Wolffram Schröer

Corresponding states analysis of the critical points in binary solutions of room temperature ionic liquids

A systematic study of liquid–liquid phase transitions in mixtures of room temperature ionic liquids (RTIL) with nonionic solvents of different dielectric constant is presented. We report phase diagrams and the location of the critical points in the binary systems. The salts are 1-alkyl-3-methylimidazolium tetrafluoroborates and hexafluorophosphates with butyl-, hexyl-, octyl- side chains. The solvents are water, n-alcohols (n = 1 − 16), branched alcohols, and dialcohols. A corresponding state analysis of the data is carried out in terms of the variables of the restricted primitive model (RPM), which considers ionic fluids as equal-sized charged hard spheres in a dielectric continuum. The reduced critical temperatures are found to depend almost linearly on the dielectric constant of the solvent. The data of all investigated systems are represented by a master plot, which indicates a continuous crossover from a Coulomb driven phase transition, reasonably described by the RPM, towards a phase transition determined by hydrophobic interactions.

The liquid-liquid phase transition in ionic solutions: Coexistence curves of tetra-n-butylammonium pricrate in alkyl alcohols

We report coexistence curves of the liquid–liquid phase transition in ionic solutions. The phase diagrams of tetra-n-butylammonium pricrate solutions in a series of alkanols (2-propanol, 1-decanol, 1-dodecanol, 1-tridecanol and 1-tetradecanol) are determined either by measuring the refractive index in the two phase region in one sample of near-critical composition as a function of the temperature, or by direct observation of the composition dependent phase separation temperatures. With the exception of the 2-propanol system, the critical points are in accordance with the predictions by the restricted primitive model. The coexistence curves are analyzed in terms of different composition variables, of which the volume fraction seems to be the most appropriate one. For the volume fraction, deviations from asymptotic Ising behavior are observed which are equally well described by a critical exponent slightly different from the Ising value or by Wegner corrections. Although the deviations are quite small, they...

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.

Near‐critical light scattering of an ionic fluid with liquid–liquid phase transition

We report on the first static and dynamic light scattering measurements of near‐critical behavior of an ionic fluid. The model system tetra‐n‐butylammonium picrate‐tridecanol exhibits a liquid–liquid phase transition with an upper critical point near 342 K. The correlation lengths and scattering intensities scale with temperature corresponding to mean‐field behavior. The data are also concordant with nonclassical 3D‐Ising exponents if suitable Wegner corrections are applied. The spherical model recently suggested to apply for ionic fluids can be ruled out, thus resolving a current controversy in the literature.

Effective suppression of multiply scattered light in static and dynamic light scattering

The evaluation of conventional light-scattering experiments in turbid media is often highly complicated because of the presence of multiple scattering contributions. The three-dimensional (3-D) cross-correlation method presented provides an effective and handy method to suppress the influence of multiply scattered light. As the time dependence of the 3-D cross-correlation function is determined solely by the singly scattered light, the evaluation of the decay constant yields reliable values for the effective diffusion coefficient and the hydrodynamic particle size of the suspended particles. Furthermore, analysis of the amplitude of the 3-D cross-correlation function permits the determination of the differential scattering cross section even for highly turbid suspensions.

Liquid–vapor interface of square-well fluids of variable interaction range

Properties of the liquid-vapor interface of square-well fluids with ranges of interaction lambda=1.5, 2.0, and 3.0 are obtained by Monte Carlo simulations and from square-gradient theories that combine the Carnahan-Starling equation of state for hard spheres with the second and third virial coefficients. The predicted surface tensions show good agreement with the simulation results for lambda=2 and for lambda=3 in a temperature range reasonably close to the critical point, 0.8</=T/T(c)</=0.95. As expected, the surface tension increases with the range of interaction and decreases monotonically with temperature. A comparison between theory and simulation results is also given for the width of the interface and for the coexistence curves for the different interaction ranges.

Ion association and electrical conductance minimum in Debye–Hückel-based theories of the hard sphere ionic fluid

We investigate the ion–ion pair association equilibrium in the hard sphere ionic fluid as predicted by several Debye–Huckel-based theories. The degree of dissociation is examined in a pairing theory based on Ebeling’s definition of the association constant, and in extensions of this theory for ion pair–free ion interactions by Fisher and Levin, and ion pair–ion pair interactions by Weiss and Schroer. The effect of an increase of the dielectric constant on the degree of dissociation is studied. In all cases, minima of the density-dependent degree of dissociation along isotherms are found at low ionic density, at least over part of the temperature range. The loci of these minima in the temperature-density plane are calculated. The results are compared with conductivity data. We discuss a possible resemblance of the system’s behavior in a region in which the degree of association changes rapidly with a tricritical scenario, in which a line of second-order insulator–conductor phase transitions intersects the ...

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 ion speciation of ionic liquids in molecular solvents of low and medium polarity

The ion speciation of ionic liquids dissolved in molecular liquids of low and medium polarity is studied experimentally and theoretically. The ion speciation in some representative systems is characterized by dielectric relaxation spectroscopy and electrical conductance measurements. A corresponding-states approach is used to compare the experimental results with predictions for a reference system of charged hard spheres in a dielectric continuum. Topics of special interest are the formation of ion pairs and larger ion clusters in dilute solutions and their fate at high concentrations, where they have to redissociate in the excess ionic liquid to form the charge-ordered structure of the fused salt. In solvents of low polarity this transition leads to electrical conductance minima and liquid-liquid immiscibilities.

The general phase behavior of mixtures of 1-alkyl-3-methylimidazolium bis[(trifluoromethyl)sulfonyl]amide ionic liquids with n-alkyl alcohols.

The liquid-liquid equilibrium (LLE) phase behavior of mixtures of 1-ethyl-3-methylimidazolium bis[(trifluoromethyl)sulfonyl]amide (C(2)mimNTf(2)) with the n-alkyl alcohols (C(n)OH; 3≤n≤8) is described. By applying the cloud-point method, the LLEs were determined over a temperature range of 275-423 K at ambient pressure. Partial miscibility with upper critical solution temperatures (UCST) was observed. The UCST increase with increasing chain length of the alcohols. The phase diagrams were analyzed numerically by presuming Ising criticality. Concepts for the description of the asymmetry of the phase diagram by presuming the validity of the rectilinear diameter rule or a nonlinear diameter requested by the theory of complete scaling were applied. The results (UCST, critical composition, width and diameter of the phase diagrams) are compared with similar systems and discussed in terms of the corresponding state behavior; they map on a single curve. From at least 45 individual phase diagrams, general aspects of the behavior of this ionic liquid-alcohol mixture type were extracted. A simple empirical relationship was formulated to allow the description of the UCST with an accuracy of about 10 K when taking the ratio of the molar volumes of the alcohol and the 1-alkyl-3-methylimidazolium cation of the ionic liquid into account.