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 co ncentration gradients developed in vertical columns of supersaturated solutions. The diffusion coefficients decreased linearly with increasi ng concentration up to the saturation point and decreased more rapidly with increasing concentration in the supersaturated region. This dras tic decrease of diffusion coefficient at supersaturated concentrations was similar to that observed in other supersaturated aqueous solution s which was attributed to the formation of solute clusters in metastab le solutions. A cluster diffusion model employing the solution viscosi ty and thermodynamic data can provide a good correlation between predi cted and experimental diffusion coefficients in undersaturated solutio ns, but fails to provide a reasonable prediction of the much lower val ues of diffusion coefficients in supersaturated solutions. The average size of the diffusing entities estimated from the diffusion coefficie nt data showed a steady growth with increasing concentration in the un dersaturated region and a faster growth with increasing concentration in the supersaturated region. The average cluster sizes in supersatura ted solutions were estimated from the data of concentration gradients in vertical columns and solution thermodynamics using the concept of t he number and weight average molecular weights. The estimated cluster size increased with increasing degree of supersaturation as well as so lution ''age'', and showed good agreement with those estimated from th e diffusion coefficient data.