Thermodynamic study of POSS-based ionic liquids with various numbers of ion pairs

Abstract

We have previously reported a polyhedral oligomeric silsesquioxane (POSS)-based room temperature ionic liquid (IL) that shows a melting temperature below 25 °C. The connection of ion pairs to POSS can significantly improve the thermal stability and decrease the melting temperatures of the ion pairs. To understand the mechanism of these changes induced by POSS, we studied the thermal properties of POSS derivatives with variable numbers of imidazolium–carboxylate ion pairs. Initially, it was found that a modified POSS with a larger number of ion pairs could be a thermally stable, room temperature IL. From the series of thermodynamic and conformational analyses, we concluded that the star-shaped distribution of ion pairs originating from the cubic framework are necessary to express the characteristic changes in the thermal properties of ion pairs connected to POSS. We have reported the polyhedral oligomeric silsesquioxane (POSS)-based room temperature ionic liquid. The connection of ion pairs to POSS improved the thermal stability and decreased the melting temperatures of the ion pairs. Herein, to understand the mechanism of these changes, we executed comparison study on thermal properties of POSS derivatives with variable numbers of ion pairs. We illustrated that the isolation of ion pairs caused by the cubic framework should be necessary to express the characteristic changes in thermal properties of ion pairs connected to POSS.