Solvent Electrostatic and Covalent Effects on Complexation Thermodynamics

A new method is suggested for estimating the electrostatic and covalent contributions to the standard Gibbs energy, enthalpy and entropy of complexation reactions upon transfer from water (W) to non-aqueous and mixed aqueous organic solvents (S). The equations derived for calculation of the electrostatic (temperature dependent) contributions are based on the temperature dependences of thermodynamic parameters of complexation in aqueous solution and the temperature dependence of the dielectric constant of water. These contributions correspond to the transfer process in water from T₁ = 298.15 K to a higher temperature (T₂) at which water has the same dielectric constant as does the solvent S at 298.15 K (εW(T₂) = εS(T₁)). The covalent (temperature independent) contributions are calculated at isodielectric conditions (transfer from water at T₂ to S at T₁) using the corresponding thermodynamic cycle. Application of the model to an analysis of solvent effects is demonstrated and discussed with an example given of a typical complexation reaction between K⁺ and 18-crown-6 in pure non-aqueous and water-acetonitrile mixed solvents.