Reduction in liquid fragility and enhancement in ionic conductivity in the glassy phase of [BmIm][NTf2] ionic liquid under nano-confinement: PALS and BDS investigations

Abstract

Positron annihilation lifetime spectroscopy (PALS) and broadband dielectric spectroscopy (BDS) have evidenced fundamental changes in the structural and dynamic properties of the room temperature ionic liquid (RTIL or IL) 1-butyl-3-methylimidazolium-bis(trifluoromethylsulfonyl)-imide ([BmIm][NTf₂]) under confinement in SBA-15 nano-pores (average pore size of ∼5 nm). It is observed that the nano-confinement transformed the fragility of the IL [BmIm][NTf₂] from high to low, resulting in a reduction in the ionic conductivity and diffusion coefficient of the ions inside the nano-pores at room temperature. However, compared with the bulk, in the glassy phase (below 200 K), an increase in both the ionic conductivity and diffusion coefficient is observed, which is due to enhanced molecular free volume under nano-confinement below 200 K. The results of this study are relevant for the fundamental understanding of the intriguing properties of [Bmim][NTf₂] (IL) confined inside nano-pores, which can be employed for several important spatial confinement applications.