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Semipermeable membranes of cellulose acetate for desalination in the process of reverse osmosis. I. Lyotropic swelling of secondary cellulose acetate

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  • Contribution No. 247 from the laboratories of the Chemical Products Division, the Von Karman Center of Aerojet-General Corporation, Azusa, California. This work was supported by the Office of Saline Water, U. S. Department of the Interior, Contract No. 14-01-0001-272. Presented in part at the 144th National Meeting, American Chemical Society, Los Angeles, April 1963.

Abstract

The role of certain inorganic salts in increasing the flux of water through semipermeable membranes of cellulose acetate is related to the capacity of the component ions to swell the cellulosic substrate. The mechanism whereby ions which are high on the Hofmeister lyotropic series swell and dissolve secondary cellulose acetate has been investigated. Swelling is effected by the formation of metastable complexes involving the highly hydrated cationic fraction of the salt and both the hydroxyl and acetate groups of the cellulose acetate. Whereas effective anions possess large ionic radii and cause a breakdown in the structure of water which results in increased hydration of the cation, ineffective anions possess higher charge densities and tend to pair with the cation and decrease the latter's electrophilicity and hence hydratability. Ionic charge density is the factor of prime importance both with respect to the hydratability of the cation and the tendencies toward water disaggregation and ion association of the anion. The mechanism involved in the swelling of secondary cellulose acetate is related to swelling phenomena in many other polymers containing polar groups.

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