Effect of molecular shape on rejection of uncharged organic compounds by nanofiltration membranes and on calculated pore radii

We examined the effects of molecular shape on the rejection of uncharged organic compounds by nanofiltration membranes and on pore radii calculated for those membranes. Molecular shape was approximated by a rectangular parallelepiped; molecular length (L) and molecular width (MWd) were used as shape parameters. The steric partition factor of a solute was calculated from L and MWd, and the Stokes radius and diffusivity were estimated from MWd. Twenty-four alcohols and polyhydric alcohols were used as probe compounds, and pore radius was estimated by both non-spherical and spherical models within a uniform pore-size model or log-normal pore-size distribution model. The uniform pore/non-spherical model gave better correspondence between the calculated and observed rejections for NTR-729HF membranes, and the calculated pore radius (0.634 nm) was larger than that from the spherical model (0.387 nm). Similar results were obtained from previous rejection data. In the case of the non-spherical model, employing a log-normal pore-size distribution model did not improve the correspondence between observed and calculated rejections. The logarithm of the product of calculated solute permeability and pore length log(B* Δx) was correlated linearly with MWd. Molecular width is thus a major factor controlling solute permeation.