Explanations of the changes of dynamics at the liquid-liquid transition in phosphonium ionic liquids

Abstract

Novel properties of ionic conductivity relaxation and structural relaxation were found in experiments on liquid-liquid transition (LLT) in phosphonium ionic liquids with the cation [P_666,14]⁺ and different anions. Structural reorganization at LLT causes the isobaric (isothermal) conductivity relaxation time τ_σ(T,P) to abruptly increase in its temperature (pressure) dependence, which is accompanied by the corresponding decrease of the exponent β_σKWW(T,P) in the Kohlrausch-Williams-Watts time correlation function. The correlation between τ_σ(T,P) and β_σKWW(T,P) indicates the former is related to or determined by the latter. This inference leads to the explanation by the Coupling Model (CM) with its signature equation predicting exactly that quantitatively. The explanation is justified by the absence of abrupt increase of the relaxation time τ_0σ(T,P) of the local primitive relaxation in the CM at LLT, as expected from its insensitivity to the structural reorganization in LLT. In addition, the CM equation also explained two other properties found in the phosphonium ionic liquids. (1) The β_σKWW(T,P) is invariant to variations of T and P while keeping τ_σ(T,P) constant. (2) The structural α-relaxation times ${\tau }_{\alpha }\left(T\right)$ in some ionic liquids are longer with a larger activation energy than τ_σ(T) of ion conductivity relaxation, and correspondingly the frequency dispersion of the former is much broader than the latter.