Heat capacity and glass transition of an ionic liquid 1-butyl-3-methylimidazolium chloride

The heat capacity Cp of 1-butyl-3-methylimidazolium chloride, bmimCl, which is a prototype ionic liquid, was measured with an adiabatic calorimeter in the temperature range 5–380 K. A single crystal of bmimCl fused at 341 K with excess entropy of 77.1 J K−1 mol−1. In the supercooled phase, a glass transition with a large Cp jump occurred at 225 K. The low temperature Cp of glassy bmimCl showed a large low-energy excitation usually called boson peak. We concluded that ionic liquid bmimCl is quite disordered and fragile as glass-forming molecular liquids from a thermodynamic point of view.