A detailed investigation of aqueous solutions of MgSO4 has been made by dielectric relaxation spectroscopy over a wide range of frequencies (0.2 ≤ ν/GHz ≤ 89) and concentrations (0.017 ≤ c/M ≤ 2.24). Detailed analysis of the spectra shows conclusively, as has long been inferred from ultrasonic absorption studies, the simultaneous presence of double solvent separated (2SIP), solvent-shared (SIP), and contact (CIP) ion pairs. The constants derived for the stepwise formation of each ion pair type and for the overall association are in excellent agreement with literature estimates based on other kinds of measurements. In addition, evidence has been obtained for the existence of a triple ion, Mg2SO42+(aq), or possibly a more aggregated species, at high electrolyte concentrations (c > 1 M). Support for the presence of CIPs, SIPs, and the triple ion is provided by Raman spectroscopy. The implications of the present findings for quantitative models of the thermodynamic behavior of higher-valent electrolytes are b...

Complexity in "simple" electrolyte solutions: Ion pairing in MgSO4(aq)

Dielectric spectra were measured for eight, mostly imidazolium-based, room temperature ionic liquids (RTILs) over a wide range of frequencies (0.2≤ν/GHz≤89) and temperatures (5≤θ/°C≤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.

Temperature dependence of the dielectric properties and dynamics of ionic liquids.

A systematic study of the dielectric relaxation spectra of aqueous solutions of NaBr, NaI, NaNO3, NaClO4, and NaSCN has been made over a wide range of frequencies (0.2 ≤ ν/GHz ≤ 89) and solute concentrations (0.05 ≤ c/M ≤ 1.5) at 25 °C. The spectra could be adequately described by a single Cole−Cole (CC) process, symmetrically broadened relative to that of pure water. However, similar quality fits were also obtained with a three-Debye-process (3D) model consisting of a small ion-pair contribution at lower frequencies and two solvent relaxations at higher frequencies. Assuming the ions to be solvent separated, the 3D model provided estimates of their association constants and their rate constants for formation and dissociation. The bulk water relaxation times obtained from both models showed almost no dependence on the nature of the anion. Nevertheless, there were subtle differences in the concentration dependences of the relaxation times which correlated with some, but not all, of the anion properties tha...

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

We study aqueous solutions of the amphiphilic osmolyte trimethylamine-N-oxide (TMAO) using broadband dielectric spectroscopy and femtosecond mid-infrared spectroscopy. Both experiments provide strong evidence for distinctively slower rotation dynamics for water molecules interacting with the hydrophobic part of the TMAO molecules. Further, water is found to interact more strongly at the hydrophilic site of the TMAO molecules: we find evidence for the formation of stable, TMAO·2H2O and/or TMAO·3H2O complexes. While this coordination structure seems obvious, the lifetime of these complexes is found to be extraordinarily long (>50 ps). The existence of these long-lived complexes leads to pronounced parallel dipole correlations between water and TMAO, reflected in enhanced amplitudes in the dielectric spectra. The strong interaction between water and TMAO also results in a red-shifted band for the O–D stretching vibration of HDO molecules in an isotopically diluted aqueous TMAO solution. This O–D stretching v...

Complex formation in aqueous trimethylamine-N-oxide (TMAO) solutions.

Results of a dielectric relaxation study of the aqueous solutions of KCl and CsCl at 25 °C are reported using measurements with a vector network analyzer (0.2 ≤ ν/GHz ≤ 20) and with waveguide interferometers (27 ≤ ν/GHz ≤ 89). Similar to previously reported data for NaCl, the spectra of both salts are well fitted by a Cole−Cole equation, although a double-Debye model is competitive at intermediate CsCl concentrations. The impact of the solute on the water structure, reflected in a decrease in the bulk-water relaxation time with rising electrolyte concentration, decreases in the sequence NaCl > KCl > CsCl and appears to be proportional to the surface-charge density of the cation. The interactions of K+ and Cs+ with their hydration shells are too weak to cause irrotational bonding of H2O molecules. It is argued that CsCl exhibits weak ion pairing, although the data are not sufficiently accurate to determine the extent of association and the nature of the ion pair formed.

Dielectric spectroscopy of aqueous solutions of KCl and CsCl

The permittivity and loss of solutions of tri(n-butyl)ammonium picrate and iodide in various polar solvents have been measured at a number of frequencies between 0.2 and 3.0 GHz. The relative static permittivities of the pure solvents used range from 3.37 to 20.7 at 25°C. For all the solutions investigated, the observed dispersion of permittivity is adequately described by a single relaxation time, which for a given solvent depends on the concentration as well as the nature of the solute. Apparent dipole moments evaluated from the dispersion amplitude by means of Bottcher's theory, on the assumption that the dipole occupies a spherical cavity, are 12.3±0.4 D and 11.2±0.6 D for the picrate and iodide respectively. These values indicate that the relaxing dipole is an ion pair. Dielectric relaxation times corrected for internal field effects by different relations are compared with those calculated from the partial molar volume of the solute and the viscosity of the solvent by means of the Debye-Stokes equation. Density data for solutions of the two salts in 1,2-dichloroethane are included and from these data, partial molar volumes of the salts have been evaluated.

E. A. S. Cavell

Papers

Dielectric relaxation in non aqueous solutions. Part 2.—Solutions of tri(n-butyl)ammonium picrate and iodide in polar solvents