Layer-by-layer assembled membranes containing hexacyclen-hexaacetic acid and polyethyleneimine N-acetic acid and their ion selective permeation behaviour

Ultrathin separation membranes were prepared upon alternating electrostatic adsorption of polyvinylamine (PVA) and 1,4,7,10,13,16-hexacarboxymethyl 1,4,7,10,13,16-hexaazacyclo-octadecane (az6ac), or PVA and poly(N-carboxymethyl iminoethylene) (LPEI-ac) on porous polymer supports. Membrane formation was optimum at pH 6 of the dipping solutions. Membranes were studied on ion permeation under conditions of diffusion dialysis, nanofiltration (NF) and reverse osmosis (RO). Membranes containing az6ac exhibit a high cation selectivity in diffusion dialysis, the separation factor α(NaCl/MgCl2) being 28, whereas the selectivity in chloride/sulfate transport is only poor. Under RO conditions (Δp = 40 bar), the PVA/az6ac membranes exhibit a high salt rejection (NaCl: 85%, CaCl2: 95%), the total flux being three times higher than for corresponding all-polyelectrolyte membranes, e.g., PVA/PVS (polyvinylsulfate). Membranes containing PVA and LPEI-ac, the polymeric open-chain analogue of az6ac, are generally less selective in ion transport and the flux is lower. The origin of the advantageous properties of PVA/az6ac membranes is discussed.