Ion Hydration and Effective Charge
Pincus, Philip A.
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Ions in water are typically understood to be hydrated, i.e. the appropriate water dipolar orientations point toward the charge (which is here taken to be of spherical symmetry). Depending on the specific charge, these oriented waters of hydration are tightly bound in up to three aqueous monolayers. The effective “hard core” radius of the ion together with its tightly bound water molecules is in the nanometer range. Because dipoles have zero net charge, the hydrated ion complex is generally viewed to have the samec harge as the unhydrated ion. We apply the Hone, Pincus1 view of hydrogen bonding in water to suggest that this may not be precisely correct and that there may be an effective charge which differs from the nominal charge. It is this effective charge that would be implicated in such phenomena as Debye screening.