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Ions are ubiquitous in nature and play a fundamental role in many essential ecological, biochemical, physiological and technological processes. Under the conditions of pressure and temperature prevailing on earth, ions would probably not play such an important role if their presence was not associated with that of large amounts of water. In the absence of water, at room temperature and ambient pressure, the extremely strong Coulombic interactions existing between oppositely-charged ions promote their aggregation into solid salts, exceptionally ionic liquids. The magnitude of these interactions is such that gas-phase ions are essentially non-existing species at equilibrium. Yet, in polar solvents, prominently water, dielectric screening of these direct Coulombic interactions by the dipolar solvent molecules represents an almost equally strong opposing force that permits, in synergy with entropic effects, the existence of solvated ions as dissociated entities. The properties of ions in solution crucially depend on the nature and magnitude of these solvation forces. Their characterization is thus an essential step towards the understanding of the properties of ions in solution. The key quantity accounting for the magnitude of ion-solvent interactions, in the simplest context of an individual ion at infinite dilution in a given solvent, is the corresponding single-ion solvation free energy, knowledge of the pressure and temperature dependence of which also gives access to all other derivative thermodynamic solvation parameters, e.g. the single-ion solvation enthalpy, entropy, volume, heat capacity, compressibility or expansivity, as well as to the corresponding partial molar variables in solution. The determination of single-ion solvation free energies and of their derivatives via experimental measurements or theoretical calculations is an issue that has preoccupied the physical chemistry community for more than one century. It is a very fundamental problem, which could well be viewed as the “hydrogen-atom problem” of ion thermodynamics and electrochemistry. Yet, this apparently simple issue hides an unexpected amount of complexity and truly represents a challenge, which cannot be claimed to have found a satisfactory solution even nowadays. The goal of Chapter 1 is to introduce the topic in the broad context of physics and chemistry, and to provide an overview of the aim, scope and content of the book.

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