Dr. Mauro Carlos Costa Ribeiro (University of São Paulo)

"Ionic liquids, i.e., molten salts at room temperature (by convention, T m < 100°C), exhibit complex phase transition phenomenology, including crystallization, polymorphism, events preceding melting (pre-melting), or vitrification and crystallization upon heating of the glass (cold-crystallization). In this talk, we will show how Raman spectroscopy has been used to study phase transitions of ionic liquids as a function of temperature and pressure. We will discuss the low-frequency regions of the Raman spectrum (as well as the far-IR spectrum), which are due to the intermolecular dynamics of the liquid, and the high-frequency regions that indicate, for example, conformational changes in the molecular structures of the ions normally...
used in the preparation of ionic liquids. In the case of a simple, symmetric anion, the dependence of vibrational frequency on pressure is reproduced using a hard-sphere model and extrapolations of empirical equations of state proposed for ionic liquids. Curves of melting and glass transition temperatures as a function of pressure, T m (P) and T g (P), were obtained by Raman spectroscopy using a diamond anvil cell (DAC) in a cryostat. The similarity dT m /dP ≈ dT g /dP observed experimentally will be discussed with arguments based on nonequilibrium thermodynamics. Examples will be given to illustrate how complex pre-melting or solid-solid transition events, which are often not apparent in calorimetric measurements, are revealed by Raman spectra of ionic liquids obtained at different rates of change in temperature and pressure.