Confinement of pyrrolidinium-based ionic liquids [CnMPyrr]+[Tf2N]− with long cationic alkyl side chains (n = 10 and 16) to nanoscale pores: Dielectric and calorimetric studies

Abstract

Ionic liquids (ILs) with long cationic side chains have shown potential applications in wide fields such as phase-change materials for seasonal energy storage, environmentally friendly herbicides, components of ideal electrode material for supercapacitor devices, and more. In this paper, we investigated the glass transition dynamics, charge transport and structure properties of two pyrrolidinium-based ILs, [CnMPyrr]+[Tf2N]− with n = 10 and 16. Differential scanning calorimetry studies demonstrated the poor glass forming ability (GFA) of the tested materials, especially [C16MPyrr]+[Tf2N]−. Temperature-dependent X-ray diffraction measurements suggested the layered structures in the crystalline and liquid phases of both ILs. Through the dielectric measurements of [C10MPyrr]+[Tf2N]−, we noticed its higher conductivity than the homologues with shorter cationic alkyl chains, in the supercooled liquid state. Additionally, the derivative spectra, εder′′ = (−π/2)[∂ε′/∂ ln (f)], of [C10MPyrr]+[Tf2N]− at temperatures above Tg were examined, and no evidence of the slow sub- α mode associated with the self-aggregation of alkyl chains is detected. Next, we checked how [C10MPyrr]+[Tf2N]− and [C16MPyrr]+[Tf2N]− behaved when geometrically confined in alumina nanopores with various sizes ranging from 10 nm to 120 nm. With respect to the bulk material, confined [C10MPyrr]+[Tf2N]− samples are featured by the drastically enhanced GFA but up to 300-fold decrease in ionic conductivity. In addition, for [C16MPyrr]+[Tf2N]−, vitrification is unfulfilled even when confined into 10 nm nanopore. Nonetheless, nanoscale constraints bring pronounced melting temperature depression and enhanced conductivity to the crystalline confined samples relative to the bulk counterpart. The results of this study contribute to a more comprehensive picture of various confinement effects on ILs and will benefit the rational design of ILs-containing nanodevices.