Effects of Feed Solution pH on Polyelectrolyte Multilayer Nanofiltration Membranes

Area of Science:

• Materials Science
• Chemical Engineering
• Membrane Technology

Background:

• Polyelectrolyte multilayer (PEM) membranes are emerging as a promising alternative to conventional polyamide membranes for nanofiltration (NF).
• Many PEM systems utilize weak polyelectrolytes, whose charge density is pH-dependent, potentially impacting membrane properties and performance across different operating pH values.
• Understanding this pH susceptibility is crucial for designing stable and efficient PEM-based NF membranes for diverse applications.

Purpose of the Study:

• To investigate the influence of feed solution pH on the structure and performance of four distinct polyelectrolyte multilayer (PEM) systems.
• To compare the pH-dependent behavior of strong/strong, weak/weak, weak/strong, and asymmetric PEM membrane configurations.
• To elucidate the relationship between polyelectrolyte type, membrane structure (swelling, surface charge), and filtration performance (permeability, MWCO, salt retention).

Main Methods:

• Fabrication and characterization of four different PEM membrane systems: PDADMAC/PSS (strong/strong), PAH/PAA (weak/weak), PAH/PSS (weak/strong), and an asymmetric PAH/PSS + PAH/PAA combination.
• Systematic evaluation of membrane structure (swelling, surface charge) and performance metrics (permeability, molecular weight cutoff (MWCO), salt retention) across a pH range (0-14).
• Analysis of the impact of pH on polyelectrolyte interactions and resulting membrane stability.

Main Results:

• PDADMAC/PSS membranes showed minimal structural and performance changes with pH variation.
• PAH/PAA membranes exhibited significant susceptibility to pH, with changes in swelling, charge density, permeability, MWCO, and salt retention.
• PAH/PSS membranes displayed less pH sensitivity than PAH/PAA due to the presence of a strong polyelectrolyte, with additional interactions stabilizing the structure.
• The asymmetric membrane (PAH/PSS + PAH/PAA) showed pH dependency, dominated by the stronger PAH/PSS component due to its higher bilayer count.

Conclusions:

• The pH susceptibility of PEM membranes is highly dependent on the types of polyelectrolytes used, with weak/weak systems being the most sensitive.
• Membrane performance parameters like salt retention, permeability, and MWCO are significantly affected by pH-induced changes in membrane structure and charge.
• Careful selection of polyelectrolyte pairs is essential for developing robust PEM nanofiltration membranes with predictable performance across various operating conditions.