Citation: ROBINSON, J.P. ... et al, 2004. Solvent flux through dense polymeric nanofiltration membranes.Journal of Membrane Science, 230 (1-2), pp. [29][30][31][32][33][34][35][36][37] Additional Information:• ABSTRACTThis work examines the flux performance of organic solvents through a polydimethylsiloxane (PDMS) composite membrane. A selection of n-alkanes, i-alkanes and cyclic compounds were studied in deadend permeation experiments at pressures up to 900 kPa to give fluxes for pure solvents and mixtures between 10 and 100 l m -2 h -1. Results for the chosen alkanes and aromatics, and subsequent modelling using the Hagen-Poiseuille equation, suggest that solvent transport through PDMS can be successfully interpreted via a predominantly hydraulic mechanism. It is suggested that the mechanism has a greater influence at higher pressures and the modus operandi is supported by the non-separation of binary solvent mixtures and a dependency on viscosity and membrane thickness. The effects of swelling that follow solvent-membrane interactions show that the relative magnitudes of the Hildebrand solubility parameter for the active membrane layer and the solvent(s) are a good indicator of permeation level. Solvents constituting a group (e.g. all n-alkanes) induced similar flux behaviours when corrections were made for viscosity and affected comparable swelling properties in the PDMS membrane layer.

The separation of organic solutes in organic solvents was assessed using dense poly(dimethylsiloxane) (PDMS) membranes with different degrees of cross-linking and varying thickness of the dense PDMS layer. The predominant rejection mechanism for low-polarity organic solutes is shown to occur via size exclusion, with the rejection also being dependent on the degree of membrane cross-linking, the swelling propensity of the membrane−feed stream, and the transmembrane pressure. It is postulated that the size-exclusion mechanism arises as a consequence of the relatively large degree of swelling of the PDMS material (up to 300%), which induces appreciable regions between the polymer chains for solvent and solute transport to take place. The degree of swelling governs the relative size of the transport regions within the membrane and, hence, the overall solvent flux and solute rejection characteristics. It is shown that solvent−solute coupling plays a major role in solute transport, with the convective element of solute flow increasing as the degree of swelling increases and the solute size decreases. Despite the existence of a size-exclusion mechanism, it is difficult to rule out the solution-diffusion model as an interpretation of the data; however, it is also demonstrated that models based on pore flow can adequately define the experimental data. The similarities between the two approaches are discussed, and potential evidence of a transition between solution diffusion and pore flow is introduced.

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