Viscosity, Plasticity, and Diffusion as Examples of Absolute Reaction Rates

TLDR

The concentration of vapor above a liquid reflects the free‑energy cost of forming molecular‑sized holes, providing a measure of such “molecular” voids. The study develops new equations for absolute viscosity, diffusion coefficients, and electrical conductance based on reaction‑rate theory, aiming to replace conventional models when ion and solvent sizes are comparable. The authors derive equations for absolute viscosity and diffusion coefficient from reaction‑rate theory, applying them to experimental data such as Orr and Butler’s heavy‑into‑light water diffusion. These equations explain the law of rectilinear diameters, reduce to successful empirical formulas, account for viscosity’s increase with shear stress, and provide a satisfactory interpretation of heavy‑into‑light water diffusion data.

Abstract

Since to form a hole the size of a molecule in a liquid requires almost the same increase in free energy as to vaporize a molecule, the concentration of vapor above the liquid is a measure of such ``molecular'' holes in the liquid. This provides an explanation of the law of rectilinear diameters of Cailletet and Mathias. The theory of reaction rates yields an equation for absolute viscosity applicable to cases involving activation energies where the usual theory of energy transfer does not apply. This equation reduces to a number of the successful empirical equations under the appropriate limiting conditions. The increase of viscosity with shearing stress is explained. The same theory yields an equation for the diffusion coefficient which when combined with the viscosity and applied to the results of Orr and Butler for the diffusion of heavy into light water gives a satisfactory and suggestive interpretation. The usual theories for diffusion coefficients and absolute electrical conductance should be replaced by those developed here when ion and solvent molecule are of about the same size.