Extrinsic compensation on separation performance of layer-by-layer nanofiltration membranes

Extrinsic compensation, the adsorption of counter ions to the excessive charges in layer-by-layer (LBL) nanofiltration (NF) membrane, has been proposed as an important factor in determining the NF separation performance. This paper reported the evidence of extrinsic compensation of positively overcompensated poly(styrene sulfonate) (PSS)/poly(allylamine hydrochloride) (PAH) LBL NF membranes by phosphoric anions. Direct observation of phosphorous (P) element by X-ray photoelectron spectroscopy in LBL membrane after being soaked in H₃PO₄ was accompanied by reduced zeta potential from positive to negative (pH = 7) and the consequently improved rejection to Na₂SO₄ with no discernable alteration in pore size. Similar changes of LBL membrane performance were observed after soaking in other strong acids including H₂SO₄, HNO₃ and HCl. This result opens a new concept for understanding the ion-membrane interaction in determining the separation performance of nanofiltration. Systematic elaboration of this factor in LBL membrane design is needed in future study.