Dual hydrogen bonds stabilize layer-by-layer membranes in highly saline environments

Layer-by-layer (LBL) nanofiltration (NF) membranes have shown advantages in separating monovalent and divalent cations. The separation process always involves highly saline solutions especially at high water recovery. Due to the non-chemical weak binding of polyelectrolytes (PEs), LBL NF membranes tend to disassemble in highly saline solutions. This work presents a new discovery of strengthening salinity stability via hydrogen bonds. The superior stability of poly(allylamine hydrochloride) (PAH)/polystyrene sulfonate (PSS) as indicated by ∼1.5 M critical salt concentration (CSC) demonstrated the effect of single hydrogen bond with PSS. The formation of the hydrogen bond was confirmed by Attenuated Total Reflectance Fourier Transform Infrared Spectroscopy (ATR-FTIR) and Raman spectroscopy. Poly(hexamethylene biguanide) (PHMB)/PSS assembly with dual hydrogen bonding capability exhibited a CSC exceeding the maximal NaCl concentration studied around 3 M. The role of hydrogen bonds in stabilizing membrane structure under saline conditions was further elucidated by Molecular dynamics (MD) simulation. The unexpected salinity stability by hydrogen bond expanded the application horizon of LBL NF membranes in much harsher environments, i.e. seawater desalination and separation of matters in highly saline solutions. Better design of the chemical structure of the polyelectrolytes opens the door for design of novel nanofiltration membranes for wider applications.