Hydration of Ions in Organic Solvent and Its Significance in the Gibbs Energy of Ion Transfer between Two Immiscible Liquids

Various kinds of cations (alkali and alkaline earth metal ions, Me 4 N +, Et 4 N +, n -Bu 4 N +, and tetraphenylarsonium ion) and anions (halide ions, SCN -, ClO 4 -, NO 3 -, and tetraphenylborate ion) have been extracted from water to nitrobenzene (NB) using several extractants: viz. tetraphenylborate and dipicrylaminate for the cations; n -Bu 4 N +, n -Pen 4 N +, n -Hep 4 N +, and tris(1,10-phenanthroline)iron(II) for the anions. The determination of water content in NB by means of the Karl Fischer method has confirmed that some water molecules are coextracted by hydrophilic inorganic cations and anions. Accurate numbers ( n ) of the coextracted water molecules per ion have been established. On the basis of these findings, a new model has been proposed for a better understanding of the Gibbs energy of ion transfer at the organic solvent/water interface. If hydrated radii of ions evaluated from n are used, conventional (Born-type) electrostatic solvation models are invalid. A new approach recognizing short-range ion−solvent interactions (e.g. hydrogen bonds) has given a better account of for the hydrated ions.