Coarse-Grained Models of Ionic Solutions

Abstract: Popular Abstract in English Typically, when we speak of ionic solutions we mean a salt (as solute) dissolved in some liquid (the solvent). In this thesis, we predominantly consider ionic solutions for which there is no solvent---room-temperature ionic liquids (RTILs). It follows that these liquids have a large concentration of charged particles, a property that causes them to be effective electrolytes in capacitors. A capacitor works as a rechargeable battery, storing a net charge (an excess adsorption of positive or negative particles) onto the surface of an electrode when a voltage is applied, and releasing them when connected to a circuit, creating a current to power some electrical components. The nature of this adsorption (or the `electric double layer') is therefore of interest to us. Another appealing feature of RTILs is the ability to control their physical properties by changing their chemical structures. Synthesizing ionic liquids is expensive, and there are vast numbers of possible compounds so there is a necessity to describe these chemicals theoretically, to explain and predict their behaviour. However, the old theories of electrolyte-electrode behaviour fail under these high concentrations. Computer simulations of RTILs with all-atom forcefields can be very time-consuming, so to save time and gain a broader understanding, we can use less detailed, `coarse-grained' models. These simple models also allow easy implementation into a classical density functional theory framework, an approximate semi-analytical theory capable of delivering results of comparable accuracy to simulation, but at a fraction of the time.