The viscosity and structure of solutions. Part 3.—Interpretation of the thermodynamic activation parameters for propan-1-ol–water–electrolyte systems

Abstract

The molar contributions of the electrolyte Δµ^⊖≠₃, ΔH^⊖≠₃ and ΔS^⊖≠₃ to the activation parameters for viscous flow of the ternary systems propan-1-ol-water–electrolyte are analysed in terms of the new approach given in the first part of this series. The thermodynamic activation parameters for the binary solvent system are first briefly discussed. The excess enthalpy for the transition state is calculated and resolved into the corresponding partial molar enthalpies; these functions are compared with the corresponding ground-state functions. The thermodynamics and quasi-thermodynamics are consistent with the retention of an aqueous structure as propanol is added to water, up to a volume fraction ϕ₂ of ca. 0.25, followed by a breakdown to a structure of lower order. The mean entropy of activation ΔS^≠₁₂ passes through a maximum at ϕ₂ of ca. 0.25 and is taken as a measure of the extent to which the transition-state mixtures are in turn broken down with respect to the ground state. Minima in Δµ^⊖≠₃ near ϕ₂≈ 0.25 are observed for all the salts at both temperatures. In solvents having an aqueous type of structure, coordination of the ions is incomplete in the ground state but increases in the less strongly bonded transition state; the increase in coordination is a maximum and Δµ^⊖≠₃ a minimum when the transition-state mixed solvent is most broken up with respect to the ground-state solvent. The minima are least pronounced and at lowest ϕ₂ for the chloride of the strongly coordinating Li⁺. For ϕ₂ > 0.25 the Δµ^⊖≠₃ increase to the values characteristic of typical non-aqueous solvents, in which ground-state coordination is complete and formation of the transition state is characterised by breaking rather than making ion–solvent bonds. The enthalpies of transfer of the electrolytes in the transition state, ΔH^⊖′_t, are related to the thermodynamics of the transition-state solvent by the theory of de Valera, Feakins and Waghorne. Though crude, the approach shows why the minima in ΔH^⊖≠₃, unlike those in Δµ^⊖≠₃, occur at widely different ϕ₂ for different electrolytes, and demonstrates the power of considering the special properties of the solvent in the transition state.