Fabrication of microporous polyaryl nanofilm at the liquid-liquid interface for selective molecular separation

Summary

Polymeric membranes with precise molecular weight cutoffs are necessary for molecular separations. Here, we present a stepwise preparation of microporous polyaryl (PAR_TTSBI) freestanding nanofilm as well as the synthesis of bulk polymer (PAR_TTSBI) and fabrication of thin film composite (TFC) membrane, with crater-like surface morphology, then provide the details of separation study of PAR_TTSBI TFC membrane.

For complete details on the use and execution of this protocol, please refer to Kaushik et al. (2022)¹ and Dobariya et al. (2022).²

Graphical abstract

Highlights

• •Polyaryl thin microporous film preparation at the liquid-liquid interface
• •Fabrication of polyaryl thin film composite membrane having surface functionality
• •Membrane performance measured through permeance and rejection of molecular markers

Publisher’s note: Undertaking any experimental protocol requires adherence to local institutional guidelines for laboratory safety and ethics.

Polymeric membranes with precise molecular weight cutoffs are necessary for molecular separations. Here, we present a stepwise preparation of microporous polyaryl (PAR_TTSBI) freestanding nanofilm as well as the synthesis of bulk polymer (PAR_TTSBI) and fabrication of thin film composite (TFC) membrane, with crater-like surface morphology, then provide the details of separation study of PAR_TTSBI TFC membrane.

Before you begin

Polymeric membranes with enhanced mass-transport and solute selectivity are highly anticipated for liquid separation, relevant to industrial applications; where mixture of two or more solutes separated from each other.^1,2,3,4,5 Unlike conventional separation processes (distillation, evaporation), membrane based liquid separation processes perform separation in liquid phase without changing the phase of desired solutes. Thus, low energy consumption, less carbon footprints, continuous operation, modular design and large-scale applicability make membrane based separation process advantageous over others.^3,4

Maturity of the process led to produce successful technological examples like sea-water desalination through reverse osmosis (RO), water purifications through nano-filtration (NF) process, organic solvent nanofiltration (OSN), and gas separation etc.^3,4,6,7 Owing to this process, the shortage of fresh water can be addressed. In this line, last two decades witnessed progressive improvement in the separation performance of NF membranes. This has been achieved by enhancing water permeance without altering solute selectivity. For enhancing the water mass-transport, first-hand principles are (i) tailor the polymer chains at molecular scale to offer greater number of interconnected micropores⁸ and; (ii) downsizing the thickness of active separating membrane layer in the nanoscale dimensions for lesser resistance^1,2,9; (iii) enhancing the hydrophilicity of the membrane surface¹⁰; (iv) tailor-made membrane surface structure.¹⁰

The prominent class that attracted significant attention in nanofiltration membranes in the family of microporous materials (pore size < 2 nm) is, “Polymer with Intrinsic Microporosity (PIMs)”, contains interconnected voids or pores of < 2 nm which have been useful to generate “free volume”.^6,7 Adopting the understanding of conformational microporosity and incorporating into the well-developed dense NF membrane may led to the development of microporous membranes with improved permeance-selectivity trade-off. According to the literature, the permeance–selectivity trade-off also requires a few augmented features, like tailor made functionality, morphology and reduced thickness of membrane.^4,5

In the present work, we prepared thin microporous polyaryl (PAR) nanofilm onto ultrafiltration (UF) polyether sulfone (PES) support to form a thin film composite membrane. The thin film was prepared at the liquid-liquid interface by reacting an aqueous layer containing rigid, contorted, spirocyclic nucleophilic monomer like spirobisindane (TTSBI) and a hexane layer containing electrophilic trimesoyl chloride (TMC). The prepared thin polyaryl nanofilm presents molecular weight cut-off greater than ∼450 gmol^-1 (molecular marker) with water permeance of ∼84 Lm^-2h⁻¹ bar^-1.

Preparation of the reagents and equipment

A complete list of reagents and equipment can be found in the “key resources table.”

• 1.
  Preparation of 13 % w/v PES dope solution:

  • a.
    Take a clean oven dried airtight 1-liter glass bottle.
  • b.
    Placed PES polymer glass beads into the clean closed glass tray and dry into the oven at 70°C for 3 h.
  • c.
    Weighed desire amount (13 gm) of PES in non-contaminated zone and transfer PES polymer beads into the airtight glass bottle using clean transfer funnel.
  • d.
    Add desire amount (100 mL) of analytical grade N,N- dimethyl formamide (DMF) into the glass bottle containing PES polymer.
  • e.
    Maintain constant stirring (Stirring rate 1,000–1,200 rpm) for 4 h at 70°C temperature. This allows dissolution of PES polymer into the DMF to yield 13 % w/v polymer dope solution.
  • f.
    Allow the 13 % w/v polymer dope solution to cool down at 25°C.
• 2.
  Preparation of polyether sulfone (PES) support via phase inversion.

  • a.
    A membrane sheet of ∼60 m length and 0.32 m wide casted on a nonwoven fabric at a speed of 4–7 cm/min using a continuous casting machine.
  • b.
    In this process, a thin layer of the polymer solution on the nonwoven fabric casted with a knife positioned at ∼130–150 μm above the nonwoven fabric.
  • c.
    The freshly casted membranes washed with DI water and kept in DI water (There are no specific recommendations for amount of water but make sure that the support should be submerged in water properly and number of washing should be more than 10 times. This allows removal of excess DMF solvent. It is recommended to store PES support at 4°C in isopropanol-water mixture (1:1) w/w%. This storage procedure wet the pores of PES and restrict the formation of biofilm.
• 3.
  Preparation of NaOH solution for TTSBI:

  • a.
    Calculated amount of NaOH was weighed.
  • b.
    Weighed amount of NaOH dissolve into 15 mL of DI H₂O for TTSBI with varied molar ratio (Table 1) to obtain different wt% of TTSBI solution as given in Table 1.
  • c.
    Weighed the calculated TTSBI monomer into 50 mL centrifuge tube and solubilize it completely in varied concentrations of NaOH solution (Table 1).

Table 1.

concentration variation of monomer TTSBI

Reagent	Final concentrations (wt%)	Amount
TTSBI	0.00066	0.1 mg
TTSBI	0.0066	1 mg
TTSBI	0.066	10 mg
NaOH	0.333	5 mg
H2O	–	15 mg

Calculations of wt% for TTSBI:

Weight % (wt%) = mass of solute (g)/mass of solution (mL) ∗100.

 For example:

 Wt% of 0.1 mg TTSBI.

 Here, mass of solute = 0.1 mg.

 Mass of solution = 15 mL of NaOH Solution.

 Wt% = 0.0001/15 ∗100 = 0.00066%.

 Similarly, wt % of 1 mg TTSBI and 10 mg TTSBI are 0.0066% and 0.066% respectively.

Key resources table

REAGENT or RESOURCE	SOURCE	IDENTIFIER
Chemicals
3,3,3,3-Tetramethyl-1,10-spirobiindane-5,5,6,6-tetraol ((TTSBI) 96.0 %)	Sigma-Aldrich	CAS Number: 77-08-7
Trimesoyl chloride (1,3,5-Benzenetricarbonyltrichloride) (TMC (98.0 %))	Sigma-Aldrich	CAS Number: 4422-95-1
Acid orange (AO)	TCI Chemicals	CAS Number: 633-96-5
Orange G (OG)	TCI Chemicals	CAS Number: 1936-15-8
Brilliant blue R (BBR)	TCI Chemicals	CAS Number: 6104-59-2
Congo red (CR)
NaCl (99.9 %)	Sisco Research Laboratories Pvt. Ltd	CAS Number: 7647-14-5
NaOH (98 %)	Sisco Research Laboratories Pvt. Ltd	CAS Number: 1310-73-2
Na2SO4 (99.5 %)	Sisco Research Laboratories Pvt. Ltd	CAS Number: 7757-82-6
Potassium ferricyanide (98 %)	Sisco Research Laboratories Pvt. Ltd	CAS Number:13746-66-2
n-Hexane (99%)	Sisco Research Laboratories Pvt. Ltd	CAS Number: 110-54-3
Acetone (99%)	Sisco Research Laboratories Pvt. Ltd	CAS Number: 67-64-1
Isopropanol, AR	Merck Millipore, India	CAS Number: 67-63-0
Methanol, AR	Merck Millipore, India	CAS Number: 67-56-1
Acetonitrile, AR	Merck Millipore, India	CAS Number: 75-05-8
Dimethylformamide (99 %)	Merck Millipore, India	CAS Number: 68-12-2
Polyethersulfone (PES) beads	Gharda Chemicals Ltd (Now Solvay Advance Polymer)	GAFONE PES
Non-woven polyester fabric	Nordlys-TS100, Polymer Group Inc., France	NA
N-type silicon wafer	University Wafer, Boston, USA	https://universitywafer.com
Other instruments
Transmission electron microscope (TEM)	JEOL, JEM-2100	https://www.jeol.com/products/scientific/tem/JEM-2100.php
Scanning electron microscope (SEM)	JEOL, JSM 7100F	https://www.jeol.com/products/scientific/tem/JEM-2100.php
Electrical conductivity meter	Eutech PC2700	http://www.eutechinst.com/pdt_para_pc_pc2700.html
Quartz cell	Light path 10 mm	–
UV−vis spectrophotometer	UV-2700, Shimadzu	https://www.shimadzu.com/an/literature/uv/jpa112014.html#:∼:text=Information%20Accept%20Cookies-,Introduction%20of%20Shimadzu%20UV%2D2700%20UV%2DVisible%20Spectrophotometer%20for%20High,can%20now%20be%20easily%20measured
FTIR spectrometer	PerkinElmer, Spectrum GX	https://www.perkinelmer.com/product/spectrum-two-ft-ir-sp10-software-l160000a
Semi-continuous casting machine	Elcometer 4320 automatic film applicator	https://www.elcometer.com/elcometer-4340-motorised-automatic-film-applicator.html
Microwave oven	Tanco P2T-125A	–

Step-by-step method details

Abbreviations

PES: Polyether sulfone.

TTSBI: 3,3,3,3 -Tetramethyl-1,10 -spirobiindane-5,5,6,6-tetraol.

TMC: Trimesoyl chloride.

PAR: polyaryl.

TFC: Thin film composite.

Step 1: Preparation of polyether sulfone (PES) support via phase inversion

Timing: Total time to prepare PES support 3 days

This section describes the preparation of polymer dope solution and casting of the solution to prepare PES support membrane and storage conditions.

• 1.
  Preparation of polyether sulfone (PES) support via phase inversion.

  • a.
    Preparation of the PES support by phase inversion process according to the reported procedure (Sarkar et al.⁹).
  • b.
    Take the freshly prepared polymer dope solution (13 %w/v PES) and cast onto the non-woven polyester fabric.
  • c.
    Cast PES support membrane onto the non-woven polyester fabric at a speed of 4–7 m/min using a semi-continuous casting machine.
  • d.
    Diffusion of Water as a non-solvent at 25°C results in the porous PES support with a thickness of 130–150 μm.
  • e.
    Wash the membranes thoroughly with DI water.
  • f.
    Initially stored all the prepared PES support membranes in DI water for 2 days for conditioning.
  • g.
    After complete washing and conditioning, store the PES support membranes at 10°C in (1:1) w/w % isopropanol-water mixture. Figure 1.

Figure 1.

Phase inversion method

1. Non-woven fabric roll, 2. Polymer dope solution reservoir, 3. Pouring polymer dope solution into reservoir, 4. Polymer casted fabric undergo phase inversion into water bath, 5. PES membrane passing through the water layer, 6. PES roll down.

Step 2: Synthesis of free-standing polyaryl nanofilm

Timing: Complete process to form free-standing film formation takes 5 min

The section defines the formation of phenoxide ion and reaction with TMC at liquid-liquid interphase to generate the polymeric film.

• 2.
  Synthesis of free-standing polyaryl nanofilm.

  • a.
    We report the step-by-step procedure involving the synthesis of free-standing polyaryl nanofilms.
  • b.
    Interfacial polymerization process is used to prepare polymer thin film at liquid-liquid interphase (Water-hexane).
  • c.
    Take freshly prepared aqueous solution of contorted monomer (TTSBI) in sodium hydroxide into a transparent bottle in different concentrations as mentioned in Table 2.
  • d.
    Preparation of 0.1 % TMC solution:

    • i.
      Take 100 mL oven dried air tight brown-colored conical flask.
    • ii.
      Weighed the 100 mg of TMC into the brown colored conical flask.
    • iii.
      Dissolve the TMC into 100 mL hexane to obtain the 0.1 %w/v TMC solution.
  • e.
    Take the 0.1 % (w/v) TMC solution in hexane (10 mL) and add it slowly from the walls to the surface of the aqueous solution of TTSBI monomer containing the bowl.
  • f.
    Allow the nucleophilic attack of TTSBI phenoxide ion on Trimesoyl chloride to happen for 1 min, to yield the free-standing polymeric nanofilm at the interface. Figure 2.
  • g.
    Free-standing polyaryl nanofilm is clearly visible with all the different concentrations at the interphase.
  • h.
    Increasing the time of interfacial polymerization resulted into the thick polyaryl nanofilm. Subsequently, transfer the resulting microporous polyaryl nanofilms from the interface with help of a suitable substrate (Si Wafer/ Cu grid/ Glass slides). Afterwards, wash it carefully with distilled water (Wash of the film on Si wafer with few drops of distilled water gently is required).
  • i.
    The formation of defect-free PAR_TTSBI nanofilm is analyzed with various Electron microscopy tools. Figure 3.

    Note: Level the surface before preparation of nanofilm at interface, for uniform film.

Table 2.

Monomer concentration variation to optimize the thickness of nanofilms

Reagents	Final concentrations (wt%)	Amount
TTSBI	0.00066	0.1 mg
TTSBI	0.0066	1 mg
TTSBI	0.066	10 mg
NaOH	0.333	5 mg
TMC	0.1	100 mg

Figure 2.

Synthetic scheme for the Polyaryl nanofilm

Figure 3.

Step-wise method for transferring film on the suitable substrate

Step 3: Synthesis of microporous polyaryl nanofilm composite membranes

Timing: Microporous nanofilm composite membrane takes 30 min

The section depicts formation of thin microporous nanofilm composite membranes. Firstly, clean the support membrane followed by addition by monomer solutions. This result into the formation of composite membrane.

• 3.
  Synthesis of microporous polyaryl nanofilm composite membranes.

  • a.
    The thin film composite (TFC) membranes for molecular separation are prepared by interfacial polymerization directly onto PES ultrafiltration supports; step-wise preparation is depicted in Figure 3.
  • b.
    Preparation of rectangular boat (active area 95 cm²) by folding the edges of the PES support.
  • c.
    Wash the membrane support boat with distilled water before use.
  • d.
    To obtain uniform coating, put the boat on glass plate after leveling.
  • e.
    Pour the aqueous basic solution containing the sodium phenoxide ions (0.1 mg TTSBI in 15 mL of NaOH solution, 1 mg TTSBI in 15 mL of NaOH solution, 10 mg TTSBI in 15 mL of NaOH solution) into the rectangular boat (Boat is prepared from 126 cm²(9∗14) polyethersulfone while active area of boat is 84 cm²(7∗12)), allow it to diffuse and adsorb on the surface of PES support boat for 2 min.
  • f.
    Gently decant the phenoxide-loaded support membranes.
  • g.
    Immediately press the support with a rubber roller to remove the excess solution and trapped air.
  • h.
    Pour 10 mL of TMC in hexane (0.1 % w/v) after 1 min, and allow it to polymerize for 1 min at the interphase onto the PES support.
  • i.
    Decant the hexane solution immediately after 1 min of polymerization.
  • j.
    Dry the resulting membranes in air for 2 min.
  • k.
    Place the membranes in the preheated hot air oven at 90°C for 10 min to complete the cross-linking reaction.
  • l.
    Store the resulting membranes immediately after cross-linking in distilled water at 4°C. Figure 4.

Figure 4.

Step-wise preparation of polyaryl nanofilm composite membrane

Step 4: Synthesis of polyarylate bulk polymer

Timing: Polyaryl bulk polymer synthesis takes 12 h

The sections depicts the preparation of bulk polymer synthesis using continuous stirring, followed by filtration and drying.

• 4.
  Synthesis of polyarylate bulk polymer.

  • a.
    Synthesize the bulk polyarylate polymer to characterize and define the structural features for further comparison with polymeric nanofilm, presented in Figure 4.
  • b.
    Dissolve 0.471 g NaOH in distilled water, and add TTSBI monomer (4:1 Molar Ratio for TTSBI (1 gm)), upon stirring the solution color changes to dark brown, this suggest the formation of phenoxide ion.
  • c.
    Add solution of 0.1 wt % TMC in hexane into the glass bottle containing the aqueous phase of phenoxide ion, yielding the formation of a film at the interface.
  • d.
    Continue the reaction mixture under vigorous stirring for 10 min on a magnetic stirrer at room temperature.
  • e.
    The formation of polyarylate polymer starts taking place.
  • f.
    Filter the polymer.
  • g.
    After filtration, the aqueous layer come to contact with the organic layer and again formation of PAR_TTSBI polymer takes place in the filtrate.
  • h.
    Again, starring the filtration under vigorous conditions (stirring at 1,200–1,400 rpm) and filter the polymer and repeat it two to three times.
  • i.
    Wash the resulting polyarylate bulk polymer with water followed by hexane (Three to four wash of 15 mL water then two wash of 15 mL hexane are required for remove impurities).
  • j.
    Dry it at 110°C for 8 h in an oven.
  • k.
    Afterwards, collect the solid amorphous polymers for further analysis. Figure 5.

Figure 5.

Preparation of polyarylate bulk polymer

Note: The concentration of TMC in the hexane phase should 10 times lower than the concentration of TTSBI phenoxide solution in the aqueous phase.

Step 5: Performance evaluation of the polyaryl thin film composite membrane by using high-pressure cross-flow cell assembly

Timing: Performance of membrane against all the molecular markers takes 7 days

The present sections depicts the evaluation of the membrane against the different molecular markers and salts. The define concentration of the molecular markers and salts prepared, pass through the membrane in cross-flow cell and the permeate and rejected stream is tested.

• 5.
  Performance evaluation of the polyaryl thin film composite membrane by using high-pressure cross-flow cell assembly.

  • a.
    Testing of polyaryl nanofilm composite membranes for molecular separation, the performance is analyzed using cross-flow filtration setup, as shown in Figure 6.
  • b.
    Cut the membrane using seizure according to the effective area of the membranes without touching the surface of the thin film composite membrane, which was stored at 4°C previously.
  • c.
    The effective area of the membranes for the cross-flow testing cell was 14.5 cm².
  • d.
    Diaphragm pump used to pump the feed solution (≈ 1 Lmin^-1) from a feed tank.
  • e.
    Pre-compact the membranes for at least 4 h under 2-bar pressure by using pure water to reach the steady-state condition.
  • f.
    Calculate Membrane permeances using the following equation:

    P = V/A.t. Δp

    Where V is the volume of the permeate in Liters, t (h) is the collection time, A (m²) is the effective area of the membrane used and Δp (bar) is the transmembrane pressure. Figure 6.
  • g.
    Feed and permeate solutions was collected to monitor the membrane performance.
  • h.
    Prepare the 2 g.L^-1 (2,000 ppm) concentration solution of NaCl and Na₂SO₄ salts, and 0.1 g.L^-1 (100 ppm) of K₃[Fe(CN)₆].
  • i.
    Analyze the separation performance of the membranes using different molecular markers (salts and dyes) detailed in Table 3 as feed solution.
  • j.
    The concentration of 7-Hydroxy-2-naphthalene sulfonic acid (HNSA), acid orange (AO), orange G (OG), acid fuchsin (AF), congo red (CR), and brilliant blue R (BBR) in the feed are 0.1 g.L^-1 (100 ppm) each.
  • k.
    The procedure of the measurement of the concentration of molecular markers in permeate and feed via electrical conductivity meter and UV-visible spectrophotometer. .
  • l.
    Calculate the rejection (R) of membranes as a percentage according to the following equation:

    • i.
      R (%) = (1 –C_p/C_f) × 100.
    Where, C_p (in g.L^-1) and C_f (in g.L^-1) are the concentrations of solutes in permeate and feed, respectively.
    The concentration of salts and dyes in permeate and the feed measured by an electrical conductivity meter and by the UV-visible absorbance values respectively. Figure 7.

Note: At least four membrane coupons should be tested to get an average value and standard deviation.

Figure 6.

High-pressure cross-flow cell assembly

Table 3.

Different molecular markers with a molecular weight

Molecular markers	Molecular weight (g/mol)
NaCl	58
Na2SO4	142
SNF	194
K3[Fe(CN)6]	329
AO	350
OG	452
AF	585
CR	696
BBR	852

Figure 7.

UV-Visible analysis of the feed (F) and permeate (P) samples of molecular markers

Expected outcomes

Structural analysis

The high reactivity of electrophilic acyl chloride functional groups in TMC and the nucleophilic hydroxyl functional group in TTSBI undergo condensation reaction to form ester group by eliminating salt (NaCl) and resulting into the polyarylate polymer. The structural features and functional group analysis performed by FTIR spectroscopy. FTIR spectra of PAR TTSBI presented in Table 4 shows the presence of C=O_COOR stretching frequency of ester groups at 1,750 cm^-1 suggested ester bond formation in polymer. The stretching frequency at 3,450 cm^-1 correspond to the O-H_COOH, and 1,625 cm^-1 for C=O_COOH correspond to the carboxylic acid (Figure 8), clearly indicates the presence of free hanging carboxyl chain end group of the polymer. The bending frequency at 1,220 cm^-1 indicates the C-O_COOR/COOH proves reveals the presence of ester/arylate polymer with the carboxyl chain end groups. C=C stretching frequency appears at 1,500 cm^-1 and aromatic sp² C-H stretching vibration and out of the plane bending frequency at 2,900 cm^-1 and 675 cm^-1 suggested the aromatic rings in the polymer chain. Figure 8.

Table 4.

Functional groups with FT-IR frequencies

Functional group	Wave number (cm-1)
O-H stretching	3,450
C-H-stretching	2,850
C=O stretching (-COOR)	1,750
C=O stretching (-COOH)	1,625
C=C stretching (Aromatic)	1,500
C-O stretching (-COOR/(-COOH)	1,220
C-H Banding (Aromatic)	675

Figure 8.

FT-IR spectra of TTSBI Bulk polymer

Morphological analysis

The morphological analysis of PAR thin film is analyzed with the help of electron microscopic tools (Figure 9). Closer look at Transmission electron microscopy (TEM) suggested the uniformity; stacking of polymer layers, folding and presence of chain-like structures in the PAR thin film (Figures 9D–9F). The thin film noticed Silicon wafer and suggested defect free surface with linear and circular wrinkles, presence of craters in the film. Linear and circular wrinkles with crumpled nature are clearly seen on the polyaryl nanofilm and does not collapse during drying and testing. The formation of crumpled morphology has a critical role to play as it enhances the active surface area for membrane, resulting into high permeance. Other observations suggested that the thin films remained continuous and devoid of cracks on the top surface. Figure 9.

Figure 9.

Electron microscopic image

(A–F) Morphological analysis of the PAR thin film with crumbled morphology, and continuous nanofilm (A–C), TEM image (D–F).

Analysis of membrane performance

High pure water permeance (PWP) of 100 Lm^-2 h^-1 bar^-1 is observed with TTSBI: TMC (0.2 mmol/L: 37.7 mmol/L) for 1 min contact time (Table 1). Increasing the monomer concentration (10 times), the PWP is decreasing from 100 to 37 Lm^-2 h^-1 bar^-1 for TTSBI:TMC (2.0 mmol/L: 37.7 mmol/L) for 1 min contact time. The significant reduction in water permeance explained based on the formation of dense thick separation layer. High concentration of monomer destabilize the interface and rapid reaction kinetics result into the thick film, thus increase the water path length. Polyaryal nanofilms with crumpled and crater (folds and wrinkles) morphology enhanced active surface area of membrane and led to the high permeance. Separation performance of polyaryl nanofilm is measured in terms of permeance and rejection against molecular markers, like monovalent, divalent, and multivalent salts (NaCl, 2 g.L^-1; Na₂SO₄, 2 g.L^-1; and K₃[Fe(CN)₆], 0.1 gL^-1) respectively and negatively charged dye molecules like (7-Hydroxy-2-naphthalene sulfonic acid (HNSA), acid orange (AO), orange G (OG), acid fuchsin (AF), congo red (CR), and brilliant blue R (BBR); 0.1 gL^-1each). Polyaryl nanofilm composite membrane [TTSBI:TMC (1 mg (0.2 mmol/L: 37.7 mmol/L) for 1 min contact time] (Table 1) revealed 4% rejection for monovalent NaCl, 29% for divalent Na₂SO₄, and 98% rejection for multivalent K₃[Fe(CN)₆]. Water permeance >75 Lm^-2 h^-1 bar^-1 was observed for mono and divalent salts and >80 Lm^-2 h^-1 bar^-1 water permeance was observed for multivalent salts. Rejection profile suggested that the polyaryl nanofilm remained active for high rejection (>98%) for multivalent salt (K₃[Fe(CN)₆]) against low rejection (29%) for divalent (Na₂SO₄) and (4%) for monovalent salt (NaCl). Here, 4% indicate the rejection of monovalent salt which suggests more selectivity of polyaryl nanofilm towards monovalent ions, it means this polyaryl nano film is more selective towards monovalent ions than di and multivalent ions and divalent ions than multivalent ions. The decrease in the water permeance for mono and divalent salts with respect to multi-valent salts is due to the limited mass transport. The polyaryl nanofilm showed high rejection profile for negatively charged molecular markers, e.g., BBR, CR, AF, OG, and AO. The ultrathin polyaryl nanofilm showed the reduction of resistance related to the thick composite membrane as a separation layer. The formation of a polyaryl nanofilm with ultrathin separating layer as a composite membrane can be used for high water permeance membranes. Moreover, the prepared polyaryl nanofilm membranes with surface morphology, and hydrophilicity represents high water permeance, and molecular weight cut-off above 452 g mol^-1. Figure 10.

Figure 10.

Membrane performance: Permeance of various molecular markers and Molecular weight cut-off (MWCO)

Limitations

It is crucial to understand and develop ultrathin separation layer (for example polyaryl nanofilm) onto ultra-porous ultrafiltration support. As this will enable superiority towards membrane performance for molecular separation.

Troubleshooting

Problem 1

Step 1: Defect formation on the PES support membrane.

Potential solution

• •Preparation of PES support is crucial.
• •Polymer dope solution should be devoid of any air bubble otherwise it creates defects at the time of support membrane fabrication.
• •For this purpose, it is necessary to use airtight glass vessel.

Problem 2

Step 2: Hydrolysis of TMC result into the change in reaction kinetics.

Potential solution

• •TMC (Trimesoyl chloride) is hydrolyzed in presence of air and converted into trimesic acid.
• •Prepare a fresh solution in a dark-colored flask as per the requirement and close it immediately after use then covered the neck of the closed flask with parafilm.

Problem 3

Step 3. Non-uniform distribution of monomer on the support boat result into non-uniform thickness polymer thin film.

Potential solution

• •For the uniform distribution of the solution, initial step to be taken care is leveling the surface of the PES boat using bubble meter before addition of monomer solution.

Problem 4

Step 5: Achieving true rejection of molecular marker is critical during membrane testing in the cross-flow cell.

Potential solution

• •Pump the molecular markers solution using a diaphragm pump in increasing molecular weight as per given in table no. 3.
• •Wash the cross-flow cell by passing DI water at least for 2 h after every molecular marker.

Problem 5

Step 5: The high flux membranes have an inherent pressure drops across porous support layer upon which active the ultrathin polyaryl nanofilm layer of composite membrane resides.

Potential solution

• •Choice of support layer with much higher water permeance is designed without altering the separation performance.

Resource availability

Lead contact

Further information and requests for resources and reagents should be directed to and will be fulfilled by the lead contact, Ketan Patel (ketanpatel@csmcri.res.in).

Materials availability

This study did not generate new unique reagents.

Data and code availability

• •Data: All data reported in this paper will be shared by the lead contact upon request.
• •Code: This paper does not report the original code.
• •Any additional information required to reanalyze the data reported in this paper is available from the lead contact upon request.