Non-aqueous nanofiltration: solute rejection in low-polarity binary systems

The separation characteristics of a dense polydimethylsiloxane (PDMS) membrane were studied using alkyl and aromatic solvents and low-polarity, sulphur bearing, organometallic and poly-nuclear aromatic (PNA) solute compounds. Rejection was found to be dependent on trans-membrane pressure, crossflow rate (hydrodynamic conditions), solute size and the degree of swelling induced by the solvent. Rejection increased progressively with pressure whilst a threshold condition was observed above which further increases in crossflow had a negligible influence on rejection. Measurements over the molecular weight range 84–612 g/mol showed the membrane to have a molecular weight cut-off in the region 350–400 g/mol to all but one of the tested PNA compounds (i.e. rubrene). An additional correlation using molecular dimensions instead of molecular weight showed the cut-off size of the membrane to be in the region of 1–2 nm, with all data falling on a well-defined rejection/size curve. Solvent type influenced membrane swelling to an extent dependent on the relative magnitude of the solubility parameters for the solvent and PDMS; similar values led to more swelling, higher fluxes and lower rejections. Results support the concept of viscous solvent flow whilst solute transport could be either predominantly viscous or a combination of viscous and diffusive. With larger molecules, a size exclusion mechanism was dominant. A new model is proposed that takes account of solute transport by a combination of viscous and diffusive mechanisms and this is shown to well represent the experimental data.