Artificial water channel based membrane for desalination

Open Access
Author:
Shen, Yuexiao
Graduate Program:
Chemical Engineering
Degree:
Doctor of Philosophy
Document Type:
Dissertation
Date of Defense:
June 01, 2016
Committee Members:
  • Manish Kumar, Dissertation Advisor
  • Manish Kumar, Committee Chair
  • Andrew Zydney, Committee Member
  • Darrell Velegol, Committee Member
  • Michael Anthony Hickner, Outside Member
Keywords:
  • Artificial water channels
  • Biomimetic membranes
  • Desalination
Abstract:
Nature provides excellent examples of highly efficient and selective material transport across cell membranes through biological membrane proteins. Bioinspired artificial water channels combine the high permeability and selectivity of biological aquaporin (AQP) water channels with chemical stability, opening the possibility to develop bioinspired channel-based materials for separations that utilize their unique transport properties. This work first focused on developing methods to characterize the molecular transport of artificial water channels. Peptide-appended pillar[5]arene (PAP) artificial water channels were found to have a single-channel water permeability of 1.0(±0.3)×10−14 cm3/s or 3.5(±1.0)×108 water molecules per second, which is in the range of AQPs (3.4∼40.3×108 water molecules per second) and their current synthetic analogs, carbon nanotubes (CNTs, 9.0× 108 water molecules per s). PAP channels were also found to self-assemble into 2D arrays in lipid membranes, with a pore density (∼2.6×105 pores per μm2) two orders of magnitude higher than that of CNT membranes (0.1∼2.5×103 pores per μm2). Another artificial water channel (imidazole-quartet channel) formed a 2.6 Å pore encapsulating oriented dipolar water-wires in a confined chiral conduit via self-assembly of imidazole-based molecules in lipid bilayers. They mimic the selective filter of M2 proton channel from Influenza A and transport ~106 water molecules per second and reject all ions except protons. PAP artificial water channels could also be incorporated into block copolymer (BCP) membranes and have shown the same high water conductance as observed in native lipid membranes. Despite of the less favorable interaction with poly(butadiene)-b-poly(ethylene oxide) (PB-PEO) BCPs due to chemical hydrophobic mismatch, PAP channels could be densely packed into polymer membranes through a controlled dialysis-based self-assembly when physical hydrophobic mismatch is minimized. The packing density is found to be of the same magnitude as observed in 2D arrays of PAP channels in lipids. This completely artificial material that possesses aquaporin-like permeability and maintains chemical and mechanical stability can be potentially used for the next-generation filtration materials. In order to be more scalable for membrane fabrication, a solvent cast-based method was developed to fabricate a polymer film with PAP artificial water channels incorporated. Using a sacrificial water-soluble layer, the PB-PEO film, with a thickness of 20-30 nm, can be floated on water and transferred to another substrate after crosslinking. The cross-sectional transmission electron microscopy images implied a possible lamellar structure and the presence of PAP channels was confirmed by time-of-flight secondary ion mass spectrometry. Future work should focused on using these channel-based BCP membranes for gas and liquid separations.