather large association constants of the latter salts.

Dielectric relaxation of nitromethane and its mixtures with ethylammonium nitrate: Evidence for strong ion association induced by hydrogen bonding

Co-reporter:Andreas Nazet, Lisa Weiß, Richard Buchner

Journal of Molecular Liquids 2017 Volume 228(Volume 228) pp:

Publication Date(Web):1 February 2017

DOI:10.1016/j.molliq.2016.09.008

•Ethylammonium nitrate is strongly associated in mixtures with nitromethane at XEAN ≤ 0.013.•At XEAN ≥ 0.4 EAN + nitromethane mixtures behave as “lubricated” ionic liquid.•Cooperative dynamics of mixtures and ion association dominated by hydrogen bonding among ions.•Ion solvation by nitromethane is weak.Binary mixtures of the aprotic protophobic solvent nitromethane (NM) and the protic room-temperature ionic liquid ethylammonium nitrate (EAN) were studied with dielectric relaxation spectroscopy in the two single-phase regions, 0 < xEAN ≤ 0.013 and 0.4 ≤ xEAN < 1, of the binary mixtures at 25 °C. All spectra were well described by a superposition of two relaxation processes whose origins were of composite nature. At low xEAN the solutions behave as a strongly associated electrolyte, displaying moderate ion solvation by nitromethane at the infinite-dilution limit. At high xEAN far from the miscibility gap, the observed dynamics represents the typically observed behaviour of a lubricated ionic liquid smoothly reaching the properties of pure EAN.Additionally, neat NM was investigated in the temperature range of 5 to 65 °C to explore its potential as a calibration standard in dielectric spectroscopy. The obtained 0.05 to 89 GHz spectra were well fitted by a single Debye equation. The dynamics of this medium-permittivity dipolar liquid is governed by rotational diffusion of NM dipoles close to slip boundary conditions with only weak dipole-dipole correlations. Unfortunately, difficulties in obtaining samples of reproducible purity discredit NM as a calibration standard for dielectric measurements.

Evidence for cooperative Na+ and Cl− binding by strongly hydrated L-proline

Co-reporter:Olga A. Dmitrieva;Marina V. Fedotova

Physical Chemistry Chemical Physics 2017 vol. 19(Issue 31) pp:20474-20483

Publication Date(Web):2017/08/09

DOI:10.1039/C7CP04335J

In nature the amino acid L-proline (Pro) is a ubiquitous and highly effective osmolyte protecting cells against osmotic stress. To understand this effect knowledge of the hydration of Pro and its interactions with dissolved salts is essential. We studied these properties by combining statistical mechanics and broadband dielectric spectroscopy and found that Pro remains strongly hydrated up to high amino-acid concentrations. This is also the case upon NaCl addition to a 0.6 M Pro solution. Here, additionally a Pro·NaCl aggregate is formed with a stability constant of K° ≈ 0.95…1.25 M−1, where Na+ and Cl− cooperatively bind to adjacent carboxylate-oxygen and ammonium-hydrogen atoms, respectively.

Urea hydration from dielectric relaxation spectroscopy: old findings confirmed, new insights gained

Co-reporter:Vira Agieienko and Richard Buchner

Physical Chemistry Chemical Physics 2016 vol. 18(Issue 4) pp:2597-2607

Publication Date(Web):24 Dec 2015

DOI:10.1039/C5CP07604H

We report results on urea hydration obtained by dielectric relaxation spectroscopy (DRS) in a broad range of concentrations and temperatures. In particular, the effective hydration number and dipole moment of urea have been determined. The observed changes with composition and temperature were found to be insignificant and mainly caused by the changing number density of urea. Similarly, solute reorientation scaled simply with viscosity. In contrast, we find that water reorientation undergoes substantial changes in the presence of urea, resulting in two water fractions. The first corresponds to water molecules strongly bound to urea. These solvent molecules follow the reorientational dynamics of the solute. The second fraction exhibits only a minor increase of its relaxation time (in comparison with pure water) which is not linked to solution viscosity. Its activation energy decreases significantly with urea concentration, indicating a marked decrease of the number of H-bonds among the H2O molecules belonging to this fraction. Noncovalent interactions (NCI) analysis, capable to estimate the strength of the interactions within a cluster, shows that bound water molecules are most probably double-hydrogen bonded to urea via the oxygen atom of the carbonyl group and a cis-hydrogen atom. Due to the increased H-bond strength compared to the water dimer and the rigid position in the formed complex the reorientation of these bound H2O molecules is strongly impeded.

Correction to “Densities, Viscosities and Conductivities of the Imidazolium Ionic Liquids [Emim][Ac], [Emim][FAP], [Bmim][BETI], [Bmim][FSI], [Hmim][TFSI], and [Omim][TFSI]”

Co-reporter:Andreas Nazet; Sophia Sokolov; Thomas Sonnleitner; Takashi Makino; Mitsuhiro Kanakubo

Journal of Chemical & Engineering Data 2016 Volume 61(Issue 1) pp:699-699

Publication Date(Web):December 24, 2015

DOI:10.1021/acs.jced.5b00999

Hydration and Ion Binding of the Osmolyte Ectoine

Co-reporter:Andreas Eiberweiser, Andreas Nazet, Sergey E. Kruchinin, Marina V. Fedotova, and Richard Buchner

The Journal of Physical Chemistry B 2015 Volume 119(Issue 49) pp:15203-15211

Publication Date(Web):November 13, 2015

DOI:10.1021/acs.jpcb.5b09276

Ectoine is a widespread osmolyte enabling halophilic bacteria to withstand high osmotic stress that has many potential applications ranging from cosmetics to its use as a therapeutic agent. In this contribution, combining experiment and theory, the hydration and ion-binding of this zwitterionic compound was studied to gain information on the functioning of ectoine in particular and of osmolytes in general. Dielectric relaxation spectroscopy was used to determine the effective hydration number of ectoine and its effective dipole moment in aqueous solutions with and without added NaCl. The obtained experimental data were compared with structural results from 1D-RISM and 3D-RISM calculations. It was found that ectoine is strongly hydrated, even in the presence of high salt concentrations. Upon addition of NaCl, ions are bound to ectoine but the formed complexes are not very stable. Interestingly, this osmolyte strongly rises the static relative permittivity of its solutions, shielding thus effectively long-range Coulomb interactions among ions in ectoine-containing solutions. We believe that via this effect, which should be common to all zwitterionic osmolytes, ectoine protects against excessive ions within the cell in addition to its strong osmotic activity protecting against ions outside.

Densities, Viscosities, and Conductivities of the Imidazolium Ionic Liquids [Emim][Ac], [Emim][FAP], [Bmim][BETI], [Bmim][FSI], [Hmim][TFSI], and [Omim][TFSI]

Co-reporter:Andreas Nazet; Sophia Sokolov; Thomas Sonnleitner; Takashi Makino; Mitsuhiro Kanakubo

Journal of Chemical & Engineering Data 2015 Volume 60(Issue 8) pp:2400-2411

Publication Date(Web):July 15, 2015

DOI:10.1021/acs.jced.5b00285

Data for the transport properties electrical conductivity, κ, and dynamic viscosity, η, of the imidazolium ionic liquids [Emim][FAP], [Emim][Ac], [Bmim][BETI], [Bmim][FSI], [Hmim][TFSI], and [Omim][TFSI] (κ only) is presented. Electrical conductivity has been studied in the wide temperature range of (273.15 to 468.15) K, whereas η was determined in the range of (273.15 to 408.15) K. The data could be well fitted by the empirical Vogel–Fulcher–Tammann equation. Additionally, the densities of these ionic liquids, showing a linear dependence on temperature, were collected from (273.15 to 363.15) K.

Ion Hydration and Association in Aqueous Potassium Phosphate Solutions

Co-reporter:Andreas Eiberweiser, Andreas Nazet, Glenn Hefter, and Richard Buchner

The Journal of Physical Chemistry B 2015 Volume 119(Issue 16) pp:5270-5281

Publication Date(Web):March 31, 2015

DOI:10.1021/acs.jpcb.5b01417

Ionic hydration and ion association in aqueous solutions of KH2PO4, K2HPO4, and K3PO4 at 25 °C up to high concentrations have been investigated using dielectric relaxation spectroscopy (DRS). The three phosphate anions were found to be extensively hydrated, with total hydration numbers at infinite dilution of ∼11 (for H2PO4–), ∼20 (HPO42–), and ∼39 (PO43–). These values are indicative of the existence of a second hydration shell around HPO42– and especially PO43–. Two types of hydrating water molecules could be quantified: irrotationally bound (ib, H2O molecules essentially “frozen” on the DRS time scale) and “slow” (loosely bound water molecules with identifiably slower dynamics than bulk water). For H2PO4– over the entire concentration range and for HPO42– and PO43– at concentrations c ≲ 1 mol L–1, only “slow” H2O was detected; however, at higher concentrations of the latter two anions, an increasing fraction of ib water appears, making up ∼50% of the total hydration number close to the saturation limit of K2HPO4. Contrary to common belief, all three salts showed significant ion pair formation, with standard association constants of the 1:1 species increasing in the order: KH2PO40(aq) < KHPO4–(aq) < KPO42–(aq). The main type of ion pair in solution shifted from solvent-shared ion pairs (SIPs) to double-solvent-separated ion pairs (2SIPs) in the same sequence.

Ultra-Broadband Dielectric and Optical Kerr-Effect Study of the Ionic Liquids Ethyl and Propylammonium Nitrate

Co-reporter:Thomas Sonnleitner, David A. Turton, Glenn Hefter, Alexander Ortner, Stefan Waselikowski, Markus Walther, Klaas Wynne, and Richard Buchner

The Journal of Physical Chemistry B 2015 Volume 119(Issue 29) pp:8826-8841

Publication Date(Web):June 19, 2014

DOI:10.1021/jp502935t

Dielectric relaxation (DR) and optical Kerr-effect (OKE) spectra of the archetypal protic ionic liquids ethyl- and propylammonium nitrate (EAN and PAN) have been measured over an unusually large frequency range from 200 MHz to 10 THz at temperatures (mostly) between 5 and 65 °C. Analysis of the low-frequency α-relaxation, associated with the cooperative relaxations of the cations (DR) and anions (OKE) and any clusters present, indicated that ion reorientation in EAN is decoupled from viscosity and occurs via cooperative relaxation involving large-angle jumps rather than rotational diffusion. Detailed consideration of the high-frequency parts of the DR and OKE spectra showed that the observed intensities were a complex combination of overlapping and possibly coupled modes. In addition to previously identified intermolecular H-bond vibrations, there are significant contributions from the librations of the cations and anions. The present assignments were shown to be consistent with the isotopic shifts observed for deuterated EAN.

Dynamics of RTILs: A comparative dielectric and OKE study

Co-reporter:Thomas Sonnleitner, David A. Turton, Stefan Waselikowski, Johannes Hunger, Alexander Stoppa, Markus Walther, Klaas Wynne, Richard Buchner

Journal of Molecular Liquids 2014 192() pp: 19-25

Publication Date(Web):

DOI:10.1016/j.molliq.2013.09.019

Ion Association of Imidazolium Ionic Liquids in Acetonitrile

Co-reporter:Marija Bešter-Rogač, Alexander Stoppa, and Richard Buchner

The Journal of Physical Chemistry B 2014 Volume 118(Issue 5) pp:1426-1435

Publication Date(Web):January 15, 2014

DOI:10.1021/jp412344a

Molar conductivities, Λ, of dilute solutions of the ionic liquids (ILs) 1-ethyl-3-methylimidazolium tetrafluoroborate ([emim][BF4]), 1-butyl-3-methylimidazolium tetrafluoroborate ([bmim][BF4]), 1-butyl-3-methylimidazolium hexafluorophosphate ([bmim][PF6]), 1-hexyl-3-methylimidazolium tetrafluoroborate ([hmim][BF4]), and 1-hexyl-3-methylimidazolium bis(trifluoromethanesulfonyl)amide ([hmim][NTf2]) in acetonitrile (AN) were determined as a function of temperature in the range 273.15–313.15 K. The data were analyzed with Barthel’s lcCM model to obtain limiting molar conductivities, Λ∞(T), and association constants, KA°(T) of these electrolytes. The temperature dependence of these parameters, as well as the extracted limiting cation conductivities, λi∞, were discussed. Additionally, dielectric spectra for [hmim][NTf2] + AN were analyzed in terms of ion association and ion solvation and compared with the inference from conductivity. It appears that in dilute solutions the imidazolium ring of the cations is solvated by ∼6 AN molecules that are slowed by a factor of ∼8–10 compared to the bulk-solvent dynamics. Ion association of imidazolium ILs to contact ion pairs is only moderate, similar to common 1:1 electrolytes in this solvent.

Do H-bonds explain strong ion aggregation in ethylammonium nitrate + acetonitrile mixtures?

Co-reporter:Thomas Sonnleitner, Viktoriya Nikitina, Andreas Nazet and Richard Buchner

Physical Chemistry Chemical Physics 2013 vol. 15(Issue 42) pp:18445-18452

Publication Date(Web):14 Aug 2013

DOI:10.1039/C3CP51773J

Binary mixtures of the protic ionic liquid ethylammonium nitrate (EAN) and acetonitrile (AN) were studied at 25 °C over the entire composition range by means of broadband dielectric spectroscopy covering 0.2 ≤ ν/GHz ≤ 89. The dielectric spectra could be decomposed into two relaxation processes, both of which proved to be composite modes. For dilute solutions the higher-frequency Debye relaxation centered at ∼60 GHz is associated with the rotational diffusion of AN molecules, whereas at higher salt concentrations ultra-fast intermolecular vibrations and librations of EAN dominate the process. For EAN-rich solutions the lower-frequency relaxation is mainly due to jump reorientation of the ethylammonium cation, whereas contact ion pairs (CIPs) dominate this mode for dilute solutions. From the relaxation amplitudes effective solvation numbers and ion-pair concentrations were determined. For vanishing EAN mole fraction, xEAN → 0, an effective cation solvation number of ∼7 was found which steeply drops until xEAN ≈ 0.2 but shows only moderate decrease later on. The obtained association constant for EAN, K0A = 970 L mol−1, exceeds that of other 1:1 electrolytes in AN by a factor of ∼30–50. This observation, as well as the fact that CIPs are formed despite strong cation solvation, indicates that ion pairing is mainly driven by the formation of strong hydrogen bonds between anions and cations.

Hydrophilic and Hydrophobic Hydration of Sodium Propanoate and Sodium Butanoate in Aqueous Solution

Co-reporter:Hafiz M. A. Rahman, Glenn Hefter, and Richard Buchner

The Journal of Physical Chemistry B 2013 Volume 117(Issue 7) pp:2142-2152

Publication Date(Web):January 24, 2013

DOI:10.1021/jp310029c

Aqueous solutions of sodium propanoate (NaOPr) and n-butanoate (NaOBu) have been studied at concentrations of 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, centered 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. Although 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.

Solvation and Association of 3:1 Electrolytes in N,N-Dimethylformamide

Co-reporter:Anna Fuchs, Richard Buchner, and Glenn Hefter

The Journal of Physical Chemistry B 2013 Volume 117(Issue 46) pp:14468-14476

Publication Date(Web):October 30, 2013

DOI:10.1021/jp4089436

A detailed study has been made of the solvation and ion association of the trifluoromethanesulfonate (Tf–) salts of aluminum(III), scandium(III), and lanthanum(III) in N,N-dimethylformamide (DMF) at 25 °C using dielectric relaxation spectroscopy over the frequency range of 0.1 ≲ ν/GHz ≤ 89. The spectra of all solutions exhibited either two (for ScTf3 and LaTf3) or three (for AlTf3) relaxation processes, a dominant mode centered at ∼15 GHz due to the solvent and one or (for AlTf3 solutions) two solute-related processes at lower frequencies (νmax ≲ 2 GHz). Effective solvation numbers, Zib, calculated from the solvent relaxation process indicated that all three cations were strongly solvated by DMF with Zib0 values at infinite dilution in the order (Al3+ ≈ Sc3+ ≈ 10) < (La3+ ≈ 13), consistent with at least partial formation of a second solvation shell around each cation. One solute-related mode for each set of salt solutions was assigned to the rotational diffusion of solvent-shared ion pairs (SIPs) of 1:1 stoicheometry; the additional slower process for AlTf3 solutions in DMF was attributed to the presence of double-solvent-separated IPs. The overall association constants at infinite dilution for the 1:1 IPs, KA°(MTf2+), were significant, but as expected from Debye–Hückel considerations, the KA values decreased rapidly with increasing solute concentration.

Dielectric Relaxation Spectroscopy Shows a Sparingly Hydrated Interface and Low Counterion Mobility in Triflate Micelles

Co-reporter:Filipe S. Lima, Hernan Chaimovich, Iolanda M. Cuccovia, and Richard Buchner

Langmuir 2013 Volume 29(Issue 32) pp:10037-10046

Publication Date(Web):July 11, 2013

DOI:10.1021/la401728g

The properties of ionic micelles are affected by the nature of the counterion. Specific ion effects can be dramatic, inducing even shape and phase changes in micellar solutions, transitions apparently related to micellar hydration and counterion binding at the micellar interface. Thus, determining the hydration and dynamics of ions in micellar systems capable of undergoing such transitions is a crucial step in understanding shape and phase changes. For cationic micelles, such transitions are common with large organic anions as counterions. Interestingly, however, phase separation also occurs for dodecyltrimethylammonium triflate (DTATf) micelles in the presence of sodium triflate (NaTf). Specific ion effects for micellar solutions of dodecyltrimethylammonium chloride (DTAC), bromide (DTAB), methanesulfonate (DTAMs), and triflate (DTATf) were studied with dielectric relaxation spectroscopy (DRS), a technique capable of monitoring hydration and counterion dynamics of micellar aggregates. In comparison to DTAB, DTAC, and DTAMs, DTATf micelles were found to be considerably less hydrated and showed reduced counterion mobility at the micellar interface. The obtained DTATf and DTAMs data support the reported central role of the anion’s −CF3 moiety with respect to the properties of DTATf micelles. The reduced hydration observed for DTATf micelles was rationalized in terms of the higher packing of this surfactant compared to that of other DTA-based systems. The decreased mobility of Tf– anions condensed at the DTATf interface strongly suggests the insertion of Tf– in the micellar interface, which is apparently driven by the strong hydrophobicity of −CF3.

Dielectric relaxation and ultrafast transient absorption spectroscopy of [C6mim]+[Tf2N]−/acetonitrile mixtures

Co-reporter:Peter W. Lohse, Nils Bartels, Alexander Stoppa, Richard Buchner, Thomas Lenzer and Kawon Oum

Physical Chemistry Chemical Physics 2012 vol. 14(Issue 10) pp:3596-3603

Publication Date(Web):07 Feb 2012

DOI:10.1039/C2CP23704K

Mixtures of the ionic liquid (IL) [C6mim]+[Tf2N]− and acetonitrile have been investigated by a combination of dielectric relaxation spectroscopy (DRS) and ultrafast transient absorption techniques using the molecular probe 12′-apo-β-carotenoic-12′-acid (12′CA). Steady-state absorption spectra of the 12′CA molecule have also been recorded. The position of the probe's S0 → S2 absorption maximum correlates linearly with the polarizability of the mixture, suggesting that the bulk composition is a good approximation to the local composition. The lifetime τ1 of the S1/ICT state of 12′CA varies rather smoothly with composition between the value for pure acetonitrile (42 ps) and neat [C6mim]+[Tf2N]− (94 ps). At low IL contents there appears to be an influence of discrete ion pairs. Employing static dielectric constants from the DRS experiments, one finds that the lifetime of the probe in the IL mixtures is shorter than that in pure organic solvents with the same polarity parameter. This suggests an increased stabilization of the S1/ICT state in IL-containing mixtures, most likely due to IL-specific Coulombic interactions between the cation and the negative end of the probe's dipole. An ultrafast solvation component is observed which is ca. 0.5 ps in pure acetonitrile, and approaches the value for the pure IL (2.0 ps) already around x(IL) = 0.3. This is interpreted in terms of an efficient perturbation of the cooperative solvation response of acetonitrile by the presence of small amounts of IL and possibly also the viscosity increase when adding IL. This view is also supported by the increase of the average longitudinal relaxation time of acetonitrile upon addition of small IL amounts extracted from the DRS experiments.

Hydration of Formate and Acetate Ions by Dielectric Relaxation Spectroscopy

Co-reporter:Hafiz M. A. Rahman, Glenn Hefter, and Richard Buchner

The Journal of Physical Chemistry B 2012 Volume 116(Issue 1) pp:314-323

Publication Date(Web):November 18, 2011

DOI:10.1021/jp207504d

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 ≲ 3.2 M and ≲ 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 ≲ 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 H2O 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 −CH3 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.

Structure and Dynamics of 1-N-Alkyl-3-N-Methylimidazolium Tetrafluoroborate + Acetonitrile Mixtures

Co-reporter:Alexander Stoppa, Johannes Hunger, Glenn Hefter, and Richard Buchner

The Journal of Physical Chemistry B 2012 Volume 116(Issue 25) pp:7509-7521

Publication Date(Web):June 10, 2012

DOI:10.1021/jp3020673

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 (xIL) ≲ 0.2 the IL behaves as a rather weakly associated conventional electrolyte while at xIL ≳ 0.2 it takes on its IL characteristics, “lubricated” by the AN.

Ion-pair or ion-cloud relaxation? On the origin of small-amplitude low-frequency relaxations of weakly associating aqueous electrolytes

Co-reporter:Andreas Eiberweiser, Richard Buchner

Journal of Molecular Liquids 2012 176() pp: 52-59

Publication Date(Web):

DOI:10.1016/j.molliq.2012.03.025

Hydration and sodium-ion binding of trifluoroacetate in aqueous solution

Co-reporter:Hafiz M.A. Rahman, Richard Buchner

Journal of Molecular Liquids 2012 176() pp: 93-100

Publication Date(Web):

DOI:10.1016/j.molliq.2012.05.017

Properties of Sodium Tetrafluoroborate Solutions in 1-Butyl-3-methylimidazolium Tetrafluoroborate Ionic Liquid

Co-reporter:Viktoriya A. Nikitina, Andreas Nazet, Thomas Sonnleitner, and Richard Buchner

Journal of Chemical & Engineering Data 2012 Volume 57(Issue 11) pp:3019-3025

Publication Date(Web):October 4, 2012

DOI:10.1021/je300603d

Electrical conductivities, κ, of sodium tetrafluoroborate solutions in 1-butyl-3-methylimidazolium tetrafluoroborate ([bmim][BF4]) ionic liquid were measured in the wide temperature range of (238.15 to 458.15) K. Additionally, their densities, ρ, viscosities, η, and molar conductivities, Λ, are reported for the temperature range (278.15 to 358.15) K, supplemented by dielectric data at 298.15 K. The values for η and κ are well-described by the Vogel–Fulcher–Tammann equation. Walden plots, log(Λ) vs log(η–1), for the NaBF4 solutions coincide with the straight line found for neat [bmim][BF4], indicating that the solute has only limited impact on the structure of the ionic liquid. This is corroborated by the similarity of the standard molar volume of NaBF4 and its intrinsic volume, which suggests that solute-induced electrostriction is weak. Within the experimental uncertainty, the dielectric properties of the most concentrated NaBF4 solution (0.1739 mol·kg–1) were found to be identical with those of pure [bmim][BF4].

Association of ionic liquids in solution: a combined dielectric and conductivity study of [bmim][Cl] in water and in acetonitrile

Co-reporter:Marija Bešter-Rogač, Alexander Stoppa, Johannes Hunger, Glenn Hefter and Richard Buchner

Physical Chemistry Chemical Physics 2011 vol. 13(Issue 39) pp:17588-17598

Publication Date(Web):05 Sep 2011

DOI:10.1039/C1CP21371G

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 KoA ≈ 60 M−1 and increasing with temperature, [bmim][Cl] is only weakly associated in water (KoA ≈ 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.

Features of ion hydration and association in aqueous rubidium fluoride solutions at ambient conditions

Co-reporter:M.V. Fedotova, S.E. Kruchinin, H.M.A. Rahman, R. Buchner

Journal of Molecular Liquids 2011 Volume 159(Issue 1) pp:9-17

Publication Date(Web):15 February 2011

DOI:10.1016/j.molliq.2010.04.009

Aqueous RbF solutions have been investigated at ambient conditions by dielectric relaxation spectroscopy (DRS) over the concentration range of 0 ≤ c/M ≤ 5 and by statistical mechanics (RISM integral equation theory) in the concentration range of 2.15–9.18 M. The behavior of the dielectric spectra and their characteristic parameters (dispersion amplitudes and relaxation times) is discussed, as well as the pair correlation functions and partial coordination numbers derived from statistical mechanics. The paper focuses on the influence of salt concentration on the features of ion hydration and association. The observed changes in the dielectric properties indicate a decrease of the effective hydration number from ∼ 15–18 at infinite dilution to ∼ 3.5 at 5 M and possibly weak association involving stable ion pairs at c ≤ 0.5 M. A major finding is that both ions, Rb+ and F−, slow down surrounding water molecules, leading to the emergence of a separate “slow water” relaxation in the spectra. According to the analysis of RISM equilibrium properties, an average number of ∼ 6 water molecules coordinates both Rb+ and F− at c = 2.15 M. This number decreases by ∼ 19% for the cation and by ∼ 10% for the anion as c growths up to 9.18 M. The RISM-data indicate a significant increase in the number of contact and solvent-separated ion pairs with increasing c. The possibility of ion clustering is discussed for concentrated RbF(aq) solutions.

Densities, Viscosities [from (278.15 to 318.15) K], and Electrical Conductivities (at 298.15 K) of Aqueous Solutions of Choline Chloride and Chloro-Choline Chloride

Co-reporter:Saadia Shaukat and Richard Buchner

Journal of Chemical & Engineering Data 2011 Volume 56(Issue 12) pp:4944-4949

Publication Date(Web):October 20, 2011

DOI:10.1021/je200856f

Data for the electrical conductivity (at 298.15 K), density, and viscosity [both at (278.15, 288.15, 298.15, 308.15, and 318.15) K] of aqueous solutions of choline chloride (ChCl) and chloro-choline chloride (Cl-ChCl) are reported for solute molalities m ≲ 2.0 mol·kg–1 (ChCl) and ≲ 2.1 mol·kg–1 (Cl-ChCl), respectively. From the densities apparent molar volumes were calculated, and the limiting partial molar volumes and Hepler's constants of ChCl(aq) and Cl-ChCl(aq) were evaluated. From the viscosities Arrhenius activation energies of viscous flow were determined. While apparent molar volumes suggest at best weak ion pairing for ChCl(aq) and Cl-ChCl(aq), the Walden product obtained for 25 °C hints at significant ion–ion interactions and/or changes in ion hydration with increasing concentration.

1-Ethyl-3-methylimidazolium Ethylsulfate in Water, Acetonitrile, and Dichloromethane: Molar Conductivities and Association Constants

Co-reporter:Marija Bešter-Rogač, Johannes Hunger, Alexander Stoppa, and Richard Buchner

Journal of Chemical & Engineering Data 2011 Volume 56(Issue 4) pp:1261-1267

Publication Date(Web):February 16, 2011

DOI:10.1021/je101130e

Molar conductivities, Λ, of dilute solutions of the ionic liquid 1-ethyl-3-methylimidazolium ethylsulfate in water, acetonitrile (AN), and dichloromethane (DCM) were determined as a function of temperature (AN: T = (273.15 to 313.15) K; DCM: T = (273.15 to 308.15) K; water: T = (278.15 to 313.15) K) in the concentration range c = (≈0.25 to ≈5)·10−3 mol·dm−3. The data were analyzed with Barthel's low-concentration chemical (lcCM) model to obtain the limiting molar conductivities, Λ∞(T), and association constants, KA°(T), of this electrolyte in the investigated solvents. From Λ∞(T) the Eyring activation enthalpy of charge transport was determined and found to be slightly larger than the corresponding value for viscous flow of the solvent. Strong ion pairing was found for the electrolyte in DCM (KA° ≈ 6·104 mol−1·dm3), whereas ion association in AN is rather weak (KA° ≈ 40 mol−1·dm3). From the temperature dependence of KA°(T) the enthalpy and entropy of the ion-pairing process were calculated, and ion association was found to be entropy-driven for both nonaqueous solvents. In water the salt is fully dissociated.

Dielectric Relaxation Study of the Ion Solvation and Association of NaCF3SO3, Mg(CF3SO3)2, and Ba(ClO4)2 in N,N-Dimethylformamide

Co-reporter:Anna Płaczek, Glenn Hefter, Hafiz M. A. Rahman, and Richard Buchner

The Journal of Physical Chemistry B 2011 Volume 115(Issue 10) pp:2234-2242

Publication Date(Web):February 22, 2011

DOI:10.1021/jp1116307

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 NaCF3SO3) 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 Mg2+ and possibly Ba2+. Detailed analysis of the solute-related processes showed that solvent-shared ion pairs (SIPs) were formed in NaCF3SO3 solutions in DMF. The data for Mg(CF3SO3)2 and Ba(ClO4)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, KA, were small for all three salts in DMF and increased in the order: NaCF3SO3 < Ba(ClO4)2 < Mg(CF3SO3)2.

Correlation between polarity parameters and dielectric properties of [Na][TOTO]—a sodium ionic liquid

Co-reporter:Oliver Zech, Johannes Hunger, Joshua R. Sangoro, Ciprian Iacob, Friedrich Kremer, Werner Kunz and Richard Buchner

Physical Chemistry Chemical Physics 2010 vol. 12(Issue 42) pp:14341-14350

Publication Date(Web):06 Oct 2010

DOI:10.1039/C0CP00840K

The ionic liquid (IL) [Na][TOTO], with sodium as the cation and an oligoethercarboxylate as the anion, shows properties that differ significantly from conventional ionic liquids, like imidazolium salts. Its polarity, determined in the temperature range of (293–333) K from measurements of Reichardt's ENT value and the Kamlet–Taft parameters is extraordinarily low and matches the dielectric constant extrapolated from frequency dependent complex permittivity measurements. The dielectric spectra also reveal split dynamics with the dominating slow mode probably associated with the reorientation of –COO−⋯Na+ ion pairs and the fast mode arising from the flexibility of the oligoethylene moieties of the anion. This assignment, supported by the derived effective dipole moments, argues against our previous hypothesis [O. Zech et al., Chem.–Eur. J., 2009, 15, 1341–1345] that salts, such as [Na][TOTO], have low melting points because of cation complexation in a pseudo crown ether-like fashion and resulting “intra-molecular” charge neutralization. The present data rule out a rigid chelate-like complex as the dominating species. Considering the present findings together with the viscosities of [TOTO]− salts, a crosslinked structure of the ionic liquid with strong –COO−⋯Na+ interactions and relatively weaker forces between cations and ether oxygen groups of the anions appears to be plausible.

How ideal are binary mixtures of room-temperature ionic liquids?

Co-reporter:Alexander Stoppa, Richard Buchner, Glenn Hefter

Journal of Molecular Liquids 2010 Volume 153(Issue 1) pp:46-51

Publication Date(Web):15 April 2010

DOI:10.1016/j.molliq.2009.05.001

Data for the densities, conductivities, viscosities and dielectric properties of binary mixtures of the tetrafluoroborate and the dicyanamide salts of 1-N-ethyl-3-N-methylimidazolium have been measured at 25 °C. The mixtures exhibit practically “ideal” (linear) mixing behaviour with respect to viscosity, molar volume, and the effective dipole moment of the observed relaxation associated with cation reorientation, indicating a smooth change in the structure of the mixtures. On the other hand, the electrical conductivity and dielectric relaxation time deviate considerably from “ideal” mixing behaviour, indicating enhanced translational and rotational dynamics in the mixtures.

Are Nanoscale Ion Aggregates Present in Aqueous Solutions of Guanidinium Salts?

Co-reporter:Johannes Hunger, Stefan Niedermayer, and Richard Buchner, Glenn Hefter

The Journal of Physical Chemistry B 2010 Volume 114(Issue 43) pp:13617-13627

Publication Date(Web):October 11, 2010

DOI:10.1021/jp101520h

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 Conductivity of Imidazolium-Based Ionic Liquids from (−35 to 195) °C. A. Variation of Cation’s Alkyl Chain

Co-reporter:Alexander Stoppa, Oliver Zech, Werner Kunz and Richard Buchner

Journal of Chemical & Engineering Data 2010 Volume 55(Issue 5) pp:1768-1773

Publication Date(Web):November 23, 2009

DOI:10.1021/je900789j

Data for the conductivity (κ) of the ionic liquids [emim][BF4], [bmim][BF4], [hmim][BF4], and [omim][BF4] from (−35 to 195) °C are reported. The data can be well-fit with the Vogel−Fulcher−Tammann equation. Additionally, molar conductivities (Λ) were determined for the limited temperature range of (5 to 65) °C. Walden plots of these data indicate that the investigated compounds can be classified as “high-ionicity” ionic liquids. It is suspected that the large difference between the present κ values and some of the literature values is mainly due to the neglect of BF4− hydrolysis.

Molar Conductivities and Association Constants of 1-Butyl-3-methylimidazolium Chloride and 1-Butyl-3-methylimidazolium Tetrafluoroborate in Methanol and DMSO

Co-reporter:Marija Bešter-Rogač, Johannes Hunger, Alexander Stoppa and Richard Buchner

Journal of Chemical & Engineering Data 2010 Volume 55(Issue 5) pp:1799-1803

Publication Date(Web):September 11, 2009

DOI:10.1021/je900531b

Molar conductivities, Λ, of dilute solutions of the ionic liquids 1-butyl-3-methylimidazolium chloride and 1-butyl-3-methylimidazolium tetrafluoroborate in methanol and DMSO were determined as a function of temperature (methanol, T = (273.15 to 313.15) K; DMSO, T = (293.15 to 318.15) K). The data were analyzed with Barthel’s low-concentration chemical model (lcCM) model to obtain the limiting molar conductivities, Λ∞(T), and association constants, KA°(T), of these electrolytes. From Λ∞(T), the Eyring activation enthalpy of charge transport and (where possible) the ion conductivity of the cation, λ∞([bmim]+), were determined.

Interactions and dynamics in electrolyte solutions by dielectric spectroscopy

Co-reporter:Richard Buchner and Glenn Hefter

Physical Chemistry Chemical Physics 2009 vol. 11(Issue 40) pp:8984-8999

Publication Date(Web):02 Jul 2009

DOI:10.1039/B906555P

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.

Broadband dielectric response of dichloromethane

Co-reporter:Johannes Hunger, Alexander Stoppa, Andreas Thoman, Markus Walther, Richard Buchner

Chemical Physics Letters 2009 Volume 471(1–3) pp:85-91

Publication Date(Web):16 March 2009

DOI:10.1016/j.cplett.2009.02.024

Abstract

A systematic study of the dielectric response of dichloromethane over a broad frequency range ( $8.5 ⩽ ν / GHz ⩽ 1000; ⩽ 4.3 THz$ at 25 °C) at temperatures from 5 to 35 °C is reported. The spectra are best described by a superposition of three contributions. The dominating loss peak at $\sim 70 GHz$ , fitted by a modified Debye equation accounting for inertial rise, represents the rotational diffusion of molecular dipoles. Additionally, two libration modes contribute, which are described by damped harmonic oscillators of resonance frequencies $\sim 0.9 THz$ and $\sim 2.1 THz$ . While the latter modes reflect the anisotropy of the dichloromethane molecule, the diffusive reorientation at long times appears to be rather isotropic.

Temperature Dependence of the Dielectric Properties and Dynamics of Ionic Liquids

Co-reporter:Johannes Hunger;Alexer Stoppa;Simon Schrödle Dr.;Glenn Hefter Dr. Dr.

ChemPhysChem 2009 Volume 10( Issue 4) pp:723-733

Publication Date(Web):

DOI:10.1002/cphc.200800483

Dipole Correlations in the Ionic Liquid 1-N-Ethyl-3-N-methylimidazolium Ethylsulfate and Its Binary Mixtures with Dichloromethane

Co-reporter:Johannes Hunger, Alexander Stoppa and Richard Buchner, Glenn Hefter

The Journal of Physical Chemistry B 2009 Volume 113(Issue 28) pp:9527-9537

Publication Date(Web):June 19, 2009

DOI:10.1021/jp9024574

Dielectric spectra over the frequency range of 0.2 ≲ ν/GHz ≤ 89 have been measured for the room-temperature ionic liquid 1-N-ethyl-3-N-methylimidazolium ethylsulfate ([emim][EtSO4], IL) and its mixtures with dichloromethane (DCM) at temperatures of 5 ≤ ϑ/°C ≤ 65 and 25 °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 (xIL ≲ 0.2), contact ion pairs also contribute to this mode. The second mode at ∼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 [EtSO4]− anions. The highest-frequency mode (located at ∼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][EtSO4]. The structure of the IL appears to be maintained up to quite high dilutions (xIL ≥ 0.2) in DCM.

Dielectric Relaxation Spectroscopy of Aliphatic Ionene Bromides and Fluorides in Water: The Role of the Polyion’s Charge Density and the Nature of the Counterions

Co-reporter:Miha Lukšič, Richard Buchner, Barbara Hribar-Lee and Vojko Vlachy

Macromolecules 2009 Volume 42(Issue 12) pp:4337-4342

Publication Date(Web):May 21, 2009

DOI:10.1021/ma900097c

Conductivities of Binary Mixtures of Ionic Liquids with Polar Solvents

Co-reporter:Alexander Stoppa, Johannes Hunger and Richard Buchner

Journal of Chemical & Engineering Data 2009 Volume 54(Issue 2) pp:472-479

Publication Date(Web):October 25, 2008

DOI:10.1021/je800468h

Data for the conductivity, κ, of selected binary mixtures of the ionic liquids [emim][BF4], [bmim][BF4], [bmim][PF6], [bmim][DCA], and [hmim][BF4] with polar solvents (water, propylene carbonate, dimethylsulfoxide, methanol, dichloromethane) at 25 °C are reported. Additionally, mixture densities, ρ, were determined to convert κ into molar conductivity, Λ. The obtained results were fitted by appropriate interpolation formulas. Where possible, data were compared with information from the literature. Electrode polarization and sample purity, including [BF4−] hydrolysis, were considered as possible sources of errors in κ. The effect of viscosity on the accuracy of ρ and thus Λ was checked.

From Ionic Liquid to Electrolyte Solution: Dynamics of 1-N-Butyl-3-N-methylimidazolium Tetrafluoroborate/Dichloromethane Mixtures

Co-reporter:Johannes Hunger, Alexander Stoppa and Richard Buchner, Glenn Hefter

The Journal of Physical Chemistry B 2008 Volume 112(Issue 41) pp:12913-12919

Publication Date(Web):September 19, 2008

DOI:10.1021/jp8045627

Dielectric spectra have been measured at 25 °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 ≲ ν/GHz ≤ 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 (xIL ≳ 0.5) in DCM. At even higher dilutions (xIL ≲ 0.3), the IL behaves as a conventional but strongly associated electrolyte.

Temperature Effects on Ion Association and Hydration in MgSO4 by Dielectric Spectroscopy

Co-reporter:Chrika Akilan;Nashiour Rohman Dr.;Glenn Hefter Dr. Dr.

ChemPhysChem 2006 Volume 7(Issue 11) pp:2319-2330

Publication Date(Web):27 SEP 2006

DOI:10.1002/cphc.200600342

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≤ν/GHz≤89), concentrations (0.02≤m/mol kg⁻¹≤2.3) and temperatures (5≤ϑ/°C≤65). Consistent with literature data from other techniques, the spectra show clear evidence at all temperatures for the simultaneous existence of double solvent-separated (2SIP), solvent-shared (SIP) and contact (CIP) ion pairs and a triple ion (TI), Mg₂SO₄²(aq). Increased formation of CIPs and especially TIs occurs with increasing temperature. The overall standard ion-association constant, K_A°, corresponding to the equilibrium: Mg²(aq) SO₄²⁻(aq)⇌MgSO₄⁰(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 Mg² and SO₄²⁻ influence solvent water molecules beyond their first hydration spheres but that MgSO₄(aq) is less strongly hydrated than the previously studied CuSO₄(aq).

Effects of hydration on the thermodynamic properties of aqueous ethylene glycol ether solutions

Co-reporter:Simon Schrödle, Glenn Hefter, Richard Buchner

The Journal of Chemical Thermodynamics 2005 Volume 37(Issue 5) pp:513-522

Publication Date(Web):May 2005

DOI:10.1016/j.jct.2004.10.006

The densities and isobaric specific heat capacities of binary mixtures of water with various open-chain and cyclic ethylene glycol ethers have been measured at 298.15 K using vibrating tube densimetry, and flow or differential scanning calorimetry, respectively. Excess molar volumes were derived over the whole composition range. Molar isobaric heat capacities and the relative apparent thermodynamic quantities were determined in the water-rich region. The data reflect the changes in the structure and hydrogen-bond dynamics of water caused by these non-ionic solutes. The observed effects are discussed in terms of the influence of hydrophobic hydration on the thermodynamic properties of aqueous solutions. Correlations are given that enable the prediction of the thermodynamic properties of open-chain and cyclic oligo(ethylene oxide) ethers in their pure liquid state and at infinite dilution in water.

Percolating Microemulsions of Nonionic Surfactants Probed by Dielectric Spectroscopy

Co-reporter:Simon Schrödle Dr. Dr.;Werner Kunz Dr.

ChemPhysChem 2005 Volume 6(Issue 6) pp:

Publication Date(Web):4 MAY 2005

DOI:10.1002/cphc.200400605

Charge exchange in aggregated micelles: Percolation phenomena in microemulsions of nonionic surfactants (see picture) are investigated by dielectric spectroscopy. For the investigated 10⁻³ M KCl(aq)/n-octane/C₁₂E₅ system the conductivity and Cole–Cole parameter follow the predictions of the dynamic percolation theory, whereas the relaxation time and the dispersion amplitude of the characteristic relaxation are specific to the sample composition.

Ion association and hydration in 3:2 electrolyte solutions by dielectric spectroscopy: Aluminum sulfate

Co-reporter:Simon Schrödle, Wolfram W. Rudolph, Glenn Hefter, Richard Buchner

Geochimica et Cosmochimica Acta (15 November 2007) Volume 71(Issue 22) pp:5287-5300

Publication Date(Web):15 November 2007

DOI:10.1016/j.gca.2007.08.026

Broadband dielectric measurements utilizing state-of-the-art coaxial reflectometry and traveling-wave interferometry have been made on aqueous solutions of the 3:2 electrolyte aluminum sulfate at 25 °C over the frequency range 0.2 ⩽ ν (GHz) ⩽ 89 and at total solute concentrations 0.012 ⩽ c (M) ⩽ 0.65. Detailed analysis of the solute contribution to the dielectric spectra revealed the simultaneous presence of double solvent-separated (2SIP), solvent-shared (SIP) and contact (CIP) ion pairs. Concentrations of the various ion-pair types and the equilibrium constants for their formation were determined using calculated dipole moments and other relevant quantities. The 2SIPs and SIPs were found to persist down to low concentrations in this notionally strong electrolyte. Good agreement was found with earlier Raman and NMR studies for CIP concentrations and with thermodynamic determinations of the overall ion association constant. In contrast to divalent metal sulfate solutions, both SIPs and, to a lesser extent, 2SIPs remain present at quite high Al2(SO4)3(aq) concentrations. The persistence of SIPs and 2SIPs was consistent with the effective hydration numbers obtained from an analysis of the cooperative H-bond relaxation mode of bulk water in the solutions. This analysis indicated that the hydration of Al3+ ions is extremely strong, with significant effects on the dielectric relaxation timescale even beyond the second hydration shell, at least in dilute solutions.

Dielectric relaxation and ultrafast transient absorption spectroscopy of [C6mim]+[Tf2N]−/acetonitrile mixtures

Co-reporter:Peter W. Lohse, Nils Bartels, Alexander Stoppa, Richard Buchner, Thomas Lenzer and Kawon Oum

Physical Chemistry Chemical Physics 2012 - vol. 14(Issue 10) pp:NaN3603-3603

Publication Date(Web):2012/02/07

DOI:10.1039/C2CP23704K

Mixtures of the ionic liquid (IL) [C6mim]+[Tf2N]− and acetonitrile have been investigated by a combination of dielectric relaxation spectroscopy (DRS) and ultrafast transient absorption techniques using the molecular probe 12′-apo-β-carotenoic-12′-acid (12′CA). Steady-state absorption spectra of the 12′CA molecule have also been recorded. The position of the probe's S0 → S2 absorption maximum correlates linearly with the polarizability of the mixture, suggesting that the bulk composition is a good approximation to the local composition. The lifetime τ1 of the S1/ICT state of 12′CA varies rather smoothly with composition between the value for pure acetonitrile (42 ps) and neat [C6mim]+[Tf2N]− (94 ps). At low IL contents there appears to be an influence of discrete ion pairs. Employing static dielectric constants from the DRS experiments, one finds that the lifetime of the probe in the IL mixtures is shorter than that in pure organic solvents with the same polarity parameter. This suggests an increased stabilization of the S1/ICT state in IL-containing mixtures, most likely due to IL-specific Coulombic interactions between the cation and the negative end of the probe's dipole. An ultrafast solvation component is observed which is ca. 0.5 ps in pure acetonitrile, and approaches the value for the pure IL (2.0 ps) already around x(IL) = 0.3. This is interpreted in terms of an efficient perturbation of the cooperative solvation response of acetonitrile by the presence of small amounts of IL and possibly also the viscosity increase when adding IL. This view is also supported by the increase of the average longitudinal relaxation time of acetonitrile upon addition of small IL amounts extracted from the DRS experiments.

Do H-bonds explain strong ion aggregation in ethylammonium nitrate + acetonitrile mixtures?

Co-reporter:Thomas Sonnleitner, Viktoriya Nikitina, Andreas Nazet and Richard Buchner

Physical Chemistry Chemical Physics 2013 - vol. 15(Issue 42) pp:NaN18452-18452

Publication Date(Web):2013/08/14

DOI:10.1039/C3CP51773J

Binary mixtures of the protic ionic liquid ethylammonium nitrate (EAN) and acetonitrile (AN) were studied at 25 °C over the entire composition range by means of broadband dielectric spectroscopy covering 0.2 ≤ ν/GHz ≤ 89. The dielectric spectra could be decomposed into two relaxation processes, both of which proved to be composite modes. For dilute solutions the higher-frequency Debye relaxation centered at ∼60 GHz is associated with the rotational diffusion of AN molecules, whereas at higher salt concentrations ultra-fast intermolecular vibrations and librations of EAN dominate the process. For EAN-rich solutions the lower-frequency relaxation is mainly due to jump reorientation of the ethylammonium cation, whereas contact ion pairs (CIPs) dominate this mode for dilute solutions. From the relaxation amplitudes effective solvation numbers and ion-pair concentrations were determined. For vanishing EAN mole fraction, xEAN → 0, an effective cation solvation number of ∼7 was found which steeply drops until xEAN ≈ 0.2 but shows only moderate decrease later on. The obtained association constant for EAN, K0A = 970 L mol−1, exceeds that of other 1:1 electrolytes in AN by a factor of ∼30–50. This observation, as well as the fact that CIPs are formed despite strong cation solvation, indicates that ion pairing is mainly driven by the formation of strong hydrogen bonds between anions and cations.

Interactions and dynamics in electrolyte solutions by dielectric spectroscopy

Co-reporter:Richard Buchner and Glenn Hefter

Physical Chemistry Chemical Physics 2009 - vol. 11(Issue 40) pp:NaN8999-8999

Publication Date(Web):2009/07/02

DOI:10.1039/B906555P

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.

Correlation between polarity parameters and dielectric properties of [Na][TOTO]—a sodium ionic liquid

Co-reporter:Oliver Zech, Johannes Hunger, Joshua R. Sangoro, Ciprian Iacob, Friedrich Kremer, Werner Kunz and Richard Buchner

Physical Chemistry Chemical Physics 2010 - vol. 12(Issue 42) pp:NaN14350-14350

Publication Date(Web):2010/10/06

DOI:10.1039/C0CP00840K

The ionic liquid (IL) [Na][TOTO], with sodium as the cation and an oligoethercarboxylate as the anion, shows properties that differ significantly from conventional ionic liquids, like imidazolium salts. Its polarity, determined in the temperature range of (293–333) K from measurements of Reichardt's ENT value and the Kamlet–Taft parameters is extraordinarily low and matches the dielectric constant extrapolated from frequency dependent complex permittivity measurements. The dielectric spectra also reveal split dynamics with the dominating slow mode probably associated with the reorientation of –COO−⋯Na+ ion pairs and the fast mode arising from the flexibility of the oligoethylene moieties of the anion. This assignment, supported by the derived effective dipole moments, argues against our previous hypothesis [O. Zech et al., Chem.–Eur. J., 2009, 15, 1341–1345] that salts, such as [Na][TOTO], have low melting points because of cation complexation in a pseudo crown ether-like fashion and resulting “intra-molecular” charge neutralization. The present data rule out a rigid chelate-like complex as the dominating species. Considering the present findings together with the viscosities of [TOTO]− salts, a crosslinked structure of the ionic liquid with strong –COO−⋯Na+ interactions and relatively weaker forces between cations and ether oxygen groups of the anions appears to be plausible.

Association of ionic liquids in solution: a combined dielectric and conductivity study of [bmim][Cl] in water and in acetonitrile

Co-reporter:Marija Bešter-Rogač, Alexander Stoppa, Johannes Hunger, Glenn Hefter and Richard Buchner

Physical Chemistry Chemical Physics 2011 - vol. 13(Issue 39) pp:NaN17598-17598

Publication Date(Web):2011/09/05

DOI:10.1039/C1CP21371G

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 KoA ≈ 60 M−1 and increasing with temperature, [bmim][Cl] is only weakly associated in water (KoA ≈ 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.

Urea hydration from dielectric relaxation spectroscopy: old findings confirmed, new insights gained

Co-reporter:Vira Agieienko and Richard Buchner

Physical Chemistry Chemical Physics 2016 - vol. 18(Issue 4) pp:NaN2607-2607

Publication Date(Web):2015/12/24

DOI:10.1039/C5CP07604H

We report results on urea hydration obtained by dielectric relaxation spectroscopy (DRS) in a broad range of concentrations and temperatures. In particular, the effective hydration number and dipole moment of urea have been determined. The observed changes with composition and temperature were found to be insignificant and mainly caused by the changing number density of urea. Similarly, solute reorientation scaled simply with viscosity. In contrast, we find that water reorientation undergoes substantial changes in the presence of urea, resulting in two water fractions. The first corresponds to water molecules strongly bound to urea. These solvent molecules follow the reorientational dynamics of the solute. The second fraction exhibits only a minor increase of its relaxation time (in comparison with pure water) which is not linked to solution viscosity. Its activation energy decreases significantly with urea concentration, indicating a marked decrease of the number of H-bonds among the H2O molecules belonging to this fraction. Noncovalent interactions (NCI) analysis, capable to estimate the strength of the interactions within a cluster, shows that bound water molecules are most probably double-hydrogen bonded to urea via the oxygen atom of the carbonyl group and a cis-hydrogen atom. Due to the increased H-bond strength compared to the water dimer and the rigid position in the formed complex the reorientation of these bound H2O molecules is strongly impeded.

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Abstract