Influence of membrane hydrophilicity on water permeability: An experimental study bridging simulations

There has been an argument on the influence of membrane hydrophilicity on permeability between experimental and simulation works. Theoretical work finds that the membranes have a characteristic called a threshold pressure drop (ΔP_T). Strongly hydrophobic membranes exhibit extremely high ΔP_T, making membranes less wettable and consequently less permeable. In order to experimentally investigate the influence of wetting states of membranes, determined by membrane hydrophilicity, on permeability, we prepared a series of membranes with progressively changing hydrophilicities by atomic layer deposition (ALD) of TiO₂ on polytetrafluoroethylene membranes while keeping the pore size unchanged. In addition, adding ethanol into the water and raising the operating pressure drops are also selected to improve the wetting degrees. The experimental results verify the presence of ΔP_T for all membranes and the hydrophobic membranes having higher ΔP_T. Since the hydrophilic membranes have better wetting degrees but higher transport resistance, there should be an optimized hydrophilicity, which is found to be determined by the feed solution and the operating ΔP. The optimized hydrophilic modification is the one when the membrane can be just adequately wetted by the feed solution at the target operating pressure drops. Highest water permeability is expected to be obtained at a moderate hydrophilicity, neither at the extreme hydrophilicity nor at the extreme hydrophobicity.