Formation of asymmetric cellulose acetate membranes

Cellulose acetate membranes were prepared from casting solutions containing dioxane as a solvent and varying concentrations (up to 6%) of maleic acid as an additive. Coagulation took place in water at different temperatures. The effect of these variables on membrane structure and membrane properties is related to two phenomena of phase separation in the system cellulose acetate/dioxane/water, viz. gelation in the toplayer and liquid-liquid phase separation in the sublayer of the membrane. We adopted a solution transport model which correlates membrane flux with the skin thickness and membrane salt rejection with the compactness (microstructure) in the skin. Effects of variables such as maleic acid concentration and coagulation temperature on the position of curves in the phase diagram and on membrane properties are discussed. It is concluded that more work should follow on the kinetics of the phase changes mentione