Synthesis of sphere-like polyelectrolyte complexes and their homogeneous membranes for enhanced pervaporation performances in ethanol dehydration

Synthesis of sphere-like polyelectrolyte complexes and their homogeneous membranes for enhanced pervaporation performances in ethanol dehydration

Author’s Accepted Manuscript Synthesis of sphere-like polyelectrolyte complexes and their homogeneous membranes for enhanced pervaporation performance...

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Author’s Accepted Manuscript Synthesis of sphere-like polyelectrolyte complexes and their homogeneous membranes for enhanced pervaporation performances in ethanol dehydration Ziqiang Tong, Xiufeng Liu, Baoquan Zhang www.elsevier.com/locate/memsci

PII: DOI: Reference:

S0376-7388(18)31646-6 https://doi.org/10.1016/j.memsci.2019.01.001 MEMSCI16771

To appear in: Journal of Membrane Science Received date: 15 June 2018 Revised date: 29 December 2018 Accepted date: 1 January 2019 Cite this article as: Ziqiang Tong, Xiufeng Liu and Baoquan Zhang, Synthesis of sphere-like polyelectrolyte complexes and their homogeneous membranes for enhanced pervaporation performances in ethanol dehydration, Journal of Membrane Science, https://doi.org/10.1016/j.memsci.2019.01.001 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting galley proof before it is published in its final citable form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.

Synthesis of sphere-like polyelectrolyte complexes and their homogeneous membranes for enhanced pervaporation performances in ethanol dehydration

Ziqiang Tong, Xiufeng Liu, Baoquan Zhang* School of Chemical Engineering and Technology, Tianjin University, Tianjin 300350, China *

Corresponding author. Tel.: +86 22 85356517. [email protected]

ABSTRACT A series of sphere-like polyelectrolyte complexes (PECs) and their homogeneous polyelectrolyte complex membranes (HPECMs) were synthesized by using sodium carboxymethyl cellulose (CMCNa) and poly(diallyldimethylammonium chloride) (PDDA). Different compositions or ionic complexation degrees (ICD) of PECs could be characterized by FT-IR spectra. The circular dichroism spectrum (CD) and field emission scanning electron microscopy (FESEM) were employed to examine the conformation and morphology of PECs, respectively. Zeta (ζ) potential was utilized to investigate the charge density of PEC particles while the surface hydrophilicity of HPECMs was studied by using contact angle (CA) meter. When subjected to ethanol dehydration, HPECMs revealed a satisfactory pervaporation performance and desired anti-trade-off phenomenon. For HPECM 0.31 and HPECM 0.57, the permeation fluxes were 2.25 kg·m-2·h-1 and 2.93 kg·m-2·h-1, and the water content in permeate reached up to 99.35 wt% and 99.38 wt%, with respect to 10 wt% water in feed under 70 °C. All the results indicated that small-sized and sphere-like PECs were truly in favor of enhancing the pervaporation performance in ethanol dehydration.

Keywords: sphere-like polyelectrolyte complex; homogeneous membrane; pervaporation

1. Introduction Polyelectrolyte complexes (PECs) are an important type of multicomponent polymeric 1 / 20

materials. By mixing solutions of polyanions and polycations, PECs are spontaneously formed accompanying the release of counterions. The driving force for PECs formation is mainly the electrostatic interaction between oppositely charged groups in polyelectrolyte chains, but other weak interactions such as hydrogen bonding, van der Waals, or hydrophobic influence could also play an additional role [1]. The structures of PECs are diverse and can be tailored by changing parameters such as mixing ratio, concentration, solution pH, ionic strength, and temperature in the process of PECs preparation [2]. Meanwhile, the specific characteristics of polyelectrolytes, such as chain rigidity/flexibility, topography, charge density, and molecular weight have important influences on defining the structure of PECs [2]. Benefiting from their versatility in chemical composition and structure, PECs have been applied in many respects such as gene/DNA delivery and drug release [35], protein adsorption [6], flocculation [7,8], surface modification [9], microencapsulation [10], pervaporation [1113], nanofiltration [14,15], and fuel cells [16]. In pervaporation field, PECs are promising in enhancing the pervaporation performance as a consequence of their ionic crosslinking and highly hydrophilic properties. Polyelectrolyte complex membranes (PECMs) are promising membrane materials in limiting excessive swelling and counterions loss, and especially suitable for the dehydration of hydrous alcohols [17]. Up to present, there are three strategies for preparation of PECMs including the interfacial complexation, the acid blending, and the protection-deprotection methods [1120]. Both the interfacial complexation method and the acid blending method cannot achieve the processability of PECs. The pervaporation performances of PECMs fabricated in these two methods are unsatisfactory due to the underutilization of charges in ionized groups [18]. Recently, Zhao et al. [19,20] proposed a new route, i.e. the protection-deprotection method, to fabricate soluble and processable PECs, as well as homogeneous polyelectrolyte complex membranes (HPECMs). There are many advantageous characteristics such as no glass transition, high hydrophilicity, tunable surface charge, and structural stability when this method is applied to design novel functional materials. The formation of PECs consists of two stages, an initial rapid formation of nanoscale primary complex particles (sized at ca. 5-20 nm) and their subsequent aggregation to secondary particles [21]. Müller et al. proposed there were three components existing in the stable PECs dispersion: (i) nanoscale soluble primary complexes, (ii) dispersed colloidal particles of aggregated secondary 2 / 20

complexes, and (iii) larger insoluble precipitate particles [22]. The PECs could be sphere-like, needle-shaped, or toroid-like, which was mainly determined by the conformation of polycations or polyanions. It was acknowledged that polyelectrolytes could exist in different conformations including α-helixe, β-sheets, or random coils, depending on some variables such as pH, temperature, and the ionic strength of the dispersant. Besides, the intrinsic properties (e.g. the number, type, and position of ionic groups as well as molecular weight) have some influences on the conformation of polyelectrolytes [2]. For example, Müller et al. [23] found that randomly coiled poly(L-lysine) would induce the formation of sphere-like PEC particles and isotropically structured PECMs while the α-helical poly(L-lysine) could cause needle-like PEC particles and anisotropic PECMs. Obviously, the morphology of PECs could be implemented by changing the conformations of polyelectrolytes [22]. Sodium carboxymethyl cellulose (CMCNa) is a kind of low cost, environmentally friendly, and widely used biochemical materials, which is extremely suitable to be polyanions. Zhao et al. used the protection-deprotection method to prepare needle-like poly(diallyldimethylammonium chloride)/sodium carboxymethyl cellulose (CMCNa-PDDA), chitosan/sodium carboxymethyl cellulose (CMCNa-CS), and poly(2-methacryloyloxy ethyl trimethylammonium chloride)/sodium carboxymethyl cellulose (CMCNa-PDMC) PEC particles and corresponding HPECMs, demonstrating better performances in the dehydration of ethanol, isopropanol, acetone, or dioxane aqueous solution [20,24,25]. Furthermore, Wang and Jin et al. [2630] modified polyanions or polycations, and prepared various needle-like PECs particles containing special ion-pairs, as well as their HPECMs. More stable pervaporation performance was acquired in the dehydration of organic solvents, even though it seemed a little bit complicated for synthetic operations. In view of particle characteristics, large and needle-like PECs with anisotropy could cause the non-uniform distribution of long-range structures and lower charge densities, which would weaken the dehydration performance as a result. In contrast, we speculate that smaller-sized and sphere-like PECs particles could avoid the above weaknesses. It is believed that small and spherical PECs should possess higher charge densities, spatial isotropy, and long-range homogeneity. All of these are beneficial to enhance the pervaporation performance of HPECMs. By changing parameters of polyelectrolytes and operating conditions, the conformation of polyelectrolytes could be regulated, therefore, the PECs with different morphologies can be 3 / 20

manufactured. Herewith, PDDA and CMCNa were selected to fabricate soluble sphere-like PECs. By tuning the ionic complexation degree (ICD), a series of PECs and HPECMs were synthesized. The satisfactory pervaporation performances and desired anti-trade-off phenomenon could be achieved in ethanol dehydration. 2. Experimental 2.1. Materials CMCNa (Mw = 250,000 g/mol, DS = 0.9) and PDDA (Mw = 100,000200,000 g/mol, 20% aqueous solution) were purchased from Aladdin, Shanghai, China. The viscosity of PDDA was in the range of 4001000 cP (25 °C). Both of CMCNa and PDDA were used without further purification. The polyacrylonitrile (PAN) ultra-filtration membrane was obtained from Shandong Megavision Membrane Technology & Engineering Co., Ltd. The PAN membrane used as the support in this study was washed thoroughly before casting. Sodium hydroxide (NaOH) and hydrochloric acid (HCl), supplied by Tianjin Guangfu Chemical Co. Ltd, were all analytical reagents. All of the water used was obtained from a water purifier (Ulupure-II-10 T, Chengdu Ultrapure Technology Co., Ltd.) with a resistivity of 18.25 M cm. 2.2. Synthesis of PECs PECs with four different compositions were synthesized by dropping PDDA into CMCNa solution at different HCl concentrations. The procedure would be illustrated as below. At first, 400 mL of 0.01 M CMCNa and 0.01 M PDDA (monomer unit mole concentration) were prepared by dissolving CMCNa or PDDA in HCl aqueous solutions with four concentrations (0.011 M, 0.009 M, 0.005 M, 0.003 M). The CMCNa solution was stirred vigorously with a rotating speed of 400 rpm in a flask at 60 °C overnight for complete dissolution. Then, the PDDA solution was added to CMCNa dropwise at a steady rate under a more vigorous stir at 600 rpm. The insolubles appeared immediately when complex reaction between PDDA and CMCNa occurred, and the system became more and more turbid. The end point would be reached when an obvious macroscopic phase separation emerged, and then the dropping of PDDA was stopped. The volume fraction of PDDA added to the whole CMCNa (which was 400 mL) was defined as ICD. It also referred to the proportion that the number of carboxylate groups on CMCNa accounted for the total number of unionized carboxylic acid groups in PECs. Then, PECs precipitate was filtrated and washed with deionized water several times to remove the free ions and parent polyelectrolytes being 4 / 20

adhered to it. Finally, the precipitate was dried at 60 °C to a constant weight for further use. For the sake of simplicity, PECs with PDDA:CMCNa value equaling X were named as PEC X, and the membranes made from PEC X were named as HPECM X. 2.3. Synthesis of HPECMs For the synthesis of CMCNa-PDDA HPECMs, 0.5 g PECs was dissolved in aqueous NaOH to prepare casting solution (1.0 wt%). Due to pre-added HCl, some unionized carboxylic acid groups belonging to CMCNa existed in PECs. These unionized carboxylic acid groups made PECs solids soluble in NaOH through their ionization. To avoid the existence of free NaOH, the needed moles were determined by the equation: [NaOH]=[CMCNa]·(1-VPDDA/VCMCNa)

(1)

where [CMCNa] was the moles of entire CMCNa monomers and VPDDA/VCMCNa referred to the ICD of PECs samples. By this way, free NaOH could be avoided as much as possible in dope solution. In fact it was observed that the pH value of the prepared solution was nearly 8, which revealed that just a little free NaOH existed. Finally, the obtained solution was cast uniformly on PAN ultra-filtration membrane and dried at 60 °C and 50 % relative humidity overnight. For clarity, the schematic diagram for preparing soluble PECs was given in Fig. 1.

Fig. 1. Schematic diagram for fabricating CMCNa-PDDA PECs.

2.4. Characterization 5 / 20

Fourier transform infrared spectroscopy (FT-IR) of solid PECs was obtained by utilizing a NICOLET 6700 FT-IR spectrometer. The solid PECs were dispersed in KBr to make a disk sample, and the scanning range was from 4000 to 400 cm-1. The zeta (ζ) potential of PEC particles was measured on a Nano ZS ζ potential analyzer (Malvern). The pH values of solutions were determined by using a digital pH meter (S2211T, Alalis). The conformation of PEC particles was identified by a Jasco J-810 circular dichroism (CD) detector. The surface and cross-sectional morphologies of HPECMs were observed with a Hitachi S-4800 Field Emission Scanning Electron Microscopy (FESEM). HPECM samples were coated with gold prior to FESEM observations. The water contact angle of membrane materials was measured on a contact angle meter (OCA15EC DataPhysics). 2.5. Pervaporation experiments Pervaporation experiments were conducted on a homemade setup with the effective membrane area of 4.02 cm2, the detailed procedure could be found elsewhere [28]. The downstream pressure was kept at about 100 Pa by using a vacuum pump (Vacuubrand RC6) with an electromagnetic controller (Vacuubrand CVC3000). The feed temperature was kept within an accuracy of 0.3 °C by an electric control thermometer. The permeate was condensed by liquid nitrogen as a cold trap and analyzed by gas chromatograph (Ke-Chuang GC9800). Permeate flux (J) and separation factor (α) were calculated according to the following equations:

J

Q A t

(2)



PW / PE FW / FE

(3)

where Q was the weight of permeate collected in cold trap within operating time t, and A referred to the effective membrane area, PW, PE and FW, FE represented the weight fractions of water and ethanol in permeate and feed, respectively. The pervaporation experiments at the same operation condition were repeated three times and averaged. 3. Results and discussion 3.1. Synthesis of PECs The ICD (VPDDA / VCMCNa) values identified at four HCl concentrations were summarized in Table 1. Obviously, the ICD was lowered with the increase of HCl concentration, indicating that 6 / 20

less PDDA was needed for neutralizing a fixed amount of CMCNa, This exactly corresponded to the mechanism shown in Fig. 1.

7 / 20

Table 1 Compositions of PEC samples determined by experimental observations. PEC samples

[HCl]

ICD

Membranes

PEC 0.23

0.011 M

0.23

HPECM 0.23

PEC 0.31

0.009 M

0.31

HPECM 0.31

PEC 0.45

0.005 M

0.45

HPECM 0.45

PEC 0.57

0.003 M

0.57

HPECM 0.57

3.2. Characterization of PECs and HPECMs Fig. 2 (a) shows the FT-IR spectra of CMCNa, PEC 0.23, PEC 0.31, PEC 0.45, PEC 0.57 and HPECM 0.31, respectively. In the curve of CMCNa, strong absorption band was observed only at 1610 cm-1 but nothing revealed at 17001800 cm-1, which indicated that carboxyl groups in CMCNa were fully ionized. For PEC samples, the coexistence of absorption bands at 1610 cm-1 and 1730 cm-1 indicated that both of ionized and unionized carboxylic acid groups existed. With the decrease of HCl concentration, the amount of carboxylic acid groups protonated by HCl reduced while ionized carboxylic acid groups increased. It meant that more PDDA was needed to complex or coordinate with CMCNa and the ICD of PECs became higher. Therefore, the band intensity at 1610 cm-1 enhanced accompanying with the reduction at 1730 cm-1. For HPECM 0.31, the intensity of absorption at 1730 cm-1 nearly disappeared and absorption at 1610 cm-1 got much stronger, which proved the complete ionization of unionized carboxyl groups by NaOH and was precisely consistent with the mechanism as shown in Fig. 1. In Fig. 2 (b), typical CD spectrum for PEC 0.31 particles at pH = 7 was given. A distinct negative intensity at around 195 nm appeared, which indicated a random coil conformation of polyelectrolytes [22].

Fig. 2. (a) FT-IR spectra of CMCNa, PEC 0.23, PEC 0.31, PEC 0.45, PEC 0.57 and HPECM 0.31. (b) CD spectrum of PEC 0.31 in the range between 180 and 250 nm at pH = 7.

Fig. 3 presents the surface and cross-section morphology of HPECM 0.31. It could be 8 / 20

observed that the membrane displayed a smooth, homogeneous and dense surface. The thickness was nearly 2.2 ± 0.2 μm. As seen from Fig. 3 (b), sphere-like PEC particles with a diameter of 30-50 nm distributed uniformly on the surface of HPECM 0.31. The size was much smaller than those needle-shaped PECs [20,2426,29,30], which accounted for some particular characteristics as would be investigated below.

Fig. 3. FESEM images of HPECM 0.31 surface (a) and cross-section (c), and their magnified morphology (b) and (d), respectively.

Actually, both needle-like and sphere-like PECs could be formed by using CMCNa and PDDA (Supporting Information). As reported earlier [2], weakness/strength, charge density, molecular weight, linear or branched topology, and monomer properties have some influences on the comformation of a polyelectrolyte. The more carboxyl groups CMCNa has, the more fully ionized it could be in aqueous solution. Thus, it tended to adopt an extended structure, which would favor the α-helical conformation to induce needle-like PECs [22]. In this experiment, due to the relatively less ionic groups, CMCNa was in less charged. This made it difficult to prevent the tendency of organic polymer chains to bridge or entangle with each other due to the hydrophobic interaction. Therefore, CMCNa was expected to adopt a more coiled conformation. As a result, sphere-like PECs was obtained. According to Müller et al. [23], the formation of needle-like PECs required a relatively high molecular weight for polyanion. It was believed that the synergistic 9 / 20

effects of ionic group number and molecular weight for CMCNa would result in different morphologies of CMCNa-PDDA PEC particles. Fig. 4 (a) shows the ζ potential of CMCNa, PEC 0.23, PEC 0.31, PEC 0.45, and PEC 0.57, respectively. It revealed that ζ potential of PEC particles increased with ICD. The reason could be explained as follow. With the addition of PDDA, the nucleation sites dramatically increased and tiny sphere-like PECs induced by coiled CMCNa were generated in large quantities. The excess CMCNa dominated the shell region through electrostatic interaction, which competed with the aggregation of primary particles resulted by hydrophobic interactions, bridging or chain entanglement. Therefore, the size distribution of PEC particles was enlarged while the average size became smaller. Moreover, the exceedingly abundant nucleation and vigorous stir would also hinder the aggregation and finally caused tiny sphere-like PEC particles. There were two significant factors determining their ζ potential, the particle size, which could be also interpreted as compactness, and the residual charge. Both the larger compactness and larger residual charge resulted in higher ζ potential of PECs. With the growth of ICD, the ionic cross-linking degree of CMCNa and PDDA in PECs should increase. As a result, the compactness of PECs increased. The ζ potential increased with ICD, indicating that the compactness of PEC particles dominated their ζ potential even though the residual charge density reduced in theory. This also meant that with the growth of ICD, the hydrophilicity of HPECMs ought to rise as well, thus, the water contact angles were expected to gradually reduce, which was consistent with the results in Fig. 4 (b).

Fig. 4. (a) The -ζ potential of CMCNa, PEC 0.23, PEC 0.31, PEC 0.45, and PEC 0.57. (b) Variation of water contact angles with ICD for HPECMs.

3.3. Pervaporation performances of HPECMs 10 / 20

3.3.1 Effect of ICD on pervaporation performance Fig. 5 shows the effect of ICD on pervaporation performance at 50 °C in ethanol dehydration with 10 wt% water in feed. The flux increased with ICD, meanwhile, the water content in permeate remained stable. This phenomenon was contrary to the trend of pervaporation performance in most HPECMs with needle-like PECs [20,25]. According to the solution-diffusion model, surface properties and aggregation structures governed the solution and diffusion steps, respectively, of polymer membranes in pervaporation process. As shown in Fig. 4 (a), the ζ potential of PECs increased with ICD and always maintained at a higher value, so that the surface of HPECMs was more attractive to water due to its higher polarity and repelled the pass of ethanol. Therefore, huge flux and ultrahigh selectivity could be obtained with this special morphology. At the same operating condition, the flux of most organic membranes was almost all less than 1 kg·m-2·h-1. In this work, the flux of HPECM 0.31 was up to 1.6 kg·m-2·h-1 with 99.58 wt% water content in permeate, which was nearly twice of organic membrane flux. 2.0

90

-1

Flux (kg·m ·h )

1.8

1.6

-2

80 1.4 70 1.2 60 Flux Water in permeate

1.0 0.2

Water in permeate (wt %)

100

0.3

0.4

0.5

50 0.6

ICD Fig. 5 Effect of ICD on flux (circles) and water in permeate (triangles) at 50 °C in ethanol dehydration with 10 wt% water in feed.

3.3.2 Effect of operating temperature on pervaporation performance Fig. 6 reveals the effect of feed temperature on pervaporation performance of HPECM 0.31 and HPECM 0.57. Both the flux increased and had a rapid growth during 30-55 °C with a very high water content in permeate. It could be attributed to the rising temperature, which caused a larger driving force in pervaporation process and led to a bigger flux. While, as seen from Fig. 6, water content in permeate remained basically unchanged and was always kept above 99.30 wt% in 11 / 20

the range of 17-55 °C. This “anti-trade-off” phenomenon should be ascribed to PECs own characteristics. The strong ionic interaction within and between PECs particles effectively limited the swelling of membrane and further contributed to a quite stable internal structure. Thus, the selectivity kept steady with the increase of temperature. Moreover, the flux of HPECM 0.57 was larger than that of HPECM 0.31 at same operating condition, which meant that compared to HPECM 0.31, the charge density and hydrophilicity of HPECM 0.57 was higher. It was also in accordance with the results of ζ potential and contact angle tests. Therefore, it could be demonstrated that the size and morphology of PECs truly had a significant influence on pervaporation performance of HPECMs.

2.0

-2

-1

Flux (kg·m ·h )

80

1.5

60

1.0

40 HPECM 0.31 HPECM 0.57

0.5 20

30

40

50

Water in permeate (wt %)

100

20 60

Temperature (°C) Fig. 6. Effect of operating temperature on flux (solid) and water in permeate (hollow) of HPECM 0.31 (triangle) and HPECM 0.57 (circle) in ethanol dehydration with 10 wt% water in feed.

Based on the data in Fig. 6, the temperature dependence of pervaporation flux was discovered to follow Arrhenius equation, as illustrated in Fig. 7, where the logarithms of permeation fluxes of water and ethanol were plotted against reciprocal temperature, respectively. The permeation activation energy of water and ethanol calculated from fitting line slope were 28.08 and 94.28 kJ·mol-1 for HPECM 0.31, and 25.85 and 37.23 kJ·mol-1 for HPECM 0.57, respectively. Despite the fact that the ICD differed in these two membranes, there was no remarkable difference in water permeation activation energy and the value retained always quite low, implying that the diffusion of water molecules was much easier within the membrane [31,32]. It also explained the stable water content in permeate with increase of temperature.

12 / 20

Fig. 7. Arrhenius plots (ln J versus 1000/T) of HPECM 0.31 (a) and HPECM 0.57 (b) in ethanol dehydration with 10 wt% water in feed.

3.3.3 Effect of feed content on pervaporation performance Fig. 8 shows the effect of water content in feed on pervaporation performance. When the feed concentration increased from 5 wt% to 20 wt%, it could be observed that the water in permeate stayed at a high level more than 98.5 wt% and 97.8 wt% for HPECM 0.31 and HPECM 0.57 with huge fluxes, respectively. A slight decrease appeared when feed concentration further rised to 30 wt%, but it still maintained relatively high. For instance, the flux reached up to astonishing 7.66 kg·m-2·h-1 with water content kept at 94.72 wt% in permeate for HPECM 0.57 in this condition. This satisfactory pervaporation performance should be attributed to the special ionic cross-linking structure and excellent hydrophilicity resulted by sphere-like PECs particles with smaller size and uniform charge distribution in HPECM.

Fig. 8. Effect of water content in feed on flux (solid) and water in permeate (hollow) of HPECM 0.31 (triangle) and HPECM 0.57 (circle) in ethanol dehydration at 50 °C. 13 / 20

3.3.4. Comparison of ethanol dehydration performance Table 2 compares the pervaporation performance of HPECMs in this study with those reported in the literature for ethanol dehydration. Some commercial available polymers, blending films and polyeletrolyte membranes prepared in different ways were selected. For HPECM 0.31 and HPECM 0.57, the permeation fluxes were 2.25 kg·m-2·h-1 and 2.93 kg·m-2·h-1, and the water concentration in permeate reached up to 99.35 wt% and 99.38 wt%, respectively, with 10 wt% water in feed at 70 °C. Under the premise of maintaining similar separation effect, the flux of the HPECMs with spherical PECs was about 10-15 times higher than that of traditional PVA and CS membranes, and was also much bigger than that of most organic membranes. Due to ultrahigh flux and excellent separation effect, the performance of this work proved to be far more superior to other membranes. Table 2 Pervaporation performances of HPECMs for ethanol dehydration and comparison with various membranes reported in the literature.

Membranes

Feed ethanol (wt%)

Feed temperature (°C)

Water in permeate (wt%)

Flux (kg·m-2·h-1)

Separation factor

References

PVA/Sericin PVA/CS PI/SPIa PEIb HA/CSc CS/CPd CS/CMCNa GACS/PCPe GACS/PDAf

90 95 85 95 90 90 90 90 90

60 60 60 75 80 80 70 80 80

92.74 80.49 97.67 97.12 98.74 96.60 99.16 99.30 86.18

0.12 0.39 2.00 0.51 1.45 1.25 1.14 1.39 2.28

115 78.4 237 641 704 256 1062 1279 56

[35] [36] [37] [38] [39] [40] [25] [41] [42]

CS-ST-GPE-Sg

90

70

99.03

0.41

919

[43]

SCMC/PDDAh CS/CMCNai SCMC/PEI PEI/TA-8j SA/PEIk CS/PSSl PEG@POSS/SAm HPECM 0.31 HPECM 0.57

82 90 80 90 90 90 90 90 90

50 70 50 76 60 70 77 70 70

97.12 95.43 98.11 99.00 99.22 99.00 99.17 99.35 99.38

2.10 0.49 0.65 1.34 1.52 0.50 2.50 2.25 2.93

153 188 208 1012 1145 904 1077 1376 1443

[44] [25] [44] [45] [46] [47] [48] This work This work

Note: the pervaporation performance of this work in the table was measured with downstream pressure maintained at 200 Pa. a

Polyimide/sulfonate polyimide blend membranes.

b

Polyelectrolyte polyethyleneimine composite membranes.

c

Polyelectrolyte hyaluronic acid/chitosan/PAN composite membranes.

d

Polyelectrolyte chitosan/polyacrylonitrile composite membranes.

e

Glutaraldehyde cross-linked chitosan/polycarbophil calcium/PAN composite membranes. 14 / 20

f

Glutaraldehyde crosslinked chitosan/polydopamine/polyethersulfone membranes.

g

Chitosan-silica nanoparticles with sulfonic acid groups blending membranes.

h

Sodium cellulose sulfate/PDDA two-ply PEC membranes.

i

CS-CMCNa two-ply PEC membranes.

j

Polyethyleneimine/tannic acid multilayer membranes.

k

Sodium alginate/polyethyleneimine multilayer membranes.

l

Chitosan/poly(4-styrenesulfonic acid) multilayer membranes.

m

Poly(ethylene glycol)-functionalized polyoctahedral oligomeric silsesquioxanes/SA composite membranes.

Table 3 demonstrates the influence of PEC particle morphology on the pervaporation performance of HPECMs. All of them were prepared by the protection-deprotection method using CMCNa as polyanions. Compared with the HPECMs with needle-like PECs, the flux of the HPECMs with sphere-like PECs was nearly doubled. It indicated that the performance of the latter were indeed much better than that of the former in ethanol dehydration, even though lager free volume contributed by large inter-particle size existed in HPECMs with needle-like PECs. Actually, in addition to the free volume, the charge density of PECs could be an important factor that might influence the pervaporation performance of their HPECMs. The ζ potential of diluted sphere-like PECs dispersion and needle-like PECs dispersion with the same ICD were tested (Supporting Information). It suggested that smaller spherical PEC particles possessed higher charge density than needle-like ones, which was conducive to the enrichment of water molecules on the surface of HPECM and the transport in the membrane to achieve a big flux. Meanwhile the spherical morphology of PECs also favored the spatial isotropy and long-range homogeneity of HPECM, which was beneficial to the improvement of separation selectivity. A schematic model was given in Fig. 9 to demonstrate the different behavior of water molecules in these three kinds of membranes. Interestingly, the HPECMs flux of this study was even much bigger than part of inorganic membranes and hybrid membranes, almost 1-2.5 times higher, and the selectivity was also more outstanding than the majority of them [33,34]. Obviously, it was the big charge density and the isotropy of sphere-like morphology of PECs particles that gave water molecules the priority to permeate through the membrane.

15 / 20

Table 3 Comparison of pervaporation performances for the HPECMs with sphere-like PECs and other HPECMs using CMCNa as polyanions.

Membranes

PECs Morphology

Feed ethanol (wt%)

Feed temperature (°C)

Water in permeate (wt%)

Flux (kg·m-2·h-1)

References

CMCNa/PDDA

Needle-like

90

40

99.12

0.56

[20]

Needle-like

90

70

99.16

1.14

[25]

CMCNa/CS CMCNa/QP4VP

a

NA

90

60

95.10

0.89

[49]

b

NA

90

70

98.70

1.76

[27]

c

Needle-like

90

70

98.86

1.32

[29]

NA

90

70

99.43

1.39

[28]

HPECM 0.31

Sphere-like

90

70

99.35

2.25

This work

HPECM 0.57

Sphere-like

90

70

99.38

2.93

This work

SCMC/PDDA

CMCNa/PEVP CMCNa/SCS

d

Note: the pervaporation performance of this work in the table was measured with downstream pressure maintained at 200 Pa. NA represented that the morphology was not available. a

CMCNa/quaternized poly(4-vinylpyridine) HPECM

b

Sulfated sodium carboxymethyl cellulose/PDDA HPECM

c

CMCNa/poly (N-ethyl-4-vinylpyridiniumbromide) HPECM

d

CMCNa/sulfated CS HPECM

Fig. 9. A schematic model of water (blue circles) and ethanol (orange circles) permeation in common polymeric membranes (a), HPECMs with needle-like PECs (b), and HPECMs with sphere-like PECs (c). The arrows are used to indicate the moving direction of molecules.

Conclusions A series of sphere-like PECs and HPECMs with good repeatability were prepared by using CMCNa and PDDA in this study. The properties of the HPECMs were strongly varied with ICD ranged from 0.23 to 0.57. The typical CD spectrum and FESEM images confirm the random coil conformation and sphere-like morphology of PECs particles. Compared with the polyelectrolyte membranes reported in the literature, some unique characteristics of HPECMs could be observed. When subjected to ethanol dehydration, the HPECMs with sphere-like PECs in this work revealed a much enhanced pervaporation performance and desired anti-trade-off phenomenon. For HPECM 0.31 and HPECM 0.57, the permeation fluxes were 2.25 kg·m-2·h-1 and 2.93 kg·m-2·h-1, and the water concentration in permeate reached up to 99.35 wt% and 99.38 wt%, respectively, with 16 / 20

respect to 10 wt% water in feed at 70 °C. It could be proved that the HPECMs with sphere-like PECs were much superior to those with needle-like PECs in ethanol dehydration. All these results implied that small-sized and sphere-like PECs particles with uniform charge distribution were truly in favor of enhancing the pervaporation performance of the HPECMs in ethanol dehydration.

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Graphical Abstract

Highlights 

Sphere-like polyelectrolyte complexes (PECs) were successfully synthesized.



Homogeneous polyelectrolyte complex membranes (HPECMs) could be fabricated.



The HPECMs showed excellent pervaporation performances for ethanol dehydration.



Water flux up to 2.93 kg·m-2·h-1 with a separation factor of 1443.

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