Glass Transitions of Molecular Liquids and Room-Temperature Ionic Liquids

Transcription

1 Glass Transitions of Molecular Liquids and Room-Temperature Ionic Liquids Osamu Yamamuro (ISSP, University of Tokyo) Coworkers Molecular liquids: T. Matsuo (Osaka Univ.), K. Takeda (Naruto Edu. Univ.), S. Takahara (Okayama Sci. Univ.), I. Tsukushi (Chiba Inst. Tech.), N. Onoda-Yamamuro (Tokyo Denki Univ.), etc. Ionic liquids: Y. Inamura, S. Hayashi, H. Hamaguchi, T. Sakakibara (Univ. of Tokyo) Recent Progress in Glassy Physics (Paris, September 2005)

2 Outline of this talk 1. Introduction What happens at a glass transition? Adam-Gibbs theory and CRR 2. Molecular liquids CRR size determined from configurational entropy 3. Ionic liquids What is RT ionic liquids? CRR size from configurational entropy Dynamics of ionic liquids (quasielastic neutron scattering) Magnetization of a magnetic ionic liquid related to spin glasses 4. Summary

3 What happens at a glass transition T m Heat capacity jump at T g τ T g Steep increase of Structural relaxation time Why drastic slowing down occurs at T g (not at 0 K) without major structural change?

4 Adam-Gibbs theory, CRR and energy landscape cooling Cooperatively Rearranging Region 1 T g S c T m 1 S c (T)k ln W Number of Basins T K 5 log(τ /s) τ = τ 0 exp (z*δµ/rt) = τ 0 exp (A/S c T) CRR size Configurational entropy T g T K

5 Heat capacities of glass-forming hydrocarbon liquids Large and sharp C p jump at T g

6 Calculation of configurational entropy S c (T) = Δ fus S T fus 0 T gl [C p T fus [C p liq (T' )- C p gl (T' )]/ T' dt' (T' )- C p cr (T' )]/ T' dt' 1st term: entropy of fusion 2nd term: entropy decrease due to ordering in liquid 3rd term: correction for vibrational entropy

7 Configurational entropy of molecular liquids S c (T) = s c * N A A / T 2 z *(T) = s c * N A / S c (T) Extrapolation of entropy at hightemperature limit (entropy of a CRR) CRR size (number of molecules) (Adam-Gibbs theory)

8 CRR size of molecular liquids z*(t g ) = 48 molecules Consistent with some hole burning, NMR, and computer simulation studies

9 What is RT ionic liquids? Low vapor-pressure Amphiphilicity Non-combusibility Useful for solvents! Example of RT ionic liquid Typical cation: Butylmethylimidazolium [bmim]+ Anions: Cl, Br, I, BF4, PF6, FeCl4 etc. Prototype ionic liquid Magnetic ionic liquid

10 Magnetic ionic liquid [bmim]fecl4 Water [bmim]fecl4 Single component magnetic liquid! Chemically stable and non-volatile Various applications!

11 Our interests on RT ionic liquids (1) Glass transitions of liquids with intermediate positional ordering due to ionic interactions (2) Low melting temperatures of liquids with strong ionic interaction and large ionic size (3) Magnetic ordering or freezing in structural glasses at low temperatures

12 Heat Capacities of [bmim]cl and [bmim] FeCl 4 crystal I glass & liquid fus 341 crystal glass & liquid g Glass transition with a large C p jump

13 Temperature dependence of configurational entropy Isopropylbenzene 1-Propanol 1-Pentene Toluene 3-Bromopentane 1-Butene Ethylbenzene 3-Methylpentane 2-Methyltetrahydrofuran Propylene carbonate Salol Orthoterphenyl [bmim]cl S c of an ionic liquid is as large as those of molecular liquids Fragile liquid!

14 Temperature dependence of CRR size Butyronitrile Isopropylbenzene 1-Propanol 1-Pentene Toluene 3-Bromopentane 1-Butene Ethylbenzene 3-Methylpentane 2-Methytetrahydrofuran Propylene carbonate Salol Orthoterphenyl [bmim]cl CRR size of ionic liquids is similar to those of molecular liquids

15 Relation between T g and T m [bmim]fecl 4 Closed circles are for molecular liquids 2/3 law is valid for RT ionic liquids

16 I ( Q) Inc,elastic = σ inc 4π Debye-Waller factor : Mean square displacement N exp ( 2W ) 4 Glass Cryst. T g < u 2 > / < u 2 >(20K) Excess <u 2 > increase around T g Onset of fast β relaxation T [K] [bmim]cl Similar to molecular and polymer glasses

17 Analysis of quasielastic scattering (1) 6 [bmim]cl liquid at K Q = 0.38 Å -1 5 Lorenzian fitting S(Q,ω) (arb. unit) Q = 2.43 Å -1 T ( Q,ω ) = 1 π Γ S ( Q) ω 2 + Γ S Q s : HWHM ( ) Q [Å -1 ] ω [mev]

18 Analysis of quasielastic scattering (2) 80x K 335K 320K 300K 300K Fitted by S = DQ 2 Γs [mev] 40 Simple diffusive motion! Q 2 [Å -2 ] 4 5 6

19 Self diffusion coefficient Diffusion Coefficient of liquid [bmim]cl Fitted Line to obtain ΔE D = D 0 exp ΔE RT ln( D [Å 2 s -1 ] ) [bmim]cl ΔE = 10.9 kjmol -1 cf. intermolecular rotational barrier of alkanes (13-15 kjmol -1 ) fitted line : ln(d) = ( ± 0.258) -( ± 83.9)*(1/T) Very flexible motion! T -1 [ K -1 ] x10-3 Similar result for [bmim]fecl 4

20 Magnetization (SQUID) measurement for [bmim]fecl 4 Magnetic moment Inverse magnetic moment Curie-Weise law Liquid and glassy [bmim]fecl 4 is paramagnetic Negative Weise temperatur antiferromagnetic interaction

21 Magnetization measurement (Faraday method) T N Glass Crystal Collaboration with Prof. Sakakibara (ISSP, Univ. of Tokyo) Antiferromagnetic transition at 2.2 K in a crystalline state A sign of saturation of M at 0.5 K in a glassy state What happens at lower temperature? A new-type spin glass?

22 Summary (1) CRR size of molecular liquids is frozen-in at 4-8 molecules at T g. (2) Ionic liquids are similar to molecular liquids (z*, fast β,.) in spite of intermediate positional ordering Orientational degrees of freedom is important (3) [bmim] ion (or butyl-group) is very flexible. Ionic liquids are stabilized entropically Origin of low T m of ionic liquids? (4) [bmim]fecl 4 is paramagnetic liquid and glass. A new type spin glass at lower temperature?

23 Future Study (1) Measurements for more cations and more anions to generalize the present discussion (2) Neutron scattering at slower time scale (NSE, etc.) to see non-arrhenius region (3) Low temperature experiments for [bmim]fecl 4 to see a new spin glass (?)