Hydrophobic polyelectrolyte multilayer membranes

Abstract

The present invention relates to a hydrophobic polyelectrolyte multilayer (PEM) membranes comprised of a polyelectrolyte layer comprised of one or more bilayer(s) of a polycation and a polyanion, preferably an nanofiltration PEM membrane. The present invention further relates to a method for providing an asymmetrical PEM membrane, or the use of the PEM membranes for the purification, decontamination or cleanup of water, including drinking water.

Description

[0001] HYDROPHOBIC POLYELECTROLYTE MULTILAYER MEMBRANES

[0002] Description

[0003] The present invention relates to hydrophobic polyelectrolyte multilayer (PEM) membranes comprised of a polyelectrolyte layer comprised of one or more bilayer(s) of a polycation and a polyanion, preferably a nanofiltration PEM membrane. The present invention further relates to a method for providing an asymmetrical PEM membrane, or the use of the PEM membranes for the purification, decontamination or cleanup of water, including drinking water.

[0004] Polyelectrolyte multilayers (PEMs) have proven to be versatile materials with a vast range of applications. One area where PEMs have shown remarkable success is membrane technology. Here, support membranes coated with PEMs have surpassed the laboratory stage and are now practically applied in commercial settings. A PEM membrane is a type of membrane composed of alternating layers of positively and negatively charged polymers, known as polyelectrolytes. These membranes are typically assembled using a technique called layer-by-layer (LbL) deposition, by alternately adsorbing oppositely charged polyelectrolytes. The buildup of PEMs can be controlled and tuned using pH, ionic strength, temperature, number of layers, polyelectrolyte molecular weight, and solvent and / or solute type. During LbL deposition, a suitable growth substrate, usually charged, is dipped back and forth between dilute baths of positively and negatively charged polyelectrolyte solutions. During each dip a small amount of polyelectrolyte is adsorbed, allowing the gradual and controlled build-up of electrostatically cross-linked films of polycation-polyanion layers. This method allows precise control over the thickness and composition of the membrane at the nanoscale. Other suitable methods for coating the substrate are for example dynamic coating or role-to-role processing. Thickness control of such films may be down to the single-nanometer scale depending on the deposition conditions. Furthermore, the properties of PEM membranes can be tailored by choosing different polyelectrolytes, adjusting the number of layers, and modifying the assembly conditions.

[0005] PEM membranes can be used for water purification, as defined by its retention, where salts and small organic contaminants, like e.g. micropollutants, are rejected (retained) by the membrane while water is allowed to pass in order to remove these contaminants or lower their concentration in e.g. drinking water. To remove e.g. pesticides, herbicides or medicines, nanofiltration (NF) membranes are being used. For chemically stable NF membranes, dense layers of polyelectrolytes, i.e. sulfonated poly(ether ether ketone) (SPEEK) are coated, typically on top of polyethersulfone (PES) supports. Other methods of obtaining dense NF membranes can be based on interfacial polymerization, where a crosslinked polymeric network of 50-500 nm is formed on top of a porous support. Currently, the densest commercially available PEM membranes have a molecular weight cutoff (MWCO) 400 Da, but there is a particular interest within the scientific community to develop denser PEM membranes. Dense PEM membranes can be employed in the removal of organic micropollutants from wastewater.

[0006] A frequently used approach to create denser PEM membranes is to increase the ionic crosslink density of the PEM. This can be done by increasing the amount of intrinsic charge compensation by using low salt concentrations or optimizing the pH in the case of pH-sensitive polyelectrolytes. Another strategy is to use polyelectrolytes with a high charge density, either by using a different chemistry or by increasing the ratio of charged monomer units. However, the most recent scientific publications highlighted that the charge density of the used polyelectrolyte pair and the MWCO of the PEM membrane are not correlated, showing that creating PEM membranes with a low MWCO is more complicated than initially assumed. The suspected cause of this is the contribution of other intermolecular interactions, besides ionic crosslinking which play a role in PEM buildup.

[0007] While electrostatic interactions are pivotal for the buildup for a polyelectrolyte multilayer, it is understood there are many more types of interactions including hydrogen bonding, hydrophobic interactions, charge transfer, host-guest coordination chemistry (i.e. 71-71 and 7i-cation interactions), and covalent bonding which can play a role during LbL assembly, though not all these interactions are well studied and clear in the context of membranes. Although many of these interactions are weaker in magnitude than the electrostatic interactions, it is important to note that this effect comes on top of the electrostatic interactions present in PEM layers and can thus strengthen the overall interaction. Especially the hydrophobic interactions can have a substantial contribution to the overall binding energy and cannot be neglected. Optimizing these interactions may prove advantageous for creating denser PEM membranes.

[0008] A dense separation layer is not the only requirement for commercially interesting PEM membranes. A very important requirement for commercial use is the chemical stability of the used polyelectrolytes and resulting PEM membrane. In industrial applications, the used membranes are frequently cleaned with hypochlorite, which is used for the oxidation of organic foulants. However, hypochlorite can be very damaging to the PEM layer over time, resulting in a decrease in the membrane properties and filtering functionality of the PEM membrane. Finally, permeability and selectivity are intrinsically linked in membrane technology. Dense membranes typically have a low permeability. This may hinder the application of those membranes in practice.

[0009] Considering the above, there is a need in the art for a PEM membrane having improved density, i.e. comprised of a dense separation layer having decreased molecular weight cutoff (MWCO), while at the same time the PEM has improved chemically stability for large scale and long-term commercial use and maintains high membrane permeability and selectivity. For example, the PEM membrane qualifies as a higher stability membrane when it shows stable performance when exposed to hypochlorite and at extreme pH conditions. In addition, there is a need in the art for a method for providing these improved PEM membranes.

[0010] It is an object of the present invention, amongst other objects, to address the above need in the art. The object of present invention, amongst other objects, is met by the present invention as outlined in the appended claims.

[0011] Specifically, the above object, amongst other objects, is met, according to a first aspect, by the present invention by a polyelectrolyte multilayer (PEM) membrane comprised of a polyelectrolyte layer, wherein the polyelectrolyte layer is comprised of one or more bilayer(s) comprised of a polycation and a polyanion, wherein said polycation is comprised of amine-based monomers wherein the amine is quatemized with an alkyl group, wherein the polycation is an amine based polycation selected from the group consisting of Poly(4-vinylpyridine) (P4VP), Poly(3-vinylpyridine) (P3VP), Poly(2-vinylpyridine) (P2VP), poly(allylamine hydrochloride) (PAH), poly(ethyleneimine) (PEI), poly(diallyl methyl amine hydrochloride) (PDAMAHC), and a copolymer of 2-propen-l-amine-hydrochloride with N-2-propenyl-2-propen-l -aminehydrochloride (CPPAHC), polyvinylamine (PVA), or copolymers thereof, preferably P4VP or copolymers thereof.

[0012] The unique requirement for both dense and stable PEM membranes are met by using a polyelectrolyte with both a quaternary amine group and a hydrophobic alkyl chain. Furthermore, the PEM membrane showed to have optimal permeability and selectivity properties. The main driver for hydrophobic interactions is entropic gain upon release of water molecules when hydrophobic polyelectrolytes interact and the additional enthalpic interactions that become possible for the released water molecules. It has been shown that more hydrophobic multilayers contain less water, and that hydrophobic polyelectrolyte complexes are less prone to swelling and this is advantageous for creating denser membranes, as membrane swelling has shown to be a better predictor for MWCO than the charge density of the used polyelectrolyte pair. To increase the strength of the hydrophobic interactions, hydrophobic side groups are added to the polyelectrolytes, preferably by attaching alkyl chains (of increasing chain length) to polyelectrolytes. P4VP is a preferred amine based poly cation due to the location of the amine situated in the aromatic pyridine ring. Due to the N placed on the aromatic ring most distant from the backbone of the monomer, there is least steric hinderance for the quatemizing reaction to be carried out and adding the alkyl to the amine based polycation. Therefore, the P4VP due to its structure is preferred and provides the most efficient and optimal PEM membranes enabling stable and dense membrane structures.

[0013] Quatemization of a polymer refers to a chemical modification process where a polymer is treated to introduce quaternary ammonium groups (-N R,. where R is an alkyl group) into its structure. This process enhances certain properties of the polymer, such as its hydrophobicity. Amine based polyelectrolytes are positively charged polyelectrolytes and can be distinguished by their degree of substitution; primary (NH2R), secondary (NHR2), tertiary (NR3), and quaternary (NR / ) amine poly cations. The quaternary substituted polycations are preferred for obtaining improved stable PEM membranes, because their charge is independent on pH and they cannot be oxidized, as the quaternary amines are already in their lowest redox state meaning that they are stable in oxidising environments such as in hypochlorite solutions, which is often present as decontaminant in water or used to clean fouled membranes. The use of strong (for example permanently charged) polyelectrolytes also guarantee a high pH stability of the PEM membrane. Thus, the use of quaternary amines in the PEM is advantageous as it yields higher stability to both hypochlorite and extreme pH conditions.

[0014] Furthermore, the addition of hydrophobic groups denser membrane systems can be produced and resulting in less water is being taken up by the PEM membrane, reducing membrane swelling, and increasing membrane capabilities such as permeability and selectivity of the PEM membrane. By using a polyelectrolyte with both a quaternary amine group and a hydrophobic alkyl chain, a dense and stable PEM membrane is achieved. Furthermore, results show that the resulting PEM membrane still has a high permeability, comparable to the non- quatemized PEM membranes, for example non-quatemized poly(4-vinylpyridine) (P4VP).

[0015] According to another preferred embodiment, the present invention relates to the PEM membrane, wherein the polyanion is selected from the group consisting of poly(styrene sulfonate) (PSS), poly(acrylic acid) (PAA) and poly(vinylsulfonic acid sodium) (PVS), preferably poly(styrene sulfonate), sulfonated polyarylene ethers, or copolymers thereof, preferably PSS or copolymers thereof. To influence the PEM membrane performance, the charge density and hydrophobicity of polyelectrolytes on the PEM membrane performance, several combinations of polyelectrolytes (polyanion-polycation) can be used.

[0016] According to a preferred embodiment, the present invention relates to the PEM membrane, wherein the polyanion is a co-polymer of polyethersulfone (PES) and partly sulfonated PES (SPES, wherein the partly sulfonated PES having a degree of sulfonation of at least 5%, 10%, 15%, 20%, 25%, 30%, 35%, 40%, 50%, 60%, 70%, 80%, 90%, 100%, preferably at least 15%, more preferably at least 25%, most preferably at least 35%. As this polymer is only partly sulfonated it is a co-polymer of polyethersulfone (PES) and SPES. Experiments have shown that a PEM membrane comprised of a polyanion of a PES and SPES co-polymer, provided excellent permeability, membrane selectivity and salt retention properties. Furthermore the molecular weight cutoff (MWCO) are exceptionally low due to the hydrophobic and densely packed structure of these membranes. According to yet another preferred embodiment, the present invention relates to the PEM membrane according, wherein the alkyl is a hydrophobic alkyl having a carbon chain length of between 1 to 20 carbons, preferably 2 to 10 carbons, more preferably 3 to 6 carbons, most preferably 4 to 5 carbons. Experiments show that with increasing carbon length the MWCO values of the resulting PEM membrane decrease thereby providing a denser membrane improving the filtering properties of the PEM membrane. However, when using alkyls having a higher carbon length (>10 C length), also the hydrophobicity of the polymer membrane will increase making it harder, and at some point, not possible anymore, for the polymer to become soluble in water since the polymer becomes too hydrophobic. In that case instead of water-based solvent an organic solvent such as acetone, ethanol or toluene may be used, however this option is less preferred due to the increasing environmental and health legislation steering towards a more environmentally friendly method of producing PEM membranes.

[0017] According to a preferred embodiment, the present invention relates to the PEM membrane, wherein the alkyl is one or more selected from the group consisting of methyl-, ethyl-, propyl-, butyl-, pentyl-, hexyl-, heptyl-, or octyl -group, preferably a methyl- or ethyl -group.

[0018] According to another preferred embodiment, the present invention relates to the PEM membrane wherein the PEM membrane has a molecular weight cutoff (MWCO) of at most 275 Da, preferably at most 250 Da, more preferably at most 220 Da, even more preferably at most 180 Da, most preferably at most 140 Da, even more preferably at most 130 Da. The chemistry of present invention used in the PEM membrane enables to provide membranes that have very low MWCO values and are at the same time very durable in view of higher stability to both hypochlorite and extreme pH conditions. At present no nanofiltration PEM membranes are known within such low MWCO values, having a dense and chemically stable structure.

[0019] According to a preferred embodiment, the present invention relates to the PEM membrane, wherein the PEM membrane has a permeability of at least 4 L m-2 h-1 bar-1, preferably at least 6 L m-2 h-1 bar-1, more preferably at least 8 L m-2 h-1 bar-1, even more preferably at least 10 L m- 2 h-1 bar-1, most preferably at least 12 L m-2 h-1 bar-1. On average, known commercially available PEM nanomembranes have a permeability of between ~7 or 8 to 20 L m-2 h-1 bar-1. However, there is a fine balance between permeability and membrane selectivity and salt retention properties. At present the commercially available nanofiltration membranes with low permeability of about 7 to 8 also have poor / strongly reduced selectivity and salt retention properties, thereby reducing the membrane filtering properties. In contrast, membranes of the present invention have both good permeability as well as membrane selectivity and salt retention properties. Due to the addition of the hydrophobic groups the membrane of the present invention can be tuned towards very low permeability values. According to another preferred embodiment, the present invention relates to PEM membrane, wherein the PEM membrane has a salt retention value of MgCE of at least 80%, preferably at least 85%, more preferably at least 90%, most preferably at least 95%. Experiments show that the PEM membrane of present invention has excellent salt retention properties, especially when compared to the commonly used PDADMAC based membranes in nanofiltration. Both the positive and negative terminated PEM membranes of present invention show improved salt retention for different single component salt solutions (NaCl, Na2SC>4, MgCE or MgSCE). Especially the negatively terminated quatemized P4VP / PSS according to present invention is much improved in view of MgCE retention. The results indicate that the contribution of dielectric exclusion to the ion retention is very much increased in comparison to the non-quatemized membranes. Dielectric exclusion takes place in the membrane matrix and is caused by the difference in the dielectric constant of the membrane and the feed solution. In this exclusion mechanism, the ion retention is mainly determined by the valency of the ions, and the charge sign of the ion plays a much smaller role. A higher contribution of dielectric exclusion is indicative of a denser membrane. This is in line with the permeability and MWCO measurements of the membranes of the present invention.

[0020] According to a preferred embodiment, the present invention relates to the PEM membrane, wherein the PEM membrane has an organic micropollutant (OMP) retention value that is at least 1.2 to 3x higher, preferably 1.3 to 2.5x higher, more preferably 1.5 to 2x higher than the OMP retention value of a comparable PDADMAC based PEM membrane. The PDADMAC based membrane that is commonly used and studied in the art is for example the PDADMAC / PSS membrane. In the present invention, the polycation terminated PEM membranes are denser than polyanion membranes, likely due to the higher ratio of hydrophobic polymers. Moreover, OMP retention increases with increasing alkyl chain length. The QP4VP membranes outperform PDADMAC / PSS membranes in selectivity and are made with more chemically stable polyelectrolytes than other dense PEM membranes. Consequently, the permeate concentration of the PEM membranes of present invention are 1.2 to 3x lower in view OMPs compared to PDADMAC.

[0021] According to another preferred embodiment, the present invention relates to the PEM membrane, wherein the PEM membrane has an organic micropollutant (OMP) retention value of at least 50%, preferably at least 55%, more preferably at least 60, most preferably at least 63%, preferably wherein the OMP is a positively charged OMP. Retention values of present invention are preferably related to OMPs that have a molecular weight of between 50 to 600 Da, preferably 100 to 500, more preferably 135 to 370 Da. The contribution of size exclusion is also reflected in the OMP retention trend for the QP4VP membranes. Generally, the OMP retention increases upon increase of the alkyl chain length, with Me-QP4VP / PSS having lower retentions than Pr- QP4VP / PSS. This clearly shows that increasing the hydrophobic interactions is a viable method for increasing OMP retention, and that especially high retentions can be obtained for positively charged OMPs.

[0022] According to yet another preferred embodiment, the present invention relates to the PEM membrane, wherein the OMP is one or more selected from the group consisting of salicylic acid, ibuprofen, naproxen, sulfamethoxazole, diclofenac, bezafibrate, pyrazole, benzotriazole, paracetamol, caffeine, isoproturon, atrazine, bisphenol A, carbamazepine, phenolphthalein, bromothymol blue, metformin, lidocaine, atenolol, metoprolol, sotalol, nadolol, amisulpride, PFAS molecules and Chlorothalonil metabolites, preferably wherein the OMP is a positively charged OMP, even more preferably wherein the OMP is one or more selected from metformin, lidocaine, atenolol, metoprolol, sotalol, nadolol and amisulpride.

[0023] According to another preferred embodiment, the present invention relates to the PEM membrane, wherein at least 70%, preferably at least 75%, more preferably at least 80%, even more preferably 90%, most preferably at least 95% of the amine-based monomers of the polyelectrolyte are quatemized with and an alkyl group, and the remaining amine-based monomers monomers of the polyelectrolyte layer are unquatemized. By having partly quatemized PEM membranes, the flexibility is the hydrobopicity of the resulting PEM and enables “tweaking” and control of the PEM membrane in view of density and retention properties, depending on the use and goal of the membrane. Furthermore, an in part quatemized membrane (i.e. 75% of the amine-based monomers of the polyelectrolyte layer are quatemized with and an alkyl group) is cheaper and less time consuming to produce and therefore may be commercially a more preferred option.

[0024] According to a preferred embodiment, the present invention relates to the PEM membrane, wherein 100% of the amine-based monomers of the polyelectrolyte layer are quatemized. The PEM membrane that is fully quatemized (i.e. substantially 100% of the amine-based monomers of the polyelectrolyte layer are quatemized with and an alkyl group) provides the highest stability due to the full protection of the monomers, which are all quaternary substituted polycations and cannot be oxidized in oxidizing environments such as in hypochlorite solutions and extreme pH conditions.

[0025] According to another preferred embodiment, the present invention relates to the PEM membrane, wherein the PEM membrane is comprised of at least 1 bilayers, preferably at least 2 bilayers, more preferably at least 5 bilayers, even more preferably at least 7 bilayers, most preferably at least 9 bilayers.

[0026] According to yet another preferred embodiment, the present invention relates to the PEM membrane, wherein the PEM membrane is comprised of a positively or negatively terminated bilayer, preferably a negatively terminated bilayer. A negatively terminated bilayer is preferred for the PEM membrane applications for its lower degree of fouling and greatly improved salt retention properties due to the improved dielectric separation properties of the PEM membrane of present invention in comparison to other known negatively terminated PEM membranes such as the PDADMAC / PSS. Positively terminated may be used in view of improved membrane permeability in contrast to negatively terminated membranes. Depending on the application a positive terminated membrane may be more suitable, for example filtering microbial pollutants, such as viruses and DNA that are often negatively charged.

[0027] According to a preferred embodiment, the present invention relates to the PEM membrane, wherein the PEM is a nanofiltration membrane, more preferably an asymmetrical nanofiltration membrane.

[0028] According to another preferred embodiment, the present invention relates to the PEM membrane, wherein the PEM membrane is an asymmetric membrane comprised of two PEM membrane sections differing in porous density, preferably wherein a first membrane section comprising a first polyelectrolyte layer has a more open porous structure than a second membrane section comprising a further polyelectrolyte layer and wherein the second membrane section is coated on top of the first membrane section providing a denser second membrane section on top. Dense membranes typically have a low permeability. This may hinder the application of those membranes in practice. One way to diminish this effect is by using asymmetric PEM coating. Here, the separation layer consists of two different PEM sections, an open bottom section to guarantee high permeability, and a dense top section to create high selectivity. It is important that the resulting PEM membrane still has a high permeability. Permeability and selectivity are intrinsically linked in membrane technology. As a consequence, dense membranes typically have a low permeability. This may hinder the application of those membranes in practice. One way to diminish this effect is by using asymmetric PEM coating thereby creating an asymmetric PEM membrane. Here, the separation layer of the membrane consists of two different PEM sections, a more “open” bottom section to guarantee high permeability, and a denser top section to create high selectivity of the PEM membrane.

[0029] According to a preferred embodiment, the present invention relates to the PEM membrane, wherein the first polyelectrolyte layer is comprised of amine based polycations that are quatemized with methyl-alkyl groups and the at least one further polyelectrolyte layer is comprised of amine based polycations that are quatemized with propyl-alkyl groups.

[0030] The present invention, according to a second aspect, relates to a method for providing a (hydrophobic) PEM membrane as disclosed herein, wherein a support membrane is coated with at least one layer of a polyelectrolyte as disclosed herein, preferably coated with at least two layers of said polyelectrolyte. The support membrane functions as membrane scaffold to be coated and is generally a porous ultrafdtration support membrane, however also other types may be used including microfdtration support membranes. As also disclosed herein, the layer of a polyelecrolyte (or polyelectrolyte layer) is comprised of one or more bilayer(s) comprised of a polycation and a polyanion.

[0031] According to a preferred embodiment, the present invention relates to the method, wherein said PEM membrane is an asymmetrical PEM membrane, and wherein the support membrane is coated with at least one polyelectrolyte layer having a more open porous structure providing a first membrane layer, wherein on top of said at least one membrane layer at least one further polyelectrolyte layer is provided providing a second membrane layer having a denser porous structure than the first membrane layer. Asymmetrical PEM membranes enable the provision of a denser membrane with improved retention and selectivity while at the same time providing optimal water permeability due to the asymmetrical build of the membrane system.

[0032] According to a preferred embodiment, the present invention relates to the method, wherein the first membrane layer comprises a polycation that is comprised of amine based monomers wherein the amine is quatemized with and an alkyl group, and wherein the second membrane layer comprises a polycation that is comprised of amine based monomers wherein the amine is quatemized with and an alkyl group that differs from the first membrane layer.

[0033] The present invention, according to a further aspect, relates to the use of a PEM membrane of the present invention for the purification, decontamination or cleanup of water, preferably drinking water. Preferably, the PEM membrane is used for the purification of water from organic micropollutants (OMP), small organic molecules, pesticides, PFAS molecules, hormones, plasticizers, heavy metals, inorganics salts, pharmaceuticals, medicine residues, bacteria, viruses and other microorganisms preferably positively charged OMPs. Harmful substances, including small molecule residue drugs or harmful pesticides can be removed from for example (drinking) water (i.e. yielding cleaned drinking water). But also surface water may be purified with the PEM membranes of present invention, including desalination or descaling of water.

[0034] The present invention will be further detailed in the following examples and figures wherein:

[0035] Figure 1: shows several examples of polycations for the PEM membranes of present invention; methyl-, ethyl- and propyl-substituted Poly(4-vinylpyridine), referred to as Me-QP4VP, Et-QP4VP and Pr-QP4VP, respectively, are indicated. These poly cations are with the alkyl groups fully quatemized poly cations. Furthermore, an in-part quatemized hydrophobic polycation was also synthesized, having about 75% methyl-substituted P4VP monomers, referred to herein as 75%Me-QP4VP.

[0036] Figure 2: shows the permeability of PEM membranes of present invention having a negatively terminated or positively terminated configuration. For all systems PSS was used as the polyanion. The quatemized PEM membranes show to have sufficient permeability to be of commercial relevance, also compared to the commonly studied PDADMAC based system.

[0037] Figure 3: shows the molecular weight cutoff value (MWCO) of PEM membranes of present invention. This MWCO data demonstrates that hydrophobic interactions applied in the PEM membranes of present invention can be used to create dense and very stable PEM membranes.

[0038] Figure 4: shows the salt retention values of PEM membranes of present invention, including negatively terminated (Figure 4A) and positively terminated (Figure 4B) PEM membranes. For the negatively terminated QP4VPs, the MgCE retention exceeds the Na2SO4 retention, indicating a negatively charged membrane with Donnan based exclusion. However, in the case of Et-QP4VP and Pr-QP4VP these differences are minor, indicating that dielectric exclusion plays a major role in the ion retention of these membranes. This is in line with the dense nature of these quatemized membranes as observed in the permeability and MWCO experiments. For the negatively terminated QP4VP membranes, the retention of Na2SO4 exceeds the retention of MgCE. However, the difference in retention between these two salts decreases with increasing alkyl chain length, which indicates the contribution of dielectric exclusion to the ion retention increases. This higher contribution of dielectric exclusion is indicative of a denser membrane. This is in line with the permeability and MWCO measurements.

[0039] Figure 5: shows the chemical stability of the PEM membranes comprised of the quatemized poly cations of present invention after exposure to sodium hypochlorite for 10 minutes, 2 hours and 23 hours. Poly(allylamine hydrochloride) (PAH), was included as positive control and PDADMAC was included as negative control. As expected, the PAH sample showed to be sensitive to the exposure to sodium hypochlorite, resulting in white precipitation already after the first visual inspection after 10 min. In contrast, the quatemized polycations showed to be highly stabile and not being affected by oxidation for at least 23 hours that was tested.

[0040] Figure 6: shows the Organic micropollutant (OMP) retention of the PEM membranes comprised of the quatemized polycations of present invention. The retention of the positively terminated membranes was measured using a feed mixture consisting of model OMPs selected to represent a wide range of properties including charge (indicated as 0 or +) and molecular weight (indicated in Da). The PDADMAC (PDADMAC / PSS) and a non-quatemized P4VP (P4VP / PSS) membranes were included as comparison membranes. In some cases, the OMP retention was above the quantification limit. This is indicated with a marker (quantification limits: a= 1%, b = 99.5%, c = 99.9%). In the case of the markers b and c, the displayed retention is the quantification limit. Actual OMP retention may be higher in these cases. Error bars represent the 95% confidence interval.

[0041] Figure 7: Shows the pure water permeability (PWP) and MWCO values of Me-

[0042] QP4VP / SPES, Et-QP4VP / SPES, and Pr-QP4VP / SPES PEM membranes according to the present invention. The membranes are comprised of quatemized poly(4- vinylpyridine) (QP4VP) derivatives: methyl-QP4VP (Me-QP4VP), ethyl-QP4VP (Et-QP4VP), and propyl-QP4VP (Pr-QP4VP) as a polycation and a copolymer of PES and SPES (referred to in the figure as SPES) as a polyanion. PWP values of the resulting membranes demonstrated relatively low permeability due to the dense multilayer structure, and the more hydrophobic Pr-QP4VP / SPES demonstrated a reduced permeability that correlated with increased hydrophobic interactions due to the longer alkyl chains. The Pr-QP4VP / SPES system also exhibited the lowest MWCO, consistent with the membranes' most hydrophobic and densely packed structure. All PEM membranes were terminated with SPES. Error bars represent the 95% confidence interval.

[0043] Figure 8: Shows the retention of single salt solutions with the membranes Me-QP4VP / SPES,

[0044] Et-QP4VP / SPES, and Pr-QP4VP / SPES PEM membranes according to the present invention. The ion retention behaviour of the membranes was evaluated using single-salt solutions comprising combinations of monovalent and divalent ions (NaCl, Na2SC>4, MgCL. and MgSCU). The multilayer membranes exhibit high monovalent and divalent salt retention, substantially higher than observed for the QP4VP / PSS membranes. This selective rejection is attributed predominantly to a dielectric exclusion mechanism, as evidenced by the consistently high retention of divalent ions regardless of charge and significant retention of monovalent NaCl. Error bars represent the 95% confidence interval.

[0045] Examples

[0046] Strong hydrophobic polycations have been synthesized by the quatemization of poly(4- vinylpyridine) (P4VP) and alkyl chains of different lengths will be used for quatemization to induce different extents of hydrophobicity. The stability of those P4VP and the quatemized poly cations in hypochlorite is compared against the golden standard for stability, PDADMAC. Also, the symmetric PEM membranes are fabricated by dip-coating onto hollow fiber support membranes and subsequently their membrane properties are determined on a crossflow setup. Overall, the examples clearly demonstrate that dense and stable PEM membranes can be fabricated through increasing the hydrophobic interactions in the PEM layer. Finally, we also coated asymmetric membranes to demonstrate that the use of hydrophobic polycations is advantageous for the production of asymmetric PEM membranes.

[0047] Chemical being used in the examples below are as follows; NaCl was obtained from Nobian (The Netherlands), 2-component epoxy glue (2K Expert) was obtained from Bison, iodoethane and ethanol (99%) were obtained from Fisher Scientific. lodomethane, 1 -iodopropane, Na2SO4, deuterium oxide, deuterium chloride, sodium hypochlorite (6-14% active chlorine), ethylene glycol and polyethylene glycol (PEG) 200, 400, 600, 1000, 1500 and 2000 were obtained from Merck. Hollow fiber support membranes (inner diameter = 0.7 mm, MWCO = 10,000 Da) were kindly donated by NX Filtration (The Netherlands). Milli-Q water was produced in-house. Poly(4-vinylpyridine) (P4VP, 200 kDa) was obtained from Scipoly (USA). MgC12, MgSO4, Poly(styrene sulfonate (PSS, 200 kDa, 30 wt% in water), glycerol (83.5-89.5%), dialysis membrane (cellulose, MWCO = 14,000 Da), diethylene glycol and triethylene glycol were obtained from Sigma Aldrich. Dimethylsulfoxide (DMSO, 99%) was obtained from Thermo Scientific.

[0048] Example 1 - Synthesis of hydrophobic polycations

[0049] Methyl-, ethyl- and propyl-substituted Poly(4-vinylpyridine) was synthesized, referred to as Me-QP4VP, Et-QP4VP and Pr-QP4VP, respectively. These polycations were fully quatemized, i.e. substituted with the alkyl groups. Furthermore, an in-part quatemized hydrophobic polycation was also synthesized, having about 75% methyl-substituted P4VP monomers, referred to herein as 75%Me-QP4VP, i.e. P4VP with 75% of monomers being quatemized with methyl, and 25% being unquatemized. See also Figure 1 for the stmctures of the synthesized poly cations. Briefly, 4 g of P4VP was dissolved in DMSO. Then, the mixture was heated, and 500% molar excess of an alkyl halide was added. The reaction temperature and added alkyl halide differed for each synthesis: iodomethane (room temperature) for Me-QP4VP, iodoethane (50 °C) for Et-QP4VP and 1-iodopropane (80 °C) for Pr-QP4VP. The solution was stirred for 24 hours. After the reaction was completed, the reaction mixture was purged with N2 for three hours to get rid of excess alkyl halide. Then, the reaction mixture was precipitated in ethanol, and the solvent was removed with vacuum filtration. The precipitate was dissolved in 1.0 M NaCl in water. For the synthesis of 75%Me-QP4VP, 2 g of P4VP was dissolved in water. Subsequently, a 500% molar excess of iodomethane was added. The solution was stirred overnight at room temperature. After the reaction was completed, the reaction mixture was purged with N2 for four hours. Finally, the mixture was freeze dried to obtain the dried product, which was redissolved in Milli-Q water.

[0050] Dialysis was performed to replace the iodide counterions with chloride counterions and to remove ions not participating in extrinsic compensation. First, dialysis was performed for 1.5 days in 0.2 M NaCl, followed by 4 days in Milli-Q water. The external solution was replaced twice daily. Then, dry polymer was obtained by evaporating the solvent in an oven at 110 °C.

[0051] To determine the quality of the synthesized products, an 1H NMR spectrum of P4VP in deuterium oxide with deuterium chloride was measured on a Bruker Ultrashield 600 MHz NMR spectrometer 1H NMR spectra of the substituted compounds in deuterium oxide (residual solvent peak at 5 4.79) were measured on a Brucker Ascend 400 MHz NMR spectrometer (Results not shown).

[0052] Example 2 - Chemical stability of the hydrophobic polycations

[0053] To assess the chemical stability of the synthesized polyelectrolytes, exposure to sodium hypochlorite was performed, followed by visual inspection. In addition a polyelectrolyte comprised of a primary amine group, more specifically poly(allylamine hydrochloride) (PAH), was included as positive control. First, polyelectrolyte and NaOCl solutions are prepared. The polyelectrolyte solutions have a concentration of 0.25 g L-l polyelectrolyte in Milli-Q, and the hypochlorite solution has a concentration of 0.08%. The pH of both solutions is set to pH 8 to ensure maximum activity of NaOCl. 0.125 g L-l polyelectrolyte solution is used as a blank. The samples consist of a 1: 1 mixture of the prepared polyelectrolyte and NaOCl solutions. Samples were inspected by eye for degradation (See figure 5) after lOmin, 2hours and 23 hours after exposure to sodium hypochlorite. As expected, the PAH sample showed to be sensitive to the exposure to sodium hypochlorite, resulting in white precipitation already after the first visual inspection after 10 min. In contrast, the quatemized polycations showed to be highly stabile and not being affected by oxidation for at least 23 hours that was tested. Example 3 - asymmetric P EM membrane by Layer by Layer production

[0054] To assess the effect of the hydrophobic coatings on membrane performance, hollow fiber ultrafiltration support membranes were coated with the layer-by-layer technique. P4VP and its quatemized variants were used as polycations, and PSS was used as a polyanion. Membranes were coated with multilayers of P4VP / PSS or QP4VP / PSS according to the present invention and were compared to PDADMAC / PSS membranes.

[0055] Briefly, the support membranes were cut to approximately 32 cm length and were immersed overnight in 10% ethanol in water to remove glycerol left from the production process. Afterwards, the fibers were immersed in Milli-Q water. Subsequently, polyelectrolyte solutions were prepared containing 0. 1 g / L polyelectrolyte and 50 mM NaCl in Milli-Q water. The pH of P4VP and 75%Me-QP4VP was adjusted to 1.5 and 6.0, respectively. The washing solution consists of 50 mM NaCl in Milli-Q water. Layer-by-layer coating is performed by a dipcoat robot. Here, the fibers are first immersed in polycation solution for 15 minutes. Then, they are submerged 3 times in a washing solution. Next, they are immersed in polyanion solution for 15 minutes, followed by 3 washing steps again. Now, one bilayer has formed on the membrane. The process is repeated until the desired number of bilayers has been deposited.

[0056] To get an impression of the number of bilayers required for membrane coating, multilayers were coated on model surfaces and their growth was studied with optical fixed angle reflectometry. The reflectometry data demonstrate that multilayers can successfully be constructed with the QP4VPs (data not shown). All PEMs constructed with QP4VPs show linear growth, indicating a low mobility of the polyelectrolytes within the multilayer. The similar growth rate meant that an equal number of bilayers could be used to coat the QP4VPs. To ensure proper transition to the layer-dominated regime, 9 (negatively terminated) and 9.5 (positively terminated) bilayers were sufficient for these polycations. The multilayer consisting of P4VP / PSS also shows linear growth, albeit with a much lower growth rate than for the QP4VPs. To get an estimate of the required number of bilayers for this membrane, the growth curve was extrapolated. It was determined that for an equal mass adsorption as for the QP4VPs, 16 bilayers should be coated for the P4VP / PSS system. Therefore, 16 (negatively terminated) and 16.5 (positively terminated) bilayers were coated for this system. For PDADMAC / PSS the growth is well studied and known to be exponential indicating a higher polyelectrolyte mobility. For PDADMAC / PSS 7 and 7.5 bilayers were selected to be within the layer dominated regime.

[0057] After coating, the membranes are deposited in Milli-Q water for 15 minutes and subsequently into an aqueous solution containing 15 wt% glycerol for a minimum of 4 hours to prevent pore collapse. The membranes are dried overnight in an unknown controlled airflow. Finally, the membranes were potted into single-fiber modules. For this, the membranes were placed into 24 cm long plastic tubing, with a hole cut at the midpoint for permeate collection. The membranes were glued into place with epoxy glue and dried overnight. Before testing, the ends of the module were cut off to achieve a clean edge, and the active length of the membrane was measured with a ruler. Four modules were produced per PEM coating type.

[0058] Example 4 - Membrane performance measurements

[0059] Permeability, molecular weight cutoff (MWCO), salt retention and organic micropollutant (OMP) retention of the PEM membranes of example 3 were measured on a custom-build crossflow setup. In the crossflow setup, 16 fibers could be measured in parallel, with four modules measured per type of PEM coating. Before the first measurement, membranes were flushed with Milli-Q water for 30 minutes to remove the glycerol. After each measurement, the membranes were flushed for 20 minutes with Milli-Q water to clean them. During all measurements, the feed temperature was set to 20 ± 1 °C, The crossflow velocity was set to 1 m s’1, the pressure was set to 1.9 bar. Four modules were measured per coating as replicates for all experiments. A Gaussian distribution was assumed for the calculation of the mean values and 95% confidence intervals.

[0060] Pure Water Permeability (PWP)

[0061] For the PWP test, Milli-Q water was used as a feed. The experiment was ran for 1 hour, and permeate was collected in tubes. Those tubes were measured before and after running the test, to determine the mass amount of the collected permeate. The permeability was then determined using formula Pw= wherein Pwis the permeability in L m’2h’1bar’1, m is the mass of the permeate in g, p is the density of pure water (assumed to be E g L’1), A is the surface area of the membrane in m2, t is the duration of the permeation in h, and AP is the transmembrane pressure in bar.

[0062] Results show that a clear trend in permeability is visible for both the negatively and the positively terminated membranes (Error! Reference source not found.2). The permeability of QP4VP membranes decreases as the alkyl chain length increases. This trend can be attributed to the enhanced hydrophobicity of the poly cation as the alkyl chain length increases. Another and surprising trend that is visible is that the positively terminated QP4VP membranes consistently show a lower permeability than the negatively terminated membranes. Especially for Me-QP4VP, the permeability decreases from 18.8 ± 1.2 L m’2h’1bar’1for [Me-QP4VP / PSS]g to 12.9 ± 1.3 L m’2h’1bar’1for [Me-QP4VP / PSS]95, a substantial difference between negative and positively terminated membranes. The positively terminated membranes do contain half a bilayer of additional coating, which increases the thickness of the separation layer and can therefore contribute to the hydraulic resistance. However, the increased thickness of the separation layer is not expected to explain the big differences in permeability which are observed between the negatively and positively terminated membranes, especially since the growth of the multilayer is linear, as was demonstrated with reflectometry. A more likely cause is that the positively terminated membranes contain more of the hydrophobic QP4VPs, resulting in less swelling.

[0063] The P4VP membranes have a permeability of 13.6 ± 1.1 L m-2h1bar1(negatively terminated) and 12.4 ± 1.7 L m-2h1bar1(positively terminated), which gives it a permeability that is in between the Me-QP4VP and the Et-QP4VP membranes. Since the P4VP membrane is less hydrophobic, it could be expected that this membrane shows a higher permeability. In contrast with the QP4VP membranes, the difference in permeability between the negatively and positively terminated P4VP membranes is not statistically significant. This can be explained by P4VP not being as hydrophobic as the QP4VPs, thus the addition of an extra layer for the positively terminated membranes is not expected to lead to a big decrease in swelling, as it would for the QP4VPs. Furthermore, the PEM membranes of the present invention show remarkable permeability values very suitable for commercial purposes and applications, where at present the average permeability being used in nanofiltration is about 6-8 m-2h1bar1. Moreover, the values are very comparable to the golden standard of chemically stable PEM systems, PDADMAC based.

[0064] Molecular Weight Cut-off (MWCO)

[0065] For the MWCO experiment, a feed mixture was prepared containing ethylene glycol, diethylene glycol, triethylene glycol and polyethylene glycol with a molecular weight of 200, 400, 600, 1000, 1500 and 2000, each at a concentration of 1 g L1. Permeate was collected for 30 minutes, and a feed sample was collected 15 minutes after the start of permeate collection. The collected samples were analyzed with gel permeation chromatography (GPC), on an Agilent 1200 / 1260 Infinity GPC / SEC series apparatus containing two columns in series (Standers Service Suprema 8 x 300 mm: 1000 A, 10 pm, followed by 30 A, 10 pm). The retention was calculated using formula R = 1 — * 100, wherein R is the retention in %, Cpis the concentration of the compound in the permeate, and Cf is the concentration of the compound in the feed. The 90% MWCO was determined by interpolation of the retention data.

[0066] A similar trend is visible for the MWCO values of the PEM membranes as observed for the permeability (Error! Reference source not found.). A connection between these properties is typical and is commonly referred to as the permeability-selectivity trade-off. Again, a dependency on alkyl chain length can be observed. For negatively terminated Me-QP4VP, a rather high MWCO of 650 ± 254 Da is observed (which was not statistically significant), which decreases to 257 ± 1 Da for Et-QP4VP and 240 ± 8 Da for Pr-QP4VP. For the positively terminated membranes this trend can be observed as well, having a MWCO of 260 ± 2 Da for Me-QP4VP, to 250 ± 2 Da for Et- QP4VP and 229 ± 2 Da for Pr-QP4VP. These values are significantly better than the MWCO found for PDADMAC, which were around 300 Da for both the negatively and positively terminated systems.

[0067] The observed MWCO’s for Et-QP4VP and Pr-QP4VP are very low. Especially when compared to other commonly used PEM membranes which were coated at similar conditions, such as a MWCO of around 300 Da for PDADMAC / PSS membranes and a MWCO of 267 Da for PAH / PSS membranes (result not shown). The Pr-QP4VP membranes have a MWCO comparable to PEI / PSS membranes coated at similar conditions (239 Da), which is one of the densest multilayers used for PEM membranes. However, PEI contains primary, secondary, and tertiary amine groups, making it sensitive to pH changes and hypochlorite degradation. These groups are not present in Pr- QP4VP. The MWCO experiments demonstrate that hydrophobic interactions can be used to create dense PEM membranes. It is expected that a PEM membrane according to present invention having an MWCO value of about at most 180 Da can be produced. By further incorporating other measures, such as covalent crosslinking, it is expected that MWCO values of about at most 140 Da can be produced.

[0068] Salt Retention

[0069] The retention of different single-component salt solutions was measured. For this, salts consisting of combinations of mono-valent and di-valent ions were used. This includes salts where both ions have the same valency (symmetric salts; NaCl, MgSO4) and salts where both ions have a different valency (asymmetric salts; Na2SO4, MgCE). This combination of salts can give an indication of the ion exclusion mechanisms which play a role in the membrane.

[0070] For the salt retention measurements, single-salt solutions (NaCl, Na2SC>4, MgCE or MgSCE) at a concentration of 5 mM were prepared. Permeate was collected over the course of 2.5 hours. The conductivity of the feed was measured before and after the measurements with a Xylem Analytics WTW Profline Cond3310 conductivity meter. The conductivity of the permeate was measured at the end of the experiment. The salt retention was then calculated using formula R = 1 — * 100, wherein R is the retention in %, opthe conductivity of the permeate and Of the average conductivity of the feed in pS cm'1.

[0071] Results show that for the negatively terminated P4VP membrane, the sequence of ion retention is Na2SC>4 (98.1 ± 0.1 %) and MgCE (13.7 ± 0.9 %) (Error! Reference source not found.). Such a retention pattern, where there is a big difference in the retention of the asymmetric salts is an indication for Donnan exclusion. Indeed, very similar performance is observed for the negatively charged PDADMAC membrane, which is well known to separate by Donnan exclusion. Donnan exclusion takes place at the surface of the membrane. Here, the charged surface excludes ions with the same charge sign, while allowing ions with an opposite charge sign to pass. This effect is especially clear for divalent ions. A striking higher retention of Na2SC>4 than MgCf therefore indicates a negatively charged membrane surface, which is in line with these membranes being terminated with a polyanion.

[0072] For the negatively terminated QP4VP membranes, the retention of Na2SC>4 always exceeds the retention of MgCF. However, the difference in retention between these two salts decreases with increasing alkyl chain length. For Et-QP4VP, the retentions for Na2SC>4 (97.8 ± 0.1 %) and MgCF (73.8 ± 1.0 %) have become much more similar. This is an indication that the contribution of dielectric exclusion to the ion retention increases. Dielectric exclusion takes place in the membrane matrix and is caused by the difference in the dielectric constant of the membrane and the feed solution. In this exclusion mechanism, the ion retention is mainly determined by the valency of the ions, and the charge sign of the ion plays a much smaller role. A higher contribution of dielectric exclusion is indicative of a denser membrane. This is in line with the permeability and MWCO measurements. Notably, the salt retention of the Me-QP4VP membranes may be slightly underestimated due to the presence of minor defects in these membranes, identified at the time of performing these experiments.

[0073] For the negatively terminated QP4VPs, the MgCF retention exceeds the Na2SC>4 retention, indicating a negatively charged surface. However, in the case of Et-QP4VP and Pr-QP4VP these differences are minor, indicating that dielectric exclusion plays a major role in the ion retention of these membranes. Once more, this is in line with the dense nature of these membranes which was indicated by the permeability and MWCO experiments, confirming the gist of the invention for use in the PEM membrane systems.

[0074] Organic Micropollutant (OMP) retention

[0075] The OMP retention of the positively terminated membranes was measured. For this, a feed mixture consisting of model OMPs was used. The model OMPs were selected to represent a wide range of properties (charge, molecular weight) and are used for a variety of different applications (pharmaceuticals, industrial chemicals, herbicides, pH indicators). Because of this variety in characteristics, a good overview can be obtained.

[0076] An OMP feed mixture was prepared containing the OMPs salicylic acid, ibuprofen, naproxen, sulfamethoxazole, diclofenac, bezafibrate, pyrazole, benzotriazole, paracetamol, caffeine, isoproturon, atrazine, bisphenol A, carbamazepine, phenolphthalein, bromothymol blue, metformin, lidocaine, atenolol, metoprolol, sotalol, nadolol and amisulpride, at a concentration of 50 pg L-l (salicylic acid and bromothymol blue 250 pg L-l because of significant degradation over the course of the experiment) in Milli-Q water with 5 mM NaCl, pH 5.8. To account for OMP adsorption to the membrane and filtration system, the OMP mixture was first filtrated for 28 hours. During this time, concentrate and permeate was recirculated to the feed solution. After this equilibration stage, feed and permeates were collected for 15 minutes.

[0077] Sample analysis was performed using a high-performance liquid chromatography - mass spectrometry (HPLC-MS) system. The HPLC system (Thermo Scientific, UltiMate 3000 RSLC) was equipped with an Acclaim RSLC Polar Advantage column (Thermo Scientific, 100x2.1 mm, 2.2 pm particle size) with AccuCore C18 guard (Thermo Scientific, 10x2.1 mm, 2.6 pm particle size). OMPs were separated over a gradient with 1.85 mM ammonium acetate and 0.15 mM acetic acid in Milli-Q water versus LC / MS grade methanol at a flow rate of 0.5 mL / min. Mass spectrometry was performed with a triple quadrupole mass spectrometer (Thermo Scientific, TSQ Quantis), in positive (5 pl injection volumes) and negative (50 pL injection volumes) ionization mode, using heated electrospray ionization (pyrazole and metformin: 2500 V, 400 °C; other compounds in positive mode: 4000 V, 400 °C; negative mode: 3500 V, 275 °C) and in single reaction monitoring (MS / MS) mode. OMP retention was quantified on the most intense precursorproduct transition of each OMP with Chrome Icon 7 software, using a calibration curve. The calibration curve was created by diluting a feed sample (1, 2, 10, 20, 100, 200 and 1000 times diluted) which was obtained during permeate collection. To account for possible sensitivity differences during the HPLC-MS analysis, the calibration series was analyzed both before and after the permeate samples in each ionization mode. The peak areas of the permeate samples were integrated and retentions were calculated from the average of the two calibration curves.

[0078] Result show that the obtained OMP retentions vary, from negligible retentions for small neutral OMPs (pyrazole, benzotriazole) to near full retention of large molecules such as amilsulpride (Figure 6). The effect of different properties of the OMPs is reflected in their retentions. OMP charge plays a substantial role in determining its retention. Substantial differences in retention can be observed between the different membranes. While the P4VP / PSS membranes show comparatively high retentions for positively and neutrally charged OMPs, the QP4VP membranes generally show higher retentions for positively charged OMPs. This difference can be explained though the difference in charge of the membranes. As could be seen with the salt retention measurements, the P4VP / PSS membranes appear to have a negative charge, facilitating retention of negatively charged OPMs. The QP4VP membranes on the other hand have a positive charge, facilitating the retention of positively charged OMPs.

[0079] Generally, higher retentions are observed for larger OMPs, suggesting a contribution of size exclusion to their retention. One notable deviation from this trend is sulfamethoxazole (253) Da, which shows a lower retention than smaller negatively charged OMPs such as ibuprofen and naproxen. While sulfamethoxazole is grouped with the negatively charged compounds, it has a pKa of 6. 16, meaning that it is only partially charged at the pH of the feed mixture. The contribution of size exclusion is also reflected in the OMP retention trend for the QP4VP membranes. Generally, the OMP retention increases upon increase of the alkyl chain length, with Me-QP4VP / PSS having lower retentions than Pr-QP4VP / PSS. This is in line with the MWCO data, which demonstrated that Pr-QP4VP is a denser membrane. This clearly shows that increasing the hydrophobic interactions is a viable method for increasing OMP retention, and that especially high retentions can be obtained for positively charged OMPs.

[0080] The OMP retention of the QP4VP membranes was compared with PDADMAC / PSS membranes. The average OMP retentions of the P4VP (80.2 ± 1.3 %), Me-QP4VP (63.7 ± 2.3 %), Et-QP4VP (66.1 ± 1.2 %) and Pr-QP4VP (71.4 ± 1.3 %) membranes are much higher than the average OMP retention of the PDADMAC membrane (39.9 ± 2.2 %). Although the PDADMAC membrane is negatively terminated while the P4VP and QP4VP membranes are positively terminated, the OMP mix consists of positive, neutral, and negative compounds. Therefore, the membrane surface charge cannot account for the substantial difference in average retention observed. The OMP retention of the QP4VP membranes is a considerable improvement with regards to the PDADMAC membrane.

[0081] Example 5 - Membrane performance measurements of partly sulfonated polyethersulfone based PEM membranes

[0082] In a further experiment PEM membranes according to present invention were prepared by coating a support membrane with five bilayers of multilayers composed of quatemized poly(4- vinylpyridine) (QP4VP) derivatives: methyl-QP4VP (Me-QP4VP), ethyl-QP4VP (Et-QP4VP), and propyl-QP4VP (Pr-QP4VP) as a polycation and partly sulfonated polyethersulfone (SPES) as a polyanion. All multilayer coatings were terminated with SPES. Partly sulfonated Polyethersulfone (SPES, Mw 146,000 g mol-1, Degree of sulfonation (DS) = 36%) was kindly provided by NX Filtration (Enschede, the Netherlands). As this polymer is only partly sulfonated it is a co-polymer of polyethersulfone (PES) and SPES, but in this example and Figure 7 and 8, it will be referred to as SPES. Ethanol (absolute 99%) was purchased from Fisher Scientific.

[0083] Briefly, quatemized poly(4-vinylpyridine) (QP4VP) derivatives: methyl-QP4VP (Me-QP4VP), ethyl-QP4VP (Et-QP4VP), and propyl-QP4VP (Pr-QP4VP) and SPES are deposited from solutions containing a 55 / 45 v / v% ethanol / water mixture and 50mM NaCl retention. In each LbL deposition cycle, membranes were alternately immersed in polycation and polyanion solutions for 15 minutes, followed by three rinsing steps of 5 minutes each in rinsing solutions matching both the ionic strength and solvent composition of the polyelectrolyte solutions. After reaching the desired number of bilayers, membranes were given a final rinse in a solution matching the solvent quality of their respective deposition process to eliminate residual salt. Subsequently, membranes were immersed in a 15 wt% glycerol solution overnight to maintain the integrity of pores during drying, followed by air drying at ambient lab temperature for 24 hours. As done for the QP4VP / PSS in previous examples, PWP, MWCO and on retention values were determined of the resulting membranes.

[0084] Pure Water Permeability and MWCO

[0085] The PWP demonstrated relatively low permeability due to the dense multilayer structure, as depicted in Figure 7. Specifically, the Me-QP4VP / SPES system exhibited a PWP of approximately 1.04 L m2h"1- bar1. A slight increase in permeability was observed for Et- QP4VP / SPES membranes, reaching 1.32 L m2h1bar1, while membranes coated with the more hydrophobic Pr-QP4VP / SPES demonstrated a reduced permeability of 0.89 L m-2li_1bar1, correlating with increased hydrophobic interactions due to the longer alkyl chains. Compared to equivalent QP4VP / PSS membranes prepared in water, SPES-based membranes showed lower permeability, attributed to increased hydrophobic interactions.

[0086] Molecular weight cut-off (MWCO) measurements, also depicted in Figure 7, further confirmed the dense nature of these nanofiltration membranes, even placing them within the reverse osmosis (RO) behaviour range of 100-200 Da. The Me-QP4VP / SPES membrane showed an MWCO of 174 ± 4 Da, while the Et-QP4VP / SPES membrane maintained a similar MWCO of 177 ± 6 Da, despite a modest increase in permeability. This indicates that subtle structural changes do not significantly alter selectivity at this scale. Conversely, the Pr-QP4VP / SPES system exhibited the lowest MWCO at 127 ± 0.4 Da, consistent with the membranes' most hydrophobic and densely packed structure. The dense multilayer structures result from enhanced hydrophobic and electrostatic interactions promoted by the ethanol-water solvent mixture and the partial hydrophobicity of SPES, leading to greater rejection of small solutes relative to QP4VP / PSS membranes.

[0087] Ion Retention

[0088] The ion retention behavior of the membranes was evaluated using single-salt solutions comprising combinations of monovalent and divalent ions (NaCl, Na2SC>4, MgCE, and MgSCU). as shown in Figure 8.

[0089] The results indicate that all three multilayer membranes, Me-QP4VP / SPES, Et- QP4VP / SPES, and Pr-QP4VP / SPES, exhibit high monovalent and divalent salt retention, substantially higher than observed for the QP4VP / PSS membranes. This selective rejection is attributed predominantly to a dielectric exclusion mechanism, as evidenced by the consistently high retention of divalent ions regardless of charge and significant retention of monovalent NaCl. The membranes were prepared using a 55% ethanol-water mixture, which reduces the dielectric constant of the medium. This process enhances hydrophobic and electrostatic interactions between SPES and the quatemized P4VP derivatives, resulting in dense multilayer structures with low water uptake and limited hydration. The resultant dielectric contrast between the membrane matrix and the aqueous phase promotes high salt retention.

[0090] The Me-QP4VP / SPES membrane quantitatively achieved a NaCl retention of 83.8 ± 1.0%, consistent with its dense structure and previously determined low permeability and MWCO. The Et-QP4VP / SPES membrane showed slightly higher NaCl retention (85.6 ± 1.0%), corresponding to increased hydrophobicity and slightly higher permeability. The Pr-QP4VP / SPES membrane, despite presenting the lowest MWCO and most compact structure, exhibited a similar NaCl retention of 85.0 ± 1.5%. Notably, all these retention values exceed those observed for previously investigated multilayer membranes based on Q-P4VP and PSS as the polyanion in an aqueous coating solution.

[0091] The present hydrophobic, partly charged SPES-based membranes thus demonstrate superior retention behavior due to their denser structure and the enhanced hydrophobic and dielectric exclusion effects. These results confirm the effectiveness of the disclosed membranes for selective rejection of monovalent and divalent salts, even outperforming QP4VP / PSS -based multilayer systems.

Claims

Claims1. A Polyelectrolyte Multilayer (PEM) membrane comprised of a polyelectrolyte layer, wherein the polyelectrolyte layer is comprised of one or more bilayer(s) comprised of a polycation and a polyanion, wherein said polycation is comprised of amine-based monomers wherein the amine is quatemized with an alkyl group, wherein the polycation is an amine based polycation selected from the group consisting of Poly(4-vinylpyridine) (P4VP), Poly(3- vinylpyridine) (P3VP), Poly(2-vinylpyridine) (P2VP), poly(allylamine hydrochloride) (PAH), poly(ethyleneimine) (PEI), poly(diallyl methyl amine hydrochloride) (PDAMAHC), and a copolymer of 2-propen-l-amine-hydrochloride with N-2-propenyl-2-propen-l -aminehydrochloride (CPPAHC), polyvinylamine (PVA), or copolymers thereof, preferably P4VP or copolymers thereof.

2. PEM membrane according to claim 1, wherein the polyanion is selected from the group consisting of poly(styrene sulfonate) (PSS), poly(acrylic acid) (PAA) and poly(vinylsulfonic acid sodium) (PV S), preferably poly(styrene sulfonate), sulfonated polyarylene ethers, or copolymers thereof, preferably PSS or copolymers thereof.

3. PEM membrane according to claim 1 or 2, wherein the polyanion is a co-polymer of polyethersulfone (PES) and partly sulfonated PES (SPES, wherein the partly sulfonated PES having a degree of sulfonation of at least 5%, preferably at least 15%, more preferably at least 25%, most preferably at least 35%.

4. PEM membrane according to claim any one of the claims 1 to 3, wherein the alkyl is a hydrophobic alkyl having a carbon chain length of between 1 to 20 carbons, preferably 2 to 10 carbons, more preferably 3 to 6 carbons, most preferably 4 to 5 carbons.

5. PEM membrane according to any one of the claims 1 to 4, wherein the alkyl is one or more selected from the group consisting of methyl-, ethyl-, propyl-, butyl-, pentyl-, hexyl-, heptyl-, or octyl -group, preferably a methyl- or ethyl -group.

6. PEM membrane according to any one of the claims 1 to 5, wherein the PEM membrane has a molecular weight cutoff (MWCO) of at most 275 Da, preferably at most 250 Da, more preferably at most 220 Da, even more preferably at most 180 Da, most preferably at most 140 Da, even more preferably at most 130 Da.

7. PEM membrane according to any one of the claims 1 to 6, wherein the PEM membrane has a permeability of at least 4 L m-2 h-1 bar-1, preferably at least 6 L m-2 h-1 bar-1, more preferably at least 8 L m-2 h-1 bar-1, most preferably at least 12 L m-2 h-1 bar-1.

8. PEM membrane according to any one of the claims 1 to 7, wherein the PEM membrane has a salt retention value of MgC’E of at least 80%, preferably at least 85%, more preferably at least 90%, most preferably at least 95%.

9. PEM membrane according to any one of the claims 1 to 8, wherein the PEM membrane has an organic micropollutant (OMP) retention value that is at least 1.2 to 3x higher, preferably 1.3 to 2.5x higher, more preferably 1.5 to 2x higher than the OMP retention value of a comparable PDADMAC based PEM membrane.

10. PEM membrane according to any one of the claims 1 to 9, wherein the PEM membrane has an organic micropollutant (OMP) retention value of at least 50%, preferably at least 55%, more preferably at least 60, most preferably at least 63%, preferably wherein the OMP is a positively charged OMP.

11. PEM membrane according to any one of the claims 9 or 10, wherein the OMP is one or more selected from the group consisting of salicylic acid, ibuprofen, naproxen, sulfamethoxazole, diclofenac, bezafibrate, pyrazole, benzotriazole, paracetamol, caffeine, isoproturon, atrazine, bisphenol A, carbamazepine, phenolphthalein, bromothymol blue, metformin, lidocaine, atenolol, metoprolol, sotalol, nadolol, amisulpride, PFAS molecules and Chlorothalonil metabolites,12. PEM membrane according to any one of the claims 9 to 11, wherein the OMP is a positively charged OMP, even more preferably wherein the OMP is one or more selected from metformin, lidocaine, atenolol, metoprolol, sotalol, nadolol and amisulpride.

13. PEM membrane according to any one of the claims 1 to 12, wherein at least 70%, preferably at least 75%, more preferably at least 80%, even more preferably 90%, most preferably at least 95% of the amine-based monomers of the polyelectrolyte layer are quatemized with and an alkyl group, and the remaining amine-based monomers monomers of the polyelectrolyte layer are unquatemized.

14. PEM membrane according to any one of the claims 1 to 13, wherein 100% of the amine-based monomers of the polyelectrolyte layer are quatemized.

15. PEM membrane according to any one of the claims 1 to 14, wherein the PEM membrane is comprised of at least 1 bilayer, preferably at least 2 bilayers, more preferably at least 5 bilayers, even more preferably at least 7 bilayers, most preferably at least 9 bilayers.

16. PEM membrane according to any one of the claims 1 to 15, wherein the PEM membrane is comprised of a positively or negatively terminated bilayer, preferably a negatively terminated bilayer.

17. PEM membrane according to any one of the claims 1 to 16, wherein the PEM is a nanofiltration membrane.

18. PEM membrane according to any one of the claims 1 to 17, wherein the PEM membrane is an asymmetric membrane comprised of two PEM membrane sections differing in porous density, preferably wherein a first membrane section comprising a first polyelectrolyte layer has a more open porous structure than a second membrane section comprising a further polyelectrolyte layer and wherein the second membrane section is coated on top of the first membrane section providing a denser second membrane section on top.

19. PEM membrane according to claim 18, wherein the first polyelectrolyte layer is comprised of amine based polycations that are quatemized with metyl-alkyl groups and the at least one further polyelectrolyte layer is comprised of amine based polycations that are quatemized with propyl-alkyl groups.

20. A method for providing a PEM membrane of any one of the claims 1 to 19, wherein a support membrane is coated with at least one layer of a polyelectrolyte according to any one of the claims 1 to 19, preferably coated with at least two layers of said polyelectrolyte.

21. Method according to claim 20, wherein said PEM membrane is an asymmetrical PEM membrane, and wherein the support membrane is coated with at least one polyelectrolyte layer having a more open porous structure providing a first membrane layer, wherein on top of said at least one membrane layer at least one further polyelectrolyte layer is provided providing a second membrane layer having a denser porous structure than the first membrane layer.

22. Method according to claim 21, wherein the first membrane layer comprises a polycation that is comprised of amine-based monomers wherein the amine is quatemized with and an alkyl group, and wherein the second membrane layer comprises a polycation that is comprised of amine-based monomers wherein the amine is quatemized with and an alkyl group that differs from the first membrane layer.

23. Use of a PEM membrane of any one of the claims 1 to 19 for the purification, decontamination or cleanup of water, preferably drinking water.

24. Use according to claim 23, wherein the PEM membrane purifies the water from organic micropollutants (OMP), small organic molecules, pesticides, hormones, plasticizers, heavymetals, inorganics salts, pharmaceuticals, medicine residues, bacteria, viruses and other microorganisms.

Images