Ion specific effects on aqueous phase separation of responsive copolymers for sustainable membranes

Hypothesis: Salt identity and concentration affects the preparation of membranes via the aqueous phase separation approach. The phase inversion process and morphology of the resultant membranes is expected to vary as function of these two parameters. Experiments: Polymeric membranes based on the responsive copolymer polystyrene-alt-maleic acid (PSaMA) are prepared using the aqueous phase separation approach and the influence of salt identity (Na₂SO₄, LiCl, NaCl, NaNO₃, NH₄Cl, MgCl₂, CaCl₂) and concentration on resultant membrane morphology and separation performance is investigated. Complementary stability experiments of PSaMA solutions are performed to help understand the intricate aqueous phase separation process. Findings: Specific ion effects are observed during membrane formation by the aqueous phase separation approach. At equal ionic strengths, Na₂SO₄ and LiCl lead to the formation of more open membrane structures compared to NaCl, NaNO₃, NH₄Cl, and MgCl₂, while CaCl₂ results in membranes with dense top layers. These ion-specific effects are likely caused by a combination of ion mobility and interaction potential between the ion and the polyelectrolyte. Overall, from this work it becomes clear that salt identity and concentration are key parameters in the APS process, and they can be optimised to tune membrane structure from open microfiltration to dense nanofiltration membranes.