Themolecular-scale properties of the room temperature ionic liquid(RTIL) 1-butyl-3-methylimidazolium bis(trifluoromethylsulfonyl) imide,[C4mim+][Tf2N–],confined in nanometer-scale carbon pores have been investigated usingsmall-angle X-ray and neutron scattering and fully atomistic moleculardynamics simulations. [C4mim+][Tf2N–] densities significantly higher than that ofthe bulk fluid at the same temperature and pressure result from thestrong affinity of the RTIL cation with the carbon surface duringthe initial filling of slitlike, subnanometer micropores along themesopore surfaces. Subsequent filling of cylindrical ∼8 nmmesopores in the mesoporous carbon matrix is accompanied by weak RTILdensification. The relative size of the micropores compared to theion dimension, and the strong interaction between the RTIL and theslit-like micropore, disrupt the bulk RTIL structure. This resultsin a low-excluded volume, high-RTIL ion density configuration. Theobserved interfacial phenomena are simulated using a molecular dynamicsmodel consisting of a linear combination of mesopore and microporeeffects. These observations highlight the importance of includingthe effects of a porous substrate’s internal surface morphology,especially roughness and microporosity, on the resulting electrolytestructural properties and performance in electrical energy storageapplications. [ABSTRACT FROM AUTHOR]