Manipulating the ionization of polyamide nanofiltration membrane for cascade separation of low-molecular-weight tobacco extracts

Nanofiltration (NF) is a promising technology capable of separating various organic molecules from mixtures. However, the application of current NF membranes for tobacco extracts with similar molecular weights (MWs) suffers from the limited separation capability due to their intrinsically wide pore size distribution, especially the inability to reject molecules with low MWs. Here, we present an electrostatic interaction regulation strategy for molecules and membranes by controlling the ionization behavior of both polyamide membranes and compound molecules to selectively separate low MW tobacco extracts with similar sizes. It was achieved by meticulously tuning the membrane chargeability and solution pH to dissociate or protonate charged groups of the membrane surface and targeted molecules. As a result, compared with the original solution, the rejection of low MW tobacco extracts—phthalic acid, benzoic acid, and ethyl vanillin—increased by 98.4, 83.0, and 82.4 %, respectively, after adjusting the feed solution pH. Furthermore, by manipulating ionization behavior, the membrane effectively rejected similarly charged molecules while allowing neutral and oppositely charged molecules to pass, successfully facilitating the separation of electronegative, electropositive and electroneutral mixtures. In the four-stage membrane cascade system, all tobacco extracts in the mixture were stepwise separated and purified, increasing the purity of each molecule (phthalic acid, benzoic acid, ethyl vanillin, nicotine, and 2-phenylethanol) from 20 % to over 70 %. Overall, this study developed a novel membrane cascade system by manipulating the molecular-membrane electrostatic interactions, providing an efficient and energy-saving method for the separation and purification of small-molecule tobacco extracts with similar MWs.