Confinement Effects on the Ionic Liquid Dynamics in Ionosilica Ionogels: Impact of the Ionosilica Nature and the Host/Guest Ratio

Abstract

Ionosilica ionogels have been lately introduced as emerging all-ionic designer materials. They consist of an ionic liquid (IL) guest trapped within a solid ionosilica support host. In this work, we investigate the influence of the (i) ionosilica nature and (ii) the ionosilica/IL ratio on the mobility of the confined IL. We report the elaboration of various ionosilica ionogels via a nonhydrolytic sol–gel process, using namely tris(3-(trimethoxysilyl)propyl)amine (TTA), methyl-tris(3-(trimethoxysilyl)propyl)ammonium iodide (MTTA), and tetrakis(3-(trimethoxysilyl)-propyl)ammonium iodide (TKTA) as ionosilica precursors, with the IL butyl-methyl imidazolium bis(trifluoromethanesulfonyl)imide ([BMIM] TFSI). Various ionogels were prepared from different ratios between the ionosilica host and the IL guest. The host/guest interactions, i.e., the change in the conformational contribution of the IL counter-anion, were explored by means of Raman spectroscopy. In addition, the transport properties of the confined species were probed via spin echo solid-state NMR experiments and Complex Impedance Spectroscopy (CIS) measurements. Raman experiments revealed different conformational equilibrium for the TFSI anion in the various ionosilica ionogels, with an increase in the cisoid form compared to the bulk IL due to confinement effects. The TFSI anion seems to experience different degrees of confinement and thus different interactions with the ionosilica scaffold as a function of the investigated parameters. Concomitantly, the 1H spin echo NMR and CIS measurements revealed a significantly higher ionic mobility in the materials synthesized from the tris-trialkoxysilylated ammonium precursor compared to the one synthesized from the tetra-trialkoxysilylated ammonium precursor. These results may reflect stronger host–guest interactions in the latter case related to the higher degree of the network reticulation. All these techniques give concordant results and highlight an effect of the chemical constitution of the ionosilica scaffold and the amount of confined IL on its dynamics within the network.