Stability study of polymer inclusion membranes (PIMs) based on acidic (D2EHPA), basic (Aliquat 336) and neutral (TOPO) carriers: effect of membrane composition and aqueous solution

The long-term use is one of the weaknesses of polymeric inclusion membranes. The present study focuses on the identification of the optimal conditions of the membrane and aqueous solutions compositions that will allow long-term use of polymeric inclusion membranes. We tried to find out a way to minimize the loss of the membrane organic phase of a polymer inclusion membrane (PIM). For this purpose, PIMs consisting of cellulose triacetate (CTA) as a base polymer and three different carriers, Aliquat 336 (basic), di-(2-ethylhexyl) phosphoric acid (D2EHPA) (acid) and trioctylphosphine oxide (TOPO) (neutral), were elaborated. The stability of the obtained membranes was studied in detail by determining their mass loss when immersed in aqueous solutions containing various salts or acids. The results showed that the stability of the PIMs is heavily influenced by the various studied parameters. The highest stability was recorded in HNO3 acidic medium (pH1) and 1 M NaNO3 medium where a mass loss < 5% wt. was observed. Besides, the PIM containing TOPO was found to be more stable than that containing Aliquat 336 which is in its turn more stable than that containing D2EHPA. The membranes based on Aliquat 336 and TOPO have excellent stability up to T = 60 degrees C; however, the membrane containing D2EHPA becomes unstable when the temperature is > 25 degrees C. One of the most relevant results is that the mass loss also depends on the saturation of the aqueous phase by carrier; a better stability is obtained in the case of a continuous immersion. Thus, the membrane can be kept stable for a period of up to 1 month. Finally, this study shows that it is possible to optimize the conditions that allow to stabilize membranes for extended use by adjusting the aqueous medium and membrane composition.