Diffusion coefficients of aqueous phenols determined by the Taylor dispersion technique. Evidence for solute adsorption on the walls of Teflon tubing

Abstract

The Taylor dispersion technique has been used to determine the diffusion coefficients of the m-alkoxyphenol and alkyl p-hydroxybenzoate homologous series in aqueous 5 × 10^–3 mol dm^–3 NaOH solutions and water–ethanol mixtures. The deviations from the expected behaviour for the higher homologues increase with the eluent water content and are attributed to solute adsorption on the walls of the Teflon dispersion tube. However, the adsorption corrections performed using Golay's equation for capillary chromatography do not account for all the diffusion coefficient deviations. The experimental solute hydrodynamic radii, calculated through the Stokes–Einstein equation, decrease as the eluent ethanol content is increased. These radii are smaller than those estimated by using atomic contributions or by adding the atomic van der Waals radii to the solute optimized geometry.