Assessing the Stability of Polymer Inclusion Membranes: The Case of Aliquat 336-Based Membranes

Leaching of the extractant from polymer inclusion membranes (PIMs) into the feed and receiving aqueous solutions shortens their life. Therefore, when a particular PIM extractant has been selected, it is important to choose a base polymer that will minimize to the greatest extent extractant leaching compared to other base polymers, thus providing the best stability of the PIM. However, comparisons of the stability of PIMs composed of the same extractant and different base polymers is usually conducted by multiple cycles of extraction and back-extraction steps, which are time-consuming and labor-intensive. An alternative approach based on thermal analysis (thermogravimetric analysis (TGA) and differential thermal analysis (DTA)) was developed and applied to PIMs containing 40 wt.% Aliquat 336, one of the most frequently used PIM extractants, and the three most frequently used PIM base polymers, i.e., poly(vinyl chloride) (PVC), cellulose triacetate (CTA), and poly(vinylidene fluoride-co-hexafluoropropylene) (PVDF-HFP). The temperatures and enthalpies associated with Aliquat 336 release were compared, with PVDF-HFP exhibiting the highest values, indicating the strongest interaction between the extractant and the polymer matrix and, thus, the highest stability. The PVC-based PIM was predicted to be the most prone to extractant leaching among the PIMs studied. This stability ranking was confirmed theoretically by quantum chemistry (DFT) calculations, which provided molecular-level insights into the likely interaction sites between Aliquat 336 and the polymer chains. An experimental validation of the above leaching order was also provided by PIM leaching experiments in aqueous 0.1 M and 0.05 M NaCl solutions, where membrane mass losses over a 24 h period were determined. The results of the current study demonstrated thermal analysis to be a fast and viable approach in comparing the stability of PIMs with the same extractant but different base polymers.