ASYMMETRIC SOLUTE TRANSPORT AND SOLVENT FLUX IN DUAL-SKINNED HOLLOW-FIBER MEMBRANES

Dual-skinned hollow fiber membranes were produced with thin skins on b oth the inner (lumenal) and outer surfaces of an annular macroporous m atrix. These dual-skinned membranes demonstrated a clear directional s electivity with sieving coefficients that were dramatically different in the two flow directions (shell-to-lumen and lumen-to-shell). Unlike the incidental directional selectivity previously reported with singl e-skinned reverse osmosis membranes, sieving coefficients in both dire ctions are controllable by varying the properties of the two skin laye rs. The directional sieving behavior of these membranes is a result of the directional nature of the convective solute transport across a me mbrane having two skin layers with different effective pore sizes. Int ernal concentration polarization significantly increases solute sievin g coefficients when flow occurs through the more open skin layer first , but is largely absent when the flow is in the other direction. The i nternal concentration polarization also has a strong effect on the sol vent flux through these membranes causing large directional difference s in flux during filtration of a dilute macromolecular solution. The u nique transport characteristics of these dual-skinned membranes thus p rovide an extraordinarily powerful tool for the design and development of novel membrane devices and processes that exploit the directional selectivity of these new membrane structures.