Effect of amine salt surfactants on the formation and performance of thin

film composite poly (piperazine-amide) nanofiltration membranes

Jun Xianga,b, Zongli Xieb, Manh Hoangb and Kaisong Zhanga,*

a Key Laboratory of Urban Environment and Health, Institute of Urban Environment, Chinese Academy of

Sciences, Xiamen 361021, China (Email: jxiang@iue.ac.cn; kszhang@iue.ac.cn )

b CSIRO Materials Science and Engineering, Private Bag 33, Clayton, Vic. 3169, Australia (Email:

zongli.xie@csiro.au; manh.hoang@csiro.au)

*Corresponding author: K S Zhang

Key Laboratory of Urban Environment and Health

Institute of Urban Environment, Chinese Academy of Sciences

Xiamen 361021, China

Fax: +86 592 6190977

Tel: +44 592 6190782

Email: kszhang@iue.ac.cn

ABSTRACT

This paper presents a systematic analysis the effect of amine salt surfactant added in aqueous

phase using interfacial polymerization (IP) to fabricate nanofiltration membrane. Four types

of amine salts with different molecular structures, molecular weights (MW) and charges,

including tetraethylammonium chloride (TEAC), tetrabutylammonium bromide (TBAB),

camphorsulfonic acid triethylamine salt (CAS-TEA), 1-butyl-3-methylimidazolium chloride

(BMIMCL), were incorporated into piperazine (PIP) solution during interfacial

polymerization with trimesoyl chloride (TMC). The formed thin film composite (TFC)

piperazine-amide polymer was supported on a polyethersulfone (PES) ultrafiltation

membrane (UF). The membranes surfaces were characterized by scanning electronic

microscopy (SEM), attenuated total reflection fourier transform infrared spectroscopy

(ATR-FTIR), and X-ray photoelectron spectroscopy (XPS). The flux and NaCl rejection were

also evaluated. The addition of amine salt containing larger steric configuration cationic

amine group in aqueous solution, resulted in a TFC membrane with high performance, and

the amine salt would not combined into the polymer, being easily washed away.

Keywords: Amine salt surfactants; interfacial polymerization; thin film composite membrane

1. Introduction

Nanofiltration (NF) has been widely used in water separation for drinking water

production and wastewater treatment/reuse as it can remove turbidity, microorganisms and

hardness, as well as a fraction of the dissolved salts at low operating pressure with high water

flux [1] . Most commonly commercial NF membranes are thin film composite (TFC)

polyamide membranes with a thin film layer fabricated onto a microporous substrate via

interfacial polymerization (IP) technique [2, 3]. By employing IP technique, the properties of

both top selective layer and bottom porous substrate can be independently controlled and

optimised to achieve disired water permeability and selectivity while offering excellent

mechanical strength [4]. The crosslinked aromatic polyamide is generally formed by

interfacial polymersation of two monomers, a polyfunctional amine (e.g.,

m-phenylenediamine) in aqueous phase and a polyfunctional acid chloride (e.g. trimesoyl

chloride) in organic solvent. Since water and the hydrocarbon solvent are immiscible,

polymerization reaction takes place at the water / hydrocarbon interface [5, 6]. In the early

development of the TFC membrane (1960s~1980s), most studies focused on researching the

IP reaction mechanism and finding a suitable material for the top barrier layer. Since 1980s,

the focus shifted to improve the properties of TFC membranes with respect to

permeability/selectivity z[3, 7, 8].

As membrane performance is mainly determined by the structure and property of the

top thin film layer, efforts have been devoted to the preparation of the ultrathin barrier layer

to fabricate membranes with high salt rejection properties. The IP process parameters, such as

type of monomer, monomer concentration, surfactant, reaction time, temperature, aqueous

solution pH, additives for cross-linker structure or pore-induced, as well as the fabrication

process and post-treatment affect the membranes performance. In a recent review by Kang et

al. [9], the use of monomers in the IP process were examined. Lau et al. [4] also investigated

monomers with different molecule weights and functional groups. These studies revealed that

functional monomers play an important role in the extent of cross-linking and different

monomers structure leads to different configuration of polymer. I1 Juhn Roh and co-workers

found that increasing acid chloride concentration resulted in thicker polyamide (PA) film with

lower flux, however changing the diamine concentration did not remarkably affect the

membrane performance [10].The curing conditions in the IP process such as reaction time,

curing temperature and the type of organic solvent used were also systematically studied [11,

12]. It was found that a higher diffusivity of functional amine reactant could lead to higher

amount of amine dissolving in the organic solvent, obtaining higher water flux, thickness, and

roughness, but less cross-linking. To increase the flux and salt rejection, more amine needs to

react with the acyl halide groups at the interface but not to diffuse into the bulk organic phase.

To obtain a membrane exhibiting both high water flux and selectivity, a higher curing

temperature is generally needed to remove organic solvent, but with the disadvantage of

increasing roughness. In addition, LiBr, a inorganic salt drew researchers’ interest [13], when

added in aqueous solution, it could produce high properties NF membranes, and by

controlling the concentration of LiBr in aqueous phase solution, the morphology of

membranes could be adjusted.

Previous researches mainly focused on selection of monomers, monomer ratios,

reaction conditions and post-treatment. Factors such as amine salt surfactant and support

membrane structure and properties, especially the role of amine salt surfactant, were not

frequently reported. The addition of surfactants is generally recommended as a necessary

component for IP process as it is believed that surfactant assists monomer in water phase

moving into organic phase, thus improves polymerization efficiency and consequently

properties of formed TFC membranes [4]. In early patents [14-17] amine salts, known as

phase-transfer catalysts (PTC), had been investigated in details, including concentration,

types and structure, etc., however the working mechanism was not examined [11, 12, 18, 19],

and little attention has been paid to their roles in IP process and the affect of their structure.

In this study, the mechanism of amine salts promoting IP process was investigated by

incorporating different types of amine salts into aqueous phase solutions before interfacial

polymerization with multi-functional acyl halide.

2. Experimental

2.1 Materials

Table 1 lists chemicals used in this study. Except lithium Chloride (LiCl) which was

heat treated to remove moisture, all other chemicals were used as received without further

purification. Deionised water (resistivity18MΩ·cm-1 @25°C) was used for preparing aqueous

solution.

2.2 Preparation of microporous PES membrane substrate

Microporous PES membrane was used as the supporting substrate of the polyamide

thin film layer. It was fabricated via phase inversion by following procedure: First, PES cast

solution was prepared by dissolving 19 wt% PES, 3.5 wt% Polyvinylpyrrolidone (K30), and

3 wt% LiCl in DMAC with a ratio of 74.5 wt% at 80. After stirring for 12h, the resulting

homogeneous solution was casted onto a non-woven fabric (thickness 100~110µm) using a

casting knife, followed by dipping into a deionised water bath for immediate phase inversion.

The thickness of wet film was controlled at ~250µm. After 30min in a gelation media, the

membrane was taken out and washed thoroughly using distilled water. The entire fabrication

membrane process was carried out at room temperature and relative humidity of ~30%.

Resulting microporous membrane with a molecular weight cut off (MWCO) of between 30k

and 50k Dalton was stored in a fridge at 4 prior to use.

2.3 Synthesis of TFC NF membranes

TFC membranes were prepared using traditional IP approach described elsewhere [2,

20-22]. Membranes fabrication were conducted using two 8mm deep polytetrafluoroethylene

(PTFE) circular rings, with outside and inner diameter of 140mm and 100mm respectively.

Four aqueous solutions were prepared, namely1.6 wt% PIP, 0.2 mol/L amine salt (where,

CAS-TEA was generated by reacting CAS with TEA in a weight ratio of 2.32 : 1), 0.15 wt%

sodium dodecyl sulphate (SLS) and 0.5 wt% sodium phosphate, while the organic phase

solution contained 0.35 wt% TMC, using N-hexane as solvent. Four different types of amine

salts including tetraethylammonium chloride (TEAC), tetrabutylammonium bromide (TBAB),

camphorsulfonic acid triethylamine salt (CAS-TEA), 1-butyl-3-methylimidazolium chloride

(BMIMCL) were used in the study with the conditions listed in Table 2. Except for using

different amine salt, a typical procedure for synthesizing poly (piperazine-amide) membranes

is described below.

First, the non-woven fabric side of PES membrane substrate was taped onto one of the

PTFE rings, and then clamped to the other PTFE ring. PIP solutions was poured onto the

dense side of the PES substrate, membrane was immersed for 1 minute, then the excess PIP

solution was removed using a soft rubber roller and dried for 2 minutes vertically at room

temperature. The TMC solution was poured over the membrane, keeping contact with TMC

solution for 20s, to allow in situ formation of a TFC layer over the surface of PES support.

Subsequently the fabricated membrane was cured for 3 minutes in an air-circulation oven at

50. Finally the membranes were thoroughly washed and stored in deionized water until the

membrane performance was evaluated.

To ensure the reproducibility, five samples were prepared for each membrane with

different amine salts.

2.4 Membrane testing

Performance of fabricated TFC NF membranes were evaluated with respect to the

water flux and salt rejection using a cross-flow PTFE cell CF042 (Sterlitech Co.) at 25 and

0.35 MPa for a feed solution contained 500 ppm NaCl and 2000 ppm MgSO4. The water flux

was calculated by measuring the weight of permeate solution passing through the membrane

over time (h) and over an exposed membrane area (m2). NaCl and MgSO4 salt rejections were

calculated from the conductivity of the feed and the permeation measured with a digital

conductivity meter (sensION + EC5 Portable Conductivity Meter, Hach Co.).

2.5 Membrane characterization

ATR-FTIR: The functional structure of TFC membrane samples was analysed using a Thermo

Scientific Nicolet 6700 iTR spectrometer. Attenuated total reflection-Fourier transform

infrared spectroscopy (ATR-FTIR) was performed on membrane samples from 600 to 4000

cm-1 wavelength with an 8 cm-1 resolution.

SEM: The surface morphology of the TFC membrane samples were imaged using a Philips

XL30 scanning electronic microscope (SEM). Before testing, all the membrane samples were

dried in vacuum oven at 80for more than 48 h. The samples were sputter coated with a

10–20 nm thin layer of iridium under vacuum.

XPS: X-ray photoelectron spectroscopy (XPS) analysis was performed using an AXIS Ultra

DLD spectrometer (Kratos Analytical Inc., Manchester, UK), with a monochromatised AlKa

source at a power of 180 W (15 kV × 12 mA), a hemispherical analyser operating in the fixed

analyser transmission mode and the standard aperture (analysis area: 0.3 mm × 0.7 mm). The

total pressure in the main vacuum chamber during analysis was less than 10-8 mbar. Survey

spectra were acquired at a pass energy of 160 eV. To obtain more detailed information about

chemical structure, oxidation states etc., high resolution spectra were recorded from

individual peaks at 40 eV pass energy (yielding a typical peak width for polymers of 1.0 – 1.1

eV).

Data processing was performed using CasaXPS processing software version 2.3.15

(Casa Software Ltd., Teignmouth, UK). All elements present were identified from survey

spectra. Binding energies were referenced to the aliphatic hydrocarbon peak at 285.0 eV.

Other details of test condition and the equipment information of software were given

elsewhere [23, 24].

3. Result And Discussion

3.1 Membrane morphologies

The surface morphologies of NF membranes were monitored by FESEM and were

presented in Fig. 1a. The pure PES membrane substrate showed a smooth surface. The poly

(piperazine-amide) layer polymerized onto the PES substrate resulted a thin film layer with a

rough surface.

When CAS-TEA, TEAC, TBAB or BMMIC (Fig. 1b~e) were added to the PIP solution,

the surface morphologies of the thin film show similar feature as can be seen from images

with 15,000 times magnification. This could be due to the similar basic element of structure

belonging to poly (piperazine-amide) structures. The surface SEM images of the membrane

show a rough film totally covering the surface of the PES substrate. The surfaces of

composite membrane formed have a “ridge-and-valley” morphology that gives quantitatively

rougher surface morphology [25-27].

The reasons why the membranes surfaces possess similar features could be because the

three former amine salts with similar symmetrical structure of cationic amine groups,

containing homologous substitute groups just with different number of carbon atoms (ethyl

and butyl), have the same catalytic property.

From the surface features, it was interesting to note that as the MW of cationic amine

group increased, the roughness of TFC layers increased. The total thickness of TFC

membranes (including the thickness of rough surface) were distributed in a relatively narrow

range of 500~850 nm. CAS-TEA resulted in a thickest membrane, followed by TEAC, TBAB,

and BMMIC, indicating that the four membranes possess different cross-linkers. It should be

noted that the CAS-TEA molecule possesses two organic groups, one containing a

super-hydrophilic bond of –SO3H, the other one with an amine functional bond. The former

may play a role as a surfactant in the formation of a smooth surface of the TFC membrane.

3.2 FTIR and XPS analysis

ATR-FTIR and XPS analyses were used to characterize the physiochemical structure of

the TFC layer and to understand the catalytic effect of four amine salts. XPS is a surface

sensitive technique and measures the element concentration (except H) and chemical binding

information for the top 1–10 nm depth of the surface region [28-30], while ATR-FTIR offers

much deeper penetration (from <200 nm to >1 um). Figure 2 shows the ATR-FTIR spectra of

PES membrane and four PES supported TFC NF membranes with different amino salts.

ATR-FTIR spectra feature information of the TFC layer in the range of 4000~2600 cm-1,

while PES and part of TFC layer in a lower range (2000~600 cm-1) [31], The peak at 1426

cm-1 (R-CH2-R) and 1664 cm-1 (-C-N) were assigned to PVP K30. Spectra of TFC

membranes with CAS-TEA, TEAC, TBAB, BMMIC added into aqueous solution are

presented in Fig. 2b and Fig. 3. FTIR spectra of these membranes show two collective

features:

1. Fig. 3 shows the peak resolve between 1520 cm-1 ~ 1750 cm-1, the peak at 1664 cm-1

for PVP that belongs to PES was overlapped by the peak of 1626cm-1 corresponding to

strong amideⅠband signal for poly (piperazine-amide). No new peak was detected in

TFC composite membranes, indicating amine salts could be washed away easily and

was not integrated into the poly (piperazine-amide) layer. This confirms that amine salt

functions as a catalyst in the aqueous solution during the IP process, consistent with

previous reports [19].

2. A broad and strong signal observed at the wavelength of 3442 cm-1 was assigned to the

carboxylic acid group or hydroxyl group on the surface of composite membranes

comparing to PES membrane. This means in the surface of NF membranes remain too

many the carboxylic acid group or hydroxyl by excess acyl groups’ hydroxylations that

make the fabricated NF membranes more hydrophilic.

Table 3 shows the result of XPS analysis for C, O, N contained on the surfaces of four

nanocomposite membranes. The bond of O-C=O represents –COOH [2], N-C=O stands for

piperazineamide, because the other reactants and additives don’t contain an aldehyde group,

the concentration of C (-C=O, N-C=O) and C (O-C=O) were assigned to C (piperazine-amide)

and C (–COOH). The presence of cross-linker in TFC layer can be analysed by comparing

the data of C (–COOH) to the sum of C (–COOH) and C (piperazine-amide). The highest

extent of cross-linker in the TFC layer occurs when TBAB was added, followed by TEAC

whilst CAS-TEA and BMMIC show a similar extent. This is mainly due to BMMIC with a

relatively long cationic amine group, making both efficacies of surfactant and catalyst as that

of CAS-TEA. It could be seen from the morphology of BMMIC, with two types of

morphologies as showed fig. 1a-e.

3.3 Membrane testing

Fig. 4 shows the membrane performance using a feed solution containing 500ppm

NaCl and 2000 ppm MgSO4. The MgSO4 rejection remained high at 96.4±1.8 %, regardless

of the type of amine salts. On the other hand, significant differences were observed for NaCl

rejection and water flux among membranes using four different types of amine salts,

BMIMCL outperformed the other three amine salt surfactant.

As can be seen from Fig. 4, when the MW of cationic amine groups increased, the

water flux for corresponding NF membrane were increased except for CAS-TEA. The lowest

cross-linking extent shown by XPS suggesting CAS-TEA makes an ion pair with the PIP

molecule and transfers it into the organic solution thus increases the diffusivity of functional

reactants. As a general observation in this study, the higher amounts of amine dissolved in

organic solvent, the higher water flux for corresponding membrane and the lesser

cross-linking.

The molecular structure of different amine salts are shown in Table 2. The three

dimensional structures show that the cationic amine group of CAS-TEA is a planar

configuration despite the ethyl group. TEAC has a tridimensional structure, and its molecular

size is much larger than the TEA group. The amine salts, when used to catalyze the IP

reaction, increase cross-linking in the layer and the porosity of TFC layer due to their large

steric configuration interfering in the membrane formation. This was also demonstrated by

BMMIC, a large asymmetric molecule with a cationic amine group, which has a similar MW

to TEA and with a tridimensional configuration [13, 19], which was thought to be a phase

transfer catalyst. By improving the transfer mechanism and increasing bond length due to

increased MW, the role of the amine salt changed to be a surfactant, increasing the

membranes water flux whilst decreasing salt rejection.

4. Conclusions

A series of TFC membranes were fabricated by IP process, using amine salts

(CAS-TEA, TEAC, TBAB, BMMIC) with different molecular structures and molecular

weights (MW) as surfactant. TFC membranes were characterised using SEM, ATR-FTIR, and

XPS. Membranes performance was also evaluated with respect to the water flux and

MgSO4/NaCl salt rejection. The addition of these ammine salts noticeably affected the

membranes properties. The extent of cross-linking was increased with increased molecular

weight of cationic amine groups. The higher performance was also found to be associated

with the surface roughness. As MW of amine group increases, the amine solubility in aqueous

solution reduces, an optimal type of amine salt in IP process amine and its role in IP process

should be considered.

5. ACKNOWLEDGE

This work was supported by CSIRO Water for Healthy Country National Research

Flagship. Dr Thomas, Dr James Mardel and Mr Mark Greaves from CSIRO are greatly

acknowledged for their assistance in XPS, FTIR and SEM analysis. Jun Xiang would like to

acknowledge the scholarship provided by the Chinese Scholar Council.

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Table 1 Materials used in this work

Table 2 The chemical structures of amine salts

Table 3 Surface composition: atomic concentrations relative to that of carbon (atomic ratios

X/C).

Fig.1. SEM micrograph of different membranes (a) PES membrane without TFC layer, (b)

TFC membrane with CAS added into aqueous solution, (c) TFC membrane with TEAC

added into aqueous solution, (d) TFC membrane with TBAB added into aqueous

solution, (e) TFC membrane with BMMIC added into aqueous solution

Fig.2. FTIR feature of PES support membrane and TFC membranes fabricated by four types

of ammine salt. (a) PES support membrane; (b) four TFC NF membranes FTIR feature

compare to PES support membrane.

Fig.3. FTIR peak resolve between 1520cm-1 to 1750cm-1 for TFC NF membranes

Fig.4. Four TFC membranes properties with different type of amine salt added in aqueous

phase

Table 1

Material CAS

number Source

Lithium Chloride (≥99%) 7447-41-8 Sigma-Aldrich Inc., St. Louis, MO,

USA Polyehersulfone (E6020) 25667-42-9 BASF (China) Co. Ltd. Guangzhou

Piperrazine (99%) 110-85-0 Sigma-Aldrich Inc., St. Louis, MO,

USA

Trimesic acid trichloride (98%) 4422-95-1 Sigma-Aldrich Inc., St. Louis, MO,

USA Tetraethylammonium chloride

(≥99%) 56-34-8

Sigma-Aldrich Inc., St. Louis, MO, USA

Tetrabutylammonium bromide (98%)

1643-19-2 Sigma-Aldrich Inc., St. Louis, MO,

USA

Camphorsulfonic acid (99%) 3144-16-9 Sigma-Aldrich Inc., St. Louis, MO,

USA 1-Butyl-3-methylimidazolium

chloride (≥98%) 79917-90-1

Sigma-Aldrich Inc., St. Louis, MO, USA

Triethylamine (≥99%) 121-44-8 Merck Pty Limited, Kilsyth, Victoria,

Australia

Sodium dodecyl sulfate (99%) 151-21-3 Sigma-Aldrich Inc., St. Louis, MO,

USA

N-haxane (99%) 110-54-3 Sigma-Aldrich Inc., St. Louis, MO,

USA

Sodium phosphate (96%) 7601-54-9 Sigma-Aldrich Inc., St. Louis, MO,

USA

Polyvinylpyrrolidone, K30 9003-29-8 Sinopharm Chemical Reagent Co.,Ltd,

China

N, N-dimethylacetamide (≥99%) 127-19-5 Jinshan Jingwei Chemical Co., Ltd,

Shanghai,China

MgSO4 (≥99%) 7487-88-9 Sinopharm Chemical Reagent Co.,Ltd,

China

NaCl (≥99%) 7467-14-5 Sinopharm Chemical Reagent Co., Ltd,

China

Table 2

Name Chemical Structure

CAS-TEA

TEAC

TBAB

BMMIC

Chemical Structure Structure of cationic amine

group

MW (cationic amine group)

101.9

130.2

242.37

139.17

Table 3

CSA-TE

A BMIMC

L TBAB TEAC

C (CHx, aliphatic or aromatic)

0.484 0.499 0.501 0.484

C (C-O, C-N) 0.329 0.277 0.304 0.285

C (C=O, N-C=O) 0.132 0.122 0.123 0.141

C (O-C=O) 0.028 0.026 0.013 0.022

O 0.176 0.169 0.161 0.176

N 0.148 0.132 0.130 0.148

Fig.1

(a)

(b)

(c)

(d)

(e)

Fig.2

(a)

(b)

Fig.3

Fig.4