Tailoring Phosphonium Ionic Liquids for a Liquid–Liquid Phase Transition

Abstract

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 selfassembly behavior.

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

“…We recently found that all tested [P 666, n ][TFSI] can be easily supercooled, and cold crystallization occurs only on slow heating from the glassy state. 33 Herein, the dielectric measurements of [P 666, n ][TFSI] were performed from the temperatures covering the normal liquid through the supercooled liquid to the glass phase. The obtained dielectric data are presented in the conductivity formalism in Figure 2 .…”

“… 29 However, this could be expected since the LLT did not show up on the DSC thermograms of [P 666, n ][TFSI] ILs. 33 To recognize the structural origin of changes in ion dynamics, temperature-dependent Raman measurements were performed on all examined ILs herein.…”

“…These samples were synthesized and purified in the QUILL Centre with a purity of more than 98%, which was reported in our previous work. 33 All studied ILs are colorless and transparent liquids at room temperature, and we used samples without further treatment. The chemical structures of the studied ILs are presented in Figure 1 .…”

“…According to recent studies, all these materials can be easily supercooled and reveal a tendency to self-assembly, as evidenced by aliphatic chain coupling due to their mutual ordering. 33 Therefore, they are good candidates for studying the effects of pressure on the nanoorganization of ILs. We found that self-assembly of cation alkyl chains taking place in [P 666, n ][TFSI] ILs affects the rate of ion dynamics both at ambient and elevated pressures, leading to the peculiar behavior of σ dc ( T –1 ) near the liquid–glass transition and the inflection point at high pressure.…”

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

“…MD simulations have been performed to verify the proposed mechanism of charge transport (see Supporting Information for details). − To mimic the architecture of the phosphonium ILs, a simple bead–spring coarse-grained model was employed, consisting of an amphiphilic molecule containing a positively charged head connected to a stiff, neutral tail and a negatively charged free counterion (Figure a). Since recently we found that the 14-carbon chain is critical to observe LLT and that shortening the other three tails makes the LLT better detectable, we have been omitted the latter in MD simulations . The morphologies at high and low reduced temperatures and various pressures have been considered for two different anion-to-cation size ratios.…”