Membrane performance in separation processes is strongly dependent on the membrane structure which must be controlled during the formation step. In this study, a mixture of NMP and acetone with different ratios has been utilized as solvent to prepare cellulose acetate membranes with different morphologies. The membranes formed by 0–25% NMP in NMP/acetone mixture had a sponge-like structure with closed pores. By increasing the NMP concentration from 25 to 50%, the morphological structure of membranes changed from sponge-like to finger-like. Results showed that the common thermodynamic and kinetic parameters of membrane formation could not predict the proportion of solvents in which the primary structural change had occurred. Moreover, the analysis of some phenomenological models pertaining to the phase separation process such as Da, η0/X and η0/ΔP showed that the recent η0/Δp model offered a rather good description of the system morphology with changing mixed solvent composition. A significant finding was that the pure water permeability of membrane was correlated well with the trend predicted by both η0/Δp and η0/X data points.

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