(376e) Fundamental Pure and Mixed Liquid Sorption Properties of Osn Membranes Based on Polybenzimidazoles

Organic solvent nanofiltration is attracting increasing interest as it could potentially replace distillation. As reported by Marchetti et al.¹, for a given separation, OSN would help save up to 3 orders of magnitude of energy compared to distillation. Moreover, since most of chemical synthesis occur in solution, solvent recovery and solute concentration is a common issue in the chemical, petrochemical and pharmaceutical industry. Although flux and rejection data are available in the literature for several materials, fundamental structure-transport property correlations for OSN membranes are still missing, with the specific role of solvent and solute sorption and diffusion coefficients being fairly unknown. Moreover, most of available experimental data refer to composite membranes, where the presence of a fabric backing makes it difficult to provide a fundamental description of solvent and solute sorption and transport in the active layer. This lack of fundamental information has hampered the development of rational methods for the design of better materials for OSN.

In this study, free standing, mechanically robust flat membranes based on Celazole^® were cast from DMAc². Pure and mixed solvent and solute sorption experiments were carried out at room temperature. Among solvents, hydrocarbons, alcohols, acetone, toluene, ethyl acetate and water were considered. N-decane and PEG400 were used as mimic standard solutes. Interestingly, molar solubility decreases by an order of magnitude with increasing penetrant molar volume, which points out that liquids sorption in Celazole^® is controlled essentially by entropic factors. Indeed, as long as penetrant size increases, it is less likely to accommodate penetrant molecules into the polymer matrix³. However, this rapid decrease is followed by a plateau, where sorption changes little with penetrant chemical nature and size.

When considering sorption of isomers, solubility starts to be affected by enthalpic effects. Specifically, 1-propanol solubility is 5 times higher than 2-propanol solubility. Such behavior can be justified by considering that, while 1-propanol can easily form hydrogen bonds with the amine groups on the polymer backbone, 2-propanol is less prone to do so, since the –OH group is much more sterically screened.

Finally, the effect of the second component on solvent and solute solubility in Celazole^® was also investigated using model mixtures, such as methanol-PEG400.

References

1. P. Marchetti et al., Chem. Rev. 2014 114 10735-10806
2. H. Borjigin et al., Polymer 2015 71 135-142
3. M. Galizia et al., J. Membr. Sci. 2012 405-406 201-211