Consideration of the Mass Effect on Diffusion in the Liquid State

Abstract

An investigation of the effects of mass on diffusion in liquids was undertaken and the results are presented herein. The diffusion of gold and silver in liquid mercury was measured over the temperature range -33.0 to 136.0 °C at solute (mole fraction) concentrations ≤ 4×${10}^{-7\mathrm{}}$, where solute-solute interactions are negligible. These systems were chosen for a study of the mass effect in the liquid state because all other properties likely to influence the diffusivity values are essentially identical, and both closely fulfill the requirements of simple liquids. The data, which were obtained using both the well-known capillary-reservoir technique and a new modification of the diaphragm-cell method, can be expressed in the conventional form $D=1.11\mathrm{}\times {10}^{-4\mathrm{}}\times e^{\frac{-1360\mathrm{}}{\mathrm{RT}}}$ and $D=0.96\mathrm{}\times {10}^{-4\mathrm{}}\times e^{\frac{-1309\mathrm{}}{\mathrm{RT}}}$ ${\mathrm{cm}}^2$/sec for the Ag-Hg and Au-Hg systems, respectively. Contrary to the predictions of most current theories of transport properties of liquids, the diffusivities were found to be independent of the mass of the solute. The probable reasons for the lack of a mass dependence are discussed.