Crystal Polymorphs and Multiple Crystallization Pathways of Highly Pressurized 1-Ethyl-3-Methylimidazolium Nitrate

Crystal polymorphs and multiple crystallization pathways of a room-temperature ionic liquid (RTIL) were observed only under high pressure (HP). The RTIL was 1-ethyl-3-methylimidazolium nitrate, [C2mim][NO3]. The HP-crystal polymorphs were related to conformations of the C2mim+ cation, and the HP-crystal pathways determined by the presence or absence of the planar′ (P′) conformation of the C2mim+ cation were switched at the bifurcation pressure (PB). Above PB, modulated crystal structures derived from the HP-inherent P′ conformer. Simultaneous X-ray diffraction and differential scanning calorimetry measurements, accompanied by optical microscope observations, confirmed the normal low-temperature crystallization of [C2mim][NO3] under ambient pressure.