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Development and application of molecular interaction models with explicit polarisation for ionic liquids and eutectic solvents

Abstract : Ionic liquids (ILs), salts that are liquid at room temperature, are promising solvation media due to their unique properties, such as low flammability, negligible vapour pressure, high conductivity, and good thermal and electrochemical stability. Deep eutectic solvents (DES), a related class of compounds with similar properties, are not purely ionic, being mixtures of a salt with a molecular compound. A huge variety of ILs and DES can be obtained via ion replacement, changes in functional groups or side chain modification and it is costly to characterize such a wide range of compounds by experimental techniques. So, theoretical methods are key in their rational design and to obtain fundamental information on how molecular structure and interactions determine their physical and chemical properties. Modelling of these systems is a challenging task due to the diversity of their interactions, and a good molecular force field is important to provide a reliable description of ILs and DES. Ionic liquids (ILs), salts that are liquid at room temperature, are promising solvation media due to their unique properties, such as low flammability, negligible vapour pressure, high conductivity, and good thermal and electrochemical stability. Deep eutectic solvents (DES), a related class of compounds with similar properties, are not purely ionic, being mixtures of a salt with a molecular compound. A huge variety of ILs and DES can be obtained via ion replacement, changes in functional groups or side chain modification and it is costly to characterize such a wide range of compounds by experimental techniques. So, theoretical methods are key in their rational design and to obtain fundamental information on how molecular structure and interactions determine their physical and chemical properties. Modelling of these systems is a challenging task due to the diversity of their interactions, and a good molecular force field is important to provide a reliable description of ILs and DES. We developed a general, transferable, polarisable force field for molecular simulation of protic and aprotic ionic liquids, deep eutectic solvents, and electrolytes, named CL&Pol. This is a major upgrade from existing fixed-charge force fields, which cannot represent with the same level of physical realism the interactions in ionic and polar media, failing to predict correctly equilibrium, structural and transport properties. In order to compensate for the addition of explicit polarisation in the form of Drude induced dipoles, the Lennard-Jones parameters of the original force field are rescaled, with the scaling factor evaluated either from computationally expensive quantum calculations or from a fast and general predictive scheme that we propose. Special damping functions were introduced in the force field to represent smearing of the interactions between charges and induced dipoles at a short range involving small, highly charged atoms, thus preventing the ``polarisation catastrophe''. The new force field gives stable trajectories and shows much improved predictions of transport properties. The fragment approach is a major feature: this is not just a specific model for a few compounds, but a framework that can be easily extended and combined with polarisable and non-polarisable force fields sharing compatible functional forms. The CL&Pol model was used to study solvation of dyes and gases in the above-mentioned solvents, to describe the interfaces of ILs with nanomaterials, and to design electrolytes for energy-storage devices.
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Kateryna Goloviznina. Development and application of molecular interaction models with explicit polarisation for ionic liquids and eutectic solvents. Theoretical and/or physical chemistry. Université de Lyon, 2021. English. ⟨NNT : 2021LYSEN077⟩. ⟨tel-03544091⟩

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