Molecular separation by poly (N-vinyl imidazole) gel-filled membranes

• Membrane permeability and molecular selectivity were tuned by solution pH.
• Molecular separation was driven by sieving, electrostatic repulsion or adsorption.
• Molecular sieving remained in high concentration salt or ethanol aqueous solution.
• Molecule mixtures can be separated based on molecule size and charge.

Here we demonstrate the tunable molecular selectivities and corresponding mechanisms of poly(N-vinyl imidazole) (PVI) gel-filled membranes prepared by UV initiated graft polymerization of N-vinyl imidazole from ultraporous polysulfone (PSf) substrate. The separation performance was investigated in detail by extensive filtration tests with various organic molecules in water of varying pH and also in salt or ethanol aqueous solution. The variation of membrane water permeability and solute rejection with the feed pH was attributed to the sensitive pore size and charge characteristics of the membranes upon reversible protonation of the confined PVI gel. The separation results discerned three main mechanisms, i.e. size exclusion (sieving), electrostatic repulsion (Donnan exclusion) and adsorption. The rejection of the neutral molecules and the large dye molecules was mostly ascribed to size exclusion, while that of the small charged solutes was dominated by the electrostatic interactions including repulsion for cationic ones and attraction for anionic ones (causing specific adsorption). Typically, at pH 3 the protonated membranes rejected ~90% of the small cationic Vitamin B1 (VB1) and 100% of the anionic Sunset Yellow (SY) before adsorption saturation, in contrast to less than ~20% rejection of the neutral Vitamin B2 (VB2) having similar molecule size. In the high concentration salt or ethanol solution the membranes still maintained molecular sieving capability. Most notably, the separations of solute mixtures were indeed achieved, depending on the membrane parameters, the solute characteristics as well as the filtration conditions. The PVI gel-filled membranes developed in this work are promising in some specified or environment-sensitive molecular level separations.

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