Urea can be transported through a supported liquid membrane (Accurel/NPOE) by carriers such as metallomacrocycles and polyaza (cleft-type) receptors. The urea flux is increased by a factor 4¿8 using polyaza receptors and by a factor 10¿15 using metallomacrocycles containing a salophene unit in which a uranyl cation is incorporated. These carriers have a high hydrophobicity and do not significantly leak from the membrane phase into the aqueous phases. The structure of the receptors and the type and number of binding sites have a pronounced influence on the transport rate. The lower urea fluxes found for the polyaza (cleft-type) carriers are most likely caused by a weaker complexation (only H-bond interactions). No transport is observed for carriers which form intramolecular H-bonds. Although lower fluxes are obtained than with a commercial haemodialysis membrane (Cuprophan), the selectivity of transport may be much higher using carrier-mediated transport.
van Straaten-Nijhuis, W. F., de Jong, F., Reinhoudt, D. N., Thummel, R. P., Bell, T. W., & Liu, J. (1993). Urea transport through supported liquid membranes using synthetic carriers. Journal of membrane science, 82(3), 277-283. https://doi.org/10.1016/0376-7388(93)85192-Y