Non-Isochronal Behavior of Charge Transport at Liquid-Liquid and Liquid-Glass Transition in Aprotic Ionic Liquids

A reversible, first-order transition separating two liquid phases of a single-component material is a fascinating yet poorly understood phenomenon. Here, we investigate the liquid-liquid transition (LLT) ability of two tetraalkylphosphonium ionic liquids (ILs), [P_666,14]Cl and [P_666,14][1,2,4-triazolide], using differential scanning calorimetry and dielectric spectroscopy. The latter technique also allowed us to study the LLT at elevated pressure. We found that cooling below 205 K transforms [P_666,14]Cl and [P_666,14][Trz] from one liquid state (liquid 1) to another (the self-assembled liquid 2), while the latter facilitates the charge transport decoupled from structural dynamics. In contrast to temperature, pressure was found to play an essential role in the self-organization of a liquid 2 phase, resulting in different time scales of charge transport for rapidly and slowly compressed samples. Furthermore, τ(P_LL) was found to be much shorter than τ(T_LL, P=atm), which constitutes the first example of non-isochronal behavior of charge transport at LLT. In turn, dielectric studies through the liquid-glass transition revealed the non-monotonic behavior of τ at elevated pressure for [P_666,14]Cl, while for [P_666,14][Trz] τ(P_g) was almost constant. These results highlight the diversity of liquid-liquid transition features within the class of phosphonium ionic liquids.