THE EFFECT OF MOLECULAR SIZE ON DIFFUSION CHARACTERISTICS IN SOIL

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Abstract
Summary: Measurements of the apparent self‐diffusion coefficients of, poly(vinyl pyrrolidone) mol. wt. 40 000, polyethylene glycol) mol. wt. 4000, and chloride ion, were used to determine the effect of molecular size on diffusion characteristics in soil. The measurements were carried out on the same soil, at a range of moisture contents, and indicated that the mobility of large molecules in soils depends, to a large degree, on the inter‐aggregate pores. Any drop in moisture content which caused these pores to drain, produced a sharp decrease in the apparent self‐diffusion coefficient of the poly(viny) pyrrolidone) 40 000. Neither poly(ethylene glycol) 4000 nor chloride, which exhibited similar diffusion characteristics, were as sensitive to variation in moisture content. The results are interpreted in terms of certain parts of the pore volume being inaccessible to the diffusing molecule. Possible reasons are outlined.