Water diffusivity in a mixed phase binary system: Effective water diffusion coefficients in tetradecanol–eicosanol mixtures

Abstract

Fourier-transform infrared attenuated total reflection spectroscopy is used to measure the effective water diffusion coefficients in pure tetradecanol between 313K and 338K and in liquid and liquid–solid mixtures of tetradecanol (C14H29OH) and eicosanol (C20H41OH) at 313K. The temperature dependent data in tetradecanol yields an activation energy for diffusion higher than that measured in pure alkanes. This is consistent with the critical role hydrogen bonds play in diffusion. Differential scanning calorimetry and infrared spectroscopy are used to construct the solid–liquid phase diagram for the tetradecanol–eicosanol system. Diffusion measurements in these mixtures are linked to the experimental phase diagram. Water diffusivity in liquid mixtures at low eicosanol concentrations (80 to 100mol percent tetradecanol) suggests that the measured diffusion coefficient of water is independent of the mixture's bulk viscosity. We find a six-fold decrease in water diffusivity when the solid fraction of the equilibrium mixture increases from zero to thirty percent. The decrease in water diffusivity results from a longer diffusion path length caused by solid portions of the mixture acting as barriers to diffusion.