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SynopsisThe determination of the diffusion coefficient for water in various porosity cellulose acetate membranes by a gravimetric method, using a humidified carrier gas, is described. It was found to be impossible to obtain meaningful results for very porous membranes, although dense membranes gave limiting values of diffusion coefficient a t high carrier gas velocities. This phenomenon is explained in terms of the dissipation of the heat of sorption by the forced convection provided by the carrier gas. The variation of diffusion coefficient with concentration of water in dense cellulose acetate is explained in terms of clustering of water molecules in the polymer a t high concentration. INTROnDUCTIONA common method for the determination of diffusion coefficients in polymers utilizes a vacuum technique in which the sample, initially evacuated, is exposed to a controlled vapor pressure source on sorption and to a vacuum again on desorption. ' We have used a carrier gas flow technique to study the interaction between water and cellulose acetate reverse osmosis membranes and have measured the effects of humidity, flow rate, membrane structure, and equilibrium concentration. EXPERIMENTAL MaterialsThe preparation and morphology of the cellulose acetate (39.8% acetyl) membranes used are described by Gittens et a1.2 The membranes were cast from acetone-formamide dope, allowing various evaporation times, then immersed in iced water (Table I). The membrane with shorter evaporation times showed a heterogeneous structure, the 30-sec evaporation membrane having a three-layer structure dominated by a porous lower layer. The 48-hr evaporation membranes were essentially dense homogeneous films.