Structural Anomalies in Ionic Liquids near the Glass Transition

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Chemical and Dynamical Processes in Solution; Polymers, Glasses, and Soft Matter

Structural Anomalies in Ionic Liquids near the Glass Transition Revealed by Pulse EPR Mikhail Yu. Ivanov, Sergey A. Prikhod'ko, Nicolay Yu Adonin, Igor A. Kirilyuk, Sergey V. Adichtchev, Nikolay V. Surovtsev, Sergei Andreevich Dzuba, and Matvey V. Fedin J. Phys. Chem. Lett., Just Accepted Manuscript • DOI: 10.1021/acs.jpclett.8b02097 • Publication Date (Web): 27 Jul 2018 Downloaded from http://pubs.acs.org on July 29, 2018

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The Journal of Physical Chemistry Letters

Structural Anomalies in Ionic Liquids near the Glass Transition Revealed by Pulse EPR Mikhail Yu. Ivanov,a,b Sergey A. Prikhod’ko,c Nicolay Yu. Adonin,c Igor A. Kirilyuk,d Sergey V. Adichtchev,e Nikolay V. Surovtsev,e Sergey A. Dzuba,f and Matvey V. Fedin*a,b a

International Tomography Center SB RAS, Institutskaya str. 3a, 630090, Novosibirsk, Russia

b

Novosibirsk State University, Pirogova str. 2, 630090, Novosibirsk, Russia

c

Boreskov Institute of Catalysis SB RAS, Lavrentiev ave. 5, 630090 Novosibirsk, Russia

d

N. N. Vorozhtsov Novosibirsk Institute of Organic Chemistry SB RAS, Lavrentiev ave. 9,

630090 Novosibirsk, Russia e

Institute of Automation and Electrometry SB RAS, Koptyug ave. 1, 630090 Novosibirsk,

Russia f

Voevodsky Institute of Chemical Kinetics and Combustion SB RAS, Institutskaya str. 3,

630090, Novosibirsk, Russia AUTHOR INFORMATION Corresponding Author [email protected] (M.V.F.)

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ABSTRACT. Unusual physical and chemical properties of ionic liquids (ILs) open up prospects for various applications. We report the first observation of density/rigidity heterogeneities in a series of ILs near the glass transition temperature (Tg) by means of pulse Electron Paramagnetic Resonance (EPR). Unprecedented suppression of molecular mobility is evidenced near the glass transition, which is assigned to unusual structural rearrangements of ILs on the nanometer scale. Indeed, pulse and continuous wave EPR clearly indicate the occurrence of heterogeneities near Tg, which exist in a rather broad temperature range of ~50 K. The two types of local environments are evidenced, being drastically different by their stiffness. More rigid one suppresses molecular mobility, whereas the softer one instead promotes diffusive molecular rotation. Such properties of ILs near Tg are of general importance; moreover, the observed density/rigidity heterogeneities controlled by temperature might be considered as a new type of tunable reaction nanoenvironments. TOC GRAPHIC

KEYWORDS: Ionic Liquids, Heterogeneities, EPR, Structural Anomalies, Glass transition


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Ionic liquids (ILs) have drawn an enormous attention during the past few decades. Huge progress has been made toward the real applications of ILs in industry, medicine and other fields.1–7 Such properties of ILs as dynamic heterogeneity and nanostructural organization play crucial role in many cases, but especially in catalysis,8–14 therefore fundamental investigations in these directions are very intensive. Although most of target applications of ILs refer to their liquid state, many structural aspects and physicochemical properties can be inferred from the studies of ILs in their glassy states. For instance, a number of approaches describe the kinetics and thermodynamics of cooperative motion at glass transition and its relation to the heterogeneous structure.15,16 E.g., such heterogeneities in imidazolium-based ILs are formed by non-polar alkyl chains and resemble micelles or cages17–23 The temperature dependences of viscosity24,25 and diffusion coefficient1 are also linked to thermodynamics of glass transition. Charge transport, a key process for energy capacitors applications of ILs, is as well accessible by correlating the glass transition dynamics with the charge transfer.26–28 In particular, quantitative agreement between the characteristic frequency of charge transport and the structural relaxation has been evidenced by a number of spectroscopic techniques,29 and the fragility parameters of corresponding ILs were obtained.30–34 Triolo and colls. investigated the temperature dependence of nanodomain size in imidazolium-based ionic liquids using X-ray diffraction and emphasized that nanodomains are mainly constructed of uncharged alkyl chains of imidazolium cations.14,17,22 A detailed thermodynamic and spectroscopic study of IL [Bmim]PF6 highlighted the existence of three temperature regions within the glassy state which differ by both spatial correlation and dynamics.35 The size and dynamics of such nanodomains depend on the length of alkyl chain and cation/anion structure.


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An efficient approach to investigate heterogeneities in ILs is provided by Electron Paramagnetic Resonance (EPR) spectroscopy. In this technique the paramagnetic molecule (most often stable nitroxide radical) is used as a probe reflecting its interaction with surrounding media via the EPR spectrum. A number of papers were focused on the study of heterogeneities in ILs using continuous wave (CW) EPR.36–42 Since CW EPR spectrum is very sensitive to the motional regime of the nitroxide, one easily identifies whether the probe is localized in the liquid (or softened) medium or in the solid one.43 However, only relatively fast and large-amplitude motions of nitroxides, e.g. diffusive rotation with characteristic times ~0.1-10 ns, are accessible using CW approach. In contrast, recently we have demonstrated that pulse EPR is sensitive to much smaller molecular motions in ILs and provides information on microscopic rigidity and heterogeneity of IL glasses.44 In this approach the stochastic molecular librations (small-angle wobbling) of dissolved radicals were analyzed, showing that librations in ILs arise at noticeably lower temperature compared to common organic glasses, to be assigned to localization of probes in micelle-like low-density heterogeneities. In general, the two approaches, CW and pulse EPR, can fruitfully complement each other to monitor probe mobility on different scales, and in this work we employ them to demonstrate for the first time the occurrence of structural anomalies in ILs near their glass transition temperatures. We investigated a series of six room-temperature ionic liquids (Figure 1, experimental details in Supporting Information (SI)). [Bmim]BF4 and [Bmim]PF6 are the most well-known and wellinvestigated ILs, and [Bmmim]BF4 and [Bmmim]PF6 are their C2-methylated analogues showing different interactions with some probe molecules.45,46 To introduce larger structural variations of cation and anion, two more ILs [BuPy]BF4 and [Bmim]C2F5BF3 were also selected.


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Table S1 (SI) provides the obtained data on glass transition temperatures (Tg) for this series of ILs and compares them with available literature values.

Figure 1. Chemical structures of studied ILs and nitroxide probes. The nitroxide probe in the glassy medium undergoes small-angle wobbling induced by its surrounding, also called the stochastic molecular librations. Such librations modulate magnetic interactions in the nitroxide and, because of their stochastic nature, effectively induce electron spin relaxation. Due to the anisotropy of interactions (mainly the hyperfine interaction between unpaired electron and

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N nucleus), transverse relaxation time (T2) turns out to be different at

different magnetic field positions of the EPR spectrum (Figure 2a). Therefore, a comparison of T2 at several (at least two) positions provides an elegant way to characterize the amplitude and characteristic time of molecular librations, as is schematically shown in Figure 2a (more details in SI). Previous studies have shown that motional parameter τc vs. temperature is the most


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informative characteristic of librations obtained by EPR, where is the mean-squared amplitude of librations and τc is their characteristic time.47–49 For comparison with previous work, below we plot the scaled functions L(T)≡1011τc. The relation (1/T2(II)1/T2(I))∝τc (Figure 2a) holds for the small-angle motion of nitroxide, what is fulfilled for T200 K the decrease of L(T) reverts to a rapid growth. We emphasize that the observed turning point is very close to the glass transition temperature Tg (Figure 2b and Table S1). Thus, the impetuous ascent of L at T>200 K indicates the change of the motional regime of spin probe. We reasonably assume that at T>Tg the small-angle librations change to the isotropic diffusive motion in softened surrounding. Thus, Figure 2b shows that stochastic molecular librations in


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[BuPy]BF4 arise at ~80 K, grow up in amplitude up to ~150 K, then become suppressed within ~150-200 K, and finally transform into diffusive rotation at T>200 K.

Figure 2. (a) Principal scheme of pulse EPR experiment (see text for details). (b) Temperature dependence of the motional parameter L≈1011τc (orange, left Y-axis) and mobile fraction M (green, right Y-axis) in [BuPy]BF4. Tg is highlighted by colored bar, whose width illustrates the scattering of literature values (Table S1), our own measurement is depicted by vertical line. To gain complementary information, we have also studied local environment in ILs using CW EPR. Radical N1 is not optimal for this task, because its mobile EPR spectrum is broadened by protons of spirocyclohexane rings.44 TEMPO is more suitable, and was used previously to reveal two different environments in ILs [Bmim]BF4 and [Bmim]PF6 at T~150-250 K, where one


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microenvironment keeps nitroxide immobile and another one allows its diffusive rotation.43 To reduce linewidth and to enhance spectral resolution and accuracy, herewith we performed similar studies for series of ILs using deuterated TEMPO-D18 (Figure 1).

Figure 3. (a) Temperature-dependent CW EPR spectra of TEMPO-D18 in [BuPy]BF4 (black) with corresponding simulations (red). (b) Temperature dependence of the mobile fraction M(T) for TEMPO-D18 in all studied ILs. Figure 3a shows representative CW EPR spectra for [BuPy]BF4, and Figure 3b plots the corresponding fraction of mobility-promoting microenvironment (≡M(T)) vs. temperature for all six studied ILs (simulation details in SI). We observe that the onset of M(T) occurs at T~160 K


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and closely corresponds to the local maximum of L(T) (Figure 2b), whereas complete conversion to a liquid phase occurs around ~270 K. Thus, there is a broad temperature range where two local microenvironments coexist with variable ratios described by M(T). The above pulse and CW EPR data fruitfully complement each other. M(T) derived from CW EPR shows the fraction of microenvironments where nitroxide exhibits slow diffusive rotation. Such fraction becomes invisible for pulse EPR, because electron spin echo cannot be detected for a rotating nitroxide. Instead, stochastic molecular librations detected by pulse EPR occur in another fraction of nitroxides, which appear ‘immobile’ in CW EPR. We emphasize that the decrease of L(T) at T~150-200 K is not a result of progressively decreasing amount of immobile nitroxides vs. temperature, because in pulse EPR approach we rely on relaxation times T2, not on signal intensities. Thus, consolidation of pulse and CW EPR results allows the following conclusions. At T~150270 K [BuPy]BF4 exhibits heterogeneous structure creating two types of microenvironments for spin probe, one ‘solid-like’ and one ‘liquid-like’. The relative fraction of the ‘liquid-like’ environment M(T) monotonically grows with temperature from zero at ~150 K to unity at ~270 K. At the same time, the fraction of the ‘solid-like’ environment correspondingly decreases. But, most strikingly, the rigidity of ‘solid-like’ environment progressively increases with temperature between 150 and 200 K. We propose that this anomalous behavior can only originate from the tightening/hardening of the local surrounding of spin probes. Such changes in local environment can refer to some specific structural rearrangements. Alternatively, we can also assume progressive formation of a fraction of nanocrystalline inclusions in this temperature range, because the stochastic molecular librations occur only in the glassy state. They are absent in the crystalline state due to the local ordering, and they are as well absent in liquids since diffusive


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motion overcomes any small-angle oscillations. Note that the occurrence of crystalline state between Tg and Tm (melting point) is known for some organic solvents (Figure S5). However, a formation of crystalline-like phase in studied ILs is rather unlikely. E.g., [Bmim]PF6 is known to have complex phase diagram with at least two crystalline phases depending on the direction of the temperature change.35,50 However, no differences in L(T) upon cooling vs. warming were detected (see Fig.S3 and below). The obtained structural anomaly (Figure 2b) seems to be a quite general phenomenon for ILs. Figure 4 shows L(T) curves obtained for six ILs with various cations and anions.

Figure 4. Temperature dependence of the motional parameter L≈1011τc for nitroxide radical N1 in ILs and common molecular solvents. The Tg temperature for each compound is depicted in the legend. All of them clearly show the same suppression of librations within ~140-200 K, with slightly different positions of local maxima and minima. One IL [Bmim]C2F5BF3 shows noticeably lower position of local maximum (~140 K) and local minimum (~170 K) compared to others, and this correlates well with the lowest Tg~170 K of this IL. Instead, another IL [Bmmim]PF6 shows


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somewhat higher position of local minimum (~210 K) and it also has higher Tg~213 K. In average, there is a good correlation of the position of anomaly in L(T) with Tg (Figure 4). Note also that the observed anomalies do not depend on the direction of temperature change (heating vs. cooling, Figure S3). An important practical consequence of this finding is that by changing temperature within ca. 140-200 K one can tune the ratio and the properties of local nanoenvironments in ILs, and in this way create new conditions for heterogeneous catalysis and other potential applications. In addition to ILs, we have applied similar libration-based pulse EPR approach to the two organic solvents which retain the glassy state in broad temperature range, namely squalane (Tg=173 K, Tm≈235 K) and sucrose octaacetate (SOA, Tg=298 K, Tm≈360 K). L(T) dependence in SOA shows a very weak monotonic ascent without any local extrema (Figure 4). L(T) in squalane can be measured till T~170-180 K, being close to Tg, and also does not show any anomalies like those in ILs. To the best of our knowledge, such anomalies (Figure 4) have never been observed before, neither in organic glasses nor in biological membranes. Additional data (Figure S4) confirms that anomaly originates from the change in global motional regime of the probe molecule. The structure of radical probe does not influence the libration characteristics, because the motion of radical is overwhelmingly dictated by local modes of the surrounding matrix, i.e. the L(T) dependence reflects inherent property of the investigated glass.44,47–49 In addition, we have recently developed an alternative approach for studying stochastic molecular librations, which employs triarylmethyl radicals instead of nitroxides, i.e. probes of drastically different size, structure and relaxation properties (Kuzhelev, A. A. et.al. submitted 2018). Remarkably, similar behavior to that shown in Figure 4 has been observed for [Bmmim]BF4 using (i) new type of


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probe and (ii) this new experimental approach, thus confirming the probe-independent nature of the observed structural anomalies. Thereby we can talk about three different and independent experimental approaches revealing the anomalous structural changes and heterogeneity in ILs near Tg: pulse EPR of nitroxide probes, CW EPR of nitroxide probes and pulse EPR of triarylmethyl probes. Interestingly, Raman spectroscopy studies indicate that the behavior of L(T) near 160 K is not due to the conformational dynamics of butyl chains (SI). More experimental techniques need to be involved in the future to elucidate the detailed structure of the observed heterogeneities. We also intend to investigate in detail the effect of the alkyl chain length in imidazolium-based ILs, as preliminary data indicates that the shape of the observed L(T) curve is chain specific. In summary, using a combination of pulse and CW EPR and monitoring the stochastic molecular librations in ILs we have observed anomalous structural behavior near the glass transition temperatures. First of all, the ILs are highly heterogeneous on the nanometer scale in the broad temperature range ~150-250 K, where solid-like nanoenvironment gets gradually replaced by a liquid-like one, the former hindering the mobility of solute molecules and the latter promoting the mobility of the solutes instead. Second, a remarkable anomaly is observed for the solid-like environment. In a range of T~150-200 K the mobility of molecules progressively decreases with temperature, being opposite to all known trends and general expectations. Such behavior was rationalized by nanoscale structural rearrangements. Finally, above glass transition at T>200 K the anomalous state vanishes, being overwhelmed by a typical softening and gradual conversion of glass into a liquid state. To the best of our knowledge, such behavior was never documented for ILs or any other glasses before. Apart from fundamental importance, we suggest that ILs near glass transition temperatures represent the unique media with nanosized heterogeneities, whose


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properties can be reversibly tuned by temperature, to be considered for various applications in catalysis and materials science. ASSOCIATED CONTENT Supporting Information. Synthesis of ionic liquids, synthesis of spin probes, experimental details, differential scanning calorimetry measurements, EPR of stochastic molecular librations in a nutshell, the influence of the direction of temperature change, temperature dependence of T2 relaxation time, librations of spin probe in common organic solvents, continuous wave EPR spectra, Raman study (PDF) The following files are available free of charge. Supporting information (PDF) AUTHOR INFORMATION Notes The authors declare no competing financial interests. ACKNOWLEDGMENT This work was supported by the Russian Science Foundation (No. 14-13-00826). The access to equipment was provided by FASO (0333-2017-0002). REFERENCES (1) (2) (3)

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TOC graphic 80x79mm (300 x 300 DPI)



Figure 1 79x89mm (300 x 300 DPI)


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Figure 2 77x112mm (300 x 300 DPI)



Figure 3 78x125mm (300 x 300 DPI)


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Figure 4 82x60mm (300 x 300 DPI)


Structural Anomalies in Ionic Liquids near the Glass Transition

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