Conformations of the alkyl side chains in l-alkyl-3-methylimidazolium cations were studied for different dihedrals along the alkyl side chain in a given cation, and for different cations of the family. The ionic liquids were represented by an all atom force field developed specifically for these compounds and the simulation results were compared with spectroscopic data. Nanometer-scale structuring in roomtemperature ionic liquids is observed using molecular simulation. The ionic liquids studied belong to the l-alkyl-3- methylimidazolium family with hexafluorophosphate or with bis(trifluoromethanesulfonyl)amide as the anions. For ionic liquids with alkyl side chains in the cation longer than or equal to butyl, segregation is observed between, on one side, the strongly charged parts of the cations and the anions, and on the other, the non-polar side chains of the cations. The alkyl chains aggregate in non-polar domains that permeate a three-dimensional network of ionic channels formed by the charged parts of the ions. The consequences of these nanostructural features on the solvation of nonpolar, polar, and associating solutes are discussed in the light of the solute-solvent radial distribution functions for systems containing between 0.1 and 0.2 mole fraction of solute.