Thermal stability and crystallization of N-alkyl-N-alkyl'-pyrrolidinium imides

A series of N-alkyl-N-alkyl'-pyrrolidinium-bis(trifluoromethanesulfonyl) imide (TFSI-) room temperature ionic liquids (RTILs) has been investigated by means of thermogravimetric analysis (TG), differential scanning calorimetry, FT-IR spectroscopy, and X-ray diffraction analysis. These compounds exhibit a thermal stability up to 548-573 K. The mass loss starting temperature, T (ml), falls in a narrow range of temperatures: 578-594 K. FT-IR spectra, performed before and after 24 h isothermal experiments at 553 and 573 K, have confirmed their great thermal stability. Below the ambient temperature, these compounds exhibit a complex behavior. N-methyl-N-propyl-pyrrolidinium-TFSI is the sole liquid which crystallizes without forming any amorphous phase even after quenching in liquid nitrogen. Its crystalline phase has a melting point, T (m), of 283 +/- A 1 K. When the amorphous solid is heated, the N-butyl-N-ethyl-pyrrolidinium-TFSI presents a glass transition temperature, T (g), at 186 K followed by a cold crystallization, T (cc), at 225 K, and a final T (m) at 262 K. The N-butyl-N-methyl-pyrrolidinium-TFSI exhibits a T (g) between 186 and 181 K, its cold crystallization leading to two different solid phases. Solid phase I has a melting point T (I,m) = 252 K and phase II, T (II,m) = 262 K. When the amorphous phase is obtained at a cooling rate of 10 K/min, its T (cc) is 204 K, and a metastable solid phase (III) is obtained which transforms into the phase II at 226 K. However, when the sample is quenched, the amorphous phase transforms into phase II at T (cc) = 217 K and phase I at 239 K. P-15-TFSI exhibits the most complicated pattern as, on cooling, it leads to both a crystallized phase at 237 K and an amorphous phase at 191 K. On heating, after a T (g) at 186 K and a T (cc) at 217 K, two solid-solid phase transitions are observed at 239 K and 270 K, the final T (m) being 279 K.