Nanofiber multilayer membranes with tailored nanochannels prepared by molecular layer-by-layer assembly for high throughput separation

Abstract

Despite the excellent potential of molecular layer-by-layer assembly (mLbL) in tuning the nanostructure and chemical functionality of the polyamide selective layer of thin film composite (TFC) membranes, practical application of mLbL is limited due to its slow film growth, low permeability and poor chlorine stability. Herein, we demonstrate the preparation of multilayer nanofiber membranes with defined nanochannels by uniformly intercalating organosilicate LAPONITE® (Lp) nanoplatelets into polyamide layers by mLbL. Incorporation of Lp nanoplatelets into the layers formed between trimesoyl chloride (TMC) and m-phenylenediamine (MPD) facilitated rapid growth of the selective layer (∼29 nm per layer for the TMC/MPD/Lp trilayer compared to ∼9 nm per layer of the TMC/MPD bilayer structure). Physicochemical characterization of the selective layer illustrated an interplanar nanochannel spacing of 0.92 nm between the Lp particles due to the spacing effect of polyamide layers. Multilayer membranes with 6 trilayers of TMC/MPD/Lp showed a water flux of 25.8 L m⁻² h⁻¹ and NaCl rejection of 89.8% at an applied pressure of 10 bar; this permeability is 1.5 times higher than that of commercial and graphene based membranes while still maintaining high NaCl rejection. Uniformly distributed nanoplatelets in the selective layer improved chlorine stability of the multilayer membranes. The high separation efficiency of the nanofiber membrane resulted from a combination of the nanochannel size in the separating layer (d-spacing of 0.92 nm) and the high porosity of the nanofiber mat. Overall, a multilayer membrane designed by mLbL is highly versatile in tailoring the distribution of nanomaterials (such as graphene, carbon nanotubes, clay nanoplatelets, etc.) and nanochannel spacing of the selective layer, and can be considered a viable approach for preparing stable membranes for many applications.