Microheterogenities and Crossover from Ising to Mean Field Criticality in Solutions of Ionic Liquids

• Applicant: Professor Dr. Wolfram Schröer
• Subject Area: Physical Chemistry of Molecules, Liquids and Interfaces, Biophysical Chemistry
• Term: from 2006 to 2010
• Project identifier: Deutsche Forschungsgemeinschaft (DFG) - Project number 29324948

• Final Report Year: 2013

Final Report Abstract

The phase behaviour of solutions of ionic liquids in aprotic solvents and water was systematically investigated. The solvents are hydrocarbons, arenes, dioxane and aryl halides. The ionic liquids are Trihexyl tetradecyl phosphonium halides (P666 (14) Cl) and such containing cations of the type 1-alkyl-3-methyl imidazolium (Cnmim+) and different anions such as halides, trifluor methyl sulfonate (Triflat, TFO) and Bis (trifluour methyl sulfonyl amide (NTF2). Depending on the length of the side chain, the size and the electronic structure of anion partial miscibility with an upper critical solution point at ambient conditions is observed. For ionic liquids with the triflat anion partial miscibility is observed in water and weakly polar solvents because of the hydrophil and hydrophobe sites of this anion. The phase diagrams are analysed presuming Ising behaviour and a non-linear diameter predicted by the theory of complete scaling. Corresponding state analysis, which is based on the model of charged hard spheres (CHS) and simulations of the pure ionic liquids, reveals general agreement with the predictions for the CHS-model. Deviations scale with the dielectric permittivity of the solvent and indicate a continous transition between Coulomb demixing in non-polar solvent and hydrophobic demixing in water. For accurate measurements of the viscosity, of the coexistence curve and the static and dynamic light scattering with mK accuracy the mixtures P666(14) Cl in n- nonane, C18mim NTf2 in c-hexane and methyl cyclohexane, C12 mim Cl in benzene, C6mim TFO in Chloro-benzene , C8mim TFO in 4-Chloro-toluene, C5mim TFO in water, are selected. Approaching the critical temperature viscosity measurements show a plateau that is quantitatively explained by shear effects. All data show Ising critical behaviour with indications of homogeneous crossover to mean-field criticality for the solutions in aprotic solvents and inhomogeneous crossover for the solutions in water. The diameter of the phase diagrams is not linear but shows the 2ß-anomaly for all concentration variables including the number density. The analysis of light-scattering measurements is complicated by the presence of various scattering contributions as due to molecular diffusion, multiple scattering. and long living concentration inhomogeneities, which must relax before the measurements can be started. With the appropriate corrections static and dynamic light scattering show Ising critical behaviour with deviations caused by the onset of crossover consistent with the analysis of the phase diagrams. The critical behaviour of the liquid – liquid phase transitions in ionic solutions belongs the 3-d Ising universality class as non-ionic systems.

Publications

• Liquid-Vapor asymmetry in mean-field theories of simple, polar, and ionic fluids. Journal of Statistical Mechanics - Theory and Experiment. Article Number: P04020 Apr 2008
  V. C. Weiss and W. Schröer
  
• The Liquid-liquid Phase transition in Solutions of Ionic liquids with Halide Anions: Criticality and Corresponding States. Pure and Applied Chemistry 80 7 1613-1630 2008
  A. Butka, V. R. Vale, D. Saracsan, C. Rybarsch, V. C. Weiss, W. Schröer
  
• Liquid-liquid phase separation in solutions of ionic liquids: phase diagrams, corresponding state analysis and comparison with simulations of the primitive model. Journal of Physics-Condensed Matter 21 424119 2009
  Schroer W, Vale VR
  
• Liquid-liquid phase transition in the ionic solutions of tetra-n-butylammonium chloride in o-xylene and ethylbenzene: Phase diagrams and corresponding state analysis. Journal of Molecular Liquids 145 116-128 2009
  Dittmar H, Butka A, Vale VR, Schröer W
  
• Lower Critical Solution Temperature in the Metastable Region of an Ionic Solution in a Non-Polar Solvent. Journal of Physical Chemistry B 113 1249-1252 2009
  Dittmar HR, Schroer W
  
• On the Chemical and the Physical Approach to Ion Association. J. Mol. Liqu. W. Schröer 164 (1-2) 3-10 2011
  Wolffram Schröer
  
• A Short History of Phase Transitions in Ionic Fluids Contrib. Plasma Phys. 52, (1) 78 – 88 2012
  W. Schröer
  
• The Ion Speciation of Ionic Liquids in Molecular Solvents of Low and Medium Polarity. Faraday Discussions 2012 154 , 391-407
  Yanping Jiang, Holger Nadolny, Stefan Käshammer, Sebastian Weibels, Wolffram Schröer, and Hermann Weingärtner