Study of the structure of asymmetric cellulose acetate membranes for reverse osmosis using electron spin resonance (ESR) method

Electron spin resonance (ESR) technique was used to study the structure of the skin layer of asymmetric cellulose acetate membranes for reverse osmosis. TEMPO (2,2,6,6-tetramethyl-1-piperridinyloxy-free radical) was used as a spin probe that was brought into the membranes either by reverse osmosis experiments with feed solutions involving TEMPO, or by blending TEMPO into casting solutions. It was found that the mobility of TEMPO in the asymmetric membrane shrunk at 90°C was the same as TEMPO in a dense homogeneous membrane prepared from the same casting solution as that used for the preparation of the asymmetric membranes. On the other hand, TEMPO was more mobile in the asymmetric membranes when they were shrunk at lower temperatures and less mobile in a dense homogeneous membrane when the latter is prepared from a casting solution without pore former (magnesium perchlorate and water). Reverse osmosis experiments were also performed using feed aqueous solutions of sodium chloride and/or TEMPO.