Melting point depression of ionic liquids by their confinement in carbons of controlled mesoporosity

The effect of ionic liquids (ILs) confinement in the nanoporosity of carbons on their low temperature transitions has been studied by differential scanning calorimetry. The ILs consisted of the [EMIm(+)] cation and [FSI-], [TESI-] or [BF4-] anions. The carbons were strictly microporous (Maxsorb) and mesoporous with a small amount of micropores, i.e. a MgO templated carbon TC-1 (L-meso = 3.4 nm), a silica templated carbon TC-2 (L-meso = 8.7 nm), and a high surface area carbon black SC2A (L-meso = 9.6 nm). All ILs encapsulated in the microporous Maxsorb demonstrated flat low temperature thermograms typical of an adsorbed state. In the narrow mesopores of TC-1, the interactions between the ions and the pore walls constrained the molecular motions, leading to a glass transition upon heating. The three confined ILs in SC2A displayed a melting peak at lower temperature (Delta T-m similar to -30 degrees C) than their neat counterpart. The Delta T-m shift was comparable for TC-2 soaked with [EMIm(+)] [FSI-], whereas only a glass transition was given by [EMIm(+)] [TESL-]and [EMIm(+)] [BF4-] confined in this carbon. Overall, the work shows that the liquid state of ILs can be extended to lower temperatures by their confinement in carbons possessing relatively wide mesopores. (C) 2020 Elsevier Ltd. All rights reserved.