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 polynuclear aromatic (PNA) solute compounds. Rejection was found to be dependent on transmembrane 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.