Phase Behavio rs of Ionic Liquids Attributed to the Dual Ionic and Organic Nature Chenyu Tang 唐晨宇12 and Yanting Wang 王延颋12

2025-05-02 0 0 1.39MB 21 页 10玖币

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Phase Behaviors of Ionic Liquids Attributed to the Dual Ionic and

Organic Nature

Chenyu Tang（唐晨宇）1,2 and Yanting Wang（王延颋）1,2*

1CAS Key Laboratory of Theoretical Physics, Institute of Theoretical Physics, Chinese Academy of

Sciences, 55 East Zhongguancun Road, P. O. Box 2735, Beijing 100190, China

2School of Physical Sciences, University of Chinese Academy of Sciences, 19A Yuquan Road, Beijing

100049, China

Abstract: Ionic liquids (ILs), also known as room-temperature molten salts, are composed of pure ions

with melting points usually below 100 ℃. Because of their low volatility and vast amounts of species,

ILs can serve as “green solvents” and “designer solvents” to meet the requirements of various

applications by fine tuning their molecular structures. A good understanding of the phase behaviors of

ILs is certainly fundamentally important in terms of their wide applications. This review intends to

summarize the major conclusions so far drawn on phase behaviors of ILs by computational, theoretical,

and experimental studies, illustrating the intrinsic relationship between their dual ionic and organic nature

and the crystalline phases, nanoscale segregation liquid phase, ionic liquid crystal phases, as well as

phase behaviors of their mixture with small organic molecules.

Keywords: Ionic liquids, phase behaviors, nanoscale segregation liquid, ionic liquid crystal

1. Introduction

Ionic liquids (ILs) are a type of salts with low melting points, often below 100 °C, meaning that they

tend to remain in the liquid phase at room temperature and are believed to exhibit some unique features

because of the strong electrostatic interactions among ions. Typical aprotic ILs are normally composed

of small anions and bulky cations with a long alkyl side-chain and a charged head group, as shown in

Fig.1, which demonstrates the chemical structure of 1-butyl-3-methylimidazolium chlorine, a typical

imidazolium-based IL. Possessing both ionic and organic features, they are believed to have

advantageous properties of both organic liquids and inorganic salts, such as good solvation ability and

tunability, low melting temperature, good conductivity, wide electrochemical window, thermal and

electrochemical stabilities, non-volatility, and non-flammability [1-4]. They are thus regarded as “green”

and “engineer” solvents that can be utilized under many industrial circumstances [5-11].

Understanding fundamental properties of ILs, particularly their phase behaviors, is apparently

essential to their applications. To investigate their phase behaviors, many computational and

experimental methods have been employed to investigate phase behaviors of ILs. Molecular dynamics

(MD) simulation has become an important means of studying the structure and dynamics of ILs [12-15]

where different modelling methods employing various software packages including GROMACS,

NAMD, LAMMPS, etc. [16-18] have been developed. All-atom force fields are commonly used in

addressing IL related problems by means of MD simulation [19-21], and the applicability of some

commonly used all-atom force fields, including the Amber force field [22], OPLS force field [23],

CHARMM force field [24], etc., to IL systems has been verified and the models have been constantly

improved to be better applied to ILs [15,25-35]. By considering the polarizable effect at the atomic level,

a polarizable model has also been developed to better quantify microscopic structures and dynamics of

ILs. [36-38]. Another modelling strategy is to apply a coarse-grained (CG) model in MD simulation,

which reduces the cost of computation and renders researchers with longer simulated times than all-atom

models. One of the CG methods that are used in the context of ILs is the Multiscale Corse-Graining (MS-

CG) method [39-41], which matches the instantaneous forces applied to atoms in the all-atom MD

simulation to determine the optimal empirical CG forces between CG sites (atomic groups). By contrast,

the Effective Force Coarse-Graining (EF-CG) method directly calculates the effective averaged force

between each pair of CG sites (atomic groups) to gain better transferability [42,43]. Other MD methods

applicable to study ILs include ab initio MD [26,44-46], MD with a polarized force field [47-54], and

MD with other CG models [55-60]. Apart from MD simulations, Monte Carlo (MC) simulations [61-63],

electronic correlation method [64-67], and Density Functional Theory (DFT) calculations [68-72] are

also applied to studying ILs, coming up with many favorable results.

As typical complex liquids, ILs usually have abundant and distinctive phase behaviors beyond the

description of simple liquid theories. Some unique phases, including the nanoscale segregation liquid

(NSL) phase [73-77], ionic liquid crystal (ILC) phases [78-83], the “partially arrested” glassy phase [84],

and the metastable crystal phase [85], were revealed by MD simulation and verified by experiment. Much

attention has been aroused since the detailed knowledge of these phases can shed light on the

understanding of not only ILs but also other amphiphilic complex liquids. It also provides guidance for

industrial utilization of ILs as novel solvents. In this review, we majorly focus on summarizing the phase

behaviors of aprotic ILs with alkyl cationic side chains whose number of atomic groups are even. We

will show that these behaviors tend to be affected highly by temperature, length of the cationic alkyl side-

chains, charge distribution, and nature of cations and anions, indicating that these unique phase behaviors

of ILs result from their dual ionic and organic nature [86]. By reviewing these phase behaviors and the

associated affecting factors and mechanisms, we aim to inspire further investigations in this direction

and invite more researchers to delve into this field to develop a thorough understanding of ILs in the

future.

Figure 1. Chemical Structure of 1-butyl-3-methylimidazolium chloride.

2. Dual Ionic and Organic Nature of ILs

One of the most significant features of ILs that broadens their industrial usage is that ILs inherit both

the ionic nature of inorganic salts and the organic nature of organic solvents. They have the advantages

of non-flammability, non-volatility, good stability, and conductivity compared to organic solvents, and

low melting temperature compared to traditional inorganic salts. By varying cations and anions, millions

of available ILs can be produced with various physical and chemical properties [7], which is highly

beneficial to meet certain requirements for designated applications. However, since selecting out an

appropriate combination of cation and anion via experiment is usually tedious or even unfeasible, it is

critical to have a good knowledge of the dual ionic and organic nature of ILs to help achieving computer-

aided systematic design of ILs. It has been well acknowledged that the competition between electrostatic

and van der Waals (VDW) interactions is the key factor characterizing the ionic and organic nature

[87,88], so analyzing these interactions in ILs by means of MD simulations with suitable force fields

should be informative.

In a recent work, Shi and Wang [86] performed a series of all-atom MD simulations for four

representative ILs (1-butyl-3-methylimidazolium nitrate ([BMIM][NO3]), 1-butyl-3-methylimidazolium

tetrafluoroborate ([BMIM][BF4]), 1-butyl-3-methylimidazolium hexafluorophosphate ([BMIM][PF6]),

and 1-butyl-3-methylimidazolium bis(trifluoromethylsulfonyl)imide ([BMIM][Tf2N])), and compared

them with three molecular systems with different charge distributions: a typical molten inorganic salt

(molten sodium chloride, NaCl), a strongly polar liquid (Dimethyl sulfoxide, DMSO), and a weakly polar

liquid (toluene). The dual ionic and organic nature of ILs can be depicted from the viewpoint of the cage

energy landscape (CEL) by the obtained forces, vibrational force constants, intrinsic electric fields,

cohesive energies, and cage energies.

By introducing the concept of ion cage [89-97], which indicates how each ion is surrounded by

several counter-ions in the first coordination shell, along with its counterpart of molecular cage [98-102]

for molecular liquids, it is possible to depict the structures and dynamics of various types of liquids since

the cage volume corresponds to density and the cage stability reflects dynamics. From cage structure, it

is then possible to determine CEL by calculating the ensemble-averaged local energy landscape as a

function of the dislocation of the central ion from the cage center. The curvature, slope, and depth of

CEL can be determined as the force constant

, force

, and activation energy

, which is the average

energy of a particle required to climb over the energy barrier and escape the cage calculated by using the

harmonic approximation (Fig. 2a).

Shi and Wang [86] employed the first moment of the vibrational density of state (VDOS) to

qualitatively describe the average characteristic frequency of intermolecular vibrational modes, defined

as [103]

( )d / ( ) d



     

   

(1)

where



is the frequency and

()I



the VDOS. The total, VDW, and electrostatic forces, respectively,

are compared in all the investigated liquids, and a liquid-phase cage energy

cage

is defined as the

average potential energy between an ion and a counter-ion in its ion cage to characterize the local ion-

ion interaction in the liquid.

Figure 2. (a) Cage energy landscape characterized by curvature, slope, and depth, corresponding to force

constant, force, and activation energy experienced by molecules, respectively. (b) Schematic illustration

of cage structures and cage energy landscape in inorganic salts, ionic liquids, and organic solvents. The

cage energy landscape of inorganic salts is deep and steep, whereas that of ionic liquids is still deep but

much more gently. Organic solvents and ionic liquids have a similar slope and curvature near the

minimum of the cage energy landscape, but the depths for the organic solvents are much lower. Reprinted

with permission from R. Shi and Y. Wang, Sci. Rep. 6, 19644 (2016).

From these approaches, a conclusion has been drawn that similar molecular size, geometry, and

component lead to comparable VDW forces in organic ions and organic molecules. They also weaken

the electrostatic interactions in ILs because of charge delocalization and charge transfer. The cage energy

of ILs induced by electrostatic interactions is drastically different from organic liquids. These findings

indicate that the VDW interactions, which dominate the intermolecular forces and vibrational force

constants, characterize the organic nature of RTILs, resulting in a similar geometry near the minimum of

the CEL to organic liquids; whilst the cage energy, or the depth of the CEL, can characterize their ionic

nature (Fig. 2b). Such a mechanism explains the similar and dissimilar characteristics between ILs and

organic liquids, and it clarifies the blurry dual ionic and molecular nature of ILs whereas the

corresponding microscopic mechanism provides new insights into their phase behaviors.

3. Nanoscale Segregation Liquid (NSL) Phase

A unique phase behavior in ionic liquids that can be explained by the dual ionic and organic nature

of ILs is the nanoscale segregation liquid (NSL) phase of ILs, which is different from either simple liquid

phase or liquid crystal (LC) phase. The structure was first discovered by MD simulation and later verified

by experiment [74,76,104,105]. The discovery of the NSL phase may possibly boost new industrial

applications of ILs, and the microscopic mechanism of NSL shall aid the engineering of ILs [76]. The

significance of this discovery is enhanced by the fact that the NSL exists in most IL systems with an

amphiphilic cation, independent of a specific choice of the anion.

In a series of MS-CG MD simulations on [CnMIm][NO3] with n = 4–12, Wang, et al. [74,76,106]

observed a tail-aggregation phenomenon and later referred to it as the nanoscale segregation liquid (NSL)

phase. It has been found that, in a certain temperature range, while the whole liquid is macroscopically

homogeneous, the tail groups of the cationic alkyl side chains form nanoscale nonpolar tail domains

microscopically segregated with the continuous polar network formed by charged anions and cationic

head groups. This phenomenon was later confirmed experimentally [104,107,108] by using optical

heterodyne-detected Raman-induced Kerr effect spectroscopy (OHDRIKES), Neutron Diffraction, X-

ray diffraction, electrospray ionization mass spectrometry (ESI-MS), and NMR measurements, all

providing solid evidence for the existence of the NSL phase.

To quantify the degree of tail aggregation in the NSL phase, the Gaussian-like Heterogeneity Order

Parameter (HOP) is defined as [76]

1exp 2

Nij















(2)

to quantify the spatial heterogeneity for identical sites, where

is the distance between sites i and j, and

1/3

/LN





with L being the side length of the cubic simulation box and N being the total number of

identical sites.

Both the radial distribution functions (RDFs) and HOPs of the simulated systems show that,

compared with ILs with short alkyl side chains, which is basically in the simple liquid phase, the tail

groups distribute quite heterogeneously in ILs with an intermediate alkyl side chain length. The charged

domain, constituted by anions and cationic head groups, is dominated by the electrostatic interactions,

whereas the neutral cationic tail groups form a nonpolar tail domain with the VDW interactions among

tail groups. It has also been discovered that such aggregation behavior is temperature-sensitive, which

can be understood through analyzing the tail domain diffusion in ILs [76]. Investigating the behavior of

tail domains with increasing temperature has also revealed that the transition of ILs from NSL to simple

liquid is characterized by the collective behavior of tail groups. Nevertheless, such a collective behavior

is passively induced by the steric repulsion from the continuous polar network, as concluded by a further

study on ILs with an external electric field applied that the repulsion from the polar part is the major

cause of the aggregation of the nonpolar tail groups [77].

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PhaseBehaviorsofIonicLiquidsAttributedtotheDualIonicandOrganicNatureChenyuTang（唐晨宇）1,2andYantingWang（王延颋）1,2*1CASKeyLaboratoryofTheoreticalPhysics,InstituteofTheoreticalPhysics,ChineseAcademyofSciences,55EastZhongguancunRoad,P.O.Box2735,Beijing100190,China2SchoolofPhysicalSciences,UniversityofChines...

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Phase Behavio rs of Ionic Liquids Attributed to the Dual Ionic and Organic Nature Chenyu Tang 唐晨宇12 and Yanting Wang 王延颋12

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