Chapter 4: Concepts and Definitions

The problem of the experimental and theoretical determination of single-ion solvation free energies, as well as of corresponding derivative thermodynamic solvation properties, is relatively complex. However, it is often further complicated by imprecisions, alternatives or ambiguities in the definitions of a number of fundamental concepts. Chapter 4 aims at clarifying these concepts and definitions within thermodynamics and electrochemistry, in the context of: (i) thermodynamics (variables, basic relationships, ideal behaviors, standard states, processes and cycles relevant to ionic solvation); (ii) electrostatics (interfacial effects, electric potentials, chemical potentials, workfunctions, electrode potentials); (iii) electrochemistry (types of measurements, relationship to thermodynamic quantities and electric potentials); (iv) single-ion properties (conventional, real, and intrinsic scales). Great attention is paid here to the critical examination of the physics underlying key concepts, as well as to the clarity and consistency of the employed terminology and mathematical notation, new terms or symbols being introduced whenever necessary. Care is also taken here to precisely define and consistently apply a unique thermodynamic standard-state convention, referred to as the bbme_T convention (1 bar reference pressure for solids and liquids, 1 bar reference pressure for gases, 1 molal reference concentration for solutes, warm-electron convention with Fermi-Dirac statistics). Due to incomplete or ambiguous specification in many literature sources, alternative conventions represent an important source of complication and mistakes in the field, and can be encountered at no less than six different levels: (i) choice of a reference pressure and of a reference solution concentration; (ii) choice of a standard or density-corrected solute standard-state variant; (iii) choice of a warm- or cold-electron convention for the standard-state ideal electron gas; (iv) choice of Boltzmann or Fermi-Dirac statistics for calculating the properties of the standard-state electron and proton; (v) choice of a reference electric potential for the ideal electron gas in the definition of absolute electrode potentials; (vi) choice of a specific anchoring point for the conventional scale of single-ion solvation parameters. These include the presence of an ambiguity in the standard-state definition for solute species, namely the existence of standard and density-corrected variants. The natural variant in which a standard quantity is determined and reported depends on the type of measurement performed. However, to our knowledge, this issue is probably raised for the first time in this book, although the ambiguity affects all standard thermodynamic quantities reported in the literature concerning processes involving solute species. Note, finally, that the physics of the solvation process is most directly characterized by what is referred to as semi-standard point-to-point solvation parameters, which characterize the transfer of the ion from a fixed point in the gas phase to a fixed point in solution and are exempt of standard-state bias, besides the specification of a reference pressure and temperature for the solvent properties.