Ionic-Strength Responsive Zwitterionic Copolymer ... - ACS Publications

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Ionic-strength Responsive Zwitterionic Copolymer Hydrogels with Tunable Swelling and Adsorption Behaviors Tao Xiang, Ting Lu, Weifeng Zhao, and Changsheng Zhao Langmuir, Just Accepted Manuscript • DOI: 10.1021/acs.langmuir.8b01719 • Publication Date (Web): 14 Aug 2018 Downloaded from http://pubs.acs.org on August 18, 2018

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Ionic-strength Responsive Zwitterionic Copolymer Hydrogels with Tunable Swelling and Adsorption Behaviors Tao Xianga,b*, Ting Luc, Wei-Feng Zhaod, Cheng-Sheng Zhaod

a

Key Laboratory of Advanced Technologies of Materials, Ministry of Education, School of

Materials Science and Engineering, Southwest Jiaotong University, Chengdu 610031, China b

State Key Laboratory of Molecular Engineering of Polymers, Fudan University, Shanghai

200433, China c

National Engineering Research Center for Biomaterials, Sichuan University, Chengdu 610064,

China d

College of Polymer Science and Engineering, State Key Laboratory of Polymer Materials

Engineering, Sichuan University, Chengdu 610065, China

ACS Paragon Plus Environment

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ABSTRACT:

In this work, we studied the swelling behavior and adsorption behavior of zwitterionic copolymer hydrogels, which were prepared via the free radical copolymerization of sulfobetaine methacrylate (SBMA) and other monomers including sodium p-styrene sulfonate (NaSS), acrylic acid, N-isopropylacrylamide, and 2-(dimethylamino) ethyl methacrylate. The PSBMA hydrogel showed increased swelling ratio with the increase of ionic strength at the same temperature, and the swelling process reflected endothermicity. Interestingly, the PSBMA-NaSS hydrogels were collapsed along with increasing the ionic strength, because the ions could weaken the repulsive interaction of the anionic groups of PNaSS. In addition, the PSBMA-NaSS showed high adsorption amount of methylene blue (760 mg/g). The zwitterionic hydrogels have potential to be used as an adsorbent in the field of wastewater treatment.

INTRODUCTION

Stimuli-responsive hydrogels exhibit dramatic changes in swelling behavior, mechanical strength, elastic modulus, permeability, and adhesion in response to external stimuli including ionic strength, pH, temperature, and magnetic field of the surrounding solution.1-5 Thus, stimuliresponsive hydrogels have potential to be used in biomedical engineering, separation, biosensors, purification and so on.6-9 As ionic strength is a common and easily controlled stimulus, ionicstrength sensitive hydrogels may have wide applications. Ionic-strength sensitive hydrogels are often synthesized by ionizable polymers such as carboxymethyl dextran,10 xanthan gum derivative,11 poly(acrylic acid)12 and poly(itaconic acid),13 poly(N,N-dimethylaminoethyl methacrylate)14 and poly(N,N-diethylacrylamide),15 which showed significantly different swelling behavior in water and salt solution. The ionization degree and charge density of the


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hydrogels have vital influences on the swelling and shrinking behaviors in the surrounding solution.16-18 In these studies, thez in the salt solution was always lower than that in water, because the repulsive interaction was weakened in salt solution, resulting in shrinking. Zwitterionic polymers possess both negative and positive charges in the repeat unit, and sulfobetaine (SB) and carboxybetaine (CB) materials are the most widely studied zwitterionic materials. The self-associations, ionic interactions and hydration of zwitterionic materials are different for CB and SB materials. By using molecular simulation, Jiang et al.19 found that SB polymers showed strong associations between the anionic and cationic groups, whereas CB polymers did not. Fennell et al.20 reported that the self-associations among zwitterionic moieties were dictated by the similarity in their charge densities. Thus, the SB polymers may show significant salt response. In our previous studies,21-23 ionic-strength sensitive membranes were fabricated by in situ crosslinked polymerization of sulfobetaine methacrylate (SBMA) in polyethersulfone solution. When the ionic strength increased from 0.0 to 0.3 mol/kg, the membranes showed a sharp decrease of flux. The result might attribute to the swelling of zwitterionic chains in solution with high ionic strength. Meanwhile, the swelling behavior differed from that of hydrogels prepared by acrylic acid12 and itaconic acid.13 Through the interactions between cationic and anionic groups, zwitterionic moieties can associate among themselves.24 In salt solution, the ions break the interchain and intergroup association of zwitterionic moieties because of the “anti- polyelectrolyte” behavior.25 Therefore, the existence of salt can promote the swelling or solubility of zwitterionic polymers. Xue et al.25 had studied the swelling behavior of poly(1-(3-sulfopropyl)-2-vinyl-pyridinium-betaine) (PSPV) hydrogel in salt solution, but the swelling degree in salt solution showed small changes between 0.7 and 0.9.


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The dynamics and thermodynamics of swelling behavior for ionic-strength sensitive zwitterionic hydrogels remain unclear. Zwitterionic hydrogels, both SB hydrogels and CB hydrogels have been used in biomedical field. Jiang et al.26-31 have done much work in studying the antifouling property of zwitterionic materials, and also exploring the applications in restraining the mesenchymal stem cells differentiation,26 antimicrobial property,30 real-time drug monitoring in blood plasma31 and so on. To broaden the application of zwitterionic hydrogels, copolymerization of zwitterionic monomers with other functional monomers is an effective way. Yao et al.32 fabricated poly(Nvinylcaprolactam-co-sulfobetaine methacrylate) hydrogel with both an upper critical solution temperature (UCST) and a lower critical solution temperature (LCST) in aqueous solutions, which exhibited switchable antifouling property and cytocompatibility by regulating temperature and ionic strength. Tunable bioadhesive hydrogels were also obtained by copolymerization of SBMA with N-isopropyl acrylamide, which was realized by changing the temperature.33 By copolymerization of acrylamide and SBMA, Yang et al.34 also prepared hydrogen bondingstrengthened self-healing zwitterionic hydrogels, which may be used in the field of enhanced oil recovery. Thus, by means of copolymerization with other functional monomers, we can endow zwitterionic hydrogels with abundant functions. Recently, the continuous growth of the world’s water population has gained much attention, and dyes are major water pollutants.35-36 Various approaches such as adsorption, membrane separation, coagulation, and chemical oxidation are used in the removal of dyes in wastewater.37 As one of the most efficient and low-cost approach, adsorption is an important separation and purification technique. Various adsorbents including silica gel, hydrogel, activated carbon, and their composites have been investigated, and the mechanism of adsorption includes


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electrostatic interaction, hydrogen-bond interaction, π-π stacking and so on.37-38 Because both anionic and cationic groups existed in zwitterionic hydrogels, the zwitterionic hydrogels may adsorb dyes by the electrostatic interaction. The aim of this study is to investigate the dynamics and thermodynamics of swelling for zwitterionic hydrogels in solution with varied ionic strength and temperature. The hydrogels including PSBMA hydrogels and copolymer hydrogels were prepared via the free radical copolymerization SBMA and monomers including sodium p-styrene sulfonate, acrylic acid, Nisopropylacrylamide and 2-(dimethylamino) ethyl methacrylate. We further studied the adsorption capacity of the zwitterionic hydrogels. Both cationic dyes and anionic dyes were used in the tests to verify the selective adsorption capacity. Then methylene blue was chosen as the model pollutant to investigate the adsorption abilities, kinetics and isotherms. EXPERIMENTAL SECTION Materials 2-(Dimethylamino)

ethyl

methacrylate

(DMAEMA,

99%,

Aladdin),

N,N'-

methylenebisacrylamide (MBA, 99%, Aladdin), sodium p-styrene sulfonate (NaSS, 90%, Aladdin), N-isopropylacrylamide (NIPAM, 98%, Aladdin) and acrylic acid (AA, 99%, Aladdin) were the monomers to synthesize hydrogels. Ammonium persulfate (APS, 99.5%, Aladdin), 1,3propanesulfonate (99%, Aladdin), potassium chloride (KCl, AR, Kelong), sodium chloride (NaCl, AR, Kelong), magnesium chloride hexahydrate (MgCl2.6H2O, 98%, Kelong), Iron (III) chloride (FeCl3, AR, Kelong), sodium sulfate (Na2SO4, AR, Kelong), sodium phosphate tribasic dodecahydrate (Na3PO4.12H2O, 98%, Kelong), methylene blue (MB, 97.5%, Kelong), methyl violet (MV, 98%, Kelong), Rhodamine B (RhB, 98%, Kelong), Congo Red (CR, 98%, Kelong)


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and bisphenol A (BPA, 98%, Kelong) were used as received. Ultrapure water was used throughout the study. Synthesis of hydrogel Sulfobetaine methacrylate (SBMA) was first synthesized according to previously reported procedure39 using DMAEMA and 1,3-propane sulfonate as the reagents. A typical procedure to synthesize PSBMA hydrogels was as follows: the monomer of SBMA was dissolved in 1 M NaCl solution with a concentration of 20-60 wt. % in glass vials. Then, 0.5-4 mol% (with respect to monomer) of cross-linker MBA and 1 mol% (with respect to monomer) of initiator APS were added in the above degassed solutions. The mixtures were stirred and bubbled with nitrogen for 15 min to remove the oxygen, after which the vials were sealed by a plastic tap and parafilm. The reaction solutions were polymerized at 60 oC for 20 h. The gels were then removed from the vials and immersed in water at room temperature for two weeks to reach swelling equilibrium. The PSBMA hydrogels are named as PSBMA-x-y, where x stands for the SBMA content and y stands for the MBA molar amount with respect to the monomer. To prepare zwitterionic copolymer hydrogels, SBMA and other monomers including NaSS, AA, NIPAM and DMAEMA were used in the copolymerization with total monomer weight percentage of 40%. MBA and APS were controlled at 1 mol% and 1 mol%, respectively, and the reaction condition was the same with that of PSBMA hydrogels. For example, the PSBMA1NaSS0.5 hydrogel represents for the poly(sulfobetaine methacrylate-co-sodium p-styrene sulfonate) (PSBMA-co-NaSS) hydrogel, which was synthesized by copolymerization of SBMA and NaSS with the mass ratio of 1:0.5. Characterization


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After the swollen hydrogels were dried by lyophilization to constant weight, Fourier transform infrared spectroscopy (FTIR) was used to determinate the functional groups in the hydrogels. FTIR spectra were obtained using a Nicolet 560 (Nicolet Co., American) Spectrometer with ATR Accessory. The spectra were recorded using at a resolution of 4 cm-1 over the range of 6754000 cm-1. The morphologies of the hydrogels were observed using a field emission scanning electron microscopy (FE-SEM, JEOL JSM-7500F, Japan) with the voltage of 5 kV. Gel samples for SEM were quenched by liquid nitrogen and fractured, freeze-dried and then sputter coated with a gold layer under vacuum. Swelling properties of the hydrogels To investigate the swelling behavior of zwitterionic hydrogels with different chemical compositions, the dry gels (with the weight of wd) were immersed in an excess of water or salt solutions (including NaCl, KCl, MgCl2, FeCl3, Na2SO4, and Na3PO4) with varied ionic strength at a certain temperature until swelling equilibrium was achieved. The wet weight (wt) of the hydrogels was weighed after moving the surface solution by filter paper, and the equilibrium weight of hydrogel was donated as we. The swelling ratio (SR) and equilibrium swelling ratio (SRe) of the hydrogels can be calculated on the basis of the weights of wet and dry hydrogels by using the following equation:

SR =

wt − wd wd

and

SRe =

we − wd wd

(1)

Adsorption experiment To study the adsorption capacity of zwitterionic hydrogels, swollen hydrogels (with dry weight of m0) were immersed in 20 mL dye solutions at the shaking speed of 120 rpm at room


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temperature. Using MB as the pollutant model, the adsorption amount (qt, mg/g) was calculated by the equation as follows:

qt =

(C0 − C1)VM m0

(2)

where C0 and C1 (mmol/L) are the concentrations of the MB solution before and after the adsorption, respectively, which are obtained by an UV-vis spectroscopy (UV-1750, Shimadzu, Japan) at the wavelength of 631 nm after diluting the solution . M (g/mol) is the relative molecular mass of the MB. V (L) is the volume of the dye solution. To study the selective adsorbability of pollutants including cationic MB, MV, RhB, anionic CR and BPA, pollutant solution with the concentration of 200 µmol/L was used and adsorption process was conducted until the equilibrium was reached, in which the concentration was determined by an UV-vis spectroscopy. The adsorption amount can be calculated by Eq. (2). We chose MB solution to investigate the kinetics and isotherms of the adsorption process of zwitterionic hydrogels. MB solutions with different initial concentrations (50, 100, 150 and 200 µmol/L) were used to verify the effect of initial concentration on the adsorption capacity, and the kinetics and isotherms were analyzed. Then, the effect of ionic strength and pH value on the adsorption was also studied by changing the values. RESULTS AND DISCUSSION Characterization Zwitterionic copolymer hydrogels were prepared by the traditional radical copolymerization of zwitterionic monomer (SBMA) and other functional monomers (NaSS, AA, NIPAM and


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DMAEMA), and the chemical structural formulas were shown in Fig. 1 (A). Because all the monomers have good solubility in water, we used water as the solvent. The chemical compositions of the hydrogels were analyzed by FTIR, and typical results were shown in Fig. 1 (B). The absorption peak of 1721cm-1 was ascribed to the ester groups for hydrogels of PSBMA40-1,

PSBMA1NaSS1 and PSBMA1NIPAM1. The absorption peaks of 1181 cm-1 and 1038 cm-1

were ascribed to the -SO3- groups in hydrogels. The absorption peaks of 1647 cm-1 was ascribed to the amide groups in PNIPAM. The results showed that by using the traditional radical copolymerization, zwitterionic copolymerized hydrogels could be synthesized.

Figure 1. (A) Chemical structural formulas of the monomers used in the synthesis of zwitterionic copolymer hydrogels. (B) FTIR spectra of hydrogels PSBMA-40-1, PSBMA1NaSS2 and PSBMA1NIPAM2. (C) SEM images of the hydrogels PSBMA-40-1 (a, b), PSBMA1NaSS1 (c, d), PSBMA1DMAEMA1 (e, f), and PSBMA1NIPAM1 (g, h).


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The SEM images of zwitterionic copolymer hydrogels of PSBMA-40-1, PSBMA1NaSS1, PSBMA1DMAEMA1, and PSBMA1NIPAM1 are presented in Fig. 1 (C). All the hydrogels have interconnected pores homogeneously distributed throughout the hydrogels. After the hydrophilic monomers such as NaSS and NIPAM were copolymerized with SBMA, the hydrophilic monomers enhanced the liquid diffusion, and the copolymer hydrogels showed larger pores than the PSBMA hydrogels. Moreover, no agglomerates were observed for the hydrogels, indicating that the zwitterionic copolymer was well dispersed within the matrix. Swelling behavior Influence of ionic strength on swelling properties at room temperature Swelling behavior is always used to investigate the properties of hydrogels. Structural factors such as hydrophilicity, cross-linking density and concentration affect the swelling behavior of hydrogels. The medium properties like the ionic strength, temperature, pH also influence the swelling behavior.40 In this study, we firstly investigated the dynamics of swelling behavior for PSBMA hydrogels with different cross-linker contents in both water and NaCl solutions, to reveal the effect of ionic strength on the swelling behavior. Then the swelling of zwitterionic copolymer hydrogels including PSBMA-AA, PSBMA-NaSS and PSBMA-NIPAM were also studied. The swelling of dry PSBMA gel synthesized with different monomer concentrations and crosslinker contents was firstly investigated and the result was shown in Fig. 2 (A). The swelling of PSBMA-40-1 in water got equilibrium in about 2 h. Nevertheless, the SR in NaCl solution (1 mol/kg) slowly increased and got equilibrium in about 40 h. Then the equilibrium swelling ratios in water (SRe,water) and NaCl solution (SRe,NaCl) were calculated by using the mass of the


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hydrogels before and after dehydration as shown Fig. 2 (B-D). The results showed that both the SRe,water and SRe,NaCl decreased with the increase of monomer concentration and cross-linker content. We could also find that the PSBMA hydrogels showed ionic-strength sensitivity, which could be expressed by the ratios of SRe,NaCl and SRe,water. The reason was that high concentration of salt weakened the self-associations among zwitterionic moieties, and the zwitterionic chains extended, resulting in a higher SR in salt solution.19-20 The ionic-strength sensitivity also decreased with the increase of monomer content and cross-linker content.

Figure 2. Swelling behaviors of PSBMA dry gels in water and NaCl solution (1 mol/kg) at room temperature. The hydrogels were synthesized with SBMA of 20 wt. %, 40 wt. % and 60 wt. %, and the cross-linker content of 0.5 mol%, 1 mol%, 2 mol% and 4 mol% with respect to SBMA. (A) Swelling kinetics of PSBMA-40-1 and PSBMA-60-1 in water and NaCl solution (1 mol/kg). SRe,water (B), SRe,NaCl (C) and SRe,NaCl/SRe,water (D) of the PSBMA hydrogels.


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Then we investigated swelling behavior of the copolymer hydrogels of PSBMA-NaSS, PSBMAAA and PSBMA-NIPAM. As shown in Table 1, the SRe of PSBMA-AA and PSBMA-NIPAM hydrogels in water and NaCl solution did not show much difference, while PSBMA-NaSS hydrogels showed obvious and stable ionic-strength sensitivity, in which the SRe,NaCl /SRe,water was ranged from 0.23 to 0.41. We also found that PSBMA1NaSS2 possessed the maximum value of SRe,water and SRe,NaCl. As PNaSS is a strong polyelectrolyte, it can be fully ionized in solution,41 and the repulsive interactions among SO3- ions promote the swelling in water. Nevertheless, when immersing in the salt solution, the counterions weaken the repulsive interaction and result in shrinking. Table 1. Equilibrium SR of zwitterionic copolymer hydrogels in water and NaCl solution (1 mol/kg) at room temperature. Sample SRe,water SRe,NaCl SRe,NaCl / SRe,water 18.68 6.21 0.33 PNaSS 18.22 7.51 0.41 PSBMA1NaSS0.5 PSBMA1NaSS1 33.27 10.22 0.30 PSBMA1NaSS2 71.94 16.39 0.23 4.35 2.59 0.59 PAA 1.52 2.18 1.43 PSBMA1AA0.5 2.05 1.66 0.81 PSBMA1AA2 PSBMA1NIPAM0.5 3.53 5.07 1.44 PSBMA1NIPAM2 4.12 4.30 1.05

We then verified the reversibility by alternatively immersing the hydrogels in water and NaCl solution. Fig. 3 (A) shows the swelling behavior of PSBMA-40-1, PSBMA-60-1 and PSBMA1NaSS1 in alternate water and NaCl solution (1mol/kg). The SR of PSBMA hydrogels increased when immersed in NaCl solution and reached equilibrium in about 10 h. Then SR decreased when immersed in water. For the second cycle, the SR showed similar variation trend.


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On the contrary, the SR of PSBMA1NaSS1 exhibited a decrease and then increase trend when immersing in NaCl solution and then in water, respectively, and also showed good reversibility. The above results indicated that both the PSBMA and PSBMA-NaSS hydrogels exhibited ionicstrength sensitivity, the question is that: does the sensitivity rely on the salt concentration and salt species? Thus, we investigated the swelling behavior in the NaCl solution with the ionic strength varied from 0 to 1.0 mol/kg and the results are shown in Fig. 3 (B). The SRe,NaCl of PSBMA-40-1 increased with the increase of ionic strength and got a stable value at about 0.5 mol/kg. On the contrary, SRe,NaCl of PSBMA1NaSS1 decreased with the increase of ionic strength. The results of swelling in different kinds of salt solutions, including NaCl, KCl, MgCl2, FeCl3, Na2SO4 and Na3PO4 were shown in Fig. 3 (C). For the salt with the same anion of Cl-, the SRe,NaCl decreased with the increase of the charge of cation, namely Na+ and K+, Mg2+ and Fe3+ for both PSBMA-401

and PSBMA1NaSS1. It is interesting to find that the change trend for salt solutions with

different anions of Cl-, SO42- and PO43- showed different change trend. The PSBMA-40-1 showed a decreasing trend and the PSBMA1NaSS1 showed an increasing trend.


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Figure 3. (A) Reversible swelling behavior of fully swelling PSBMA-40-1, PSBMA-60-1 and PSBMA1NaSS1 in alternate water and NaCl solution (1mol/kg). (B) SR versus ionic strength for PSBMA-40-1 and PSBMA1NaSS1. (C) SRe,NaCl/SRe,water for PSBMA-40-1 and PSBMA1NaSS1. NaCl, KCl, MgCl2, FeCl3, Na2SO4 and Na3PO4 solutions were used. Swelling dynamics analysis It is well known that water diffuses into hydrogels when dry hydrogels contact with water, resulting in the swelling. The swelling of hydrogels mainly involved two processes. In the first process, water diffuses into preexisting spaces and dynamically formed spaces of the hydrogel. After that, the further swelling leads to an increase in the separation distance among the hydrogel chains.42 The initial swelling rates are fitted to the exponential heuristic equation:43-44


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F=

Qt = Kt n Qe

or

ln F = ln K + n ln t

(

Qt < 0.6 ) Qe

(3)

where Qt (g/g) and Qe (g/g) represent the absorbed water amounts by the hydrogel at time t and at equilibrium state, respectively. K is a characteristic exponent of the hydrogel. n is a characteristic constant of the swelling which represents solvent diffusion modes inside hydrogels and provide information about the mechanism of swelling kinetics. When n ≦ 0.5, the diffusion of water corresponds to Fickian diffusion, and the rate of diffusion is much lower than the rate of relaxation; when n = 0.5~1, the diffusion corresponds to non-Fickian diffusion; when n ≧ 1, the diffusion is fast and corresponds to super-case-II diffusion.40 The results of initial swelling process of hydrogels are shown in Fig. 4 and Table 2. The values of n for PSBMA-40-1 and PSBMA-60-1 in water and NaCl solution were between 0.5~1, indicating that the initial swelling process (

Qt < 0.6 ) fits non-Fickian diffusion. Qe


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Figure 4. (A) ln(F) versus ln(t) and (B) t/Qt versus t curves of PSBMA-40-1 and PSBMA-60-1 in water and NaCl solution (1mol/kg). The above equation only describes the initial swelling process of hydrogels, the Schott secondorder dynamic equation can be used to describe the extensive swelling process as follows:40 dQt = k S (Q∞ − Qt ) 2 dt

(4)

t = A + Bt Qt

(5)


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where Q∞ (g/g) represent the maximum theoretical amount of water absorbed by hydrogel. The constant of A (A=1/ksQ∞2=1/(dQt/dt)0) and B (B=1/Q∞) can be calculated from the above equations, and the results of Q∞ are shown in Table 2. The temperature also has effect on the swelling behavior of both PSBMA and PSBMA-NIPAM hydrogels, which shown in Fig. S1 and Table S1. Table 2. The results of n, correlation coefficient (R2), and the maximum theoretical absorbed water amount (Q∞) absorbed by hydrogels calculated by Eq. (3) and Eq. (5), respectively. Samples Non-Fickian diffusion equation Schott second-order dynamic equation 2 R n R2 Q∞ (g/g) PSBMA-40-1 (water) 0.9850 0.6742 0.9846 1.3488 PSBMA-40-1 (NaCl) 0.9976 0.5903 0.9967 7.7220 PSBMA-60-1 (NaCl) 0.9961 0.5982 0.9991 5.1282

Adsorption of zwitterionic hydrogels for different pollutants The selective adsorption of pollutants PSBMA chains possess negative sulfonate and positive ammonium charges in the repeat unit. Thus we proposed that PSMA hydrogels may adsorb pollutants. We choose pollutants including cationic MB, MV, RhB, anionic CR and BPA. Unfortunately, the results in Fig. 5 showed that the PSBMA-40-1 could not adsorb any of the above pollutants. The reason might be that strong interactions existed between cationic and anionic groups in PSBMA, we would verify the assumption by measuring the adsorption in the salt solution with various ionic strength. Interestingly, after the introduction of the negatively charged polymer of PNaSS, the PSBMANaSS exhibited obvious adsorption for cationic dyes including MB, MV and RhB as shown in Fig. 5. For example, the adsorption amount of MB for PNaSS was 323 mg/g, and the adsorption amount increased with increasing the amount of PNaSS in the PSBMA-NaSS hydrogels, which


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was 279, 348 and 802 mg/g, respectively. The results illustrated that by introducing PNaSS in the hydrogels of PSBMA-NaSS, the adsorption capacity for cationic dyes was enhanced through the electrostatic interaction, but could not adsorb anionic dyes. Moreover, the PSBMA1NaSS2 showed better adsorption capacity than PNaSS. The reason might be that the PNaSS weakened the electrostatic introduction between the anionic and cationic groups in PSBMA, resulting in higher adsorption amount.

Figure 5. The equilibrium adsorption amounts of various pollutants for hydrogels of PSBMA-401,

PNaSS, PSBMA1NaSS0.5, PSBMA1NaSS1 and PSBMA1NaSS2. The initial concentration of

pollutants was 200 µmol/L. The inserted figures were the images of solution before and after adsorption for PSBMA1NaSS1. The effect of initial MB concentration on the adsorption We mainly studied the adsorption of MB with the hydrogels of PNaSS, PSBMA1NaSS0.5, PSBMA1NaSS1 and PSBMA1NaSS2. Many factors may affect dye adsorption, such as the initial


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concentration, pH and ionic strength.35 Thus the effect of these parameters is to be taken into account. The results of adsorption amounts with different initial MB concentrations were shown in Fig. 6; and the adsorption process reached the equilibrium state in about 90 h. The adsorption amount increased with the increase of initial MB concentration. For PNaSS, the adsorption capacities were 70.3, 155.5, 232.4 and 321.2 mg/g respectively, when the MB solutions with the initial concentration of 50, 100, 150 and 200 µmol/L were used. A similar trend was also found for PSBMA1NaSS0.5, PSBMA1NaSS1 and PSBMA1NaSS2.

Figure 6. The influence of initial MB concentrations on the adsorption capacities for hydrogels (A) PNaSS, (B) PSBMA1NaSS0.5, (C) PSBMA1NaSS1 and (D) PSBMA1NaSS2. The effect of pH and ionic strength on the adsorption


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The adsorption of mechanism of MB by zwitterionic hydrogels could be mainly attributed to electrostatic attraction. The alteration of pH and ionic strength may change the internal electrostatic between anionic and cationic groups of polymer chains, and the interactions between polymer chains and dyes. Fig. 7 (A) shows the influence of pH on adsorption amount. It was found that pH variation had little influence on the adsorption capacity of both PSBMA and PSBMA-NaSS hydrogels. The reason might be that the anionic groups in hydrogels were -SO3-, and pH had little on the ionization. As shown in Fig. 3 (B), both PSBMA-40-1 and PSBMA1NaSS1 exhibited ionic strengthsensitivity when the ionic strength ranging from 0 to 1.0 mol/kg, in which the electrostatic attraction between the cationic and anionic groups of polymer chains was changed. Therefore, the ionic strength might have a significant effect on the adsorption capacity. Fig. 7 (B) shows the effect of ionic strength on the adsorption of PNaSS and PSBMA1NaSS0.5. It was found that the adsorption amount for PNaSS increased from 77 to 215 mg/g, and that for PSBMA1NaSS0.5 increased from 37 to 364 mg/g. The result was interesting, and the reason might be that by increasing the ionic strength, the ions of salt can weaken the repulsive interaction of anionic groups for PNaSS; on the other hand, the electrostatic attraction of cationic and anionic groups of PSBMA was also weakened by ions of salt. Thus, the adsorption capacity of PNaSS and PSBMA1NaSS0.5 increased with the increase of ionic strength.


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Figure 7. (A) The effect of pH on the adsorption capacity of zwitterionic copolymer hydrogel. The initial concentration of MB was 200 µmol/L with no NaCl added in the solution. (B) The effect of ionic strength on the adsorption capacity of zwitterionic copolymer hydrogel. The initial concentration of MB was 50 µmol/L with pH~7. Adsorption kinetics study To explore the detailed adsorption process of MB for zwitterionic hydrogels, three kinetic models were applied. The pseudo-first-order kinetic model fits with low solute concentration and


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reflects the adsorption rate and the pseudo-second-order kinetic model reflects the adsorbed amount at equilibrium and the adsorbed amount on the surface of the adsorbent. The two models can be expressed by the following equations, respectively:

ln(qe − qt ) = ln qe − K1t

(6)

t t 1 = + 2 qt K 2 qe qe

(7)

where qt (mg/g) is the adsorption amount of MB at time t, qe (mg/g) is the equilibrium adsorption amount, K1 and K2 are the first order and second order rate constant, respectively. For the equation, a straight line was obtained by the plots of ln(qe-qt) versus t by using the pseudo-firstorder kinetic model as shown in Fig. 8 (A-D). Similarly, a straight line was obtained by the plots of t/qt versus t by using the pseudo-first-order kinetic model as shown in Fig. 8 (E-H). The values of qe, K1, K2 and the correlation coefficient (R2) can be calculated by the slope and intercept of the lines, which were listed in Table S2. The data showed that all the values of R2 were larger than 0.98. Meanwhile, the values of equilibrium adsorption amount calculated by pseudosecond-order kinetic model are closer to the experiment values. Thus, the pseudo-second-order kinetic model was more suitable to describe the kinetics of the MB adsorption for the zwitterionic hydrogels.


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Figure 8. Pseudo-first-order kinetics and pseudo-second-order kinetics models for the adsorption of MB for hydrogel (A, E) PNaSS, (B, F) PSBMA1NaSS0.5, (C, G) PSBMA1NaSS1 and (D, H) PSBMA1NaSS2, respectively.


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Although the pseudo-first-order kinetic model and the pseudo-second-order kinetic model can be used to describe the kinetics of the adsorption of MB for zwitterionic hydrogels, they failed to identify the diffusion mechanism of the process. The intraparticle diffusion model helps to study the diffusion mechanism, which is expressed by the following equation:

qt = K pt1/2 + C

(8)

where C (mg/g) is a constant and Kp is the intraparticle diffusion rate constant. By the plots of qt versus t1/2, a straight line was obtained as show in Fig. S2. Most of the R2 values were larger than 0.98, indicating the adsorption process fitted this model well. In each curve, three slopes existed indicated that at least three diffusion steps included in each adsorption process, namely, the external surface adsorption or diffusion in macro-pores, the gradual adsorption step and the final equilibrium step. Adsorption isotherm After identifying the adsorption kinetics, we further studied the equilibrium curves by using the Langmuir adsorption isotherm and Freundlich adsorption isotherm. Based on several hypothesizes including no interaction between adsorbate molecules, dynamic balance and monolayer adsorption, the Langmuir adsorption isotherm is useful to study many adsorption processes in liquid solution, the equation can be expressed by:45

ce c 1 = + e qe K L qmax qmax

(9)

where KL is the rate constant, ce (mg/L) is the equilibrium concentration of the solution, qe (mg/g) and qmax (mg/g) are the equilibrium and maximal adsorption amount, respectively.


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The Freundlich adsorption isotherm was used to describe the multilayer adsorption on the heterogeneous surface, and expressed by:45

1 ln qe = ln K F + ln ce n

(10)

where KF is the rate constant, qe (mg/g) is the equilibrium adsorption amount, ce (mg/L) is the equilibrium concentration of the solution, and 1/n reflect the adsorption intensity. By using the Eq. (9) and Eq. (10), qmax, constants of KL and KF and the correlation coefficients of RL2, and RF2 were calculated and the values were listed in Table S3. The results showed that most of the correlation coefficients calculated by the Langmuir adsorption isotherm were larger than 0.98, suggesting that the Langmuir adsorption isotherm was more suitable fitted for MB adsorption process of zwitterionic hydrogels. CONCLUSION Ionic-strength responsive zwitterionic copolymer hydrogels were fabricated by copolymerization of the zwitterionic monomer of SBMA and other functional monomers including NaSS, AA, NIPAM and DMEMA. The PSBMA hydrogels exhibited ionic-strength responsibility. After other functional monomers were copolymerized with SBMA, the resulting hydrogels showed alterative swelling behavior. Interestingly it was found that PSBMA-NaSS hydrogels were collapsed when increasing the ionic strength, in which the ions could weaken the repulsive interaction of anionic groups for PNaSS. PSBMA showed no adsorption capacity with both cationic and anionic dyes; while the PSBMANaSS hydrogels showed high adsorption amount of MB, with the maximum value of about 760 mg/g. The MB adsorption results suggested that the pseudo-second-order kinetic model and the


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Langmuir adsorption isotherm were suitable to describe the adsorption process with correlation coefficients greater than 0.98. Multi-stimuli responsive zwitterionic copolymer hydrogels were prepared by copolymerization of SBMA and other monomers, and the PSBMA-NaSS hydrogels showed controlled swelling property by changing the ionic strength. Moreover, PSBMA-NaSS hydrogels showed excellent dye adsorption capacity, which opens up a rout for application of zwitterionic hydrogels as an adsorbent in the field of waste water treatment.

ASSOCIATED CONTENT Supporting Information. The Supporting Information is available free of charge on the ACS Publications website Figure of intraparticle diffusion models for the adsorption of MB for hydrogels. Tables of parameters of the kinetic models and isotherms of MB for zwitterionic hydrogels (PDF) AUTHOR INFORMATION Corresponding Author *E-mail: [email protected] or [email protected] Author Contributions The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript. Notes The authors declare no competing financial interest.


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ACKNOWLEDGMENT This work was financially sponsored by the Open Research Project of the State Key Laboratory of Molecular Engineering of Polymers, Fudan University (No. K2018-10) and the Fundamental Research Funds for the Central Universities (2682018CX50). We also thank our laboratory members for their generous help. ABBREVIATIONS SBMA, sulfobetaine methacrylate; NaSS, sodium p-styrene sulfonate; AA, acrylic acid; NIPAM, N-isopropylacrylamide; DMAEMA, 2-(dimethylamino) ethyl methacrylate; ∆H, enthalpy; SB, sulfobetaine; CB, carboxybetaine; MB, methylene blue trihydrate; APS, ammonium persulfate; MBA, N,N'-methylenebisacrylamide; SR, swelling ratio; qt, adsorption amount; Qt, absorbed water amount. REFERENCES (1) Ulijn, R. V.; Bibi, N.; Jayawarna, V.; Thornton, P. D.; Todd, S. J.; Mart, R. J.; Smith, A. M.; Gough, J. E. Bioresponsive hydrogels. Mater. Today 2007, 10, 40-48. (2) Chaterji, S.; Kwon, I. K.; Park, K. Smart polymeric gels: redefining the limits of biomedical devices. Prog. Polym. Sci. 2007, 32, 1083-1122. (3) Caccavo, D.; Cascone, S.; Lamberti, G.; Barba, A. A. Hydrogels: experimental characterization and mathematical modelling of their mechanical and diffusive behaviour. Chem. Soc. Rev. 2018, 47, 2357-2373. (4) Shigemitsu, H.; Hamachi, I. Design Strategies of Stimuli-Responsive Supramolecular Hydrogels Relying on Structural Analyses and Cell-Mimicking Approaches. Acc. Chem. Res. 2017, 50, 740-750.


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