Influence of zwitterionic structure design on mixed matrix membrane stability, hydrophilicity, and fouling resistance: A computational study

Abstract

Zwitterion-based mixed matrix membranes (MMMs) with designed characteristics of enhanced water flux, selectivity, and fouling mitigation have emerged as a new class of advanced membranes for oilsands process-affected wastewater (OSPW) treatment. Zwitterions (ZW) characterized by super-hydrophilicity and excellent fouling resistance have gained increasing attention in membrane modification research. In general, zwitterion properties are determined by the chemistry and structural properties of its constituents, including the polymer backbone, charged moieties, spacers, as well as molecular configuration. This study used molecular dynamics simulation (MDS) to investigate the effects of polymer backbone (PB), spacer length (SL), and spacer chemistry (SC) on ZW-based MMM properties such as stability, hydrophilicity, and oil-antifouling potentiality. Membrane performance was also assessed at high temperatures (50, 70, and 90 °C. The results suggest PB, SL, and SC all influence the resultant MMM performance, with SL being the most impactful structural parameter on stability and hydrophilicity. Variation of SL was suspected to alter the ionic association and partial charges of zwitterionic moieties, which affect their ability to interact with the polymer network and water molecules. Spacer chemistry (i.e., hydroxyl (-OH) groups) can initiate self-association between zwitterionic charged groups having short SL, lessening their inter-molecular networking ability. However, for ZWs with long SL, the presence of hydroxyl groups on the spacer can result in the formation of hydrogen bonds and/or electrostatic interactions with other ZW molecules and polyvinylidene difluoride (PVDF) polymer chains or water molecules, improving membrane stability and hydrophilicity. High temperatures reduced membrane stability but to a lesser extent for MMMs compared to unmodified PVDF membrane. While temperature greatly influenced membrane hydrophilicity, the impacts were membrane-specific. The oil-fouling propensity of pristine PVDF membrane increased with temperature but of MMMs appeared stable across the temperature range studied.

Keywords: Hydrophilicity; Membrane fouling; Membrane stability; Mixed matrix membrane; Nanoparticles; Zwitterion.