Free volume in imidazolium triflimide ([C_3MIM] [NTf _2]) ionic liquid from positron lifetime: Amorphous, crystalline, and liquid states

摘要

Positron annihilation lifetime spectroscopy (PALS) is used to study the ionic liquid 1-methyl-3-propylimidazolium bis(trifluoromethylsulfonyl)imide [C3 MIM] [NTf2] in the temperature range between 150 and 320 K. The positron decay spectra are analyzed using the routine LifeTime-9.0 and the size distribution of local free volumes (subnanometer-size holes) is calculated. This distribution is in good agreement with Fürth's classical hole theory of liquids when taking into account Fürth's hole coalescence hypothesis. During cooling, the liquid sample remains in a supercooled, amorphous state and shows the glass transition in the ortho-positronium (o-Ps) lifetime at 187 K. The mean hole volume varies between 70 ?~3 at 150 K and 250 ?~3 at 265-300 K. From a comparison with the macroscopic volume, the hole density is estimated to be constant at 0.20× 1021 g-1 corresponding to 0.30 nm-3 at 265 K. The hole free volume fraction varies from 0.023 at 185 K to 0.073 at T_m +12 K=265 K and can be estimated to be 0.17 at 430 K. It is shown that the viscosity follows perfectly the Cohen-Turnbull free volume theory when using the free volume determined here. The heating run clearly shows crystallization at 200 K by an abrupt decrease in the mean 〈 τ_3 〉 and standard deviation σ_3 of the o-Ps lifetime distribution and an increase in the o-Ps intensity I_3. The parameters of the second lifetime component 〈 τ_2 〉 and σ_2 behave parallel to the o-Ps parameters, which also shows the positron's (e+) response to structural changes. During melting at 253 K, all lifetime parameters recover to the initial values of the liquid. An abrupt decrease in I_3 is attributed to the solvation of e- and e+ particles. Different possible interpretations of the o-Ps lifetime in the crystalline state are briefly discussed.