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Investigation of Ionic Liquids by Positron Annihilation Lifetime Spectroscopy

Investigation of Ionic Liquids by Positron Annihilation Lifetime Spectroscopy. G. Dlubek 1† , Yang. Yu 2 , R. Krause-Rehberg 2 , W. Beichel 3 and I. Krossing 3 1 ITA Institut für Innovative Technologien, Köthen, Germany

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Investigation of Ionic Liquids by Positron Annihilation Lifetime Spectroscopy

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  1. Investigation of Ionic Liquids by Positron Annihilation Lifetime Spectroscopy G. Dlubek1†, Yang. Yu2, R. Krause-Rehberg2, W. Beichel3 and I. Krossing3 1ITA Institut für Innovative Technologien, Köthen, Germany 2Martin-Luther-Universität Halle, Institut für Physik, 06099 Halle(Saale) Germany 3Institut für Anorganische Chemie, Albert-Ludwigs-Universität Freiburg, Albertstraße 21, D-79104 Freiburg i. Br., Germany Sep. 5th. 2011

  2. Outline • Free volume influence to molecular transport property • Fürth theory • Ionic Liquids • Experiment results and discussion • Conclusion

  3. Free volume influence to molecular transport property • Permeation properties (small molecules in polymer), viscosity, viscoelasticity, glass transition, volume recovery, mechanical properties • Fluidity: Doolittle: • Mobility: Cohen-Turnbull Equation: • Permeability coefficient: • Selectivity: • Ionic conductivity:

  4. Fürth’s hole theory: P • Ts Ref: Dlubek, G., Yu, Yang, et al., Free volume in imidazoliumtriflimide ([C3MIM][NTf2]) ionic liquid from positron lifetime: Amorphous, crystalline, and liquid states. The Journal of Chemical Physics, 2010. 133(12): p. 124502-10. The energy required for the formation of a hole of spherical shape of radius r in a continuum is equal to the sum of the work to be done against the surface tension and the work to be done against the pressure. Relation between hole volume and surface tension. • [Fürth, R. Mathematical Proceedings of the Cambridge Philosophical Society, 1941.]

  5. Ionic Liquids (ILs): • Definition: organic salts with melting points below 100 oC or even room temperature(RTILs). • Structure: organic cations paired with organic or inorganic anions. [BMIM]+ [BF4]- [NTf2]- [OTf]- [PF6]- [Cl]- [B(hfip)4]- Ionic formulae of the ionic liquids studied in this work.

  6. Experiment results and discussion • [BMIM][BF4]: The mean, <3>, and the standard deviation, 3, of the o-Ps lifetime distribution as a function of temperature T during cooling and heating of [BMIM][BF4]. Tg indicates the glass transition temperature and Tk the “knee” temperature. The intensity I3 of the o-Ps lifetime as a function of temperature T during cooling and heating of [BMIM][BF4].

  7. [BMIM][BF4]: Plot of the specific volume from PVT experiment under 0 MPavs the mean hole volume at supercooled liquid state (between Tg and Tk). The line is a linear fit of the data. Number-weighted mean <vh> (spheres) and standard deviation sh (squares) of the hole size calculated from positron lifetime. Nh’ = 0.442  1021g-1; Vocc = 0.7574 cm3/g.

  8. [BMIM][NTf2]: The mean, <3> (squares), and the standard deviation, 3(spheres), of the o-Ps lifetime distribution as a function of temperature T during cooling and heating of [BMIM][NTf2]. The o-Ps intensity I3as a function of temperature during cooling and heating of [BMIM][NTf2]

  9. [BMIM][NTf2]: Plot of the specific volume from PVT experiment under 0 MPavs the mean hole volume at supercooled liquid state (between Tg and Tk). The line is a linear fit of the data. Nh’ = 0.179 x1021 g-1 Vocc= 0.6405 cm3/g.

  10. [BMIM][OTf]: The mean, <3>, and the standard deviation, 3, of the o-Ps lifetime distribution as a function of temperature T during cooling and heating of [BMIM][OTf]. Tcrand Tmshow the temperatures of crystallization (during cooling) and melting. The o-Ps intensity I3.

  11. [BMIM][PF6]: The mean, <3>, and the standard deviation, 3, of the o-Ps lifetime distribution as a function of temperature T during cooling and heating of [BMIM][PF6]. 4shows an additional o-Ps lifetime, which appears after transformation of the cr-II into the cr-I phase. • The two o-Ps intensities I3 and I4.

  12. [BMIM][PF6]: Plot of the specific volume from PVT experiment under 0 MPavs the mean hole volume at supercooled liquid state. The line is a linear fit of the data. Nh’ = 0.376 x1021 g-1 Vocc = 0.6670 cm3/g.

  13. [BMIM][Cl]: The mean, <3>, and the standard deviation, 3, of the o-Ps lifetime distribution as a function of temperature T during cooling and heating of [BMIM][Cl]. 4shows an additional o-Ps lifetime which appears after crystallization. The two o-Ps intensities I3 and I4.

  14. [BMIM][Cl]: Plot of the specific volume from PVT experiment under 0 MPavs the mean hole volume at supercooled liquid state. The line is a linear fit of the data. Nh’ = 0.584 x1021 g-1 Vocc= 0.8822 cm3/g.

  15. [BMIM][B(hfip)4]: The mean, <3>, and the standard deviation, s3, of the o-Ps lifetime distribution as a function of temperature T during cooling and heating of [BMIM][B(hfip)4].

  16. Summarized parameters from experiment results for the ionic liquids.

  17. Hole volumes comparison with molecular volume The hole volumes of various ILs in the liquid (filled circles) and in the glass (140 K, empty circles) states as function of the molecular volume Vm = V(A+X). The straight lines are linear fits constrained to pass zero, the dashed line shows a quadratic fit.

  18. Hole volume comparison with Fürth theory • Comparison of the mean hole volumes <vh> for the liquid or supercooled liquid and glassy states of the ionic liquids under investigation. Filled symbols: cooling, empty symbols: heating. Free volume calculated by Fürth theory is shown as line in the graph. • [Fürth, R. Mathematical Proceedings of the Cambridge Philosophical Society, 1941.]

  19. Viscosity and conductivity

  20. Conclusion • Important information of the local free volume in the amorphous (glass, supercooled liquid, true liquid) and crystalline phases of ionic liquids as well as the corresponding phase transitions can be obtained from PALS. • The o-Ps mean lifetime <3> shows different behaviour indicating different phases (smaller values in crystalline phase due to dense packing of the material). • The parameters I3also responds to phase transition by sharp value change. Low value in supercooled and true liquid, due to solvation of e+, precursor of Ps. • The knee temperature Tkcoincidentswith melting temperature of corresponding crystalline structure for [NTf2], [PF6] and [Cl] samples. • The local free volume from PALS displays a systematic relationship with molecular volume. • Fitting result of viscosity and conductivity by CT equation shows the free volume influence to molecular transport property.

  21. More Results:http://positron.physik.uni-halle.de/Thanks for your time and patience!

  22. Structural dynamic: • Vogel-Fulcher-Tamman (VFT) equation: = -29.7, B = 1339 and T0 = 140.8. T(=max_o-Ps=2.85 ns)=274 KTk=280 K = -25.8, B= 731 and T0 =156. T(=max_o-Ps=3.5 ns)=271 K Tk=270 K

  23. = -34.0, B = 2250 and T0 = 132. T(=max_o-Ps=3 ns)=289 KTk=285 K = -26.7, B = 1561 and T0 = 128. T(=max_o-Ps=2.5 ns)=354 KTk=335 K

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