Molecular dynamics ionic liquid modelling

Abstract Theoretical investigations of ionic liquids are reviewed. Three main categories are discussed, i.e., static quantum chemical calculations (electronic structure methods), traditional molecular dynamics simulations and first-principles molecular dynamics simulations. Simple models are reviewed in brief. 

A well-known effect of water impurities in ionic liquids is the viscosity decrease. " However, also ionic liquids were reported, in which water impurities induces gelation. Spohr and Patey have shown with ionic liquid model systems that water tends to replace the counter ions from the ion solvation shell in ionic liquids with small ion size disparity, leading to a faster diffusion of the lighter ion-water clusters. However, water can increase viscosity of ionic liquids if the ion size disparity is too large, or if strong directional ion pairs are found. Spohr and Patey attributed this behavior to extended water-anion chains and strongly bound water-anion-cation clusters. A classical molecular dynamics study by Raju and Balasubramanian observed that the anion diffuse faster than the cation in water ionic liquid mixtures in contrast to neat ionic liquids. The larger... 

The highly detailed results obtained for the neat ionic liquid [BMIM][PFg] clearly demonstrate the potential of this method for determination of molecular reorienta-tional dynamics in ionic liquids. Further studies should combine the results for the reorientational dynamics with viscosity data in order to compare experimental correlation times with correlation times calculated from hydrodynamic models (cf [14]). It should thus be possible to draw conclusions about the intermolecular structure and interactions in ionic liquids and about the molecular basis of specific properties of ionic liquids. 

The first recognition of ionic liquids as nanosegregated structures came from molecular dynamics (MD) studies reported in the sequence of the development of a systematic force-field to model systems containing ionic liquids [4, 5],... 

It can be easily shown that the value of K is inversely proportional to the value of K and that K is dependent on both the cation and the anion of the IL Hence, it is entirely consistent with this model that the difference made by changing the anion should depend on the hydrogen bond acidity of the cation. A recent molecular dynamics study has confirmed this interpretation [56]. This study also suggests that nonspecific interactions between the dye molecule and the ionic liquid are entirely local in nature and dependent on the nature of the first solvation shell of the solute. 

Yockel S, Schatz GC (2010) Modeling 0(3P) and Ar scattering frrun the ionic liquid [emim] [N03] at 5 eV with hybrid QM/MM molecular dynamics. J Phys Orem B 114 14241-14248... 

In contrast, del Popolo et al. presented at COIL-1 a model based on ab initio molecular dynamics (MD) simulation of ionic liquids [16]. This is a departure from the more traditional approaches mentioned in the previous paragraph, with enormous potential for development both in terms of accuracy and applicability of the model. However, only a handful of rather simple ionic liquids have been modelled so far, due to the higher conputational costs and parameterisation complexity associated to the model. 

In 2001, Hanke, Price and Lynden-BelP were the first to conduct an atomistic simulation of compovmds that can be called ionic liquids under our definition. They used molecular dynamics to model the crystalline state of 1,3-dimethylimidazolium chloride ([Cimim][Cl]), 1,3-dimethylimidazolium hexafluorophosphate ([CimimllPFg]), l-ethyl-3-methylimidazolium chloride ([C2mim][Cl]), and l-ethyl-3-methylimidazolium hexafluorophosphate ([C2 mimJlPFg]). They also modeled the liquid state of [Cimim][Cl] and [Cimim] [PFg], both of which are relatively high melting substances. Because of this (and the need to speed dynamics and thus limit computation times), the liquid simulations were carried out at temperatures between 400 and 500 K. The form of the potential function they used was... 

The application of first-principles molecular dynamics to the study of organome-tallic systems or homogeneous catalytic processes in nmiaqueous solvents is certainly less extended than in water. Nevertheless, a survey of the literature shows that the number of AlMD-based studies in nonaqueous solvents is increasing. In many cases, due to the larger size of the complexes investigated and of the solvent molecules compared to in-water studies, the hybrid QM/MM approach is taken to simulate realistic model systems at a reduced computational cost. Simulations by means of first-principles molecular dynamics on ionic liquids [130] or frustrated Lewis pairs in organic solvents [131] are not covered here. 

Logotheti, G.-E., Ramos, J. Economou, I. G. (2009). Molecular modeling of imidazolium based [Tf2N ] ionic liquids Microscopic structure thermod)mamic and dynamic properties and segmental d5mamics, /. Phys. Chem. B 113 7211. 

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