Effect of oxygenated functionalities on the phase transitions of an ionic liquid confined in a mesoporous carbon host

The effect of surface functionalization on the carbon walls-ionic liquid (IL) interactions is analyzed by combining differential scanning calorimetry (DSC) and Raman spectroscopy. Four mesoporous carbons (Lmeso -9.0 nm) with a gradually increasing oxygenated surface functionality have been soaked with the 1-ethyl-3-methylimida-zolium bis(fluorosulfonyl)imide ([EMIm+][FSI-]) IL at filling ratios R approximate to 0.8 (almost complete pore fullness) and R approximate to 1.2 (fully filled pores with excess liquid). The low temperature behavior of the IL molecules confined in mesopores depends on their distribution between two populations: i) located close to the pore center, or ii) near the pore walls. In the surface clean material soaked with R approximate to 0.8, the molecules in the proximity of the pore center are involved in freezing/melting, whereas the species located in the vicinity of the pore walls are involved in a glass transition upon cooling, followed by cold crystallization and melting upon heating. When applying the functionalized carbon hosts, all the species located near the pore surface freeze already upon cooling, owing to their aggregation to the walls through the formation of hydrogen bonds with the surface oxygenated function-ality. The amount of [EMIm+][FSI-] exhibiting exclusively freezing/melting can be as well enhanced by completely filling the pores (R approximate to 1.2) to strengthen the IL-IL and IL-carbon interactions. The information derived from this study is crucial for designing an optimal porous carbon-IL system intended to be implemented in e.g., electrochemical capacitors (ECs) operating at low temperature.