Hydration number

A sodium cation is solvated by water molecules with their partially negative charged lone pairs pointing inwards towards the positively charged sodium ion

The hydration number of a compound is defined as the number of molecules of water bonded to a central ion, often a metal cation. The hydration number is related to the broader concept of solvation number, the number of solvent molecules bonded to a central atom. The hydration number varies with the atom or ion of interest.

In aqueous solution, solutes interact with water molecules to varying degrees. Metal cations form aquo complexes, wherein the oxygen of water bind to the cation. This first coordination sphere is encased in further solvation shells, whereby water bonds to the coordinated water via hydrogen bonding. For charged species, the orientation of water molecules around the solute dependent on its radius and charge,[1] with cations attracting water’s electronegative oxygen and anions attracting the hydrogens. Uncharged compounds such as methane can also be solvated by water and also have a hydration number. Although solvation shells can contain inner and outer shell solvent-solute interactions, the hydration number generally focuses on the inner shell solvent molecules that directly interact with the solute.[2]

A variety of definitions exist for hydration number. One such approach counts the number of water molecules bound to the compound more strongly (by 13.3 kcal/mol or more) than they are bound to other water molecules.[3] Hydration number estimates are not limited to integer values (for instance, estimates for sodium include 4, 4.6, 5.3, 5.5, 5.6, 6, 6.5, and 8), with some of the spread of estimated values being due to differing detection methods.[4]

  1. ^ Vaslow, Fred (1963). "The Orientation of Water Molecules in the Field of an Alkali Ion". The Journal of Physical Chemistry. 67 (12): 2773–2776. doi:10.1021/j100806a063.
  2. ^ Rempe, Susan B.; Pratt, Lawrence R. (2001). "The hydration number of Na+ in liquid water". Fluid Phase Equilibria. 183–184: 121–132. arXiv:physics/0006026. doi:10.1016/s0378-3812(01)00426-5. S2CID 1282292.
  3. ^ Zavitsas, Andreas A. (2016). "Some opinions of an innocent bystander regarding the Hofmeister series". Current Opinion in Colloid & Interface Science. 23: 72–81. doi:10.1016/j.cocis.2016.06.012.
  4. ^ Mähler, Johan; Persson, Ingmar (2 January 2012). "A Study of the Hydration of the Alkali Metal Ions in Aqueous Solution". Inorganic Chemistry. 51 (1): 425–438. doi:10.1021/ic2018693. PMC 3250073. PMID 22168370.