Dynamic and structural evidence of mesoscopic aggregation in phosphonium ionic liquids

Cosby, Tyler; Vicars, Zachariah; Heres, Maximilian; Tsunashima, Katsuhiko; Sangoro, Joshua

doi:10.1063/1.5009765

Cited by 21 publications

(17 citation statements)

References 42 publications

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“…In this representation, it is evident that an additional relaxation is present in each experiment at rates slower than that of the primary structural relaxation in agreement with prior studies. [27,42,43] In neat C n MIm BF 4 ionic liquids, the emergence of the slow dynamics was found to coincide with the onset of solvophobic aggregation, as evidenced by the development of the x-ray scattering pre-peak, and by a comparison between the relaxation rates with those previously obtained by neutron spin echo spectroscopy. [27,[29][30][31] Therefore, the slow relax-ations were attributed to fluctuations of the mesoscale aggregates at timescales longer than the structural relaxation.…”

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confidence: 56%

“…In neat C n MImBF 4 ionic liquids, the emergence of the slow dynamics was found to coincide with the onset of solvophobic aggregation, as evidenced by the development of the X-ray scattering prepeak and by a comparison between the relaxation rates with those previously obtained by neutron spin echo spectroscopy. 27,[29][30][31]45,46 Therefore, the slow relaxations were attributed to fluctuations of the mesoscale aggregates at time scales longer than the structural relaxation. This attribution is further substantiated by Yamaguchi's recent computational work, which shows that a cross-correlation exists between the shear stress relaxation and the slow relaxation of the domain structure corresponding to the scattering prepeak.…”

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confidence: 99%

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Mesoscale Organization and Dynamics in Binary Ionic Liquid Mixtures

Cosby

Kapoor

Shah

et al. 2019

J. Phys. Chem. Lett.

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The impact of mesoscale organization on dynamics and ion transport in binary ionic liquid mixtures is investigated by broadband dielectric spectroscopy, dynamic-mechanical spectroscopy,x-ray scattering, and molecular dynamics simulations. The mixtures are found to form distinct liquids with macroscopic properties that significantly deviate from weighted contributions of the neat components. For instance, it is shown that the mesoscale morphologies in ionic liquids can be tuned by mixing to enhance the static dielectric permittivity of the resulting liquid by as high as 100% relative to the neat ionic liquid components. This enhancement is attributed to the intricate role of interfacial dynamics associated with the changes in the mesoscopic aggregate morphologies in these systems. These results demonstrate the potential to design the physicochemical properties of ionic liquids through control of solvophobic aggregation.Elucidating the influence of mesoscale organization on the dynamics and transport properties of ionic liquids is critical to developing design criteria for their applications in chemical synthesis, nanoparticle growth, biomass processing, batteries, solar cells, and supercapacitors.[1-10] In the past decade, the formation of mesoscale polar and non-polar domains in ionic liquids with substantial non-polar, alkyl side groups was recognized in detailed x-ray scattering, neutron scattering, and molecular dynamics (MD) simulation studies. [11][12][13][14][15][16] Mesoscale organization has been used to qualitatively explain numerous experimental findings which imply spatially and temporally distinct regions within bulk ionic liquids. [11,[17][18][19][20][21][22][23][24][25][26] Recent studies suggest that the existence and dynamics of the aggregates in neat ionic liquids, associated with fluctuations of the polar and non-polar regions, correlate strongly with many of the physicochemical properties of ionic liquids including transport properties such as zero-shear viscosity, dc ionic conductivity, and static dielectric permittivity. [27][28][29][30][31][32] However, no efforts to exploit the mesoscale organization to design novel ionic liquids with unique physical and chemical properties have been reported. In this work, it is demonstrated that by mixing ionic liquids with varying degrees of solvophobic aggregation, it is feasible to design distinct liquids with macroscopic properties that significantly deviate from weighted contributions of the neat components.The local organization, or morphology, of the mesoscale aggregates is in part determined by the relative volume fractions of the polar and non-polar groups of the component ions.[11] In amphiphilic imidazolium, pyrrolidinium, piperidinium, quaternary phos-

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confidence: 56%

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confidence: 99%

Mesoscale Organization and Dynamics in Binary Ionic Liquid Mixtures

Cosby

Kapoor

Shah

et al. 2019

J. Phys. Chem. Lett.

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“…This relaxation emerges around 220 K, corresponding to TL−L observed in the WAXS and Raman data. The slow relaxation in ILs has been linked to interfacial polarization arising from mesoscale ordering of the polar and nonpolar domains in the liquid structure (31,32). This process that appears in the low-temperature liquid II state is directly connected to the increase in εs shown in Fig.…”

Section: Significancementioning

confidence: 99%

Evidence of a liquid–liquid transition in a glass-forming ionic liquid

Harris

Kinsey

Wagle

et al. 2021

Proc. Natl. Acad. Sci. U.S.A.

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A liquid–liquid transition (LLT) is a transformation from one liquid to another through a first-order transition. The LLT is fundamental to the understanding of the liquid state and has been reported in a few materials such as silicon, phosphorus, triphenyl phosphite, and water. Furthermore, it has been suggested that the unique properties of materials such as water, which is critical for life on the planet, are linked to the existence of the LLT. However, the experimental evidence for the existence of an LLT in many molecular liquids remains controversial, due to the prevalence and high propensity of the materials to crystallize. Here, we show evidence of an LLT in a glass-forming trihexyltetradecylphosphonium borohydride ionic liquid that shows no tendency to crystallize under normal laboratory conditions. We observe a step-like increase in the static dielectric permittivity at the transition. Furthermore, the sizes of nonpolar local domains and ion-coordination numbers deduced from wide-angle X-ray scattering also change abruptly at the LLT. We independently corroborate these changes in local organization using Raman spectroscopy. The experimental access to the evolution of local order and structural dynamics across a liquid–liquid transition opens up unprecedented possibilities to understand the nature of the liquid state.

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“…Hereafter, in the present paper, the Qa and Qp cations with four normal alkyl groups are denoted as N abcd + and P abcd + , respectively, where a , b , c , and d represent the number of carbon atoms of the alkyl groups. The bulk structure of these ILs has been studied by small-angle X-ray scattering, − wide-angle X-ray scattering, ,, NMR, Raman scattering spectroscopy, − broadband dielectric spectroscopy, , and MD simulation. ,,− In contrast to the bulk studies, a limited number of studies have focused on the surface structure of QaILs, ,,,− ,− some of which used cations having more than two alkyl chains. ,,,,,− A comparison between N 6444 + and C 4mim + has been made by Aliaga and Baldelli using SFG, revealing that the butyl chain in both the cations has similar orientational preferences at the surface. The same research group also studied the surface of a series of [N 11 i ‑Pr n + ][TFSA – ] ( i -Pr = isopropyl, n = 3, 4, 6, 10), which revealed that the orientation of the alkyl chain strongly depends on the length and that the longer alkyl chains have gauche conformation.…”

Section: Introductionmentioning

confidence: 99%

Surface Structure of Quaternary Ammonium-Based Ionic Liquids Studied Using Molecular Dynamics Simulation: Effect of Switching the Length of Alkyl Chains

et al. 2019

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Electric double layer structure at the electrode interface has been studied by using molecular dynamics simulation on four quaternary ammonium-based ionic liquids (QaILs) to investigate the effect of switching the alkyl chain length of the Qa cation. These four QaILs are composed of a common anion, bis(trifluoromethanesulfonyl)amide (TFSA −) and different cations: butyltrimethylammonium (N + 1114 , k = 1), dibutyldimethylammonium (N + 1144 , k = 2), tributylmethylammonium (N + 1444 , k = 3), and tetrabutylammonium (N + 4444 , k = 4), where k represents the number of butyl chains. The difference in k affects the potential dependence for the composition of the first ionic layer and the orientation of butyl chains in the layer. For the case of k = 1, 2, 3, the butyl chains parallel to the interface increases as the potential becomes negative, but further negative potential results in the increase in perpendicular ones. In the case of k = 1, all the cations in the first ionic layer show the perpendicular orientation at the negative potentials, forming a honeycomb lattice consisting of only cations. On the other hand, in the case of k = 4, no change in orientation has been observed due to the geometrical restrictions. The difference in k also affects the differential capacitance. The potential dependence of differential capacitance shows bell shape for the smaller two (k = 1, 2) and camel shape for the larger two (k = 3, 4). The camel shape for larger IL cations agrees with the prediction from the mean-field lattice gas model and recent experimental results. The differential capacitance at negative potentials deviated to the values higher than the model prediction and the discrepancy becomes greater for smaller k. The results indicate that the potential dependence of ionic orientation significantly affects the differential capacitance. Even for k = 4, which does not show the orientational change, the discrepancy has been observed, indicating that not only the orientational change but also the densification of ions in the first ionic layer are the factors we should take into account beyond the lattice gas model.

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Dynamic and structural evidence of mesoscopic aggregation in phosphonium ionic liquids

Cited by 21 publications

References 42 publications

Mesoscale Organization and Dynamics in Binary Ionic Liquid Mixtures

Mesoscale Organization and Dynamics in Binary Ionic Liquid Mixtures

Evidence of a liquid–liquid transition in a glass-forming ionic liquid

Surface Structure of Quaternary Ammonium-Based Ionic Liquids Studied Using Molecular Dynamics Simulation: Effect of Switching the Length of Alkyl Chains

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