Classical density functional theory & simulations on a coarse-grained model of aromatic ionic liquids.

A new classical density functional approach is developed to accurately treat a coarse-grained model of room temperature aromatic ionic liquids. Our major innovation is the introduction of charge-charge correlations, which are treated in a simple phenomenological way. We test this theory on a generic coarse-grained model for aromatic RTILs with oligomeric forms for both cations and anions, approximating 1-alkyl-3-methyl imidazoliums and BF4(-), respectively. We find that predictions by the new density functional theory for fluid structures at charged surfaces are very accurate, as compared with molecular dynamics simulations, across a range of surface charge densities and lengths of the alkyl chain. Predictions of interactions between charged surfaces are also presented

Coarse-Grained Models of Ionic Solutions

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'

Classical Density Functional Theory of Ionic Solutions

The basic structure of classical density functional theory (DFT) is reviewed from a rather general perspective. The treatment is then specialized to ionic solutions, describing the various possible extensions beyond the Poisson–Boltzmann level, that DFT offers, such as excluded volume effects, non-electrostatic interactions, connectivity (polymers) and ion correlations. The last effects are discussed rather thoroughly, with several explicit illustrations

Capillary Condensation of Ionic Liquid Solutions in Porous Electrodes

We use classical density functional theory to investigate room temperature ionic liquid + solvent mixtures in porous electrodes. We consider those mixtures that display a miscibility gap in the bulk fluid and hence have the capacity to undergo capillary condensation in pores which preferentially attract the ionic liquid component. A novel aspect of this transition is that, when the system is at or near the critical point for this transition, we find an extraordinary increase in the capacitance, which derives from the fact that the capacitance is a thermodynamic response function, diverging at spinodal points

On the stability of aqueous dispersions containing conducting colloidal particles.

We use a combination of simulations and a simple theoretical approach to investigate interactions between neutral conducting surfaces, immersed in an electrolyte solution. The study is conducted at the primitive model level, which necessitates the use of multiple image reflections. Our approximate theory is based on a classical density functional formulation of Poisson-Boltzmann theory. The same approach can in principle also be imported to more advanced treatments, where ion correlations are accounted for. An important limiting result that guides our treatment of the image forces, is that the repulsive salt-induced interactions cancel the attractive zero frequency van der Waals attraction at long range. That is, at vanishing frequency, the van der Waals interaction between the conducting surfaces is, at large separations, perfectly screened by the intervening salt solution. The simulations are computationally intensive, due to a strong dependence upon the number of image reflections used, with especially poor convergence when an odd number of images is used. We demonstrate that our approximate density functional approach is remarkably accurate, even in the presence of a 2 : 1 salt, or when the surfaces preferentially adsorb one ion species. The former observation was rather unexpected, given the lack of ion correlations within our mean-field treatment, and is most likely due to a cancellation between two opposing effects, both of which are generated by ion correlations

Theoretical Prediction of the Capacitance of Ionic Liquid Films

We use classical density functional theory to investigate ionic liquid + solvent mixtures against smooth, model electrodes. We consider those mixtures that display a wetting transition at the electrodes. We find that a wetting film of an ionic liquid at the electrodes has similar properties to the neat liquid. A novel aspect of this transition is that, close to a surface critical point, we find an extraordinary increase in the capacitance, which is derived from the critical behavior