2.7.4 Interaction of Water with Ions and Ionic Groups
Ions and ionic groups of organic molecules hinder mobility of water molecules to a greater degree than do any other types of solutes. The strength of water-ion bonds is greater than that of water-water hydrogen bonds, but is much less than that of covalent bonds.
The normal structure of pure water (based on a hydrogen-bonded, tetrahedral arrangement) is disrupted by the addition of dissociable solutes. Water and simple inorganic ions undergo dipole-ion interactions. The example in Figure 8 involves hydration of the NaCl ion pair. Only first-layer water molecules in the plane of the paper are illustrated. In a dilute solution of ions in water, second-layer water is believed to exist in a structurally perturbed state because of conflicting structural influences of first-layer water and the more distant, tetrahedrally oriented “bulk-phase” water. In concentrated salt solutions, bulk-phase water would not exist and water structure would be dominated by the ions.
There is abundant evidence indicating that some ions in dilute aqueous solution have a net structure-breaking effect (solution is more fluid than pure water), whereas others have a net structure-forming effect (solution is less fluid than pure water). It should be understood that the term “net structure” refers to all kinds of structures, either normal or new types of water structure. From the standpoint of “normal” water structure, all ions are disruptive.