AbstractThe microstructure of cellulose acetate reverse osmosis membranes which have been prepared by the Kesting dry process has been elucidated with the aid of scanning and transmission electron microscopy. Thedry‐ROmembranes consist of three layers: skin, transition layer, and substructure. The gel morphology of each of these layers originates in its sol precursors within the nascent membrane which manage to survive the sol → gel transition intact. The dense skin layer is composed of aggregates of tiny, slightly ellipsoidal nodules which are believed to be paracrystalline in nature. Immediately beneath the skin lies the transition layer, a narrow band of intermediate density consisting of widely separated closed cells. Below the latter lies the porous substructure which is composed of micrometer‐sized, open‐celled voids. Because of the size of their substructural voids, dry‐RO membranes are able to reversibly undergo wet–dry cycling without densification and loss of p
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