Editing Intermolecular force (section)

==Salt bridge==
{{Main|Salt bridge (protein and supramolecular)}}

The attraction between cationic and anionic sites is a noncovalent, or intermolecular interaction which is usually referred to as ion pairing or salt bridge.<ref>{{cite book | veditors = Ciferri A, Perico A |title=Ionic Interactions in Natural and Synthetic Macromolecules |date=2012 |publisher=John Wiley & Sons, Inc. |location=Hoboken, NJ |isbn=978-0-470-52927-0}}</ref>
It is essentially due to electrostatic forces, although in aqueous medium the association is driven by entropy and often even endothermic.  Most salts form crystals with characteristic distances between the ions; in contrast to many other noncovalent interactions, salt bridges are not directional and show in the solid state usually contact determined only by the van der Waals radii of the ions.
Inorganic as well as organic ions display in water at moderate ionic strength I similar salt bridge as association ΔG values around 5 to 6 kJ/mol for a 1:1 combination of anion and cation, almost independent of the nature (size, polarizability, etc.) of the ions.<ref>{{cite journal | vauthors = Biedermann F, Schneider HJ | title = Experimental Binding Energies in Supramolecular Complexes | journal = Chemical Reviews | volume = 116 | issue = 9 | pages = 5216–5300 | date = May 2016 | pmid = 27136957 | doi = 10.1021/acs.chemrev.5b00583 }}</ref> The ΔG values are additive and approximately a linear function of the charges, the interaction of e.g. a doubly charged phosphate anion with a single charged ammonium cation accounts for about 2x5 = 10 kJ/mol. The ΔG  values depend on the ionic strength I of the solution, as described by the Debye-Hückel equation, at zero ionic strength one observes ΔG = 8 kJ/mol.