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Chapter 7 provides concluding remarks, as well as a perspective on unsettled issues and expected progresses in the field. The authors are of the opinion that the experimentally-elusive nature of the quantities H,svt, H,svt and χsvt is not a mere consequence of a high level of technical difficulty involved in their measurement, although this difficulty certainly contributes to the challenge, but rather from a fundamental problem: bulk and surface effects are not unambiguously partitionable. Accepting that this ambiguity is a fundamental restriction imposed by the laws of physics, the concept of extra-thermodynamic assumption takes a different dimension, namely that of an arbitrary definition for this partitioning. The merit of theoretical approaches is to permit the formulation of this definition in the most fundamental context possible, i.e. as an ultimate extra-thermodynamic assumption. Adopting this line of thinking, an exact first-principles quantum-mechanical computation or a fully consistent application of the atomistic-consistency assumption in classical simulations cannot deliver assumption-free estimates for the experimentally-elusive quantities H,svt, H,svt and χsvt, i.e. there must still be some extra-thermodynamic assumption involved in the interpretation of the corresponding results. This assumption usually takes the form of a choice made concerning whether the electric potential is probed by a test charge without or with excluded volume or, equivalently, whether atoms or molecules are viewed as the source entities for this potential. The latter choice (test charge with excluded volume, molecules as sources of electric potential), referred to as the external definition, is recommended as the most “reasonable” one, because it corresponds to the assumption implicitly sustained by continuum-electrostatics calculations as well as intuitive chemical reasoning. Two other important and relatively intricate issues are also addressed in this chapter, namely whether: (i) the pressure or/and temperature derivatives of experimentally-elusive properties are also elusive; (ii) differences in experimentally-elusive properties between different solvents are also elusive. After a brief discussion of these two issues, the authors express the opinion that the answer to both questions is probably affirmative. The chapter closes on a discussion of the three main field-specific plagues of ionic thermodynamics, namely: (i) the incomplete or ambiguous definitions commonly adopted for some central concepts; (ii) the existence of numerous redundant parameters encompassing the same physical information in a different format; (iii) the plethora of alternative standard-state definitions.

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