Hofmeister effect of anions on the layer-by-layer nanofiltration membranes: Assembly kinetics and micropollutant removal

The impact of anions on the assembly process, subsequent physicochemical characteristics and separation performance of layer-by-layer (LBL) nanofiltration (NF) membranes has not been fully elucidated. In this study, model polyelectrolyte (PE) pairs of poly(sodium 4-styrenesulfonate) (PSS) and poly(diallyldimethylammonium chloride) (PDADMAC) were chosen and monovalent anions in the Hofmeister series—kosmotropes (F⁻, Ac⁻), Cl⁻, chaotropes (Br⁻, NO₃⁻)—were paired with Na⁺ as the background salt. Membranes from chaotropic anions had narrow distributions of small pores, relatively low permeance and nearly electroneutral, even slightly electropositive separation layer; fewer coating bilayers were required to reach the plateau in MgSO₄ rejection, corresponding to faster PE adsorption and thicker coating. Dynamic light scattering (DLS) technique revealed that the diffusion coefficients of PEs in solution were several magnitudes higher than the site diffusion coefficients, implying that PE adsorption was mainly enhanced by fast site diffusion. Thickness measurement by ellipsometry on silicon wafers demonstrated that the increment of adsorption was greater for PDADMAC than for PSS in chaotropic anions, which explained the difference in the overall membrane charge. Membranes from chaotropic anions performed better in removing pharmaceuticals and personal care products (PPCPs) and per- and poly-fluoroalkyl substances (PFASs) due to small pore size, highlighting the importance of steric exclusion. The role of background anions in modulating PE adsorption, tuning the pore structure, charges and separation performance is of paramount importance for future design of LBL NF membranes for a broad spectrum of separation needs.