Rapid fabrication of selective barriers through layer-by-layer self-assembly of non-stoichiometric polyelectrolyte complexes for salt and dye fractionation

Abstract

Layer-by-layer (LbL) self-assembly of oppositely charged polyelectrolytes (PEs) is a flexible and environment-friendly route for the construction of functional surfaces and membranes. However, LbL assembly is time-consuming and requires multiple layers to fabricate a separation membrane. In this context, we report the LbL self-assembly of non-stoichiometric PE complexes (PECs) for the rapid fabrication of molecular selective barrier layers. PECs containing strong charge groups and relatively large dimensions compared to those of uncomplexed PEs facilitate the rapid formation of barrier layers on base substrates. The Mem-PEC3 membrane prepared by the LbL self-assembly of the PECs (total three layers) shows 99.4% to >99.9% interception of several dyes in the presence or the absence of monovalent/divalent salts. Mem-PEC3 can fractionate salt (50 g L⁻¹) and DR-80 (1 g L⁻¹) with a salt-to-dye separation factor of ∼3300 at 1.38 bar of applied pressure. This membrane exhibits >99% flux recovery after the fractionation of salt and dye. The membrane exhibits stable performance at high salt concentrations and at a pH range of 3–12. PECs form strong ion pairs during LbL assembly starting from the first layer, which tightly hold the entire assembly on the base substrate. This work provides rapid electrostatic LbL assembly of premade PECs, which is hitherto unknown to create a molecular selective barrier.