Dynamics of ionic liquids under confinement in disordered mesopores

The molecular dynamics and phase transitions of ionic liquids confined within mesoporous structures are studied by X-ray diffraction (WAXS), temperature-modulated differential scanning calorimetry (TMDSC), and broadband dielectric spectroscopy (BDS). BMIM-TFSI is introduced into a hybrid ionosilica scaffold via a one-pot synthesis, resulting in cylindrical mesopores of average radius between 2 and 3 nm. The thermal properties are monitored as a function of the confinement state, controlled by the amount of incorporated ionic liquid, allowing the construction of an effective phase diagram under confinement. WAXS and TMDSC experiments show that, upon heating, the bulk ionic liquid undergoes cold crystallization followed by melting. Under nanoconfinement, the crystallization temperature shifts to higher values, while the melting temperature shifts downward, until crystallization is completely suppressed under strong confinement. Suppression of crystallization leads to significantly higher conductivity compared to the bulk in the temperature range between cold crystallization and melting, whereas outside this range confinement reduces ionic conductivity. BDS measurements further reveal a strong correlation between ionic conductivity and structural dynamics. The latter is significantly slowed down under confinement and accompanied by an increase in T _g. These results conceptually pave the way to control charge transport by confinement.