Study of the rejection of various solutes in OSN by a composite polydimethylsiloxane membrane: Investigation of the role of solute affinity

• The rejection rate cannot be directly related to the swelling degree of the dense membrane.
• For a given solute, the rejection rate can vary with the solvent used from negative to positive rejection.
• Negative rejection can be obtained even with a low swelling degree of the dense membrane.
• Almost total rejection of Alphazurine FG can be obtained from toluene with a highly swollen PDMS membrane.
• The membrane–solute affinity appears to be a important parameter for the solute rejection.

This paper reports the separation results of small molecules, three dyes and three aliphatic molecules of similar molecular weights obtained in organic solvent nanofiltration with a supported PDMS membrane. Diluted feed solutions were prepared with several solvents: ethanol, dimethylcarbonate, hexane, heptane and toluene. These solvents were chosen to vary the affinity with the membrane. The main results obtained indicate that the retention of the solute is highly dependent on the solvent used. Contrary to some popular ideas, the extent of membrane swelling is not found to govern the retention rate with a PDMS membrane. Indeed, it was shown that a negative retention rate could be obtained with a low amount of swelling, whereas high retention rates (99%) could be obtained with a highly swollen membrane (114 wt%). In fact, the membrane–solute affinity appears to be a decisive parameter for the mass transport of the solute together with its diffusion ability. For all of the studied systems, without exception, the rejection rate increases when the solute–membrane affinity decreases. These results were verified in the nanofiltration pressure range of 5–20 bar.

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