M. Kleemeier

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...

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.

Critical behaviour of ionic solutions in non-polar solvents with a liquid - liquid phase transition

Turbidity measurements showing crossover from mean-field to Ising criticality have been reported by Narayanan and Pitzer for the liquid - liquid phase transition in ionic solutions of alkyl-ammonium picrates in higher alcohols. The Ising region was found to increase with the dielectric permittivity D for solvents with 4 < D < 8. It was conjectured that the Ising region becomes too small to be observed for lower values of D, which is in accordance with the finding of mean-field criticality in the system triethylhexylammonium triethylhexylborate in diphenyl ether , where . In order to check this hypothesis, we investigate solutions of salts in non-protonating solvents with D<2.5. The systems are tetrabutylammonium naphthyl sulphonate in toluene and tributylheptylammonium dodecyl sulphate in cyclohexane. The location of the critical points in the corresponding state diagram is in general agreement with the model system of charged hard spheres in a dielectric continuum, i.e. the restricted primitive model (RPM). However, changes of by minute variations of the salt and of the solvent (toluene, xylene, ethylbenzene) cannot be explained by the RPM. We report measurements of the phase diagram and light-scattering results. The amplitudes of the correlation length are up to an order of magnitude larger than those typically found in non-ionic fluids. For the new systems, but also for the solution of in , Ising criticality is found in the region of .

A comparison of 3D static light-scattering experiments with Monte Carlo simulations

The problems arising in conventional static light-scattering experiments with turbid samples due to multiple scattering of light can be overcome by the cross correlation technique. We recently reported results of 3D cross correlation experiments in which the angle dependence of the experimentally determined values of the singly scattered light intensity agrees with that of the scattering cross section of the suspended particles. In this paper we present measurements for various optical path lengths of the scattered light and compare the experimental results with Monte Carlo simulations. We demonstrate that the dependences of the singly and multiply scattered light intensities on the optical path length obtained by the 3D cross correlation method are in good agreement with the data obtained from Monte Carlo simulations. These results further confirm that the 3D cross correlation technique indeed allows the direct determination of the singly scattered light intensity by experimental means without any need for the detailed information about the experimental set-up that is necessary in Monte Carlo simulations.

Critical behavior of the ionic system n-Propyl-tri-n-butylammonium iodide + water near its upper and lower consolute points

Abstract Aqueous solutions of tetraalkylammonium salts, R4NX, with sufficiently large chain length of the alkyl groups (R≥3) show liquid-liquid immiscibilities at elevated temperatures. In general, upper consolute points are observed, but the shapes of the miscibility gaps indicate the existence of lower consolute points suppressed by crystallization. In aqueous solutions of the unsymmetrically substituted salt n-propyl-tri-n-butylammonium iodide (PrBu3NI) the lower consolute point is shifted above the crystallization curve, so that the closed miscibility gap can be monitored completely. We have performed accurate measurements of the coexistence curve near both the lower and upper consolute points of this system. The upper critical point lies at approximately 345.73 K and 0.438 in weight fraction of the salt. The lower critical point lies at 331.72 K and 0.446 in weight fraction. In the vicinity of the critical points an Ising-like critical behavior of the coexistence curve is found. The correct critical exponent of the Ising model is however only observed, if account is made for the peculiarities of re-entrant phase transitions with upper and lower critical points close to one another. The results are discussed with regard to recent observations of an apparent meanfield-like behavior of some ionic fluids.

Experimental determination of singly scattered light close to the critical point in a polystyrene–cyclohexane mixture

In turbid media the presence of multiple scattering constitutes a major complication for the analysis of the intensity and of the intensity correlation functions of the scattered light. The 3D-cross-correlation technique provides an effective means to determine the single scattering intensity and to suppress the influence of multiple scattering to the time dependence of correlation functions. The technique is applied to study the temperature dependence of the critical fluctuations of a solution of polystyrene (Mw=1.11×105 g mol-1) in cyclohexane. We show that the single scattering intensity determined for a scattering angle of ϑ=90° can be described by the Ornstein–Zernike function over the entire temperature range of 313.15–293.49 K. Good agreement between experiment and Monte Carlo simulations of the scattering processes is found for the ratio of singly scattered light to the total scattering intensity.

Critical fluctuations in electrolytes with a liquid-liquid phase transition

Near-critical fluctuations of ionic solutions with a liquid-liquid phase transition are investigated by measurements of turbidity and static and dynamic light scattering. Phase diagrams are also presented. The systems are substituted ammonium salts (picrate, nitrate, iodide) dissolved in higher alcohols and water. The correlation lengths, determined from the time correlation functions using the Kawasaki mode coupling theory, also describe the temperature dependence of scattering intensity and turbidity. Depending on the specific properties of the system mean-field or Ising behaviour is observed.