Comparison of the Activation Free Energies for Viscous Flow and for Diffusion in Dilute Solutions, Derivation of the Expression for the Effect of the Solute on the Activation Free Energy of Diffusion of the Solvent, and Application to Solutions of N,

摘要

The effect of a solute on the viscosity of a solution is determined by both its intrinsic rate of motion and its effect on the rate of flow of surrounding solvent molecules. Thus, the contribution of the solute to the activation free energy for viscous flow is the sum of tow terms, one for each of these. In the viscosity experiment it is not possible to resolve these contributions unambiguously. In contrast it is possible to measure both the diffusion coefficient of the solute and the effect of the solute on the diffusion coefficient of the solvent. In this paper we report viscosity B coefficients and diffusion data for N,N-dimethylformamide in water, methanol, and acetonitrile and solvent diffusion for each of these as a function of solute concentration. The solvent diffusion data are found to obey an equation analogous to that of Jones and Dole for the solution viscosity. We also present a relationship for calculating the solute contribution to the activation free energy for diffusion of the solvent. Those for the diffusion of the solvent and for the solutes contribution to the activation free energy for viscous flow are calculated in the usual ways. Comparison of these activation free energies shows that the solute contribution to the free energy of activation for viscous flow is approximately equal to the sum of those for diffusion of the solute and that for the solute's effect on the diffusion of the solvent molecules. It is also found that the solute's affect on the motion of the solvent plays a major role in the aqueous system but makes only a minor contribution in the nonaqueous solvents.