Coarse-Grained Models of Ionic Solutions

Abstract: Room-temperature ionic liquids (RTILs) are compounds composed entirely of ions, which are liquid at temperatures below 100 degrees Celsius. Their high ionic strength and strong coupling make them useful for a number of applications, e.g. as electrolytes in supercapacitors. These properties also make them interesting subjects for theoretical research, as the canonical theories of the electrical double layer fail under these conditions. We implement coarse-grained models of RTILs into Monte Carlo (MC) simulations and Classical Density Functional Theory (DFT) to investigate their behaviour at electrode surfaces. A prewetting transition is found for a dilute RTIL and solvent mixture, with a large differential capacitance spike found in the sub critical region. Investigation of capillary condensation for a similar mixture in a pore yields another differential capacitance spike, which decreases away from critical conditions. DFT and MC are compared for a new model RTIL with the former containing a new innovation in describing charge-charge correlations, which gives good agreement for fluid structures at a surface. Pressure-distance curves are computed with DFT for a homologous series of aromatic RTILs, showing the range of interactions to increase with alkyl chain length. Increasing surface charge density causes the amplitude of the interaction free energy curves to decrease. Image charge interactions are examined for a primitive model electrolyte using MC and a new image-corrected Poisson-Boltzmann DFT formulism (iPB), that record good agreement. Increasing salt concentration enhances the desolvation repulsion, and incorporating a basic model for specific adsorption predicts that a colloidal dispersion can be stabilised in this way. The final paper discusses DFT formulism in detail for ionic systems, including new results. We find that increasing the surface exclusion zone reduces the differential capacitance, raising the temperature can enhance the differential capacitance and specific adsorption of charged components diminishes the characteristic minimum of the 'camel-hump'.