Modeling of Ionic Liquids

Patricia A. Hunt, Edward J. Maginn, Ruth M. Lynden-Bell, and Mario C. Del Pdpolo 

This chapter is divided into four key sub-sections. The first is a very brief and basic introduction to the methods used in, and information that can be obtained firom, computational studies of ionic liquids. It is primarily aimed at orientating 

Many of the ionic liquids that have been examined computationally have been based around the imidazolium cation. Fig. 4.2-1. 

Liquids are difficult to model computationally because the individual molecules (or ions) that make up the liquid are not isolated (as in the gas phase), but are interacting with each other. These interactions are not symmetric and static (as in solids) but are randomized and dynamic. Thus, errors are introduced when attempts are made to extract a portion of the liquid for study. This problem can be maneuvered around by taking a large portion and by controlling the boundary conditions of the sample. The techniques for extracting a sample and for treating the boundary of a sample are well established within the molecular dynamics methodologies. 

The high viscosity of ionic liquids is also problematic computational methods that aim to determine thermodynamic properties rely on sampling a large number of configurations or snap-shots of the constituent molecules (or ions) in different positions or orientations within the liquid [10]. In a viscous liquid this motion is limited and thus it can take a very long time, and be computationally expensive, to build up the required number of snap-shots that wiU produce accurate predictions [11,12]. 

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The model is certainly complex, but perhaps no more so than previous ionic liquid models described in Sections 5.5.1 and 5.5.2. The initial experience (Selleby 1996) suggests that the number of terms needed to describe a ternary system such as Fe-Mn-S is quite similar for both ionic two-sublattice liquid and associate models (see next section). The modelling of ionic liquids is, in the main, complex and the advantages of the various techniques can only become apparent as they become more commonly used. 

Some of the special features of molten oxides must now be described for it is these features that do not permit the hole model of ionic liquids to be applied to fused oxides... 

Nooruddin NS, Wahlbeck PG, Carper WR (2009) Semi-Empirical molecular modeling of ionic liquid tribology ionic liquid-hydroxylated silicon surface interactions. Tribol Lett 36 147-156... 

Modeling of Ionic Liquid Electrolytes Room-Temperature Ionic Liquid-Based Binary Electrolytes... 

Scheme 19.1 Corrosion inhibition mechanism and adsorption model of ionic liquids on mild steel surface in aqueous sulphuric acid medium [32]...

A special mention must also be made at this point to enpirical potential structure refinement (EPSR) models that are usually used to interpret X-ray diffraction and extended X-ray absorption fine structure (EXAFS) spectroscopy data. MD simulations of ionic liquids using these models were presented at COIL-1 by Hardacre and co-workers [17]. These are not traditional force fields, in the sense that they are developed to match structural data and thus generally lack information concerning intramolecular parameterisation (which is already provided by the structural data itself). Their contribution to the modelling of ionic liquids can be very important as far as structural properties are concerned but are limited otherwise. 

Paduszynski K, Domanska U (2012) Thtamodynamic modeling of ionic liquid systems development and detailed overview of novel methodology based on the PC-SAFT. J Phys ChemB 116 5002-5018... 

A general feature in the diffusive dynamics of supercooled or viscous liquids is that particles are trapped in a cage for a long time because the thermal motions are not activated enough. This is also the case of our model of ionic liquids at low temperatures an ion exhibits merely oscillatory motions, occasionally interrupted by significant movements. We monitor such large motions of each ion and thereby quantify local dynamics in the ionic liquid. In this study, local excitation events refer to the instances Ij, l2, h, , where the displacement of an ion i exceeds a threshold distance, i.e., r (ti) — ri(0) > d, r,(t2) — r,(ii) > d, r,(f3) — r,(f2) > d, , etc (Hedges et al., 2007). The more local excitation occurs frequently, the more the ion is mobile. The cut off distance d should be chosen appropriately in order to probe the local dynamics. We display the results for d = 3.0 A, for example, and note that other choices of d on the order of the inter-ion distances do not alter our results qualitatively. 

Xiaoyan J. Adidharma H. (2009). Thermodynamic modeling of ionic liquid density with heterosegmented statistical associating fluid theory. Chem. Eng. Science 64, 1985-1992. 

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