Anomalous phase behaviors near the multiphase coexistence point in 1-alkyl-3-methylimidazolium ionic liquids

The complex phase behaviors of ionic liquids near the multiphase coexistence point (MCP) were examined using synchrotron small- and wide-angle x-ray scattering. The cations utilized in this study were 1-alkyl-3-methylimidazolium [Cnmim]+, where n represents the alkyl chain length, and the anions were Cl−, Br−, I−, and [NO3]−. In proximity to the alkyl chain length corresponding to the MCP (nMCP), crystal polymorphism or multiple phase transition pathways were induced under low-temperature (LT) or high-pressure (HP) conditions. Within the [C8mim][X] series, [C8mim]Cl and [C8mim]Br were crystallized under HP conditions. The multiphase coexistence in [C10mim]Br and [C10mim][NO3] was sensitively dependent on the heating rate and was explained by the generalized Gibbs phase rule. With increasing deviation from nMCP, the LT and HP phase behaviors of [Cnmim][X] considerably changed.