Selectivity control in organic solvent nanofiltration–membranes or process?

This study investigates how hydrodynamic conditions, solute concentration, and solute molecular architecture influence organic solvent nanofiltration (OSN) performance using polyethylene glycols (PEGs) of different molecular weights and structures. First, a fixed PEG concentration (10 g/L) in acetonitrile was used to evaluate the impact of hydrodynamic conditions by varying feed spacer thickness in a SEPA cell at 10 bar. Rejections ranged from near 0 % (open channel) to nearly 100 % (with spacers) using the same membrane, attributed primarily to differences in concentration polarisation. Film theory-based models closely matched experimental data, confirming mass transfer limitations as key rejection determinants. Additional experiments with varying flow rates and spacer-membrane combinations reinforced these findings. Next, the effect of PEG concentration was examined. As concentration increased, flux became highly non-linear and approached limiting values above 20 g/L, consistent with concentration polarisation-driven limiting flux. Mass transfer coefficients decreased with increasing PEG molecular weight and revealed a stronger-than-expected dependence on viscosity, reinforcing the significance of viscosity variation in the boundary layer. Finally, the impact of PEG molecular architecture was studied by comparing linear, 4-arm, and 8-arm star-shaped PEGs of equivalent molecular weight (∼10 kDa). While intrinsic membrane rejections were similar, branched PEGs exhibited significantly higher apparent rejections and reduced sensitivity to concentration polarisation. This improved performance was attributed to lower solution viscosity and more compact molecular structure, which enhanced mass transfer. Overall, the study highlights the dominant role of mass transfer and solute properties in OSN membrane separations and emphasizes the need to control flow conditions and experimental design. • The impact of hydrodynamics on OSN performance was evaluated using PEGs. •Concentration polarisation, altered by flow conditions, dominate observed rejection. •Higher PEG concentrations induce limiting flux due to viscosity and polarisation. •Branched PEGs show higher apparent rejection and less sensitivity to polarisation. •Film theory modelling confirms mass transfer limitations govern PEG rejection.