Relating the rejections of oligomeric ethylene glycols and saccharides by nanofiltration: Implication for membrane pore size determination

Size exclusion is the predominant mechanism of nanofiltration (NF) for the removal of organic pollutants in water. Saccharides and polyethylene glycols (PEGs) are most widely used as probe solutes for the determination of average pore radius and molecular weight cut-off (MWCO) of NF membranes, respectively. The rejection behaviors of saccharides and PEGs are however different due to the difference in molecular geometry, making the average pore size and MWCO be hardly interchangeable. This study was devoted to associate the rejection of PEGs with that of saccharides by introducing the concept of effective solute radius which is different from the Stokes radius primarily in the aspect of preferential orientation. The rejection data of three saccharides and three PEGs by four commercial NF membranes were utilized. The lower rejection of PEGs than saccharides was more apparent for the tight membranes than for the loose membranes. The effective radii of a same PEG were different for the four membranes. An explicit equation was successfully established based on a geometric model which relates the effective radius of a solute with its width, length to width ratio and the effective orientation angle through a membrane. The PEGs always took a more horizontal orientation relative to the membrane pores during permeation. A smaller pore size and a longer molecule chain resulted in a higher orientation angle. The solute effective orientation was further related to the ratio of solute Stokes radius to membrane pore radius by an empirical formula with a same fitting coefficient for the three PEGs. This study provides valuable implications on the interpretation and prediction of the rejection of linear solutes by NF membranes.