Pressure-induced liquid-liquid transition in a family of ionic materials

Spiers Memorial Lecture: From cold to hot, the structure and structural dynamics of dense ionic fluids

The structure of ionic liquids (ILs), which a decade or two ago was the subject of polite but heated debate, is now much better understood. This has opened opportunities to ask more sophisticated questions about the role of structure in transport, the structure of systems with ions that are not prototypical, and the similarity between ILs and other dense ionic fluids such as the high-temperature inorganic molten salts; let alone the fact that new areas of research have emerged that sprung from our collective understanding of the structure of ILs such as the deep eutectic solvents, the polymerized ionic liquids, and the zwitterionic liquids. Yet, our understanding of the structure of prototypical ILs may not be as complete as we think it to be, given that recent experiments appear to show that in some cases there may be more than one liquid phase resulting in liquid-liquid (L-L) phase transitions. This article presents a perspective on what we think are key topics related to the structure and structural dynamics of ILs and to some extent high-temperature molten salts.

Crystallization Kinetics of Phosphonium Ionic Liquids: Effect of Cation Alkyl Chain Length and Thermal History

The effects of alkyl chain length on the crystallization kinetics and ion mobility of tetraalkylphosphonium, [P666,n][TFSI], (n = 2, 6, 8, and 12) ionic liquids were studied by differential scanning calorimetry (DSC) and broadband dielectric spectroscopy (BDS) over a wide temperature range. The liquid-glass transition temperature (Tg) and ion dynamics examined over a broad T range were almost insensitive to structural modifications of the phosphonium cation. In contrast, the crystallization kinetics were strongly affected by the length of the fourth alkyl chain. Furthermore, the thermal history of the sample (cold vs melt crystallization) significantly impacted the crystallization rate. It has been found that the nature of crystallization phenomena is the same across the homologous series, while the kinetic aspect differs. Finally, electric conductivity in supercooled liquid and crystalline solid phases was measured for all samples, revealing significant ionic conductivity, largely independent of the cation structure.

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), [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, τσ(PLL) was found to be much shorter than τσ(TLL, 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 τσ at elevated pressure for [P666,14]Cl, while for [P666,14][Trz] τσ(Pg) was almost constant. These results highlight the diversity of liquid-liquid transition features within the class of phosphonium ionic liquids.

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

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, [P666,n][TFSI] (n = 2, 6, 8, 12). We found that both temperature and pressure dependence of dc-conductivity (σdc) 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 σdc(T-1) data. On the other hand, isothermal measurements performed at elevated pressure revealed a unique concave-convex character of σdc(P) dependences, resulting in a clear minimum in the pressure behavior of activation volume. Such an inflection point characterizing the pressure dependence of σdc in [P666,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.

High-pressure X-ray photon correlation spectroscopy at fourth-generation synchrotron sources

A new experimental setup combining X-ray photon correlation spectroscopy (XPCS) in the hard X-ray regime and a high-pressure sample environment has been developed to monitor the pressure dependence of the internal motion of complex systems down to the atomic scale in the multi-gigapascal range, from room temperature to 600 K. The high flux of coherent high-energy X-rays at fourth-generation synchrotron sources solves the problems caused by the absorption of diamond anvil cells used to generate high pressure, enabling the measurement of the intermediate scattering function over six orders of magnitude in time, from 10-3 s to 103 s. The constraints posed by the high-pressure generation such as the preservation of X-ray coherence, as well as the sample, pressure and temperature stability, are discussed, and the feasibility of high-pressure XPCS is demonstrated through results obtained on metallic glasses.

Liquid Structure of Ionic Liquids with [NTf2]- Anions, Derived from Neutron Scattering

The liquid structure of three common ionic liquids (ILs) was investigated by neutron scattering for the first time. The ILs were based on the bis(trifluoromethanesulfonyl)imide anion, abbreviated in the literature as [NTf2]- or [TFSI]-, and on the following cations: 1-ethyl-3-methylimidazolium, [C2mim]+; 1-decyl-3-methylimidazolium, [C10mim]+; and trihexyl(tetradecyl)phosphonium, [P666,14]+. Comparative analysis of the three ILs confirmed increased size of nonpolar nanodomains with increasing bulk of alkyl chains. It also sheds light on the cation-anion interactions, providing experimental insight into strength, directionality, and angle of hydrogen bonds between protons on the imidazolium ring, as well as H-C-P protons in [P666,14]+, to oxygen and nitrogen atoms in the [NTf2]-. The new Dissolve data analysis package enabled, for the first time, the analysis of neutron scattering data of ILs with long alkyl chains, in particular, of [P666,14][NTf2]. Results generated with Dissolve were validated by comparing outputs from three different models, starting from three different sets of cation charges, for each of the three ILs, which gave convergent outcomes. Finally, a modified method for the synthesis of perdeuterated [P666,14][NTf2] has been reported, with the aim of reporting a complete set of synthetic and data processing approaches, laying robust foundations that enable the study of the phosphonium ILs family by neutron scattering.

Phase transitions and dynamics in ionic liquid crystals confined in nanopores

We investigate the phase-transition behavior of ionic liquid crystals, namely 1-methyl-3-alkylimidazolium tetrafluoroborate, [Cnmim]BF4, confined in cylindrical nanopores using differential scanning calorimetry, x-ray scattering, and dielectric relaxation spectroscopy. Here, n is the number of carbon atoms in the alkyl part of this ionic liquid crystal. For n = 10 and 12, the isotropic liquid phase changes to the smectic phase and then to a metastable phase for the cooling process. During the subsequent heating process, the metastable phase changes to the isotropic phase via crystalline phases. The transition temperatures for this ionic liquid crystal confined in nanopores decrease linearly with the increase in the inverse pore diameter, except for the transitions between the smectic and isotropic phases. In the metastable phase, the relaxation rate of the α-process shows the Vogel-Fulcher-Tammann type of temperature dependence for some temperature ranges. The glass transition temperature evaluated from the dynamics of the α-process decreases with the decrease in the pore diameter and increases with the increase in the carbon number n. The effect of confinement on the chain dynamics can clearly be observed for this ionic liquid crystal. For n = 10, the melting temperature of the crystalline phase is slightly higher than that of the smectic phase for the bulk, while, in the nanopores, the melting temperature of the smectic phase is higher than that of the crystalline phase. This suggests that the smectic phase can be thermodynamically stable, thanks to the confinement effect.

A Second Glass Transition Observed in Single-Component Homogeneous Liquids Due to Intramolecular Vitrification

On supercooling a liquid, the viscosity rises rapidly until at the glass transition it vitrifies into an amorphous solid accompanied by a steep drop in the heat capacity. Therefore, a pure homogeneous liquid is not expected to display more than one glass transition. Here we show that a family of single-component homogeneous molecular liquids, titanium tetraalkoxides, exhibit two calorimetric glass transitions of comparable magnitude, one of which is the conventional glass transition associated with dynamic arrest of the bulk liquid properties, while the other is associated with the freezing out of intramolecular degrees of freedom. Such intramolecular vitrification is likely to be found in molecules in which low-frequency terahertz intramolecular motion is coupled to the surrounding liquid. These results imply that intramolecular barrier-crossing processes, typically associated with chemical reactivity, do not necessarily follow the Arrhenius law but may freeze out at a finite temperature.

Do Ionic Liquids Slow Down in Stages?

High impact recent articles have reported on the existence of a liquid-liquid (L-L) phase transition as a function of both pressure and temperature in ionic liquids (ILs) containing the popular trihexyltetradecylphosphonium cation (P666,14+), sometimes referred to as the "universal liquifier". The work presented here reports on the structural-dynamic pathway from liquid to glass of the most well-studied IL comprising the P666,14+ cation. We present experimental and computational evidence that, on cooling, the path from the room-temperature liquid to the glass state is one of separate structural-dynamic changes. The first stage involves the slowdown of the charge network, while the apolar subcomponent is fully mobile. A second, separate stage entails the slowdown of the apolar domain. Whereas it is possible that these processes may be related to the liquid-liquid and glass transitions, more research is needed to establish this conclusively.

Self-Assembled Nanostructures in Aprotic Ionic Liquids Facilitate Charge Transport at Elevated Pressure

Ionic liquids (ILs), revealing a tendency to form self-assembled nanostructures, have emerged as promising materials in various applications, especially in energy storage and conversion. Despite multiple reports discussing the effect of structural factors and external thermodynamic variables on ion organization in a liquid state, little is known about the charge-transport mechanism through the self-assembled nanostructures and how it changes at elevated pressure. To address these issues, we chose three amphiphilic ionic liquids containing the same tetra(alkyl)phosphonium cation and anions differing in size and shape, i.e., thiocyanate [SCN]-, dicyanamide [DCA]-, and tricyanomethanide [TCM]-. From ambient pressure dielectric and mechanical experiments, we found that charge transport of all three examined ILs is viscosity-controlled at high temperatures. On the other hand, ion diffusion is much faster than structural dynamics in a nanostructured supercooled liquid (at T < 210 ± 3 K), which constitutes the first example of conductivity independent from viscosity in neat aprotic ILs. High-pressure measurements and MD simulations reveal that the created nanostructures depend on the anion size and can be modified by compression. For small anions, increasing pressure shapes immobile alkyl chains into lamellar-type phases, leading to increased anisotropic diffusivity of anions through channels. Bulky anions drive the formation of interconnected phases with continuous 3D curvature, which render ion transport independent of pressure. This work offers insight into the design of high-density electrolytes with percolating conductive phases providing efficient ion flow.

Conductivity of Ionic Liquids In the Bulk and during Infiltration in Nanopores

The conductivity of ionic liquids (ILs) in nanopores is essential when considering their application as materials for energy. However, no consensus has been reached about the influence of confinement on the mobility of the ions. A series of ILs bearing the same cation, 1-butyl-3-methylimidazolium ([BMIM]+), and six different anions ([Cl]-, [Br]-, [I]-, [BF4]-, [PF6]-, and [TFSI]-) with radii from 0.168 to 0.326 nm were investigated with respect to their self-assembly, the thermodynamics, and the ionic conductivity in the bulk, during flow and under confinement in cylindrical nanopores with sizes in the range from 400 to 25 nm. In the bulk, the [BMIM]+[X]- exhibits weak ordering as a result of cation-anion correlations (charge alteration peak), and nanophase separation of polar/apolar groups. Liquid-to-glass temperatures were found to differ by ∼50 K, their viscosities by a factor of ∼270, and their conductivities by a factor of 24 (all at a temperature of 303 K). Electrostatic interactions were largely responsible for variations in the glass temperature, the viscosity, and the conductivity. Confined ILs behave differently from the bulk. The majority of ILs in the bulk were prone to crystallization during heating but were unable to crystallize in the smaller pores. Changes in dc-conductivity were used as markers of the phase state. This allowed the construction of the effective phase diagrams under confinement. The ILs penetrate the pores with an effective viscosity of the order of their viscosity in their bulk state. However, within the pores the dc-conductivity was reduced relative to bulk, indicating the immobilization of ions at the pore walls. Hydrophobization of the pore walls by hexamethyldisilazane could partially restore the conductivity. ILs are model systems where the phase state and ion mobility can be controlled by confinement.

Pressure-induced nonmonotonic cross-over of steady relaxation dynamics in a metallic glass

Relaxation dynamics, as a key to understand glass formation and glassy properties, remains an elusive and challenging issue in condensed matter physics. In this work, in situ high-pressure synchrotron high-energy X-ray photon correlation spectroscopy has been developed to probe the atomic-scale relaxation dynamics of a cerium-based metallic glass during compression. Although the sample density continuously increases, the collective atomic motion initially slows down as generally expected and then counterintuitively accelerates with further compression (density increase), showing an unusual nonmonotonic pressure-induced steady relaxation dynamics cross-over at ~3 GPa. Furthermore, by combining in situ high-pressure synchrotron X-ray diffraction, the relaxation dynamics anomaly is evidenced to closely correlate with the dramatic changes in local atomic structures during compression, rather than monotonically scaling with either sample density or overall stress level. These findings could provide insight into relaxation dynamics and their relationship with local atomic structures of glasses.

Tailoring Phosphonium Ionic Liquids for a Liquid-Liquid Phase Transition

The existence of more than one liquid state in a single-component system remains the most intriguing physical phenomenon. Herein, we explore the effect of cation self-assembly on ion dynamics in the vicinity of liquid-liquid and liquid-glass transition of tetraalkyl phosphonium ([P_mmm,n]⁺, m = 4, 6; n = 2-14) ionic liquids. We found that nonpolar local domains formed by 14-carbon alkyl chains are crucial in obtaining two supercooled states of different dynamics within a single ionic liquid. Although the nano-ordering, confirmed by Raman spectroscopy, still occurs for shorter alkyl chains (m = 6, n < 14), it does not bring calorimetric evidence of LLT. Instead, it results in peculiar behavior of ion dynamics near the liquid-glass transition and 20-times smaller size of the dynamic heterogeneity compared to imidazolium ionic liquids. These results represent a crucial step toward understanding the nature of the LLT phenomenon and offer insight into the design of efficient electrolytes based on ionic liquids revealing self-assembly behavior.

Effect of bulky anions on the liquid-liquid phase transition in phosphonium ionic liquids: Ambient and high-pressure dielectric studies

Although the first-order liquid-liquid phase transition (LLT) has been reported to exist in various systems (i.e., phosphorus, silicon, water, triphenyl phosphite, etc.), it is still one of the most challenging problems in the field of physical science. Recently, we found that this phenomenon occurs in the family of trihexyl(tetradecyl)phosphonium [P_666,14]⁺ based ionic liquids (ILs) with different anions (Wojnarowska et al in Nat Commun 13:1342, 2022). To understand the molecular structure-property relationships governing LLT, herein, we examine ion dynamics of two other quaternary phosphonium ILs containing long alkyl chains in cation and anion. We found that IL with the anion containing branched -O-(CH₂)₅-CH₃ side chains does not reveal any signs of LLT, while IL with shorter alkyl chains in the anion brings a hidden LLT, i.e., it overlaps with the liquid-glass transition. Ambient pressure dielectric and viscosity measurements revealed a peculiar behavior of ion dynamics near T_g for IL with hidden LLT. Moreover, high-pressure studies have shown that IL with hidden LLT has relatively strong pressure sensitivity compared to the one without first-order phase transition. At the same time, the former exposes the inflection point indicating the concave-convex character of logτ_σ(P) dependences.

Pretransitional of behavior of electrooptic Kerr effect in liquid thymol

Melting/freezing are canonical examples of discontinuous phase transitions, for which no pretransitional effects in the liquid phase are expected. For the solid phase, weak premelting effects are evidenced. This report shows long-range, critical-like, pretransitional effects in liquid thymol detected in electrooptic Kerr effect (EKE) studies. Notably is the negative sign of EKE pretransitional anomaly. Studies are supplemented by the high-resolution dielectric constant temperature-related scan, which revealed a weak premelting effect in the solid phase. Both EKE and dielectric constant show a 'crossover' change in the liquid phase, ca, 10 K above the freezing temperature. It can be recognized as the hallmark of the challenging liquid-liquid transition phenomenon.