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Research of Condensed Matter Chemistry on Ionic Liquids

Yawei Liu , Xiaochun Zhang , Kun Dong , Suojiang Zhang

Progress in Chemistry ›› 2022, Vol. 34 ›› Issue (7) : 1509 -1523.

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Progress in Chemistry ›› 2022, Vol. 34 ›› Issue (7) : 1509-1523 DOI: 10.7536/PC220347
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Research of Condensed Matter Chemistry on Ionic Liquids
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Abstract

Ionic liquids are new solvents that can replace traditional solvents to achieve efficient, low-carbon, clean, recycling, novel processes and technologies, and have important application values in completing the “double carbon” goal. Meanwhile, ionic liquids are a kind of typical “soft condensed matter (soft matter)”, and their fundamental understandings and applications strongly depend on the study of their inner multi-scale microstructures, which requires the idea of “condensed matter chemistry” as the future research direction, that is, performing multi-level studies on the composition, structures, properties, functions and their relationships, and thus to regulate the transport and reaction processes in the real application systems. In this paper, we briefly review the research on ionic liquids from the perspective of “condensed matter chemistry”. At first, we introduce the chemical structures and physicochemical properties of ionic liquids, pointing out that it is necessary to study their inner structures to understand the changes in these properties. We then introduce the structures of ionic liquids from the molecular level to the nano-/micro-scale, including ion pairs, hydrogen bonds, hydrogen bond networks, clusters, interfacial structures, and nano-confined structures. Finally, the future of “condensed matter chemistry” research on ionic liquids is discussed.

Contents

1 Introduction

2 Chemical structure and physicochemical properties of ionic liquids

3 Ion pairs

4 Hydrogen bonds and Hydrogen bond networks

5 Clusters of ionic liquids

6 Interfacial ionic liquid

7 Nano-confined structures

8 Conclusion and outlook

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Keywords

ionic liquid / condensed matter chemistry / hydrogen bond / cluster / interface / nano-confinement

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Yawei Liu, Xiaochun Zhang, Kun Dong, Suojiang Zhang. Research of Condensed Matter Chemistry on Ionic Liquids. Progress in Chemistry, 2022, 34(7): 1509-1523 DOI:10.7536/PC220347

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1 Introduction

Ionic liquids are a class of "salts" that are liquid at room temperature and consist of organic cations (such as imidazolium ions, pyridinium ions, pyrrolidine ions, and tetraalkylphosphonium ions) and organic/inorganic anions (such as hexafluorophosphate ions [PF6]-, tetrafluoroborate ions [BF4]-, bis-trifluoromethanesulfonimide ions [TFSI]-, halide ions [X]-)[1,2]. Ionic liquids are considered to be safe alternatives and upgrades to traditional solvents because of their many excellent characteristics, such as high electrochemical stability, high thermal stability, negligible vapor pressure and non-flammability, which show the green characteristics of high efficiency, low carbon, cleanliness and recycling in achieving energy saving, resource saving and environmental protection. For example, ionic liquids have the function of increasing CO2 solubility and synergistic catalysis, which are very promising solvents and electrolytes for CO2 capture and conversion technology[3⇓~5]. Ionic liquids instead of traditional organic or aqueous electrolytes can enable electrochemical energy storage devices (such as batteries, supercapacitors, etc.) to work stably at higher potentials, thus achieving higher device energy density[6⇓~8]. Ionic liquids are also designable materials, and their physicochemical and electrochemical characteristics can be manipulated by preparing anions and cations with different structures and interactions. In theory, under the combination of various anions and cations, numerous ionic liquids and their mixed solutions with different properties can be obtained to meet the needs of various traditional and emerging technologies.
On the other hand, ionic liquid is a typical complex fluid, which is a kind of "soft condensed matter (soft matter)" from the perspective of condensed matter physics, and its constituent units (i.e., anions and cations) are brought together by non-covalent bonding interactions (such as Coulomb force, dipolar force, hydrogen bond, van der Waals force, etc.)[9]. Because these interactions are equivalent to the structural entropy effect, like other soft substances, ionic liquids can produce Large response functions under small environmental changes, show a variety of condensation forms under different conditions and systems, and have complex multi-level structures at different time and space scales[10][11]. For example, there are ion pairs, hydrogen bond networks, clusters and other microstructures in ionic liquids. Due to the asymmetric interaction, ionic liquids in the interfacial (such as gas-liquid interface, solid-liquid interface) region and nano-confinement have different structures and properties from the host ionic liquid. The composition of ionic liquids, the condensed state and the corresponding multi-level structure determine the transport process and chemical reaction in the system (including reaction conditions, process, mechanism, results and functions of products). More importantly, because "More is different", condensed systems exhibit new structures and properties at each level of aggregation, and cannot be explained by simple extrapolation based on the properties of secondary structures[12]. Understanding and regulating the multi-level nanostructure and dynamic behavior of condensed matter system is also an important scientific direction for the future development of physics, chemistry, materials and other fields[13]. Therefore, we need to systematically study the hierarchical structure of ionic liquid systems, understand the formation mechanism of each structure, and establish the dependence relationship of ionic liquid composition-structure-property-function, so as to guide the directional design and development of ionic liquid materials to meet the needs of specific applications.
The above idea of "condensed matter physics" to study the chemical problems of ionic liquids, that is, to study the composition, structure, properties, functions of ionic liquids and the relationship between them at multiple levels, is also the core idea of "condensed matter chemistry", a new development field in chemistry in recent years[14,15]. The study of ionic liquids is also one of the important directions in the development of "condensed matter chemistry". In this paper, we first introduce ionic liquids with different chemical structures and their physicochemical properties in different environments. Although the dependence of chemical structure on physical properties can be obtained from experimental measurements, the microscopic mechanism behind these relationships is not entirely clear, and the microstructure of ionic liquids must be understood in depth. Then, we focus on the microstructure of ionic liquids at different scales and in different environments, including ion pairs, hydrogen bonds and hydrogen bond networks, cluster structures, as well as the structures of interfacial ionic liquids and confined ionic liquids. Finally, the condensed matter chemistry of ionic liquids is prospected.

2 Chemical structure and physicochemical properties of ionic liquids

The properties of ionic liquids are essentially determined by the chemical structure and interaction of their anions and cations, so exploring the ionic structure and interaction between ions is the basis for understanding the various physicochemical characteristics of ionic liquids.
Fig. 1 shows the chemical formula structures of cations and anions in some typical ionic liquids. In general, the cations of ionic liquids are some larger organic ions (Fig. 1A ~ C), and the anions are smaller organic/inorganic ions with better symmetrical structure (Fig. 1D). Ionic liquids are generally divided into three categories according to the cation composition: aprotic (Figure 1A), protic (Figure 1b), and metal-containing (Figure 1C)[16]. Aprotic and protic ILs have similar properties, but protic ILs have labile protons (such as triethylammonium ion). Metal-containing ionic liquids can meet the demand for liquid-phase metal ion environment, and their cations are metal-containing complexes, such as lithium crown ethers, which can be used as electrolytes for lithium batteries[17][18]. The anions and cations in ionic liquids can interact with each other through Coulomb force, dipolar force, π - π, hydrogen bonding, van der Waals force, steric hindrance and so on, and ionic liquids in other solvents (such as water) may also be affected by hydrophobic interaction and solvation[19]. These interactions together determine the unique physicochemical and electrochemical characteristics of ILs.
The liquid-phase temperature range of ionic liquids is an important physical property indicator for their replacement of other solvents in practical applications. Compared with traditional organic solvents, most ionic liquids can maintain liquid phase in a wide range of temperatures. For example, at atmospheric pressure, [C4mim][TFSI]([C4mim]+ is 1-butyl-3-methylimidazolium ion) maintains a stable liquid phase at - 89 ~ 450 ° C, while ethanol only maintains a stable liquid phase at - 114 ~ 78 ° C[20]. The liquid phase temperature range of ionic liquids is limited by their glass transition temperature or melting point and thermal degradation temperature, and depends on the micro-chemical structure of anions and cations, the flexibility of carbon chains on ions, and the charge distribution[21]. In general, the longer the alkyl chain on the cation, the higher the melting point[22]; In the case of the same charge, the larger the anion, the lower the melting point[23,24]. The study on the glass transition temperature and (Tg),Xu of ionic liquids shows that the Tg decreases first and then increases with the increase of the molar volume (ionic size) of ionic liquids[25]. ILs often have very high thermal degradation temperatures (Td)[21]. Huddleston et al. Have shown that the thermal degradation temperature of alkylimidazolium ionic liquids mainly depends on the type of anions, and the T d of common anions show the following order :[PF6]->[BF4]->[TFSI]->X-[26]. In addition, pyrrolidine-based ILs are generally more heat-resistant than imidazoles, pyridines, and acyclic tetraalkylammonium[20,27].
In addition, ILs often exhibit different melting points (Tm) from their host phases in confined space. The variation of Tm depends on the size of confined space and the relative strength of solid-liquid/liquid-liquid interaction[28]. For example, the melting point of the host phase of [C2mim][PF6]([C2mim]+ is 1-ethyl-3-methylimidazolium ion) is 6 ° C, but the melting point can increase to more than 200 ° C in multi-walled carbon nanotubes[29]. In contrast, the melting point of most ILs (such as [C2mim][TFSI] and [C2mim][PF6]) will decrease significantly in nano-glass porous materials and the decrease is inversely proportional to the size of nano-pores[30].
The practical application of ionic liquids is often limited by their low mass transfer efficiency due to their high viscosity. For example, the low ionic conductivity caused by high viscosity greatly limits the performance of electrochemical energy storage devices with ionic liquids as electrolytes during low temperature and rapid operation[8]. The Coulomb and van der Waals forces between anions and cations, as well as the mass and shape of ions, all affect the viscosity of ionic liquids. In general, using larger anions and cations, longer alkyl chains, or reducing the charge of the ions can reduce the activation energy for the ions to flow through each other, thus effectively reducing the viscosity of the ionic liquid[31]. At the same time, increasing the temperature can also reduce the viscosity of ionic liquids and increase the number of free ions, thereby improving the mobility of each component[32][33].
A common method to reduce the viscosity of ionic liquids is to mix them with other low-viscosity solvents, such as acetonitrile (MeCN), ethylene carbonate (EC), diethyl carbonate (DEC) and dimethyl carbonate (DMC)[34]. Among them, the mass transfer efficiency is significantly improved after the mixture of ionic liquid and acetonitrile, so it is widely used in practice, such as the electrocatalytic reduction of CO2[35]. It should be noted that mixed ionic liquids and organic solvents can reduce the electrochemical stability of the liquid and impair the thermal stability and non-flammability of the liquid[36]. Another way to reduce that viscosity of ionic liquid is to mix two ionic liquids with different cation. For example, [C2mim][TFSI] containing mole fraction 10%[BenzC1im]+(1- benzyl-3-methylimidazolium ion) has lower viscosity and higher ionic conductivity than pure [C2mim][TFSI][37]. In addition, mixing different ILs can also adjust the melting point of ILs. For example, [BMP][PF4]([BMP]+ is 1-butyl-4-methylpyridinium ion) and [C4mim][PF4] can form a eutectic mixture, which can inhibit the arrangement of anions due to the different sizes of cations, thus reducing the melting point of ionic liquids[38]. In addition, the confinement effect may also improve the ion migration rate of ionic liquids. For example, Berrod et al. Used nuclear magnetic resonance (NMR) measurements to show that the mobility of [C8mim][BF4]([C8mim]+ is 1-octyl-3-methylimidazolium ion) in 4 nm carbon nanotubes is 2 to 3 times higher than that of its host phase[39].
ILs at the interface are also important basic structures involved in numerous practical applications, such as the gas-liquid interface in gas absorption, the liquid-liquid interface in extraction, and the solid-liquid interface in electrochemistry[40⇓⇓⇓~44]. The interfacial structure of ionic liquids affects the transfer process and chemical reaction at the interface, which determines the performance of ionic liquids in applications[45⇓⇓~48]. Surface tension is a simple but important physical parameter to characterize the surface structure of ionic liquids. Experimental measurements show that the surface tension of most ILs is less than that of water and decreases with increasing temperature and the growth of the cationic alkyl chain[49,50].
The physicochemical and electrochemical characteristics of ionic liquids with different chemical structures in different environments (such as host phase, interface region, confined space) are given in the above study.The dependence of these physical properties on the chemical structure characteristics of anions and cations (such as alkyl chain length, ion size) and environmental characteristics (such as temperature, solvent, confinement space size) are given. However, the microscopic mechanism of these dependencies is not clear. The diverse physicochemical and electrochemical characteristics of different ionic liquid systems are closely related to the multi-level microstructure of each ionic liquid in different environments. The study of these microstructures is of great significance to understand the properties and behavior of ionic liquids and to develop advanced technologies and materials based on ionic liquids. In the next few sections, we will focus on the unique microstructures exhibited by ILs at the molecular level and nanoscale.

3 Ion pair

The anion-cation pair is the simplest repeating unit in ILs, and thus is an important starting point for predicting higher-level structures and properties of ILs.
The structure and interaction details of the ion pair can be obtained by quantum mechanical calculations[51⇓⇓⇓~55]. For example, in the [C4mim][Cl] ion pair, the calculation shows that Cl- prefers to adsorb on the upper and lower sides of the imidazole ring due to Coulomb interaction; At the same time, due to the formation of C2-H…Cl hydrogen bonds, the Cl- in particular tends to be located in front of the C2-H group. This result reveals the complexity of competing forces in ionic liquids, including Coulomb forces, van der Waals forces, hydrogen bonds, and steric hindrance[51]. The anion-cation binding energy ( Δ E) obtained by quantum mechanical calculations is an important measurement parameter for the stability of an ion pair: a larger Δ E means that a more stable ion pair is easily formed. For compounds containing different anions (including [CF3CO2]-, [BF4]-, [CF3SO3]-,The calculation of the [C2mim]+ ionic liquid of [TFSI]-, [PF6]-) shows that theThe Δ E is not significantly dependent on the size and symmetry of the anion, but is dependent on the charge distribution on the anion. For example, the —CO2 group is more negatively charged than the —SO3 group, resulting in a greater Δ E for [C2mim][CF3CO2] compared to [C2mim][CF3SO3][56]. At the same time, ILs that can form “C2-H… anion "hydrogen bonds also have larger Δ E, which also explains the quasi-molecular state of ion pairs connected by hydrogen bonds in [C2mim][X] observed by NRM in the experiment."[57].
Several studies have shown that there is a charge transfer between the anion and cation in an ion pair, resulting in a decrease in the charge of a single ion[58,59]. For example, Krossing et al. Showed by high-resolution XRD measurements that the charge of the anion and cation in [C1mim][C1SO3] (1,3-dimethylimidazole methanesulfonate) was reduced to ± 0.90 e, and molecular calculations showed that its charge was reduced to ± 0.87 e[60]. Schmidt et al. Reported a greater charge reduction, with the charge of the anions and cations in [C1mim][Cl] being ± 0.63 e[61]. The decrease of ionic charge caused by the charge transfer between ionic pairs has an important impact on the Coulomb force between them, which affects the accurate description of the properties of ionic liquids. For example, the molecular simulation of ionic liquids found that using 0.8 times of charge for anions and cations can better describe the surface tension, self-diffusion and other properties of ionic liquids.
It is worth noting that the concept of ion pairs in host ionic liquids is still a controversial topic[62]. Dielectric spectroscopy measurements reveal that the [EtNH3][NO3] (ethylamine nitrate) ionic liquid is a mixture containing 8 % ion pairs and 92 % free ions, where the ion pair survival time is on the order of hundreds of picoseconds at 298 K[63]. However, dielectric spectroscopy measurements for pyrrolidine, pyridine, tetraalkylammonium, and triethylsulfonium ionic liquids show that there is no stable ion pair in these systems in the picosecond-nanosecond time scale, and the corresponding NMR measurements do not detect a stable ion pair in the millisecond time scale[64]. Molecular simulation studies also show that the time of ion-ion contact and cooperative motion in ionic liquids is about picosecond scale[65⇓~67]. Therefore, the ion pair is a transient structure that is continuously formed and dissociated, and it is controversial to define these structures as electrically neutral Discrete pairs. However, it is generally accepted that there are Dynamic pairs in ionic liquids[62]. As shown in fig. 2, cations attract anions to form a solvent layer (also known as an "ion cage") around them, and within τ I P time, the cations and one of the anions in the ion cage are relatively close to each other to form an ion pair, and after a period of stable existence, the cations move away from the anion cage to dissociate the ion pair, and begin to form an ion pair with another anion in the ion cage.
Nordness et al. Described the dissociation degree of ion pairs in ionic liquids based on experimental data and theoretical analysis, and found that for pure ionic liquids, except for some highly charged anions, the dissociation degree of ions decreases with the increase of the length of the alkyl chain on the cation, which is due to the fact that the growing alkyl chain can effectively enhance the competition of van der Waals force against the strong Coulomb force[62]. The degree of ion dissociation is roughly inversely related to the cation electron acceptor capacity (Lewis acidity) and increases with decreasing anion polarity, also due to decreasing Coulomb forces. In principle, the dissociation of ion pairs is beneficial to the production of free ions and the improvement of ionic conductivity of ionic liquids, which is very important for ionic liquids as electrolytes in electrochemistry. The dissociation of ion pairs has an important effect on other properties of ionic liquids, such as density, vapor pressure, solubility, chemical activity[62].

4 Hydrogen bond and hydrogen bond network

As mentioned above, the existence of hydrogen bonds has a very important influence on the formation and survival time of ion pairs in ionic liquids. Hydrogen bonds exist widely in ionic liquids, especially in protic ionic liquids. This fact has been confirmed by a large number of experimental means, such as Fourier transform infrared spectroscopy (FTIR), Raman spectroscopy, nuclear magnetic resonance (NMR), X-ray, neutron scattering, and computational simulations[68⇓⇓⇓~72][73⇓~75]. In contrast to the molecule-molecule or molecule-ion hydrogen bonds formed in other liquids (such as H2O…H2O and H2O…S O 4 -), hydrogen bonds in ILs are formed between charged ions and are therefore commonly referred to as "diionic hydrogen bonds"[76]. The strength of this type of hydrogen bond depends on the type of ionic liquid and is anti-correlated with the second stability energy ( Δ E 2) in the calculation. Ludwig et al. Found that the Δ E 2 of ILs [C4mim][BF4], [C4mim][BF4], [C4mim][NO3] and [PrAm][NO3]([PrAm]+ is propylammonium ion) increased gradually, and the general protic ILs had stronger hydrogen bonding effect relative to the aprotic ILs[77,78]. In addition, the hydrogen bonds of ionic liquids exist not only between anions and cations, but also between cations and even anions. Multiple hydrogen bonds with different lengths and donor/acceptor atoms can also be formed simultaneously between anions and cations (Fig. 3)[79,80]. These characteristics reflect the complex interactions between anions and cations and the rich designability of ionic liquids.
It was found that when imidazolium ionic liquids (such as [C2mim][PF6], [C4mim][BF4], and [C4mim][Cl]) were solidified to form ordered crystal structures, hydrogen bonds connected the anions and cations so that the ions were arranged in turn to form a zigzag ordered structure (Figure 4A), which was different from the simple linear hydrogen bond arrangement in other crystals (such as ice), reflecting the particularity of hydrogen bonds in ionic liquids[81⇓~83]. On the basis of a large number of studies on hydrogen bonding in such ionic liquids, our team introduced the concept of "Z bond", pointing out that the hydrogen bond in ionic liquids has its own particularity in geometry, energy, vibration, electronics and so on, which determines the unique structure and properties of ionic liquids[84].
Because a cation or anion can form multiple hydrogen bonds, the hydrogen bonds in ionic liquids can extend in space and form a complex three-dimensional hydrogen bond network. Dong et al. Used quantum mechanical calculation to study the hydrogen bond network in [C2mim][BF4] ionic liquid, and found that hydrogen bonds could connect ion clusters formed by multiple pairs of anions and cations (Fig. 4 B), and the calculated structure was consistent with the FTIR spectrum measurement[75]. Brela et al. Studied the [C2mim][TFSI] ionic liquid by ab initio molecular simulation, and found that the cation can form a hydrogen-bonding network structure with up to five anions at the same time, which is dominated by H — O hydrogen bonds. At the same time, Brela et al. Found that the molecular simulation based on the classical force field only obtained a few hydrogen bonds, and the H — F hydrogen bonds were dominant, indicating that the classical molecular simulation force field may have problems in accurately describing the complex hydrogen bonds and hydrogen bond network structure of ionic liquids[85].
The hydrogen bond (Z bond) of ionic liquids is closely related to the ionic structure. In principle, the hydrogen bond of ionic liquids can be changed by introducing functional groups, and the structure (such as hydrogen bond network, interface structure, confined structure) and properties (such as melting point, vapor pressure) of ionic liquids can be directionally controlled. Therefore, the study of hydrogen bond (Z bond) is an important basis for understanding the properties of ionic liquids and an effective means to regulate the structure and function of ionic liquids.

5 Ionic liquid cluster

Compared with single ion and ion pair, the ion cluster structure caused by the formation of hydrogen bond network is a microstructure on a larger scale. In fact, due to the large size, complex structure, strong Coulomb force and hydrogen bond (Z bond), the anions and cations of ionic liquids often aggregate to form large-scale complex heterogeneous structures, that is, cluster structures.
The cluster structure of ionic liquids can be observed in pure ions. Early Wang et al. Found through coarse-grained molecular simulation that the nonpolar alkyl chains of cations in [C2mim][NO3] ILs would aggregate and then form spatial heterogeneity in ILs[86]. Lopes et al. Used all-atom molecular simulation to study a variety of ionic liquids containing [PF6]- or [TFSI]- anions and imidazole cations with different chain lengths, and found that when the alkyl chain is greater than or equal to 4 carbon atoms, these side chains can aggregate to form a non-polar region, which is distributed in a three-dimensional ion channel grid composed of anion and cation imidazole rings[87,88]. As the chain length increases, these non-polar regions grow and form more connections, gradually changing from a dispersed phase to a continuous phase, similar to the occurrence of a microscopic phase separation process (fig. 5A). Various experimental methods have also confirmed the existence of heterogeneous structures in pure ionic liquids. For example, OHD-RIKES spectrum detection indicates the existence of density heterogeneity in imidazolium ionic liquids, which is consistent with the image of alkyl chain aggregation to form non-polar regions and anion-cation imidazole ring aggregation to form ion grids observed in the simulation[88]. Wide-angle X-ray scattering, NMR, electrospray ionization mass spectrometry have also been used to determine and explore the aggregate structure in pure ILs[89][90][91]. Collectively, the polar/nonpolar domains within these ILs are affected by the structure (e.g., alkyl chain length) and interactions (e.g., Coulomb forces) of the anions and cations, so the aggregation structure in ILs can be tailored by combining different kinds of cations and anions, varying the ratio of charged to nonpolar domains.
Ionic liquids have more distinct and complex cluster structures in their aqueous solutions. The change of system structure caused by the addition of water is very dependent on the concentration of the solution. Bernardes et al.'s Systematic study on [C2mim][EtSO4]([EtSO4]- is ethyl sulfate ion) aqueous solution showed that with the change of water molar concentration ( x H 2 O), the system can present four typical structural characteristics: when x H 2 O < 0.5, water molecules are dispersed in the polar grid of ionic liquid in the form of independent or short chains[92]; When 0.5 < x H 2 O < 0.8 时, the chain of water molecules grows and crosslinks with the polar network of ionic liquid. When the 0.8 < x H 2 O < 0.95, a bicontinuous structure with the coexistence of water molecular grids and ionic liquid polar grids appears in the system; When x > 0.95, the ionic liquid polar network is destroyed and the anions and cations are dispersed in the water molecular network in the form of small clusters. Jiang et al. Found a similar phenomenon in the molecular simulation of [C8mim][NO3] aqueous solution (Fig. 5 B)[93]. Ionic liquids dispersed in water are prone to form micelles, even vesicles. This is mainly due to the "amphiphilic nature" of ionic liquids, whose non-polar alkyl chains are significantly attracted by hydrophobic attraction in aqueous environment, and have similar phase separation behavior with surfactants. For example, Liu et al. Used all-atom molecular simulation to show that in the initial randomly dispersed [C12mim][Br]([C12mim]+ is 1-dodecyl-3-methylimidazolium ion) aqueous solution, the ionic liquid would first form several small micelles, and then fuse to form larger spherical vesicles (Figure 5C)[94]. Depending on the concentration of the IL solution, IL micelles may also exhibit rod-like structures[95]. Experimentally, it was also observed by transmission electron microscopy that with the increase of [C12mim][Br] concentration from 0.06 to 0.96 mol L-1, the ionic liquid clusters exhibited the structures of spherical micelles, rod-like micelles and spherical vesicles (Fig. 6)[96].
In addition, ILs also aggregate to form cluster structures in other solvents, such as organic solvents, other ILs, and supercritical CO2. For example, small-angle neutron scattering and nuclear magnetic resonance experiments found aggregates in methanol, acetonitrile, and benzene solutions of both [C2mim][Cl] and [C2mim][TFSI][97]. Small angle X scattering, polarized optical microscopy observed that [C16mim][Cl]([C16mim]+ is 1-dodecyl-3-methylimidazolium ion) in [EtNH3][NO3]([EtNH3]+ is ethylammonium ion) ionic liquid can form various cluster structures such as micelle, hexagon, layer and inverse bicontinuous cubic[98]. Zhang et al. Prepared ionic liquid-supercritical CO2 microemulsion using N-EtFOSA surfactant and guanidinium ionic liquid for dissolving different salts such as methyl orange, CoCl2 and sodium hydrogen chloride[99]. Chandran et al. Demonstrated the self-aggregation process of ionic liquids in supercritical CO2 by molecular simulation, and gave direct evidence that stable ionic liquid clusters can be formed in CO2 by amphiphilic surfactants[100].
The cluster structure of ionic liquids has reached nanometer or even micrometer scale, which is comparable to the current nano-microfluidics and nano-microarchitecture. Therefore, the cluster structure of ionic liquids has more direct application value. For example, Su et al. Used the [C2mim][Br] cluster as a template to assemble mesoporous silica materials in one step, and at the same time realized the preparation of ionic liquid encapsulated confined materials, which achieved higher catalytic efficiency[101]. Ishii et al. Used functional ionic liquids to assemble and construct periodic nanochannels to regulate the transport of water and ions, which can be used as candidate materials for water treatment membranes[48].

6 Interfacial ionic liquid

The formation of ionic liquid cluster structure often means the appearance of interface, that is, the local properties change continuously in a certain transition region. The force on the ionic liquid at the interface is not symmetrical, so the ionic liquid in the interface region often has different structural characteristics from the uniform host phase. For example, in ionic liquid micelles in aqueous solution, the nonpolar hydrophobic alkyl chains are buried inside the micelle to avoid interaction with water, while the polar hydrophilic ionic groups are exposed to water[102]. The existence of the interface greatly affects the thermodynamic and kinetic behavior of the system. The traditional theory usually assumes that the interface thickness is zero, the properties of the fluid outside the interface are equal to the corresponding bulk properties, and discontinuous mutations occur at the interface, and introduces statistical physical quantities such as "surface free energy" and "boundary slip length" to describe the influence of the interface on the behavior of the fluid and particles in the system. A large number of IL application systems (e.g., IL-based CO2 trapping, CO2 electrocatalytic reduction, lithium-ion batteries) are multiphase systems with various interfaces, such as gas-liquid interface and solid-liquid interface[40⇓⇓⇓~44]. As people's understanding of matter has entered the nano-micro scale and even the molecular level, there are more and more studies on the interfacial microstructure of ionic liquids.
At the gas-liquid interface, ionic liquids to minimize the free energy on the surface, the surface ions will expose the atomic groups with weaker interactions in the outermost layer. Wang et al. Used coarse-grained molecular simulation to study the imidazolium ionic liquid-vacuum interface, and found that the side chains of cations tend to align perpendicular to the surface, while the imidazole rings tend to align parallel to the surface[86,103]. At the same time, molecular modeling observed significant density fluctuations within the interfacial region, especially in systems with longer side chains (Figure 7 a). For common ionic liquids, the van der Waals force between cationic alkyl chains is weaker than the electrostatic interaction between the head group and the anion, so the alkyl chains are usually exposed on the outside and aligned parallel to each other and perpendicular to the surface under the action of van der Waals force. This trend is more obvious for ionic liquids with longer side chains, which also reveals the microscopic mechanism of the decrease of surface tension of ionic liquids with longer alkyl chains in general[49,50]. Experimentally, X-ray reflectivity (XRR) measurements show that the thickness of the gas-liquid interface of [C4mim][PF6] and [C4mim][BF4] ILs is 6 – 7 7 Å, and the electron density is 10% – 12% higher than that of the host phase. Further analysis shows that there is at least one obvious ion density fluctuation at the interface[105,106]. The subsequent molecular simulation study found that there are layered structures and density oscillations of ions at the interface, but the amplitude of the electron density curve in the interface region is small due to the cancellation effect of anionic and cationic charges. The electron density enhancement in the interface region is 12% and mainly comes from the contribution of anions, which is in excellent agreement with the experimental measurements[106].
Ionic liquids at the solid-liquid interface exhibit a very rich variety of interfacial structures due to the influence from the solid matrix. The distribution and orientation of anions and cations in the solid-liquid interface region are the most basic structural information in ionic liquids at the solid-liquid interface. When the solid matrix is charged or there are special groups, the ionic liquid has a strong electrostatic interaction or hydrogen bonding with the solid surface, which can lead to the enrichment of anions and cations on the solid surface, forming a multi-layer structure, showing fluctuations in ion density and charge density (Fig. 7B)[104,107]. Early experimental measurements of the structure of ionic liquid solid-liquid interfaces on solid substrates such as mica, silica, quartz, and metals confirmed the presence of ion and electron density fluctuations[108,109]. The period of density fluctuation is comparable to the size of anions and cations, which extends to the host phase to different degrees in different ionic liquids. For example, atomic force microscopy (AFM) measurements show that [C2mim][TFSI] can form a ∼ 0.4 nm thick [C2mim]+ layer on the negatively charged mica surface, while VSFS studies indicate that the imidazole ring of the cation only slightly favors the tilted attitude distribution of the solid surface[110]. Molecular modeling also shows that the imidazole ring tends to distribute vertically on the surface, the alkyl chain tends to distribute parallel on the surface, and the longer alkyl chain also distributes vertically on the surface (Fig. 7B)[104].
The study of the solid-liquid interface structure of ionic liquids is of great importance in electrochemical applications. The anions and cations of ionic liquids will be attracted or repelled on the charged solid surface, forming an "electric double layer structure" (EDL). The structure of the EDL determines and controls the energy stored in the EDL capacitor; For redox processes, the driving force is the voltage difference between the electrode surface and the location in the EDL where the electrochemical reaction occurs; The potential distribution in the EDL determines the electrochemical kinetics of the cell. Therefore, the EDL structure is central to numerous electrochemical applications. Due to the special ionic structure, strong electrostatic interaction and high concentration of ions, the structure of EDL composed of ionic liquids is very different from that in ordinary dilute electrolyte solution, and it shows complex changes with the charge on the solid surface. A typical process is as follows (Fig. 8A): On an uncharged solid surface, the ionic liquid forms alternating cation-rich and anion-rich layers to some extent[111,112]; The surface negative charge increases slightly, the cations are enriched on the surface to form an adsorption layer, the EDL reaches an overscreening structure, and the anions are adsorbed on the second layer to neutralize the charge[113]; The surface negative charge further increases, and the cations continue to enrich and form an ordered structure[114,115]; When the surface negative charge is enough, the cation adsorption dominates the two-layer structure of the solid surface, and the EDL reaches the crowding structure, and the anion adsorption neutralizes the charge in the third layer, resulting in the increase of the EDL thickness[116]. The complex structural changes of ionic liquid EDL on the electrode surface have a very important impact on the performance of electrochemical energy storage devices[8].
Ultrathin ionic liquid membranes on solid surfaces have received a great deal of attention in recent years[117]. The ultrathin ionic liquid membrane has the advantages of fast diffusion, less ion amount and high chemical/thermal stability, and does not have the interference of the host ionic liquid,Therefore, ultra-high vacuum experimental methods (such as electron microscopy) can be used to detect the solid-liquid and liquid-vacuum interfaces of ionic liquids with higher accuracy, and it is also a feasible system to use various quantitative calculation and molecular simulation methods to carry out research. At present, with the help of physical vapor deposition and scanning electron microscopy, researchers have constructed ordered two-dimensional structures of [C1mim][TFSI] and [BMP] [TFSI] on Ag (111) surface and Au (111) and generated disordered two-dimensional structures of [BMP] [TFSI] on Cu (111) surface (Fig. 8 B)[118⇓~120]. Studies have shown that the phase state (liquid, partial solid, full solid) and ordered structure of the ionic liquid membrane can be adjusted by changing the solid substrate, temperature, and external electric field. Therefore, ultrathin IL membranes on solid surfaces have their unique properties and advantages. Based on the previous related research, our team proposed a new concept of "Quasi-liquid" to describe the ionic liquid film on the solid surface, and pointed out that "quasi-liquid" is an ideal platform for future research on the structure-activity relationship of ionic liquids and the development of related applications[80,121,122].

7 Nano-confined ionic liquid

The ultrathin IL film on the above solid surface can be seen as an IL confined between the solid and the gas phase/vacuum, and thus is a typical nanoconfined IL. In recent years, with the development of nanotechnology and nanomaterials, the research and construction of matter have entered the nanoscale, and various nanoscale confined space systems have been formed. Ionic liquids in one-dimensional nanopores (such as carbon nanotubes), two-dimensional nanopores (such as the space between graphene sheets) and three-dimensional nanoporous materials (such as metal-organic framework materials MOF) all have nanoscale size characteristics, which are typical nanoconfined ionic liquids[123].
When the ionic liquid is in a nanoconfined space, the anions and cations will be attracted or repelled by the solid surface, and the components will be enriched or emptied near the solid-liquid interface, showing strong heterogeneous characteristics.As a result, the confined ionic liquid solution has unique structure and properties, and is also a common system in which ionic liquids exist in the form of "quasi-liquid", with typical ordered layering, density oscillation and structural phase transition. Chen et al. Characterized the morphology of a zinc-containing quaternary ammonium ionic liquid ( [Ch][ZnCl3]) in single-walled carbon nanotubes, and found that with the increase of nanotube size, the ionic liquid showed four different structures of single-chain, double-helix, zigzag pipeline and random structure pipeline in turn (Fig. 9 a)[124]. Singh et al. Used molecular simulation to study the [C4mim][PF6] ionic liquid in one-dimensional multilayer carbon nanotubes, and observed that the cation was preferentially adsorbed by the wall, and its imidazole ring was distributed parallel to the solid surface[125]; On the whole, more anions and cations are enriched near the wall, and an obvious layered structure is formed; As the nanotube radius increases, the orientation of the cations near the wall becomes weaker and the overall density oscillation amplitude becomes smaller (Fig. 9 B). Wang et al. Used molecular simulation to study the distribution of [C2mim][TFSI] between bilayer graphene, and also confirmed that the confined ionic liquid showed the structural characteristics of ordered layering and density oscillation[126]; As the layer spacing increases, the density oscillation amplitude becomes smaller (Fig. 9 C). Vicent-Luna et al. Used molecular simulation to reveal the distribution of [C2mim][SCN] in a variety of MOF materials, showing that ionic liquids present dispersed ionic liquid clusters and cross-linked ionic liquid grids in porous materials due to changes in pore connectivity (Figure 9D)[127].
The formation of the "quasi-liquid" structure of ionic liquids under confinement effect is very dependent on the hydrogen bonding (Z bonding) in ionic liquids. The zigzag arrangement observed by Chen et al. In carbon nanotubes indicates that the Z bond plays an important role in the nanoconfinement structure of the ionic liquid[124]. In addition, Dou et al. Used molecular dynamics simulation to study the [C4mim][PF6] confined to chiral carbon nanotubes, and found that 5 to 7 hydrogen bonds were formed between anions and cations on average at 300 K, which decreased monotonically with the increase of temperature and the radius of nanotubes (Fig. 10a)[128]. For the [C2mim][TFSI],Wang confined between two-dimensional graphene sheets, it is found that there are three hydrogen bonds between anions and cations on average[126]. Due to the structural change of the ionic liquid from monolayer to bilayer, the number of hydrogen bonds shows a significant non-monotonic change with increasing spacing (fig. 10B). The above results fully illustrate that the hydrogen bonding (Z bonding) interaction is closely related to the packing structure of confined ionic liquids.
The change of the structure of nano-confined ionic liquids significantly affects the performance of ionic liquids in the space. For example, Wang et al. Studied the [C4mim][BF4] in the graphene oxide channel by molecular simulation, and found that the ionic liquid was mainly distributed on the inner surface of the graphene oxide channel, showing an ordered layered feature, and the viscosity of the confined [C4mim][BF4] was 43.89 cP lower than that of the bulk phase, indicating that the confinement effect can improve the mass transfer efficiency of ionic liquid[129]. In terms of experiments, Ren et al. Reported a confinement method of ionic liquid in porous Ni-N catalyst to achieve high faradic efficiency, high current density and long-term stable operation in CO2→CO electrocatalytic reaction.It is considered that this is related to the reduction of activation energy barrier of CO2 and the increase of solubility of CO2 by ionic liquid, and the improvement of mass transfer and conductivity of ionic liquid electrolyte by confinement effect[130]. Wu et al. Found that the absorption of [C6mim][TFSI] in P25 to CO2 was about 10 times higher than that in the host phase[131]. Peng et al. Found that confining ionic liquids in multilayer materials can greatly improve the CO2 and have a good separation effect, exceeding the "Robeson upper limit" in 2008[132⇓~134]. In these practical applications, the relationship between the performance of ionic liquids improved by confinement effect and the microstructure of confined ions needs further study.

8 Conclusion and prospect

Ionic Liquids are a kind of "soft condensed matter" characterized by multiple interactions (including Coulomb force, dipolar force, hydrogen bond, Van der Waals force, π - π, etc.) And multiple structural features (including asymmetric structure, amphiphilic structure, acid-base structure, liquid crystal structure, chiral structure, etc.).It provides a broad platform for the development of various advanced technologies and materials based on it, and also brings great challenges to understand the microscopic nature behind the properties and behavior of ionic liquids. In recent years, with the development of experimental detection technology and computational simulation methods, people have gradually revealed the unique multi-level microstructure of ionic liquids (including ion pairs, hydrogen bonds, hydrogen bond networks, clusters, interfaces and confined structures from molecular level to nano-micro scale).It brings us a deeper understanding of the thermodynamic and kinetic behavior of ionic liquids, and also discovers the novel structures and phenomena of ionic liquids at the microscopic level, such as cluster structure and surface ordered structure. Based on the multi-scale simulation and experimental characterization of hydrogen bonding in ionic liquids and the semi-ordered structure of interfacial/confined ionic liquids, our team proposed the concepts of "Z bond" and "quasi-liquid", which provided a common theoretical basis for understanding the rich microstructure of ionic liquids. It is also the core content of the development of "condensed matter chemistry" to study and propose generic theories with multi-scale methods.
Although substantial progress has been made in the "condensed matter chemistry" of ionic liquids in recent years, there are still considerable challenges in further improving the basic understanding of ionic liquids and realizing the directed design of ionic liquids for practical applications. In order to promote the study of "condensed matter chemistry" of ionic liquids, many problems still need to be solved. For example, multi-scale computational simulation is an important means to study the multi-level microstructure of ionic liquids. However, for ionic liquids with complex interactions and structures, there are still many problems in establishing accurate computational models and achieving large space/long time scale simulation. It is still very difficult to correctly understand the influence and interaction of the microstructure of ionic liquids at all levels, which also brings challenges to the establishment of relevant theoretical and computational models. Experimental observation and characterization means for IL microstructure (e.g., Z-bond, quasi-liquid) in situ/real time are also important discoveries for future development. In addition, the multi-scale problem and application of ionic liquids is a typical interdisciplinary problem, involving chemistry, physics, biology, mathematics, engineering and other disciplines, and future research will inevitably be based on multidisciplinary cooperation.

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Funding

National Natural Science Foundation of China(21978293)

National Natural Science Foundation of China(21878296)

Beijing Municipal Natural Science Foundation(2202051)

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