Chapter 2: Fundamental Experimental Problems

The experimental determination of single-ion solvation free energies, as well as of corresponding derivative thermodynamic solvation parameters, is complicated by two fundamental problems: (i) the local electroneutrality of macroscopic matter at equilibrium and its corollary, the absence of free gas-phase ions in equilibrium situations; (ii) the presence of a surface polarization at air-liquid interfaces. Chapter 2 discusses these two problems in sequence, and provides a summary of their practical implications. The local electroneutrality constraint implies that single-ion solvation parameters are only directly accessible, via calorimetry and gas-phase spectroscopy-based statistical mechanics, in the form of sums over neutral ion combinations or, equivalently, as conventional (relative) values, i.e. with the proton parameters set to zero by definition. Absolute single-ion solvation parameters can also be obtained via Voltaic cell experiments and workfunction measurements. However, the presence of surface polarization effects at air-liquid interfaces implies that these determinations lead to real values, i.e. including the reversible work of interface crossing. The absolute single-ion solvation parameters that are of greatest theoretical relevance are intrinsic, i.e. solely characterizing the interaction of the ion with its polarized solvent environment, without contamination from surface effects. They cannot be accessed on the sole basis of experimental data and require the introduction of some apparently reasonable but formally unprovable postulate or model, called in this context an extra-thermodynamic assumption. The introduction of a specific assumption permits to anchor the conventional scale by providing estimates for either of three equivalent experimentally-elusive solvent-dependent quantities, namely the intrinsic solvation free energy _H,svt of the proton, the intrinsic absolute potential _H,svt of the reference hydrogen electrode, or the air-liquid interfacial potential χ_svt, along with their pressure or/and temperature derivatives. Unfortunately, considering the case of water, the use of different experimental approaches along with distinct extra-thermodynamic assumptions leads to a very large uncertainty range on the order of 0.5−1.0 V (potentials) or 50−100 kJ·mol⁻¹ (free energy) in the estimated values for the three above quantities.