The Ionic Cavity. Current and Future Work

At the same time, Levin demonstrated that the discrepancy in theoretical modeling of surface tension could be attributed to the energy of the cavity required to be cut out of water in order to accommodate the ion. The cavity energy provides the force simulated by Jungwirth and Tobias that pushes large ions toward the air-water interface. Duignan et al. combined the cavity energy with the cavity-based theory of the ionic van der Waals energy developed by Sambale et al., reproducing experimental solvation energies well. Significantly, the model was able to successfully reproduce the anomalous solvation energies of Cu+ and Ag+.

It is also significant that this cavity-based model did not require the addition of an explicit hydration shell for cosmotropic ions like Li+ or Na+. Ion-water interactions are expressed via the cavity and van der Waals energies. The cavity van der Waals model also reproduced experimental partial molar volumes and ion entropies and osmotic coefficients as well as surface tension increments, including that of acids.

Duignan’s model incorporated the finite ion size by way of the cavity radius but applied a point polarisability that required multipolar dispersion energies to be included. More recently Fiedler et al. presented a new cavity van der Waals model, which incorporates the finite size of the ion as well as the finite size of the cavity. Polland and Beck have developed a description of close range ion-water interactions, which improve the modeling of the cavity energy. These models may well provide a more robust foundation for theoretical modeling of Hofmeister effects.

The ionic van der Waals interactions and cavity energies discussed here contribute to nonelectrostatic physisorption, leading to the conventional Hofmeister series which correlates with ion size (or polarisability), e.g. Cl^- < Br^- < I^- and Li+ < Na+ < K+ < Rb+ < Cs+. But anomalous series are sometimes observed, for instance in haemoglobin aggregation, glucose oxidation, or BSA protein diffusivity, where Cs+ is displaced with the series Li+ ~ Cs+ < Na+ < K+. This can be attributed to chemisorption of ions, with a reverse chemisorption series competing with the conventional physisorption series.

It is clearly important to take into account all of the complementary aspects discussed here for further progress. Ion polarisability, ionic van der Waals energies, finite ion size and ion cavity, all are key for a correct understanding of Hofmeister effects. It is likely that the ion cavity should be taken as nonrigid. In biochemical contexts, a specific kind of Hofmeister effect - specific buffer effects - must be borne in mind, with buffer leading to protein charge reversal in some cases.

The use of Hofmeister effects to develop switchable surface coatings is a current active area of application. It is also worth remarking that until now no theoretical work has included the effect of dissolved gasses properly. These gases are known to influence colloid interactions, bubble-bubble interaction, conductance, pH, and most likely the surface tension of salt solutions. It is not only CO₂ that is important but also other dissolved gasses.

New gas applications may be found in dewatering and sterilization. Finally, while we have focussed here on Hofmeister effects in water, we note that they also manifest in nonaqueous solvents. The interested reader can find more details about Hofmeister effects in different reviews.

Current and Future Work. It is worth mentioning that not only ions but also charged headgroups and even polar parts of molecules can be included in Hofmeister series. What is more, a specific property of salts, namely their solubility enhancing or diminishing behavior for organic molecules can be extended to a whole bunch of very relevant other molecules such as sugars.

Further, cryptates and extremely polarizable macroions or nanoparticles, such as polyoxometalate anions are found to be “su- perchaotropic" and hence are at the extreme end of the Hofmeister series. Together with proteins, they bridge the gap to charged nanoparticles that come into the scope of Hofmeister “objects" now. So, one of the exciting current trends with specific ion effects is their extension far beyond “simple" single charged ions even toward mesoscalic “superions.’’