Solvents in membrane synthesis and their effect on NF/RO performance: from conventional organic solvents to ionic liquids

Membrane technology has grown significantly over the last decades and is used in a broad range of applications nowadays. Nanofiltration (NF) and reverse osmosis (RO) are applied for the separation of low molecular weight components (< 1000 Da) and salts from the feed stream. The main part of the commercial NF and RO membranes are either integrally skinned asymmetric (ISA) or interfacially polymerized thin film composite (TFC) membranes. Polyamide (PA) TFC membranes are the standard in aqueous NF and RO applications, thanks to their very thin, dense top layer, able to form hydrogen bonds with water. For solvent-resistant nanofiltration (SRNF) applications, mainly ISA membranes are applied currently, which are very simple and fast to prepare. Unfortunately, they often suffer from rather low solvent permeances, associated with their thicker selective layer compared to that of TFC membranes. Therefore, the application of TFC membranes in SRNF is currently intensively investigated.The solvents used in the synthesis and post-treatment of (SR)NF and RO membranes have an impact on several aspects of the preparation process, like monomer and polymer solubility, monomer diffusion coefficients, solvent exchange rate and degree of swelling of the membrane. Therefore, they largely influence the chemical and morphological properties of the resulting membranes, and can thus significantly improve their performance. However, in interfacial polymerization, very similar solvents have always been applied to prepare the top layer, limiting the potential to obtain an optimized performance. Solvent post-treatments are often applied to improve this performance after synthesis, but the mechanism behind these treatments is still largely unclear. Therefore, in this PhD, the importance of the solvent type in interfacial polymerization and in post-synthesis solvent treatments was investigated. This resulted in important improvements in both the synthesis procedures and in membrane performance.
The first part of this thesis focused on the potential to use ionic liquids (ILs) as reaction medium in interfacial polymerization, by replacing either the standard hexane or aqueous phase by an IL. As the physicochemical properties of ILs differed largely from those of conventional solvents, their use affected top layer formation in several ways. The replacement of hexane by an IL led to multiple advantages in the synthesis process. Not only the concentration of the amine monomer used for top layer formation could be reduced drastically, also the addition of commonly used additives could be omitted. By recycling the IL for use in consecutive interfacial polymerization cycles, the mass intensity of the top layer formation process decreased with 64%, resulting in a 52% lower mass intensity compared to the conventional interfacial polymerization. Also the residual acyl chloride monomer in the IL after top layer formation could be recycled, as the IL protected it from hydrolysis by lowering the reactivity of dissolved water molecules. Since the top layers formed via the IL-based interfacial polymerization were thinner, smoother, more hydrophilic, and showed a higher free volume size, they obtained a higher permeance and a significantly lower colloidal and organic fouling tendency.
In the second part of this research, post-synthesis solvent treatments of both TFC and ISA membranes were studied in detail to further enhance membrane performances. Solvent activation of TFC membranes is a frequently used technique to improve the RO and SRNF performance of this type of membranes. It generally results in a drastic increase in permeance, while no decrease in selectivity is observed. Despite the clear benefits of this solvent treatment, the mechanism behind it still has been unclear. In this work, the occurrence of PA oligomer leaching from the top layer during solvent activation was proven, and an attempt was made to further optimize the leaching process. Since a similar treatment could possibly have a comparable effect on other types of membranes than these TFC membranes, the influence of a solvent treatment on the morphology and performance of ISA polyimide (PI) membranes was also investigated. The membrane was first cross-linked chemically to enable the use of harsh organic solvents. As this type of membranes is totally composed of preformed, high molecular weight polymers, no oligomeric fragments could leach during the treatment, and therefore, no increase in permeance was observed here. Instead, the permeance drastically decreased and the retention increased after immersion in DMF, caused by densification of the membrane skin layer. The degree of densification was related to the polymer-solvent affinity, resulting in a varying degree of swelling and subsequent reorganization of the polymer chains. Besides the possibility to establish more energetically favorable interchain interactions during this reorganization, densification was also driven by extra cross-linking during immersion, due to a facilitated contact between the solvated, flexible polymer chains and partly unreacted cross-linker molecules. This simple treatment could transform ultrafiltration membranes into highly permeable membranes with selectivities in the (SR)NF range, showing an up to 400% higher solvent permeance compared to commercial SRNF membranes.