Theoretical Studies Using Simple Ionic Liquid Models

Abstract: Room Temperature Ionic Liquids (RTILs) are organic salts that melt below 100 degrees Celsius. The advantages of RTILs, such as high ionic concentrations and low melting points, promote several potential applications. For instance, RTILs could be utilized in manufacturing energy storage devices, and as efficient solvents. In this work, an Asymmetric Restricted Primitive Model (ARPM) of electrolytes is proposed as a simple three parameter (charge q, diameter d and charge displacement b) model of ionic liquids and solutions. It should be noted the charge is actually not an adjustable parameter for ionic liquids, since they only contain monovalent. In contrast to the well-known RPM model, this newly added charge displacement allows electrostatic and steric interactions to operate between different centres, so that orientational correlations contribute to ion-ion interactions. Based on the ARPM model, our first work utilized Monte Carlo (MC) and Molecular Dynamics (MD) simulations to explore the ion pair formation mechanism and the corresponding phase behavior in bulk systems. By adjusting the parameter b, the relative concentration of paired and free ions will change. This has consequences for the cohesive energy, and the tendency to form a fluid or a solid phase. We also investigate dielectric behaviours of corresponding liquids, composed of purely dipolar species. As one of the most important results, we found that many basic features of ionic liquids appear to be remarkably consistent with those of our ARPM at ambient conditions, when b is around 1 Ångstrom (d = 5 Å). In two subsequent works, we mainly focused on ARPM (b = 1 Å) ionic liquids in a heterogeneous system, i.e. near an electrode surface. Both MC simulations and Classical Density Functional Theory (DFT) were employed to probe the electrochemical properties, such as charge density distribution, surface potential and differential capacitance. We used both explicit and implicit solvent models. Results show that the capacitances, achieved by different combinations of solvent model and method, display a symmetric (Bactrian) camel-hump profile. In the final work, we use DFT to study structures, phase transitions, and electrochemical behaviours of two other simple and coarse-grained ionic fluid models, in the presence of a perfectly conducting electrode. In both models, the cation is able to approach the electrode interface more closely than the anions, which renders the former to be preferentially adsorbed at a neutral electrode surface. However, for a positively charge electrode, there is an effective competition between the two components at the interfaces. This is, according to our DFT formulation, sufficient to generate a structural phase transition, which has a large impact on the electrochemical behaviour.

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