The mechanism for formation of "skinned" membranes. II. Equilibrium properties and osmotic flows determining membrane structure

The critical water concentrations required for initiating precipitation of cellulose acetate from 20% solutions in acetone, dioxane, acetic acid, triethyl phosphate (TEP), dimethyl formamide (DMF) and dimethyl sulfoxide (DMSO) have been determined. The direction and magnitude of volume flow as well as the contribution of solvent and water to this osmotic flow through a porous cellulose acetate membrane separating pure water from a 2:3 water-solvent mixture have been measured. A mechanism based on the assumption that the density of each layer of the membrane is determined by the concentration of the corresponding layer of the cast polymer solution at the precipitation point has been suggested. The volume concentration of the polymer at the precipitation point has been correlated to: 1. the concentration of non solvent required for precipitating cellulose acetate 2. the direction and magnitude of osmotic flows of liquids into or out of the cast solution during leaching.