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Amino acid choline-based ionic liquids (AACBILs) have high biodegradability, low toxicity, availability, low cost, and high thermal stability compared to the traditional ionic liquids (ILs). In this work, the volumetric, structural, and dynamical properties of three AACBILs, that is, choline alanine ([CH][Ala]), choline β-alanine ([CH][β-Ala]), and choline phenylalanine ([CH][Phe]) were investigated using the quantum mechanical calculations and also molecular dynamics simulations in both gas and liquid phases. The density functional theory calculations, noncovalent interactions, and also the quantum theory of atoms in molecules methods have been used to investigate the hydrogen bonds, interaction energies, and also charge transfers between the ions of the studied ILs. Density, isobaric expansion coefficient, mean square displacement (MSD), self-diffusivity, viscosity, electrical conductivity, and transference numbers have been computed for the studied AACBILs in different temperatures and at 0.1 MPa. There is a satisfactory agreement between the calculated data with the corresponding experimental values where they were available. Structural properties including radial distribution functions and spatial distribution functions of cations and anions were investigated. The results showed that because of the presence of an amine group away from the carboxylate group and also the absence of the planar phenyl group in the anion, the interactions between ionic pairs in [CH][β-Ala] are stronger than interactions between ions in [CH][Ala] and [CH][Phe]. The results showed that the order of diffusions and electrical conductivities is [CH][Ala] > [CH][β-Ala] > [CH][Phe], which can be interpreted by different electrostatic, van der Waals, and hydrogen interactions in these ILs. Our study provides considerable molecular insight into the structural features and dynamics of these biodegradable ILs.

In this work, the structural and dynamical properties of two imidazolium-based geminal dicationic ionic liquids (GDILs), i.e. [C(mim)][NTf] with n = 3 and 5, have been studied to obtain a fundamental understanding of the molecular basis of the macroscopic and microscopic properties of the bulk liquid phase. To achieve this purpose, molecular dynamics (MD) simulation, density functional theory (DFT) and atoms in molecule (AIM) methods were used. Interaction energies, charge transfers and hydrogen bonds between the cation and anions of each studied GDIL were investigated by DFT calculations and also AIM. The mean square displacement (MSD), self-diffusion coefficient, and transference number of the cation and anions, and also the density, viscosity and electrical conductivity of the studied GDILs, were computed at 333.15 K and at 1 atm. The simulated values were in good agreement with the experimental data. The effect of linkage alkyl chain length on the thermodynamic, transport and structural properties of these GDILs has been investigated. The structural features of these GDILs were characterized by calculating the partial site-site radial distribution functions (RDFs) and spatial distribution functions (SDFs). The heterogeneity order parameter (HOP) has been used to describe the spatial structures of these GDILs and the distribution of the angles formed between two cation heads and the middle carbon atom of the linkage alkyl chain was analyzed in these ILs. To investigate the temporal heterogeneity of the studied GDILs, the deviation of the self-part of the van Hove correlation function, G(r[combining right harpoon above],t), from the Gaussian distribution of particle displacement and also the second-order non-Gaussian parameter, α(t), were used. Since, the transport and interfacial properties and ionic characteristics of these GDILs were studied experimentally in our previous studies as a function of linkage chain length and temperature, in this work, we try to give a better perspective of the structure and dynamics of these systems at a molecular level.

Three imidazolium-based linear tricationic ionic liquids (LTILs) have been simulated to study their structural and dynamical properties and obtain a fundamental understanding of the molecular basis of the microscopic and macroscopic properties of their bulk liquid phase. The effects of temperature and alkyl chain length on the physiochemical, transport, and structural properties of these LTILs have been investigated. A nonpolarizable all-atom force field, which is a refined version of the Canongia Lopes and Paudua force field, was adopted for the simulations. Densities, mean square displacements, self-diffusivities, viscosities, electrical conductivities, and transference numbers have been presented for various ions from MD simulations. The detailed microscopic structures have been discussed in terms of radial distribution functions and spatial distribution functions. The results show that, similar to that in monocationic and dicationic ILs (MILs and DILs, respectively), the anions are mainly organized around the imidazolium rings. The diffusion coefficients of the studied LTILs are smaller than those of both MILs and DILs, with comparable viscosities. Unlike those of MILs and DILs, the diffusion coefficients of the cations and anions of the studied LTILs increase with an increase in the length of the alkyl chain between the rings for LTIL-1 and LTIL-2 but then decrease for LTIL-3, which is in a good agreement with the trend of viscosity data. The calculated transference numbers show that, similar to that in MILs and DILs, cations have a major role in carrying electric current in LTILs, but this role increases from MILs to LTILs.