Exploring the phase diagram of the ionic liquid 1-butyl-1-methylpyrrolidinium bis(trifluoromethanesufonyl)imide

Abstract

X-ray diffraction and Raman scattering measurements of the ionic liquid 1-butyl-1-methylpyrrolidinium bis(trifluoromethanesufonyl)imide, [Pyrr1,4][NTf2], were performed in a wide range of temperature and pressure. Pressure dependence of glass transition Tg(p), cold-crystallization Tcc(p), and melting Tm(p) temperatures were obtained from atmospheric pressure up to ca. 2.0 GPa. The temperature dependence of Raman spectra within the Tcc < T < Tm range at atmospheric pressure unveils conformational dynamics of [Pyrr1,4]+ while [NTf2]- remains in the transoid conformation characteristic of the crystal. The different pressure dependence of Tcc(p) and Tm(p) implies that these curves merge at ca. 330 K and 1.0 GPa. Upon the assumption that the Tg(p) curve belongs to isochronous states, Tg(p) is discussed in terms of thermodynamic scaling using low-temperature and high-pressure extrapolations of an empirical equation of state. The ratio between constant volume and constant pressure activation energies for viscous flow, EV/Ep, increases only slightly in the pressure range investigated in this work. The results obtained under simultaneous variation of temperature and pressure highlight the complex interplay between glass transition and crystallization of [Pyrr1,4][NTf2].