The thermodynamics of solvation of ions. Part 1. - The heat capacity of hydration at 298.15 K

Values of the standard partial molar heat capacities of aqueous electrolytes have been critically selected from the literature and have been divided into ionic contributions using the assumption that C̄_p^∞ (Ph₄P⁺) = C̄_p^∞ (BPh₄^-). Combination of the single-ion C̄_p^∞ values with C°_p values for gaseous ions then yields single-ion values for the standard molar heat capacities of hydration, Δ_hydC°_p. Ions of various classes are considered: univalent and multivalent, monoatomic and polyatomic, hydrophilic and hydrophobic. The ionic values of Δ_hydC°_p are analysed in terms of a model in which an ion is surrounded by a first layer of immobilised solvent, a possible second layer, and then the bulk structured solvent. The Δ_hydC°_p values are broken down into a neutral term, N, an electrostatic term, E, and a configurational term, C. Two possible modes of summation are discussed, stressing in the term C the effects of the orientation of the solvent near the ion, and the effects on the fluidity of the solvent in the second and outer layers. Our analysis is consistent with previous discussions by the authors on the standard molar entropies of hydration of ions, and with other parameters that contain structural information, such as viscosity B-coefficients and partial molal volumes of ions in water.