Invariance of conductivity relaxation under pressure and temperature variations at constant conductivity relaxation time in 0.4Ca(NO₃)₂-0.6KNO₃

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

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 τσ and τν respectively. From the isothermal and isobaric conductivity relaxation data found on varying thermodynamic conditions (i.e. T and P) at constant τσ 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, τσ/τν. This co-invariance of τσ, τν, and (1 - n) at constant τσ 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 τσ and τν are functions of ργ/T with a single exponent γ = 0.58. The Coupling model is the only theory predicting the co-invariance of τσ, τν, and (1 - n) as well as the ργ/T scaling of both τσ and τν. It is applied herein to address and explain the data of the ionic liquid-water mixture.

Experimental evidence of high pressure decoupling between charge transport and structural dynamics in a protic ionic glass-former

In this paper the relaxation dynamics of ionic glass-former acebutolol hydrochloride (ACB-HCl) is studied as a function of temperature and pressure by using dynamic light scattering and broadband dielectric spectroscopy. These unique experimental data provide the first direct evidence that the decoupling between the charge transport and structural relaxation exists in proton conductors over a wide T-P thermodynamic space, with the time scale of structural relaxation being constant at the liquid-glass transition (τ_α = 1000 s). We demonstrate that the enhanced proton transport, being a combination of intermolecular H⁺ hopping between cation and anion as well as tautomerization process within amide moiety of ACB molecule, results in a breakdown of the Stokes-Einstein relation at ambient and elevated pressure with the fractional exponent k being pressure dependent. The dT_g/dP coefficient, stretching exponent β_KWW and dynamic modulus E_a/ΔV^# were found to be the same regardless of the relaxation processes studied. This is in contrast to the apparent activation volume parameter that is different when charge transport and structural dynamics are considered. These experimental results together with theoretical considerations create new ideas to design efficient proton conductors for potential electrochemical applications.

Recent progress on dielectric properties of protic ionic liquids

Protic ionic liquids (PILs) are key materials for a wide range of emerging technologies. In particular, these systems have long been envisioned as promising candidates for fuel cells. Therefore, in recent years special attention has been devoted to thorough studies of these compounds. Amongst others, dielectric properties of PILs at ambient and elevated pressure have become the subject of intense research. The reason for this lies in the role of broadband dielectric spectroscopy in recognizing the conductivity mechanism in protic ionic systems. In this paper, we summarize the dielectric results of various PILs reflecting recent advances in this field.