Immobilization and melting point depression of imidazolium ionic liquids on the surface of nano-SiOx particles

Abstract

Four kinds of imidazolium-based ionic liquids (ILs) were immobilized onto the surface of nano-SiO_x particles (d∼ 20 nm) by grinding in an agate mortar to produce a series of weight ratios of ionic liquid to nanoparticles. The physicochemical properties of immobilized ILs were investigated by differential scanning calorimetry, powder X-ray diffraction and Raman spectroscopy. It was found that the melting points (T_m) of the immobilized ILs depressed significantly in comparison with the bulk ionic liquids. The T_m depressions are 10, 12, 13 and 41 °C for [EMIM][PF₆], [PMIM][PF₆], [PHMIM][BF₄] and [EMIM][I], respectively, for a loading amount of 35 wt% ionic liquid. The T_m depression of [EMIM][PF₆] was independent of the weight proportion of immobilized ionic liquid up to 50 wt%, indicating that nano-SiO_x has a large capacity for immobilized ILs. The T_m depression of [EMIM][I] is particularly significant because the H-bonding interactions of iodine anions with surface silanol groups of nano-SiO_x particles is much weaker than that of fluorine anions with silanol groups of other investigated ionic liquids.