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This work demonstrates a more accurate method for calculating structural parameter (S) of asymmetric osmotic membranes using experimental data and a theoretical flux model which encapsulates all significant boundary layer phenomena. External boundary layer effects on the porous side of the membrane have been neglected in many current models. In these models, external concentration polarization (ECP) effects get combined with the internal concentration polarization (ICP), resulting in inflated S values. In this study, we proposed a new flux model in which ECP effects are accounted for so that S can be more accurately measured. This model considered the in-series resistances for solute transport based on intrinsic properties of the membrane as well as boundary layers at membrane surfaces and within the support layer. The results indicate that ICP is less severe than previously predicted and that cross-flow velocity, temperature and concentration of the draw and the feed solutions impact both external and internal concentration polarization. Our calculations also surprisingly show that changes in cross-flow velocity impact internal concentration polarization due to induced mixing within the support layer. Also, we suggest that it is critical to consider the "residence time" of solutes in the vicinity of the selective layer in determining the membrane selectivity.

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Hydrophilic electrospun nanofibers supported thin film composite (TFC) membranes have recently been considered for forward osmosis (FO). However, our previous work has shown that the high degree of swelling in hydrophilic nanofibers compromises fiber strength and membrane integrity. To reduce swelling, we propose to modify a non-swelling hydrophobic fiber to make it hydrophilic and fully wettable without negatively impacting the structural properties of the material. The method chosen for this work involves the interfacial polymerization of 1,6-hexane diamine and adipoyl chloride to form nylon 6,6 directly onto electrospun PVDF fibers. The result is a dual benefit of hydrophilization and strengthening of nanofibers to improve their wettability. The modified nanofibers exhibited significantly lower swelling propensity than intrinsically hydrophilic nylon 6,6 nanofibers due to the hydrophobic core fiber, and are thus able to retain strength in an aqueous environment. These modified PVDF supports were used in making TFC membranes that exhibited excellent flux performance and one of the lowest structural parameters reported in open literature.