Decoupling of conductivity relaxation from structural relaxation in protic ionic liquids and general properties

The main focus of this work is the study of conductivity relaxation of amorphous lidocaine hemisuccinate near the glass transition at ambient pressure. Measurements have been made using broadband dielectric spectroscopy (BDS) and temperature-modulated differential scanning calorimetry. Our study shows that the ion conductivity relaxation becomes increasingly faster than the structural relaxation as the glass transition temperature Tg is approached. At Tg the structural relaxation time is longer than the conductivity relaxation times by three decades, i.e. the decoupling index Rτ is about 3. Decoupling is accompanied by the ion conductivity relaxation which narrows in its frequency dispersion with decreasing temperature. This abnormal behavior is identical to that found in two other protic ionic liquids (PILs), procaine HCl and procainamide HCl. Considering that the phenomenon has been found before in several inorganic ionic glass-formers and now in three protic ionic liquids, it could be a general property of ionically conducting glass-forming substances, although more cases have to be studied before a definitive conclusion can be made. We show that it can be rationalized within the framework of the Coupling Model.