Local coordination and dynamics of a protic ammonium based ionic liquid immobilized in nano-porous silica micro-particles probed by Raman and NMR spectroscopy

Abstract

Room temperature ionic liquids confined in a solid material, for example, nano-porous silica, are particularly propitious for energy related applications. The aim of this study is to probe the molecular interactions established between the protic ionic liquid diethylmethylammonium methanesulfonate (DEMA-OMs) and silica, where the latter consists of nano-porous micro-particles with pores in the size range of 10 nm. The changes in the local coordination and transport properties induced by the nano-confinement of the ionic liquid are investigated by a combination of Raman and solid-state NMR spectroscopy. In particular, one-dimensional (1D) (1)H and (29)Si and two-dimensional (2D) (29)Si{(1)H} HETOCR solid-state NMR are combined to identify the sites of interaction at the silica-ionic liquid interface. Pulsed field gradient (PFG) NMR experiments are performed to estimate the self-diffusion of both bulk and nano-confined DEMA-OMs. Complementary information on the overall coordination and interaction scheme is achieved by Raman spectroscopy. All these advanced experimental techniques are revealed to be crucial to differentiate between ionic liquid molecules residing in the inter- or intra-particle domains.

Achieving enhanced ionic mobility in nanoporous silica by controlled surface interactions

“…22 In this contribution we present a new type of ion-conducting gel that builds upon recent results obtained with nanoporous silica micro-particles filled with an ionic liquid, where the silica structure is designed to provide extremely well-connected nanopores, very good mechanical properties, and a high surface area. 20 We here demonstrate by different experimental techniques that the ionic mobility is significantly enhanced by chemical functionalization of the silica surface, and we also present a molecular level explanation based on a flipped-ion effect. By validating these results for two very different ionic liquids, one protic short-chained and one aprotic long-chained, we demonstrate that the positive effect on ionic mobility is of general validity.…”

“…19 Diffusion NMR experiments have revealed that compared to the previous (sol-gel derived) ionogels, in these gels the mobility loss due to confinement is significantly reduced from 58% to 25% of the bulk value. 20 We have also verified by detailed spectroscopic studies including nuclear magnetic resonance (NMR), Raman and infrared, that the origin of this loss is related to surface interactions established at the ionic liquid/silica interface, i.e. at the pore walls.…”

“…As expected, the selfdiffusion coefficient decreases with fSiO 2 , that is with a higher degree of confinement. 16,20 For the ionic liquid C 6 C 1 ImTFSI, we observe that the decrease in self-diffusion is less pronounced in this type of gel than in the gels prepared via the sol-gel route…”

“…19 A SEM image of this kind of particle has already been provided by us in ref. 20. Then, calculated amounts of nanoporous silica and DEMAOMs or Table S1 in the ESI †).…”

“…Line narrowing effects. It is well known that the line width in 1 H NMR spectra reflects the mobility of the probed molecular species, 20,29 and in general narrower lines indicate a greater mobility as a result of weaker dipole-dipole interactions. 30 The strength of these interactions can be probed by varying the magic angle spinning (MAS) rate in solid-state NMR experiments, but cannot be averaged out in solid-like materials under static conditions.…”