4. Structure of Ionic Hydration Shells 1351 4.1. Results from Diffraction Experiments 1351 4.1.1. X-ray Diffraction 1351 4.1.2. Neutron Diffraction 1351 4.2. Results from Computer Simulations 1352 4.3. Results from Spectroscopic Measurements 1354 4.3.1. Vibrational Spectroscopic Measurements 1354 4.3.2. EXAFS Spectroscopy 1354 4.3.3. NMR Relaxation Studies 1355 4.3.4. Dielectric Relaxation Studies 1355 4.4. Summary of the Structure of Ionic Hydration Shells 1355

Effect of ions on the structure of water: structure making and breaking.

Hofmeister phenomena: an update on ion specificity in biology.

Selectivity of ion exchange membranes: A review

Despite prolonged scientific efforts to unravel the effects of ions on the structure and dynamics of water, many open questions remain, in particular concerning the spatial extent of this effect (i.e., the number of water molecules affected) and the origin of ion-specific effects. A combined terahertz and femtosecond infrared spectroscopic study of water dynamics around different ions (specifically magnesium, lithium, sodium, and cesium cations, as well as sulfate, chloride, iodide, and perchlorate anions) reveals that the effect of ions and counterions on water can be strongly interdependent and nonadditive, and in certain cases extends well beyond the first solvation shell of water molecules directly surrounding the ion.

https://science.sciencemag.org/content/sci/328/5981/1006.full.pdf

Cooperativity in Ion Hydration

The study of the interactions of salts and osmolytes with macromolecules in aqueous solution originated with experiments concerning protein precipitation more than 100 years ago. Today, these solutes are known to display recurring behavior for myriad biological and chemical processes. Such behavior depends both on the nature and concentration of the species in solution. Despite the generality of these effects, our understanding of the molecular-level details of ion and osmolyte specificity is still quite limited. Here, we review recent studies of the interactions between anions and urea with model macromolecular systems. A mechanism for specific ion effects is elucidated for aqueous systems containing charged and uncharged polymers, polypeptides, and proteins. The results clearly show that the effects of the anions are local and involve direct interactions with macromolecules and their first hydration shell. Also, a hydrogen-bonding mechanism is tested for the urea denaturation of proteins with some of these same systems. In that case, direct hydrogen bonding can be largely discounted as the key mechanism for urea stabilization of uncollapsed and/or unfolded structures.

/pdf/chemistry-of-hofmeister-anions-and-osmolytes-4bblt2re7s.pdf

Kim D. Collins

Papers

Ions in water: characterizing the forces that control chemical processes and biological structure.