Hermann Weingärtner

Coulombic and non-Coulombic contributions to the criticality of ionic fluids. An experimental approach

The recent discovery of liquid-liquid phase separations in electrolyte solutions with critical points near room temperature enables the systematic study of the critical behavior of ionic fluids. Depending on the nature of the molecular interactions, either sharp mean-field or Ising behavior is obtained in the temperature range down tot=(T−Tc)/Tc=10−4 or less. Mean-field-like criticality is obtained with systems which in the framework of a simple corresponding states model are fairly close to the critical point of the “restricted primitive model” (RPM) of equally-sized charged spheres in a dielectric continuum. In these cases the phase separation is driven by the Coulombic forces (so-calledCoulombic phase separations). This type of unmixing occurs for 1∶1 electrolytes in solvents of low dielectric constant. Simple mechanisms for unmixing suggested in the literature are discussed in relation to the available data. Some evidence for departures from the simple RPM prediction is found. The presence of additional short-range interactions leads to sharp Ising behavior. Examples are solutions of tetraalkylammonium salts in water and other highly structured solvents, where phase separation results from the peculiar solvophobic nature of ions (solvophobic phase separations). Previous speculations that this type of unmixing shows the tendency toward closed loops are confirmed by the first direct observation of a lower consolute point in an aqueous solution of propyl-tributylammonium iodide. By light scattering studies and measurements of the coexistence curve near the upper and lower consolute points Ising criticality is confirmed. A new mechanism for phase separation is reported for the system ethylammonium nitrate+octanol, where ion pairs are stabilized by hydrogen bonding beyond what is expected from the RPM. This comparatively subtle additional interaction (so-calledstricky ions) already changes the behavior of otherwise RPM-like systems from mean-field to Ising criticality. The results are discussed with particular emphasis on their implications for possible scenarios for explaining a mean-field critical point or crossover from mean-field to Ising behavior beyond the accessible temperature range.

Light-scattering investigations of the liquid-liquid phase transition of the ionic system: Trimethylethyl-ammonium bromide in chloroform

Phase diagrams and light-scattering measurements of solutions of trimethylethyl-ammonium bromide in chloroform (ε=4.72 at 25 C) with an ethanol content of 1% are reported. The system has a lower critical point nearT = 25°C. The critical mole fraction is xc,=0.0503 ± 0.0002, which corresponds to the reduced variablesTc*=0.036 andcc*=0.029 of therestricted primitive model (RPM) and is slightly below the values predicted by the RPM for the critical parameters. The analysis of the scattering intensity at critical composition gives v = 0.631 ± 0.003 for the critical exponent of the correlation length Σ with an amplitude of 0.87± 0.01 nm. The system, a solution of a salt of essentially spherical ions of almost equal size in a simple low-dielectric polar liquid, with critical parameters very close to predictions of the RPM, nevertheless has an Ising critical point.

The mechanism of 7Li relaxation in a supercooled aqueous LiI solution

Abstract 1H, 2D, and 7Li relaxation time measurements in a 6-aquamolal aqueous Lil solution were performed as a function of temperature. By isotopic H/D substitution the 7Li relaxation rate was separated into the dipolar and quadrupolar contributions. At about 180 K, maxima for 1/T1 plotted vs 1/T were obtained. The maximum of the quadrupolar contribution to 7Li relaxation occurs at the same temperature as the maxima obtained for 1H and 2D relaxation, which means that 7Li relaxation is determined by reorientational Brownian motions of the species. This result is compared with the predictions of several recent theories for the quadrupolar relaxation of monoatomic ionic nuclei in electrolyte solutions. The results give support to an electrostatic model.

129Xe NMR as a new tool for studying gas diffusion in liquids : self-diffusion of xenon in water

Abstract We report on a new application of 129 Xe NMR for measuring gas diffusion in liquids. Specifically, we present data for the translational diffusion coefficient of xenon in water. The values range from about 1.3×10 −9 to 3×10 −9 m 2 s −1 in the temperature range 5–55°C, and are much higher than literature data for xenon diffusivities measured by other techniques. Also, they are roughly of the same order of magnitude as the self-diffusion coefficient of water (1.90×10 −9 versus 2.30×10 −9 m 2 s −1 ), as is also predicted by computer simulation. The activation energy of about 12.8 kJ mol −1 for xenon diffusion is however surprisingly low, if compared with 18.3 kJ mol −1 observed for water in this temperature range. An unexpectedly strong influence of an added salt on the self-diffusion coefficient of xenon in water is noted.

Transference numbers of aqueous NaCl and Na2SO4 at 25°C from EMF measurements with sodium-selective glass electrodes

Transference numbers in aqueous solutions of NaCl and Na2SO4 were obtained by measuring the emf of cells with liquid junction using sodium-selective glass electrodes. Transference numbers in the NaCl−H2O system from about 0.1 to 4 mol-kg−1 (m) are in good agreement with literature data obtained by various experimental techniques, but are markedly lower than recent data obtained from emf measurements with Ag,AgCl-electrodes. Transference numbers in the Na2SO4−H2O system from about 0.03 to 1.9 m show a small decrease with increasing salt concentration, which is near the limit of experimental uncertainty. Sodium-selective electrodes are well suited to perform determinations of transference numbers and may provide an alternative to amalgam electrodes.

Liquid-liquid phase separations and critical behavior of electrolyte solutions driven by long-range and short-range interactions

The discovery of liquid-liquid phase separations in electrolyte solutions with critical points near room temperature enables the systematic study of the critical behavior of ionic fluids. Depending on the details of the molecular interactions mean-field-like criticality, Ising criticality or crossover from mean-field to Ising criticality is observed. Mean-field behavior occurs in systems which in the framework of a simple corresponding states model are fairly close to the theoretical critical point of the“restricted primitive model (RPM)” of equally-sized charged spheres in a dielectric continuum (so-called Coulombic phase separations). In these cases the phase transition is driven by the long-range Coulombic forces and may be modeled by a Bjerrum-type theory for ion pair association plus an additional term for the interaction between the ion pairs and free ions. The presence of additional short-range interactions shifts the phase transitions towards higher reduced temperatures and/or densities in the corresponding states plot. Two major examples are solvophobic unmixings in aqueous solutions of tetraalkylammonium salts and unmixings of salts with hydrogen-bonds between cations and anions (sticky ion pairs). Such systems exhibit Ising-like criticality. In the intermediate range crossover is observed. Possible scenarios for explaining the mean-field-Ising dichotomy are discussed, indicating a major role of the dielectric constant of the solvent in determining critical behavior.

High-Pressure Liquid-Liquid Immiscibility in Aqueous Solutions of Tetra-n-butylammonium Bromide Studied by a Diamond Anvil Cell Technique

By use of a diamond anvil cell, we show that at high pressures aqueous solutions of tetra-n-butylammonium bromide form a closed liquid–liquid immiscibility loop with a critical concentration of about 2.0 mol-kg−1. We report data for a near-critical isopleth, which describes the pressure dependence of the upper and lower consolute points. At about 700 MPa and 373 K both consolute points coincide to form a hypercritical point, which characterizes the minimum pressure to achieve immiscibility. We show that this immiscibility can be rationalized in terms of Pitzers ion-interaction theory. We determine the ion-interaction parameters of Pitzers theory up to 423 K at normal pressure. Inclusion of volumetric data gives the correct trend toward a high-pressure immiscibility. We discuss the results in terms of the interplay between ionic and hydrophobic forces in this system.

Determination of transference numbers with ion-selective electrodes. Transference numbers and activity coefficients of concentrated aqueous solutions of potassium fluoride

An experimental method has been developed to determine transference numbers by simultaneous emf measurements of cells with liquid junction using cation- and anion-responsive electrodes. As a corrolary of the transference measurements, activity coefficient data have been obtained. The method is illustrated by reporting transference and activity coefficient data of aueous KF solutions at 25°C from about 0.05 to 3 m obtained with potassium selective glass electrodes and fluoride selective solid state electrodes. The activity coefficients are in good agreement with literature data, which reflects the excellent Nernstian response of these electrodes in solutions up to 3m. The transference numbers show the typical behavior expected for a non-associated 1:1 electrolyte.

A Nuclear Magnetic Relaxation Study of the Structure of Binary Liquid Mixtures of Hexafluorobenzene with Benzene and Cyclohexane

— C6H6 hetero-association, which could be expected from the behaviour of the thermodynamic excess functions in this system. In contrast, the magnetic relaxation data for C6F6 —C6H12 mixtures confirm the presence of weak self-association of the components, which is expected from the behaviour of the thermodynamic quantities. An analysis of the thermodynamic quantities in terms of the Kirkwood-Buff theory of solutions supports the conclusions drawn from the analysis of the magnetic relaxation data. The results for the C6F6 —C6H6 system seem however to be at variance with the results from X-ray and neutron scattering experiments.

Anisotropic molecular reorientation of hexafluorobenzene in binary liquid mixtures with benzene and cyclohexane. A nuclear magnetic relaxation study

Abstract 19 F magnetic relaxation times T 1 of hexafluorobenzene in binary liquid mixtures with benzene and cyclohexane at 25°C were studied at resonance frequencies of 19.5 and 83 MHz. The data indicate a frequency dependence of T 1 caused by relaxation through chemical shift anisotropy, enabling the correlation time τ ⊥ for the tumbling motion of C 6 F 6 to be determined. Combination with correlation times τ i for the in-plane motion derived from 13 C relaxation times makes it possible to extract information on the anisotropic reorientation of C 6 F 6 . Reorientation of C 6 F 6 in the pure liquid is markedly anisotropic, with τ ⊥ /τ ∥ ≈ 2.7. Dilution in benzene increases the reorientational anisotropy, while dilution in cyclohexane produces no change.