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: [email protected]; [email protected] )
b CSIRO Materials Science and Engineering, Private Bag 33, Clayton, Vic. 3169, Australia (Email:
[email protected]; [email protected])
*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: [email protected]
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