No abstract

SynopsisDiffusion and distribution coefficients of water and sodium chloride have been meaaured in cellulose acetate osmotic membranes. These coefficients have been found to vary with the degree of acetylation of the cellulose ester. The diffusion coefficient of water varies from 5.7 X 10-6 to 1.3 X 10" cm.2/sec., and the diffusion coefficient of salt varies from 2.9 X 10-8 to 3.9 X lo-" as the acetyl content is increased from 33.6 to 43.2 wt.-%. A homogeneous diffusion model is proposed which describes the observations in terms of Fick's law. INTRODUCTIONRecent work done by Reid and Breton' and by Loeb and Sourirajan2 has shown that cellulose acetate films act M highly selective membranes in the separation of water from salt solutions by reverse osmosis. The present study was undertaken in an attempt to obtain quantitative data on the permeability of such membranes to water and to sodium chloride under the conditions of interest in water desalination, and'to interpret these data in terms of a reasonable membrane permeation model. EXPERIMENTALThe membranes used in this work were prepared from various grades of cellulose acetate supplied by Eastman Chemical Products, Inc. The membranes referred to as "normal" were cast with a Gardner film-casting knife on glass plates at room temperature and were stripped from the glass after they were quite dry. A solvent was chosen for each material which gave optically clear membranes, apparently free of imperfections. The casting solutions for the 37.6, 39.5, and 39.8 wt.-yo acetyl cellulose acetates were prepared by dissolving a weighed quantity of the acetate powder in three times its weight of reagent-grade acetone. The solution of the 43.2 wt.-yo acetyl material was made similarly but with pdioxane as solvent, and the 33.6 wt.-yo acetyl powder was added to three times its weight of absolute ethyl alcohol, and sufficient water was then added to effect complete solution. The "modified" membranes were prepared b y a procedure essentially similar to that described by Loeb and Sourirajan.2 In this case, the casting solution consisted of 22.2 wt.-% cellulose acetate (39.8 wt.-% acetyl, Eastman No. 39&3), 1.1 wt.-% Baker anhydrous Mg(C10&, 66.7 wt.-% reagent-grade acetone, and the balance distilled water. The solution was cooled to -5OC. and cast on cold glass plates in a deep freeze maintained at this same temperature. The membranes were allowed to dry for 4 min. after casting and were then immersed in ice water and kept there for at least 1 hr. Finally, the membranes were annealed in 81OC. water for 30 min.The gross transport properties were measured in twb types of experiments, one involving normal osmosis, the other reverse osmosis. I n the normal osmosis experiments, a piece of the membrane of interest was supported between two coarse Monel grids in a circular hole (-15 cm. diameter) through a central dividing member in a rectangular plastic box. An O-ring seal was made around the perimeter of the hole. Aqueous solutions of different salt concentrations were then placed in e...

No abstract

No abstract