Modelling the effect of pressure on the rates of ionic and polar reactions

Abstract

The rate k_P of ionic and polar reactions in solutions is known to be sensitive to the applied pressure P. In the present work, structural and dielectric contributions to k_P are expressed explicitly as functions of P. The pressure dependence of the volume and the dielectric constant of the solvent are described by the Tait equation. The model predicts the relation ln (k_P/k₀)=–νP/B–µ ln(1 +P/B) where ν, µ and B are adjustable parameters. This expression fits remarkably well the kinetic data of 11 ionic and polar reactions up to 45 kbar. The resulting activation volume is ΔV^{[graphic omitted]}_P=ΔV^{[graphic omitted]}_∞+(ΔV^{[graphic omitted]}₀–ΔV^{[graphic omitted]}_∞)/(1 +P/B) where the limiting values of ΔV^{[graphic omitted]}_P at zero and infinite pressures are expressed in terms of intrinsic volume changes in the reactants and in the surrounding solvent, and by the strength of the electrostatic interaction. The results suggest that for ionic and polar reactions, the structural and the dielectric contributions to the activation volume can be separated objectively, using a single solvent. The meaning and the significance of the various parameters are discussed.