EFFECT OF PORE SIZE ON DIFFUSION COEFFICIENTS IN POROUS MEDIA

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Abstract
The effect of pore size on the diffusion coefficient of 2,4-dichlorophenoxyacetic acid was measured. Porous media were made up of glass beads, separated into different size fractions by sieving to obtain desired pore sizes. The ratio of actual porous system diffusion coefficient K′ to the free solution diffusion coefficient Ko was K’/Ko = 1 − e−0.13r2, where r is the pore radius. These results suggest that for small pores as in closely packed particle matrices of less than fine silt size, the rate of diffusion is significantly reduced. The observed diffusion hindrance effect was attributed to a viscous drag caused by the proximity of the pore wall. The phenomenon appears to be in agreement with literature reports. Similar observations with small pores in thin membranes have indicated that the structure of water in small pores is more orderly than that in free solutions.The results suggest that a porous medium diffusion coefficient can be obtained from the free solution diffusion coefficient, but the relation needs to be evaluated for a range of ions and molecules before generalizations can be inferred. The effect of pore size on the diffusion coefficient of 2,4-dichlorophenoxyacetic acid was measured. Porous media were made up of glass beads, separated into different size fractions by sieving to obtain desired pore sizes. The ratio of actual porous system diffusion coefficient K′ to the free solution diffusion coefficient Ko was K’/Ko = 1 − e−0.13r2, where r is the pore radius. These results suggest that for small pores as in closely packed particle matrices of less than fine silt size, the rate of diffusion is significantly reduced. The observed diffusion hindrance effect was attributed to a viscous drag caused by the proximity of the pore wall. The phenomenon appears to be in agreement with literature reports. Similar observations with small pores in thin membranes have indicated that the structure of water in small pores is more orderly than that in free solutions. The results suggest that a porous medium diffusion coefficient can be obtained from the free solution diffusion coefficient, but the relation needs to be evaluated for a range of ions and molecules before generalizations can be inferred. © Williams & Wilkins 1974. All Rights Reserved.

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