Structural\nInsight into Ionogels: A Case Study of\n1‑Methyl-3-octyl-imidazolium Tetrafluoroborate Confined in\nAerosil

Ionogels were obtained by impregnating\nAerosil A380 with 1-methyl-3-octyl-imidazolium\ntetrafluoroborate (OMIM BF<sub>4</sub>) ionic liquid (IL). The IL\ncontent of the ionogels varied from 16.3 to 79.9 mol %. There was\nevidence of the confinement of the IL in silica in the shift and broadening\nof the BF<sub>4</sub><sup>–</sup> <sup>19</sup>F NMR signal\nand in the noticeable (∼50 °C) decrease in the temperature\nof IL decomposition. For both the ionogels and the pure IL, the frequencies\nof IR vibrations were different, providing further evidence of the\nconfinement effect. An analysis of textural characteristics revealed\nthat, upon its addition to Aerosil, the IL sequentially adsorbed in\nthe micropores, mesopores and interparticle space. SAXS measurements\nshowed that, in the confined IL, the size of nonpolar correlations\nsubstantially increased, from 21.5 Å in the bare IL to 25.6 Å\nin the ionogel containing 28.1 mol % IL. Unexpectedly, for the ionogel\nwith the lowest IL content (16.3 mol %), no nonpolar correlations\nwere observed, indicating the strong distortion of the structure of\nthe confined ionic liquid. To the best of the authors’ knowledge,\nthis is the first report on regular changes in nonpolar correlations\nin ionic liquids upon confinement in a porous solid. These structural\ncorrelations can easily be tuned by simply changing the IL content\nin the ionogel material.