By including excess ion polarizability into Poisson-Boltzmann theory, we show that the decrease in differential capacitance with voltage, observed for metal electrodes above a threshold potential, can be understood in terms of thickening of the double layer due to ion-induced polarizability-holes in water. We identify a new length which controls the role of excess ion polarizability in the double layer, and show that when this is comparable to the size of the effective Debye layer, ion polarizability can significantly influence the properties of the double layer.

A simple field theory approach is developed to model the properties of charged, dielectric bodies and their associated counterions. This predictive theory is able to accurately describe the properties of systems (as compared to computer simulation data) from the weak coupling limit, where the Poisson-Boltzmann theory works well, through to the strong coupling limit. In particular, it is able to quantitatively describe the attraction between like-charged plates and the influence of image charge interactions.

A theory is developed to model the behavior of mobile ions around a fixed charged distribution in the presence of dielectric bodies. By treating the short and long-wavelength fluctuations of the electric potential within different approximation schemes, this approach combines the strengths of the mean field approximation and the strong coupling expansion, while retaining the simplicity of the commonly used Poisson-Boltzmann theory. It is capable of accurately describing ion-correlation induced phenomena, such as the attraction between two like charged plates, the repulsion between two oppositely charged plates, and overcharging. The theory is compared with Monte Carlo simulation data for various systems of charged plates with their associated counterions and added electrolyte. Good agreement is found for nearly all conditions examined

Recent experiments of translocation of double-stranded DNA through nanopores [M. Wanunu et al., Nature Nanotech. 5, 160 (2009)] reveal that the DNA capture rate can be significantly influenced by a salt gradient across the pore. We show that osmotic flow combined with electrophoretic effects can quantitatively explain the experimental data on the salt-gradient dependence of the capture rate.

The dielectric interiors of colloidal particles are responsible for dispersion (van der Waals) interactions. However, these dielectric regions also alter the manner in which charges, such as on ions or other colloidal particles, interact with each other, due to the induction of charges at the dielectric interfaces. The impact of these induced charges can be represented in terms of 'image charges'. These image charges result in an ion depletion layer in the vicinity of low dielectric bodies. This depletion layer is responsible for the increase in the surface tension of water upon the addition of electrolytes. In the case of colloidal particles, this depletion layer also leads to an 'electrostatic depletion force' with a range of the order of a Bjerrum length. The relevance of this force to the salting out of proteins is discussed. This electrostatic depletion force is directly analogous to the entropically driven depletion force (due to excluded volume). Although image charge effects have been known, their influence on the behavior of colloidal systems, especially in the presence of mobile ions, has generally not been accounted for (e.g., DLVO theory). We review the previous theoretical and simulation studies of how image charges influence the properties of electrolyte and colloidal systems and discuss the relevance of these effects on experimental system