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Movement of Water as Effected by Free Energy and Pressure Gradients III. Restriction of Solutes by Membranes¹

ABSTRACT

The validity and method of using the self-diffusion coefficient of water to estimate movement in response to free energy gradients are outlined.

The dominant mechanism of water transport through these membranes is viscous flow, even when osmotic pressure differences are causing the flow. Osmotic pressure and hydraulic pressure differences are equally effective in moving water through a membrane when the solutes are completely restricted by the membrane. When solutes are not completely restricted, osmotic pressure differences are less effective than hydraulic pressure differences in moving water through membranes. Equations including the sizes of the solute molecules and the pores are developed which predict the rate at which osmotic pressure differences move water through uncharged porous media.

¹ Contribution from Soil and Water Conservation Research Division, Agricultural Research Service, USDA, and Colorado Agr. Exp. Sta., Ft. Collins. Authorized for publication as Colorado Agr. Exp. Sta. Sci. J. Series No. 780. Portions of this work were supported by the National Science Foundation and Regional research project W-68. Portions of this work are from the Ph. D. thesis of the junior author.

² Soil Scientist, USDA, and Associate Professor (Agronomy), Colorado State University; and Professor, Agricultural Engineering Department, Colorado State University, Fort Collins.

Received for publication August 27, 1962. Accepted for publication May 2, 1963.

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