Diffusion and cluster formation in aqueous solutions of potassium aluminum sulfate

The property and structure of aqueous KAl(SO4)(2) . 12H(2)O solutions were investigated by the measurements of diffusion coefficients and concentration gradients developed in vertical columns of supersaturated solutions. The diffusion coefficients decreased linearly with increasing concentration up to the saturation point and decreased more rapidly with increasing concentration in the supersaturated region. This drastic decrease of diffusion coefficient at supersaturated concentrations was similar to that observed in other supersaturated aqueous solutions which was attributed to the formation of solute clusters in metastable solutions. A cluster diffusion model employing the solution viscosity and thermodynamic data can provide a good correlation between predicted and experimental diffusion coefficients in undersaturated solutions, but fails to provide a reasonable prediction of the much lower values of diffusion coefficients in supersaturated solutions. The average size of the diffusing entities estimated from the diffusion coefficient data showed a steady growth with increasing concentration in the undersaturated region and a faster growth with increasing concentration in the supersaturated region. The average cluster sizes in supersaturated solutions were estimated from the data of concentration gradients in vertical columns and solution thermodynamics using the concept of the number and weight average molecular weights. The estimated cluster size increased with increasing degree of supersaturation as well as solution ''age'', and showed good agreement with those estimated from the diffusion coefficient data.