Unravelling the solvent flux behaviour of ceramic nanofiltration and ultrafiltration membranes

In ordertoincreasetheunderstandingoftheunderlyingprocessesinorganicsolventnanofiltration
(OSN), astudyhasbeenundertakenaimedatclarifyingthesolvent flux behaviourofceramic
nanofiltration andultrafiltration membranes.Ceramicmembraneswerechosenfortheirnon-swelling
character.Purewaterandavariationof11differentorganicsolventsweremeasuredonaseriesof
different ceramicmembraneswithpore-sizediametersrangingfrom0.9nmupto100nm.Toavoidany
historical effects,each flux measurementwascarriedoutonanewmembrane.
The flux resultswereanalysedinaphenomenologicalway,andacommonverysimplelinear
relationship wasobservedbetweentheproductof flux andviscosityofthesolvent,andthetotalHansen
solubility parameterofthesolvent.Thelinearrelationshipwasfoundforallmembranes,independentof
the membraneporesizeandthemembranematerial.Theslopeofthelinearrelationshipwasfoundto
depend exponentiallyonthepore-sizediameterandonthepolarityofthemembranesurface.Thisresult
emphasizes theimportanceofviscosityinthesolventtransport,butalsoofthepolaritydifference
between membranesurfaceandsolvent.Theverysimple flux modeldeduced,allowsastraightforward
prediction ofthe flux ofanysolventorsolventmixture,oncethewater flux ofthemembraneisknown.
At thehighpore-sizeend,thephenomenologicalmodelnaturallytransformsintotheviscous-flow or
pore-flow behaviourasrequired.Atentativephysicalexplanationofthemodeltakesintoaccountthe
presence andextensionofawaterlayeradsorbedtothetotalporesurfaceofthesemembranes.
This workalsoshowsthatthewater flux ofahydrophilicmembranegivesagoodindicationofits
molecular weightcut-off(MWCO),andthereforeofitsseparationperformanceinwater.