Kosmotropes and chaotropes: Specific ion effects to tailor layer-by-layer membrane characteristics and performances

So-called specific ion effects strongly impact physicochemical phenomena in among others, morphologies of polymeric membranes. This can be correlated to the identity of the ions, rather than just to their charge or concentration. We use this effect to tune the properties of layer-by-layer membranes prepared with the polyelectrolytes polydiallyldimethylammonium chloride and polystyrene sodium sulfonate. To systematically study this, membranes were prepared using different salts (NaBr, NH₄Cl, MgCl₂, NaCl, LiCl, MgSO₄, NaH₂PO₄, (NH₄)₂SO₄, Na₂SO₄). The specific ion effects were evaluated thanks to the classification of salts and PEs into kosmotropes (well hydrated) and chaotropes (poorly hydrated) following the law of matching water affinities (LMWA). The impact of the type of anions and cations on the multilayer formation and membrane performances was evaluated. Membranes prepared with kosmotropic salts achieved stable performances after only 4 bilayers, and almost 6 times higher water permeabilities than membranes prepared with chaotropic salts without compromising and often even improving the membrane's size exclusion behaviour. The salts used tuned the membranes charge: Kosmotropic salts delivered negatively charged membranes with Na₂SO₄ and MgCl₂ retentions till 97% and 10% respectively, while chaotropic salts formed more positively charged membranes with reversed behaviour with Na₂SO₄ and MgCl₂ rejection of 20 and 89%.