Close Packing Existence of Short-Chain Ionic Liquid Confined in the Nanopore of Silica Ionogel

Abstract

We used small/wide-angle X-ray scattering and nitrogen gas sorption to explore the structural confinement effects of confining hydrophobic ionic liquid (IL), 1-hexyl-3-methylimidazolium hexafluorophosphate ([C6mim][PF6]), in the hydrophilic nanopores of silica ionogels. A nonhydrolytic sol–gel route with tetraethyl orthosilicate (TEOS) as precursor and formic acid solvolysis was adopted to prepare the studied silica ionogels. The results revealed that all these ionogels had fractal silica frameworks, but three distinct nanopores (H2-type nanopores with various radiuses, H3-type board slit-shaped capillary nanopores, and H4-type narrow slit-like nanopores) would be respectively formed with increasing nIL/nTEOS molar ratios. Moreover, neat [C6mim][PF6] formed self-organized nanoscale segregation in the bulk. However, when [C6mim][PF6] was confined within nanopores, narrow H2 and H4 nanopores would destroy this nanoscale segregation; inversely, confined [C6mim][PF6] further formed a more close packing in the H3 nanopores. The results from solid-state 29Si NMR and infrared spectra also identified that such close packing was caused by the press of confined ILs, imposed by the repulsive interfacial interactions between anionic [PF6]− and silanol. Finally, minimizing the [PF6]−/silanol repulsive interfacial energy and IL release entropy gain were postulated as the driving forces to explain the formation mechanism of distinct pore morphologies embedded within the ionogels with various nIL/nTEOS ratios.