Origin of ργ/T scaling of primary and secondary conductivity relaxation times in mixture of water with protic ionic liquid

Abstract

Murali et al. [J. Phys. Chem. Lett., 2024, 15, 3376–3382] made ambient and high pressure dielectric measurements of a supercooled aqueous mixture of an acidic ionic liquid to find the presence of the primary (σ) conductivity relaxation together with the secondary (ν) conductivity relaxation originating from the water clusters confined by the cations and anions with relaxation times τ_σ and τ_ν respectively. From the isothermal and isobaric conductivity relaxation data found on varying thermodynamic conditions (i.e. T and P) at constant τ_σ are the invariance of (i) the frequency dispersion or the Kohlrausch function exponent (1 − n) of the primary conductivity relaxation, and (ii) the ratio of the primary and secondary conductivity times, τ_σ/τ_ν. This co-invariance of τ_σ, τ_ν, and (1 − n) at constant τ_σ was observed before in non-aqueous ionic liquids, but it is found for the first time in aqueous ionic liquids. The new data together with PVT measurements enable Murali et al. to show additionally that both τ_σ and τ_ν are functions of ρ^γ/T with a single exponent γ = 0.58. The Coupling model is the only theory predicting the co-invariance of τ_σ, τ_ν, and (1 − n) as well as the ρ^γ/T scaling of both τ_σ and τ_ν. It is applied herein to address and explain the data of the ionic liquid–water mixture.