Systematic Molecular Simulation Investigation of the Influence of the Chemical Structure of Anion and Cation to the Properties of Imidazolium-Based Ionic Liquids and to the Thermodynamic Properties of Light Gases Dissolved Therein

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This work focuses on the computational study of the physical properties of pure imidazolium-based tricyanomethanide Ionic Liquids (ILs) , [Cnmim+][TCM-] , and bis-trifluoromethylosulphonyl ILs , [Cnmim+][Tf2N-] , and on the prediction of solubility and diffusivity of light gases (e.g. Ar , CO2 , N2) therein over a wide temperature range at atmospheric pressure. Molecular simulation plays a vital role in designing ILs with controlled permeability properties for use in a number of industrial processes , such as CO2 capture , by revealing a wealth of microscopic information for the underlying mechanisms that are responsible for materials’ macroscopic properties.Here , long molecular dynamics (MD) simulations were performed in a wide temperature range and at atmospheric pressure in order to predict the thermodynamic , structural and dynamical properties of the pure ILs. A non-polarizable atomistic force-field was optimized in order to accurately predict the density and the self-diffusion coefficient. The dynamical heterogeneity exhibited by the ILs was investigated through the calculation of the non-Gaussian parameter and the deviation of the self-part of the van-Hove distribution function from the expected normal distribution. Subsets of “fast” and “slow” ions form individual clusters that consist of either mobile or immobile ions in the bulk. The structural relaxation process was analyzed by the self-part of the intermediate scattering function which exhibits strong wave number and temperature dependence. The distinct behavior of anions and cations at various length scales was thoroughly investigated as a function of the alkyl tail length. Detailed analysis of the ions’ diffusion reveals preferential motion along the direction of the alkyl tail for the cation and the S-S vector for the [Tf2N-] anion. As the alkyl tail length increases , the heterogeneity in the dynamics becomes more pronounced and is preserved for several nanoseconds at low temperatures. Simulation of light gases (e.g. Ar , CO2 , N2) in the ILs at infinite dilution was performed in order to calculate the solubility and diffusivity of the gas molecules in the ILs. The solubility was estimated based on the Widom test particle insertion technique. In all cases , the agreement between experimental data and molecular simulation is excellent. The temperature effect was thoroughly studied for all the properties of the pure ILs and the dynamic properties of light gases dissolved in the above mentioned systems. Moreover , the influence of the cation's alkyl chain length and the choice of the anion were investigated in order to study the underlying molecular mechanism that governs the macroscopic behavior of these systems.  Diffusivity was estimated through the mean square displacement of the ions. In conclusion , strong spatial organization with an indication of alkyl tail aggregration phenomena was determined. This organization leads to an inhomogeneous bulk structure and is connected to heterogeneity in the dynamics of these systems which has been also reported by previous studies of similar systems.  This inhomogeneity affects the macroscopic physical properties of the system.
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