(607f) Reformulation of the Solution-Diffusion Theory of Organic Solvent Nanofiltration and Reverse Osmosis: Effect of Sorption, Diffusion, Pressure and Molecular Interactions

While recent OSN-OSRO research efforts mostly focused on the synthesis and fabrication of solvent resistant composite polymer membranes, fundamental understanding of chemical and physical aspects that govern solvent and solute transport in OSN-OSRO membranes remains largely unexplored [1]. The transport mechanism itself in OSN-OSRO membranes is poorly understood: some researchers have hypothesized a solution-diffusion mechanism, others a pore-flow mechanism. Finally, others have considered a combination of the previous two mechanisms [2,3]. A peculiar feature of organic solvent separations is the flux non-linearity vs. Δp, that is, the negative departure of flux from linearity observed starting from Δp = 10 atm. Despite membrane compaction under pressure has been invoked to explain this phenomenon, this hypothesis has no experimental and theoretical support.

In this study, we critically discuss the hypothesis of membrane compaction, to demonstrate that the molecular origin of flux non-linearity is purely thermodynamic. We propose, for the first time, a thermodynamic-diffusion framework which describes solvent transport in OSN-OSRO membranes in terms of the concentration gradient produced by the applied pressure across the membrane. Interestingly, solvent diffusion coefficient in the membrane increases with increasing Dp, which further confirms that flux decline is not related to membrane compaction. The developed framework indicates that, if properly corrected for the effect of the frame of reference and non-ideal thermodynamic effects, the solution-diffusion model provides a satisfactory description of small molecule transport in OSN-OSRO membranes, without the need to resort to pore-flow or more complicated transport models [4].

Interestingly, although it is believed that OSN and OSRO are mainly controlled by diffusion, the developed theory indicates that sorption plays a role as important as diffusion, if not even more important in some cases. Driven by this discovery, we performed a systematic study of pure- and mixed-fluid transport in OSN-OSRO membranes, including aliphatic and aromatic hydrocarbons, alcohols, ketones and water, to elucidate how enthalpic and entropic effects influence solvent and solute solubility and permeability in the membrane [5].

Finally, a novel experimental methodology, based on the combined use of FTIR spectroscopy in the transmission mode and transport measurements, has been developed to study the molecular mechanism of OSN-OSRO membranes plasticization and the membrane structural evolution upon exposure to organic solvents. A variety of molecular phenomena, directly affecting the membrane performance in organic separations, has been described, and the role of solvent-membrane interactions on the membrane selectivity has been elucidated [6].

In conclusion, this study helps shed fundamental light on OSN and OSRO, which creates the basis for a more mature and efficient use of these processes.

References

[1] P. Marchetti, M.F. Jimenez-Salomon, G. Szekely, A.G. Livingston, Chem. Rev. 2014, 114, 10735.

[2] A. Volkov, D. Stamatialis, V. Khotimsky, V. Volkov, M. Wessling, N. Plate, J. Membr. Sci. 2006, 281, 351.

[3] M. Galizia, K.P. Bye, Frontiers in Chemistry 2018, 6, 511.

[4] K.P. Bye, M. Galizia, J. Membr. Sci 2020, 603, 118020.

[5] K.P. Bye, V. Loianno, T. Pham, R. Liu, J.S. Riffle, M. Galizia, J. Membr. Sci. 2019, 580, 235.

[6] V. Loianno, K.P. Bye, M. Galizia, P. Musto, J. Polym. Sci. 2020, 58, 2547.