Multiscale modelling of ionic liquids: from quantum methods to coarse-grained models
- Agilio Padua (Blaise Pascal University, France)
- Jose Nuno Canongia Lopes (Technical University of Lisbon, , Portugal)
Ionic liquids are salts consisting of large organic ions with low melting point below. Salts that are liquid at room temperature have been known for a long time, but compounds stable in air and water are recent. These ionic liquids at room temperature are composed of large, asymmetrically shaped, flexible ions, with delocalization of the electrostatic charge, or a combination of these factors. These features contribute to hinder crystallisation.
Many ionic liquids are very stable thermally and chemically, are liquid on a wide temperature range and have a large electrochemical window, up to 6 V. Being salts they are non volatile (their vapour pressure is virtually zero at moderate temperatures). It is possible to prepare ionic liquids by combining cations and anions of different families and by modifying functional groups in their chemical structures. Therefore, an enormous number of ionic liquids can be created and their properties are adjustable by all these possibilities of structural variation. The salts are essentially insoluble in weakly polar organic phases and some ionic liquids are also hydrophobic. These properties have led these compounds to be considered as "designer solvents" with tuneable properties in view of applications. They also offer recycling options because of their stability, non-volatility and immiscibility in other solvents.
Ionic liquids, as molten salts, are highly structured in comparison with most molecular liquids. The ionic charges impose a characteristic structure dictated by electroneutrality. In an ionic liquid in each ion is surrounded by ions of opposite sign, four to six, and then by successively less ordered layers of ions of alternating charge. These local order effects imposed by Coulomb forces have a longer range than what is generally found in molecular liquids. But ionic liquids contain not only charged groups, but also non-polar alkyl chains that can reach significant lengths and induce medium and long-range heterogeneities.
The field of room temperature ionic liquids is new (10 years) and no large body of experimental data existed. As such, molecular simulation and computational chemistry have since the beginning played major roles both in the discovery of new features and improving our understanding of ionic liquids.
The defining properties of room temperature ionic liquids result from a subtle balance of Coulomb and van der Waals interactions, H-bonds, conformational flexibility, shape asymmetry, etc. If we reduce the simulation models to "spheres with charges", we may capture the essence of the physics of a "simple" molten salt, but we will not be able to describe the complex structural and solvation behaviour of a room temperature ionic liquid. On the other hand, the heterogeneity and slow dynamics of ionic liquids demand for long trajectories.
Therefore, one of the main difficulties in the molecular modelling of ionic liquids (as for many other complex fluids) is to find the right balance between detail and efficiency. Hence the pertinence of a multiscale approach: quantum chemistry can describe electronic effects such as polarisation or charge transfer, but correctly parameterised coarse-grained models are necessary to reach long time and size scales. Atomistic models provide the link between the electronic and mesoscopic levels.
Among the main challenges today are:
- The description of dynamic quantities, since fixed-charge models tend to give too slow dynamics and the discussion is still open concerning the best way to represent polarisation (Drude, fluctuating, scaled charges). The description of H-bonds remains an issue, as in other applications of force fields.
- The study of dynamic and structural heterogeneities and understanding their links. This demands long simulations on large systems. Structural heterogeneities are understood as resulting from a segregation of charged and nonpolar domaines in ionic liquids with nonpolar moieties. These have consequences on the dynamic behaviour. But ionic liquids also exhibit dynamic heterogeneities related to their glass-forming character.
- The interactions with materials, for which very few bottom-up studies have been done. Most groups pick Lennard-Jones parameters from the literature for the atoms of the ions and of the material and assume traditional combining rules for the unlike interactions, in many cases without including explicit polarisation of conducting or semiconducting materials. It is important to evaluate the soundness of this approach and to devise methods to develop more accurate interaction models between the ions and materials.
The field has been evolving quickly and has not been the subject of a general review. Most of the researchers (not all) have met at international conferences or symposia on ionic liquids, but rarely in a workshop format for more in-depth and specialised discussions on methodology. There was one event organised at the CECAM-IE node in 2009 on the subject of computational modelling of ionic liquids, with a more general scope than the one of the present proposal. Also, the five years since that event have witnessed many significant developments (polarisation, interactions with materials) and new applications.
Several research groups have worked on the quantum, atomistic and coarse-grain levels of description for ionic liquids, and developed models that are used by a wider community of physical chemists studying solvation, spectroscopy, structure and dynamics, or interfaces.
A few groups have produced trajectories using quantum molecular dynamics with periodic conditions, on a few 10s of ion pairs over ca. 10 ps [1,2]. This type of study is most pertinent for protic ionic liquids or reactive systems. Mostly, quantum methods have been used to investigate specific phenomena on clusters of ions  such as H-bonding.
Atomistic force fields have aimed at transferability in order to represent easily different families of ionic liquids. Several fixed-charge models that are still widely used [4,5]. Explicit charge polarisation has been introduced [6,7], a major development allowing a better representation of dynamics. Atomistic models have been used for trajectories of the order of 10 ns with up to 1000 ion pairs.
Longer space and time scales have been studied using united-atom or coarse-grained models [8,9]. There have been few integrated efforts to develop a truly multiscale model, enforcing consistency across the different levels .
Interactions of ionic liquids with nanoparticles and materials can also be complex due to polarization effects, notably on carbon materials (graphite/graphene/nanotubes ) and metals. Studies have focused quite different applications of ionic liquids, from suspensions of nanoparticles for catalysis , dissolution of cellulose , electrolytes for supercapacitors  or lubricants .
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