The mechanism for formation of “skinned” membranes I. Structure and properties of membranes cast from binary solutions

Abstract

The structure and desalination properties of cellulose acetate membranes cast from binary solutions and leached in water depend upon the nature of the casting solvent. Membranes cast from acetone and dioxane are dense and structurally homogeneous. Those cast from triethyl phosphate (TEP) are also structurally homogeneous, but porous and do not reject salt. Membranes cast from acetic acid, dimethyl formamide (DMF) or dimethyl sulfoxide (DMSO) are all “skinned”, meaning that they consist of a dense cellulose acetate layer which is supported by a porous substructure. The location of the skin depends upon the nature of the solvent. When acetic acid is used as the solvent, the skin forms at the air-cast-solution interface, while when DMF or DMSO are used as solvents the skin is located at the supported surface. The porosity of cellulose acetate membranes decreases considerably when annealed at temperatures higher than 90°C. This is reflected by a considerable reduction in water permeability and a considerable increase in selectivity after annealing.