Do Ionic Liquids Slow Down in Stages?

In this study, we employed dielectric spectroscopy to investigate the effect of temperature and pressure on the ion dynamics of phosphonium ionic liquids (ILs) differing by the length of an alkyl chain, [P 666,n ][TFSI] (n = 2, 6, 8, 12). We found that both temperature and pressure dependence of dc-conductivity (σ dc ) determined for all examined ILs herein exhibit unique characteristics, unusual for aprotic ILs. Two regions differing by ion self-organization have been identified from the derivative analysis of σ dc (T −1 ) data. On the other hand, isothermal measurements performed at elevated pressure revealed a unique concave−convex character of σ dc (P) dependences, resulting in a clear minimum in the pressure behavior of activation volume. Such an inflection point characterizing the pressure dependence of σ dc in [P 666,n ][TFSI] ILs can be considered an inherent feature of ion dynamics governed by structural self-assembly. Our results offer a unique perspective to link the ion mobility at various T−P conditions to the nanostructural organization of ionic systems.

Section: Introductionsupporting

confidence: 71%

“…The same behavior has also been observed for the temperature evolution of the viscosity (η) of [P 666,14 ][DCA] and the ion dynamics of other ILs with evidence of liquid–liquid transition. 28 , 29 …”

Section: Introductionmentioning

confidence: 99%

Inflection Point in Pressure Dependence of Ionic Conductivity as a Fingerprint of Local Structure Formation

Koymeth,

Yao,

Paluch

et al. 2024

“…A fascinating phenomenon we discussed in a recent article is the disruptive effect of low valency cations such as Na + on multivalent salt structure. It is not a stretch of the imagination to think of NaCl the way one thinks of the apolar domains in room-temperature ionic liquids (ILs). − In the case of ILs one commonly finds strands of positive–negative charge alternation “spaced” by apolar domains. The smoking gun in such systems is the appearance of a scattering prepeak or first sharp diffraction peak linked with the spacing between charge strands when these are separated by an apolar domain. ,,, NaCl does just this when mixed with LaCl 3 ; it partitions the La 3+ networks, thereby causing the appearance of a new low-q scattering peak that is absent in both neat LaCl 3 and NaCl.…”

Section: Introductionmentioning

confidence: 99%

Heterogeneous Structure, Mechanisms of Counterion Exchange, and the Spacer Salt Effect in Complex Molten Salt Mixtures Including LaCl₃

Emerson,

Ivanov,

Gallington

et al. 2024

Self Cite

Complex molten chloride salt mixtures of uranium, magnesium, and sodium are top candidates for promising nuclear energy technologies to produce electricity based on molten salt reactors. From a local structural perspective, LaCl3 is similar to UCl3 and hence a good proxy to study these complex salt mixtures. As fission products, lanthanide salts and their mixtures are also very important in their own right. This article describes from an experimental and theory perspective how very different the structural roles of MgCl2 and NaCl are in mixtures with LaCl3. We find that, whereas MgCl2 becomes an integral part of multivalent ionic networks, NaCl separates them. In a recent article (J. Am. Chem. Soc. 2022, 144, 21751–21762) we have called the disruptive behavior of NaCl “the spacer salt effect”. Because of the heterogeneous nature of these salt mixtures, there are multiple structural motifs in the melt, each with its particular free energetics. Our work identifies and quantifies these; it also elucidates the mechanisms through which Cl– ions exchange between Mg2+-rich and La3+-rich environments.

“…Very recently, LLTs have been discovered in ionic liquids (ILs) − defined as molten salts that melt below 100 °C. What is more, genuine LLTs were found not in one isolated ionic liquid but in an entire set of supercooled ILs based on a common trihexyl(tetradecyl)phosphonium cation, [P 666,14 ] + , and several anions, which allowed for an in-depth study of structure-related properties governing LLT .…”

Section: Introductionmentioning

confidence: 99%

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

Koymeth,

Yao,

Paluch

et al. 2024

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), [P666,14]Cl and [P666,14][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 [P666,14]Cl and [P666,14][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 σ (P LL) was found to be much shorter than τ σ (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 σ at elevated pressure for [P666,14]Cl, while for [P666,14][Trz] τ σ (P g) was almost constant. These results highlight the diversity of liquid–liquid transition features within the class of phosphonium ionic liquids.