The Solution-Diffusion Model for Swollen Membranes

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Abstract
It has been widely recognized for many years that gases, vapors, liquids, and solutes permeate through non-porous polymer membranes by a solution-diffusion mechanism1–5. However, it was only ten years ago that it was first proposed6 that reverse osmosis might be quantitively described by this mechanistic model. The solution-diffusion model first published by Lonsdale, Merten and Riley6 has been applied with considerable success to describe a variety of systems but the cellulose acetate-water-inorganic salt system has received most attention due to its importance in desalination. It is widely accepted that this model not only describes the process of reverse osmosis but also represents an accurate mechanism in those cases where the membrane structure is truly dense, i. e. non-porous, with no imperfections - this condition refers only to the “active layer” in Loeb-type6 membranes.

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