How does the flexibility of pyrrolidinium cations affect the phase behaviour of 1-alkyl-1-methylpyrrolidinium bis(trifluoromethanesulfonyl)imide homologues under stressful conditions?

Abstract

We conducted high-pressure Raman spectroscopy measurements on a series of 1-alkyl-1-methylpyrrolidinium bis(trifluoromethanesulfonyl)imide ([Pyr_1n][TFSI], n = 3, 4, 6 and 8) homologues that have different alkyl chain lengths, n, at room temperature. The results showed that all [Pyr_1n][TFSI] samples formed a glassy state in which the glass transition pressure (p_g) slightly increased with an increase in n. This tendency is similar to prior results of high-pressure glass formation of [C_nmim][TFSI], although the p_gs for [Pyr_1n][TFSI] are larger than those for [C_nmim][TFSI] with corresponding n by ∼0.5 GPa. We discuss the local structural changes occurring in [Pyr_1n][TFSI] in view of the conformational changes of the Pyr⁺_1n cation and TFSI⁻ anion.