Charged surfaces are omnipresent in nature and ion-water interactions at an interface play a decisive role in various physico-chemical and biological processes. Consequently, the distribution of ions at charged interfaces defines a central theme of Colloid and Interfaces Science.   Gouy and Chapman were the first who tackled this problem in a quantitative fashion. The ions were treated as point charges embedded in a continuum with given dielectric constants while the surface charge was considered to be continuously smeared out. The prevailing charge distribution generates a mean electrical potential in which the ions adopt a Boltzmann distribution. The combination of the Boltzmann distribution with the Poisson equation leads to a non-linear second order differential equation for the electric potential. The solution of the so-called Poisson-Boltzmann (PB) equation yields the number density of the counter-ions as a function of the distance to the interface. The oversimplification of the Gouy-Chapman approach were obvious from the beginning and Stern was the first who pointed out that this theory predicts unrealistic high concentration of counter-ions in the vicinity of the interface due to a neglect of the geometrical dimensions of the ions. Since then, many extension of the theory have been put forward to account for the finite size of the ions , image forces   and the dependence of the dielectric constant on the electric field or ion correlation. One striking deficiency of the treatment on the pure electrostatic level is the prediction that ions of the same valence produce the same results, independent of their chemical nature. In contrast, experiments reveal pronounced differences between different ions and any realistic theory must account for this experimental fact.

Ion specific effects in aqueous salt solutions

The most simple ion specific effect manifest in the surface tension of simple aqueous electrolyte solutions. In general, ions increase the surface tension in a specific manner. The effects are not dramatic; however, due to the simplicity of this system it is crucial for testing the theories. The traditional picture of the interface of aqueous electrolyte solutions is based on a thermodynamic analysis of the equilibrium surface tension isotherm. The increase in the equilibrium surface tension is then interpreted as an interfacial zone depleted by ions. Recently this picture has been challenged by molecular dynamics simulations using polarizable force fields which predicted that soft ions such as halides are enriched at the interface with a non-monotonic ion profiles. The key to an understanding of this apparent contradiction lies in a reconsideration of the meaning of thermodynamics. There is no a priori prediction of a profile and thermodynamics can accommodate several conflicting interfacial models provided that the integral excess or depletion is in accordance to Gibbs equation. Therefore, direct experimental observations of molecular structure and energetic of ions in the interfacial region are required.

They also give access to the vibrational features of the interfacial water which are affected by the presence of the ions. Polarization dependent measurements have been used for a determination of the orientation of the pseudo-halide anion. The combined data gives a picture of the interfacial architecture on a molecular scale. We believe our current study contributes towards better understanding of this biologically relevant chaotropic ion and water interactions at the interface. Further our work shows that the orientation of the anion is relevant and needs to be taken into account to get a full picture on the interfacial architecture.