Specific ion effects in physicochemical and biological systems: Simulations, theory and experiments

Charged surfaces and ion–water interactions at an interface play a decisive role in many physico-chemical and biological processes. The classical treatment of ions at charged interfaces is the Poisson–Boltzmann (PB) theory. Despite severe simplifying assumptions it describes surprisingly well univalent ions not too close to the interface for low electrolyte concentrations in the mmol regime. However, it breaks down in the vicinity of the interface at higher surface charge densities. Consequently the list of decorations and modifications of the original PB equation is long aiming for a more realistic picture. 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. Specific ion effects can be found everywhere in chemistry and biology and there are many reports of pronounced differences in the properties of charged monolayers, micelles, vesicles, dispersions or polyelectrolyte multilayers using different identically charged counterions. The so-called “counterion effect” is usually discussed in terms of the Hofmeister series for cations or anions which are the result of a subtle balance of several competing evenly matched interactions. The complex interplay of electrostatics, dispersion forces, thermal motion, polarization, fluctuations, hydration, ion size effects and the impact of interfacial water structure makes it hard to identify a universal law. The diversity of specific ion effects is a direct consequence of this subtle interplay of forces and imposes a true challenge for the theories. The decisive information for an assessment of the theories is knowledge of the prevailing ion distribution. Hence a considerable amount of work has been applied to develop suitable model systems and to push surface characterization tools such as (resonant) X-ray reflectivity, total reflection X-ray fluorescence or X-ray standing waves to new limits. These techniques give direct information on the ions and on the interfacial architecture. A second alternative to complement these studies is infrared–visible sum frequency spectroscopy allowing to record surface vibrational spectra of the water as it is perturbed in the presence of the salts. The paper is organized in sections describing various facets of ion specific effects discussed within the network.