Influence of organic solutes on the self-diffusion of water as studied by nuclear magnetic resonance spectroscopy

Abstract

The self-diffusion coefficient of water and of the organic solute have been measured for aqueous solutions of tertiary butanol and tertiary alkyl ammonium chlorides (C_nH_2n+1)₄NCl, n= 1–4, as a function of solute concentration at 25 °C with the pulsed field gradient Fourier-transform n.m.r. technique. The decrease in the water diffusion coefficient with increasing solute concentration is interpreted with a model which includes the obstruction from the solute particles to the diffusion of water and the hydration of the solute particles. The decrease of the obstruction effect due to particle motion is accounted for by a correction which gives the model the proper limiting behaviour. Hydration numbers for the organic solutes have been calculated. The reduction of the water diffusion coefficient with increasing solute concentration can be explained by assuming that less than a monolayer of water molecules are associated with the surface. For solutions of (C₃H₇)₄NCl and (C₄H₉)₄NCl inclusion of the obstruction effects from the solute particles is essential to explain the concentration dependence of the water diffusion coefficient. The solute diffusion coefficient at infinite dilution agrees with predictions of the Stokes–Einstein equation for the hydrated solute particle.