Abstract
HypothesisSalt 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. ExperimentsPolymeric 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. FindingsSpecific 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.
Highlights
Hypothesis: Salt identity and concentration affects the preparation of membranes via the aqueous phase separation approach
The influence of different salts on the precipitation of polystyrene-alt-maleic acid (PSaMA) for sustainable membrane production is investigated in three parts: 1. Ion specific effects were studied in a model system using turbidity measurements; 2
Turbidity is measured in formazin nephelometric units (FNU) which is based on scattered light from dispersed colloidal particles of a formazin standard [33]
Summary
Using a larger concentration of H3O+ results in a larger driving force to protonate and precipitate the polymer and vice versa This can be compared to the solvent to non-solvent ratio in NIPS, where the addition of solvent to the coagulation bath can be used to slow down the kinetics of the phase separation. From the perspective of well-defined colloidal systems [18] such as negatively charged silver iodide sols [19] and anionic polystyrene latex particles, [20,21,22] individual cations and anions can be ordered from those that have the greatest salting-out effect (i.e. lowest critical coagulation concentration, CCC, for the colloidal solutions) to those that have the least (i.e. highest CCC) This leads to the following series for cations: Ca2+ ~ Mg2+ > NH4+ > K+ > Na+ > Li+, and anions: SO42À > FÀ > ClÀ > NO3À > SCNÀ. By investigating how salt affects the phase separation, additional control over membrane morphology can be achieved which in turn leads to membranes with improved performance
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