Colloids and Surfaces B: Biointerfaces · Colloids and Surfaces B: Biointerfaces 88 (2011) 279–286 Contents lists available at ScienceDirect Colloids ... [26,27]. Other researchers

Page 1

Eg

Xa

b

c

a

ARRAA

KSPGHS

1

dlia[Ihwiseapin

RL

0d

Colloids and Surfaces B: Biointerfaces 88 (2011) 279– 286

Contents lists available at ScienceDirect

Colloids and Surfaces B: Biointerfaces

jou rn al h om epage: www.elsev ier .com/ locate /co lsur fb

ffect of surfactant on porosity and swelling behaviors of guarum-g-poly(sodium acrylate-co-styrene)/attapulgite superabsorbent hydrogels

iao-Ning Shia,b, Wen-Bo Wanga,c, Ai-Qin Wanga,c,∗

Key Laboratory of Chemistry of Northwestern Plant Resources, Lanzhou Institute of Chemical Physics, Chinese Academy of Sciences, Lanzhou 730000, PR ChinaGraduate University of the Chinese Academy of Sciences, Beijing 100049, PR ChinaR&D Center of Xuyi Attapulgite Applied Technology, Lanzhou Institute of Chemical Physics, Chinese Academy of Science, Huai’an 223003, PR China

r t i c l e i n f o

rticle history:eceived 23 January 2011eceived in revised form 17 June 2011ccepted 4 July 2011vailable online 12 July 2011

eywords:urfactantorosity

a b s t r a c t

Novel fast-swelling porous guar gum-g-poly(sodium acrylate-co-styrene)/attapulgite (GG-g-P(NaA-co-St)/APT) superabsorbent hydrogels were prepared by simultaneous free-radical graft copolymerizationreaction of guar gum (GG), partially neutralized AA (NaA), styrene (St) and attapulgite (APT) usingN,N′-methylenebisacrylamide (MBA) as a crosslinker and ammonium persulfate (APS) as an initiator inaqueous solution and the surfactant self-assembling templating pore-forming technique. Fourier trans-form infrared (FTIR) spectroscopy confirmed that the surfactant could be removed from the final hydrogelproduct by methanol/water (8:1, v/v) washing process and the surfactant only act as micelle template toform pores. The effect of surfactant type on the porous microstructure of the hydrogel was assessed by

uar gumydrogelwelling behavior

field emission scanning electron microscope (FESEM). It was shown that incorporation of proper amountof anionic surfactant sodium n-dodecyl sulfate (SDS) in the gelling process of the hydrogel can obviouslyenhance the swelling capacity and initial swelling rate. The salt-sensitivity of the SDS-added hydrogelin distilled water and 15 mmol/L NaCl, CaCl2 solution or 15 mmol/L NaCl and CaCl2 solution was investi-gated, and it was found that the swelling–deswelling capability is quite reversible. A similar reproducibleon–off switching behavior was observed in the 1 mmol/L solution of phosphate buffer at pH 2.1 and 7.4.

. Introduction

Superabsorbent hydrogels are slightly cross-linked three-imensional hydrophilic polymers that are capable of absorbing

arge quantities of water or other biological fluids without dis-ntegrating, and have found extensive applications in variousreas such as medicine/pharmacy [1–3], chemical engineering4–6], agriculture [7–9] and other environmental fields [10–12].n the design and development of new superabsorbent hydrogels,igh swelling capacity, fast swelling rate and good gel strengthere especially desired. Among these properties, swelling rate

s important because it determines the application properties ofuperabsorbent hydrogel in almost each field. Recently, manyfforts have been made to improve the swelling rate of the super-bsorbent hydrogel for improving its applicability [13–16]. In

rinciple, the initial swelling rate of a superabsorbent is primar-

ly due to the penetration of water molecules into the polymericetwork through diffusion and capillarity [17]. Higher porosity in

∗ Corresponding author at: Key Laboratory of Chemistry of Northwestern Plantesources, Lanzhou Institute of Chemical Physics, Chinese Academy of Sciences,anzhou 730000, PR China. Tel.: +86 931 4968118; fax: +86 931 8277088.

E-mail address: [email protected] (A.-Q. Wang).

927-7765/$ – see front matter © 2011 Elsevier B.V. All rights reserved.oi:10.1016/j.colsurfb.2011.07.002

© 2011 Elsevier B.V. All rights reserved.

the superabsorbents can increase the contact area between poly-meric network and external solution that facilitate to speed upthe diffusion rate [15]. So, the swelling rate of superabsorbenthydrogels can be enhanced through the creation of porosity struc-ture. Various methods, such as the phase inversion technique[18], freeze-drying and hydration technique [19], the water-solubleporogens [20] and the foam technique [21,22], have been usedto create the porous structure of hydrogels. However, the contra-diction between the controllable porosity and the convenience ofthe pore-forming technique is now still existing in the hydrogelpolymerization.

Surfactant can self-assemble to form micelles in aqueous envi-ronment, which act as a template in the polymerization reactionprocess to produce pores [23]. A kind of porous alginate hydrogelsthat pore size can be controlled by varying the surfactant concen-tration has been developed [24]. In addition, the gelation of aqueoussolution of cellulose derivatives and surfactants is also studiedby Su et al. [25]. The resultant hydrogel holds much microporewithin its network structure and can be widely applied in pharma-ceuticals, cosmetics and food industry [26,27]. Other researchers

have extended the surfactant to develop chitosan-based hydro-gel for enhancing its adsorption capacity on the waste dye andother organic compounds, and this approach is economical andtechnically feasible [28–30,15]. Thus, it is expected that the porous

Page 2

2 ces B:

sb

oaacfasbbpggPhtihtnappaagbgsowi

2

2

((wACamR(pSrt

2s

ariatst

80 X.-N. Shi et al. / Colloids and Surfa

tructure and swelling properties of a hydrogel could be improvedy introducing surfactant.

In response to the fully petroleum-based polymers with seri-us environment impact, polysaccharide-based hydrogels havettracted much attention due to their renewable, biocompatiblend biodegradable advantages. Polysaccharides such as starch,hitosan, cellulose and many kinds of gum have been utilizedor fabricating eco-friendly hydrogels [31]. Guar gum (GG) is

hydrophilic and non-ionic polysaccharide extracted from theeed endosperm of the plant Cyamopsis Tetragonalobus, which cane facilely modified by grafting vinyl monomers onto its back-one to derive GG-based hydrogels with improved structure anderformance. Based on a series of work for GG-g-PNaA/clay hydro-els, we have successfully synthesized the swelling enhanceduar gum-g-poly(sodium acrylate-co-styrene)/muscovite (GG-g-(NaA-co-St)/VMT) hydrogel by introducing appropriate amount ofydrophobic styrene as co-monomer to alter the physical proper-ies of the polymeric network [32]. As a part of efforts to furthermprove the network structure of the GG-g-P(NaA-co-St)/clayydrogel and expand its application fields, in the present study,hree different surfactants including anionic surfactant sodium-dodecyl sulfate (SDS), cationic surfactant dodecyltrimethylmmonium bromide (DTAB) and non-ionic surfactant p-octyloly(ethylene glycol)phenyl ether (OP) were introduced into therocess of graft copolymerization and crosslinking of NaA, Stnd APT onto GG backbone to form guar gum-g-poly(sodiumcrylate-co-styrene)/attapulgite (GG-g-P(NaA-co-St)/APT) hydro-el and investigated their effects on the porosity and swellingehaviors of the hydrogel. The porous microstructure of the hydro-els with different surfactants was evaluated by field emissioncanning electron microscope (FESEM). The effects of the dosagef anionic surfactant SDS on equilibrium swelling of the hydrogelere investigated. The swelling kinetic and environment sensitiv-

ty, salt and pH sensitivity, were extensively investigated.

. Materials and methods

.1. Materials

Guar gum (GG) was supplied by Wuhan Tianyuan Biology Co.Wuhan, China, Mw = 220,000 D). Attapulgite clay micropowderAPT, Jiuchuan Nano-material Technology Co., Ltd., Jiangsu, China)as milled and passed through a 200-mesh screen prior to use.crylic acid (AA) and styrene (St, chemically pure, Shanghai Wulianhemical Factory, Shanghai, China). Ammonium persulfate (APS,nalytical pure, Xi’an Chemical Reagent Factory, Xi’an, China). N,N′-ethylenebisacrylamide (MBA, analytical pure, Shanghai Chemical

eagent Corporation, Shanghai, China). Sodium n-dodecyl sulfateSDS), dodecyltrimethyl ammonium bromide (DTAB) and p-octyloly(ethylene glycol) phenyl ether (OP) were all supplied by thehanghai Chemical Reagent Corp. (Shanghai, China) and used aseceived. Other agents used were of analytical grade and all solu-ions were prepared with distilled water.

.2. Preparation of the porous GG-g-P(NaA-co-St)/APTuperabsorbent hydrogels

GG (1.20 g) was dissolved in 34 mL pH = 12.5 NaOH solution in 250 mL round-bottomed flask equipped with mechanical stirrer,eflux condenser, gas inlet tube and thermometer. GG slurry wasnitially heated at 60 ◦C for 0.5 h under nitrogen atmosphere. The

ppropriate amount of surfactant was then added to the flask, andhe system was heated at 60 ◦C for 0.5 h. After that, the aqueousolution of initiator APS (5 mL, 0.10 g APS) was added to the mix-ure and stirred for 15 min to generate radicals. After the reaction

Biointerfaces 88 (2011) 279– 286

system was cooled to 40 ◦C, the mixture solution containing AA(7.20 g, neutralization degree of 70%), St (0.125 g), MBA (0.0126 g)and APT (0.93 g) was added. The reaction system was slowly heatedto 70 ◦C and kept for 3 h to complete the polymerization. Uponcompletion of the reaction, the gel products were firstly washedthoroughly with methanol/water mixture (8:1, v/v) for severaltimes to remove surfactant and then immersed into absolutemethanol for 24 h to dehydrating. The products were filtratedand wiped off the surface methanol using filter paper and driedovernight at room temperature. Finally, the dried samples wereground and passed through 40–80 mesh sieve (180–380 �m).

The comparable non-porous GG-g-P(NaA-co-St)/APT hydrogelwas prepared by a similar procedure with the porous hydrogelsexcept without the addition of surfactant. The non-porous, SDS-added, DTAB-added and OP-added hydrogels were designated asM, SM, DM and OM, respectively.

2.3. Measurement of the equilibrium swelling degree andswelling rate

The equilibrium swelling degree of the porous superabsorbentswas determined by a conventional gravimetric method: 0.050 g drysamples were immersed in 200 mL distilled water and 0.9 wt.% NaClsolution at room temperature for 3 h to reach swelling equilibrium.The swollen samples were then filtered over unabsorbed water by a100-mesh sieve and drained for 10 min to remove redundant water.The equilibrium swelling (Qeq, g/g) was determined according to Eq.(1).

Qeq = ws − wd

wd(1)

where wd is the mass of dried sample (g) and ws is the mass ofswollen hydrogel (g). Each experiment was repeated three times toobtain a mean value of Qeq, the error limits is less than 3%.

Swelling rate of the superabsorbents was measured as follows:0.050 g samples were contacted with 200 mL aqueous solution. Theswollen gels were filtered out using a 100-mesh sieve at consecu-tive time intervals, and the swelling degree (Qt) of superabsorbentsat a given time t was measured by weighing the swollen (ws) andthe dry samples (wd) according to Eq. (1).

2.4. Measurement of swelling in salt solution

The equilibrium swelling of the sample SM in various concentra-tion of salt solutions (NaCl, CaCl2 and AlCl3) and its salt sensitivityin terms of swelling and deswelling of the final product for dis-tilled water and 15 mmol/L NaCl or CaCl2 solution at consecutivetime intervals were investigated according to the above methoddescribed in Section 2.3 and the error limits is also less than 3%.

2.5. Measurement of swelling in pH solution

To investigate the swelling behaviors of the samples (M, SM,TM and OM) at various pHs, individual solutions were prepared bydiluting NaOH (pH = 13.0) and HCl (pH = 2.0) solutions, respectively.The pH values were precisely checked by a pH-meter (MettlerToledo, accuracy 6 ± 0.1). The equilibrium swelling ratio (Qeq) invarious pH solutions was determined by a method similar to thatin distilled water. To study the pH-responsiveness of the sampleSM, 1 mmol/L phosphate buffer solution with pH 2.1 and 7.4 wereused.

2.6. Instrumental analysis

FTIR spectroscopy was measured on a Nicolet NEXUS FTIRspectrometer in 4000–400 cm−1 region using KBr pellets. The

Page 3

ces B:

mci

3

3

Ohasatiam1vssw

X.-N. Shi et al. / Colloids and Surfa

orphologies of the different surfactant-added GG-g-P(NaA-o-St)/APT hydrogels were examined using a JSM-5600LV SEMnstrument (JEOL) after coating the sample with gold film.

. Results and discussion

.1. Infrared spectra

Fig. 1a and b shows FTIR spectra of M, non-washed SDS, DTAB,P-added hydrogels and washed SM, DM, OM superabsorbentydrogels, respectively. For the FTIR spectra of M, the broadbsorption peaks at about 3450 cm−1 can be assigned to the O Htretching vibration of GG and the bonding water molecules, thebsorption peaks at 2952 cm−1 and 1287 cm−1 can be ascribed tohe C H stretching and bending vibration. The peak at 1726 cm−1

s ascribed to C O stretching vibration, and the peak at 1571 cm−1

nd 1456–1410 cm−1 is attributed to the asymmetric and sym-etric stretching vibration of the COO− groups. The peaks at

169 cm−1 and 1030 cm−1 correspond to C C and C O stretching

ibration. The above information is in accordance with the FTIRpectra of GG-g-P(NaA-co-St)/MVT hydrogel [32]. From the FTIRpectra of non-washed SDS, DTAB, OP-added samples (1a) andashed SM, DM, OM samples (1b), the typical peak appearing

Fig. 1. FTIR spectra of (a) M, non-washed SDS, DTAB, O

Biointerfaces 88 (2011) 279– 286 281

around 2850–2860 cm−1 which can be attributed to the CH2symmetric stretching vibration of surfactant molecules [33], isobviously weaken after washing process. In addition, the profilesof the characteristic absorption peaks for M, washed SM, DM,OM samples are almost identical; this implies that surfactantmolecules have been removed from the hydrogels network inthe washing process, and the surfactants only acted as a micelletemplating for the formation of pore structure.

3.2. Morphological analysis

Fig. 2 shows the FESEM micrographs of M, SM, DM and OMsuperabsorbent hydrogels. It can be clearly seen that the type ofsurfactant has great influence on the morphology of the GG-g-P(NaA-co-St)/APT hydrogels. The samples M and DM presented acompact surface without any pores (Fig. 2a and c). In contrast, aporous structure was clearly observed in the samples SM and OM(Fig. 2b and d), but the pore for SM hydrogel is more dense anduniform than OM hydrogel. Moreover, the hydrogel with a better

pore distribution showed higher initial swelling rate (see Section3.5), which confirms that the anionic surfactant SDS is the superiorpore-foaming agent and its micelle template can be formed well inan anionic polymer hydrogel.

P-added samples; (b) SM, DM and OM samples.

Page 4

282 X.-N. Shi et al. / Colloids and Surfaces B: Biointerfaces 88 (2011) 279– 286

f (a) M

3

Sicsocoba

FP

Fig. 2. FESEM micrographs o

.3. Effect of the type of surfactant on the equilibrium swelling

As shown in Fig. 3, the type of surfactants (anionic surfactantDS, cationic surfactant DTAB, non-ionic surfactant OP) has greatnfluence on the equilibrium swelling capacity of GG-g-P(NaA-o-St)/APT hydrogels. The equilibrium swelling order for differenturfactant-added samples is: SM > OM > M > DM, the introductionf anionic and non-ionic surfactant may enhance the swelling

apacity, while cationic surfactant would produce negative effectsn the equilibrium swelling of the hydrogel. This can be explainedy the following reason: (i) in aqueous environment, the self-ssembling of surfactant may form non-spherical micelles that

ig. 3. Effect of the type of surfactant on the equilibrium swelling of the GG-g-(NaA-co-St)/APT hydrogel.

, (b) SM, (c) DM and (d) OM.

acted as a template for the formation of pores in the hydrogelnetwork [23,24]. The porosity plays the multiple role in enhancingthe swelling capacity and the responsive rate of the hydrogels[34]; (ii) the hydrophobic interaction between the alkyl moietiesof the surfactant molecule could decrease the hydrogen-bondinginteraction between polymeric chains, and also expand the size ofthe network pores, and accordingly, more water molecules wouldbe accommodated within the polymeric network of hydrogel[35]. However, the equilibrium swelling capacity of the hydrogelwith the addition of cationic surfactant DTAB was lower than thesurfactant-free hydrogel. This may be attributed to the electro-static attraction between cationic surfactant molecules with thecounter ionized groups (e.g. COO−) of the reaction system in thegelling process, which induced a slight physical crosslinking in thehydrogel network and reduced the network pore size [25], andthis was confirmed by the SEM micrographs (Fig. 2). As shownin Fig. 3, the equilibrium swelling capacity of anionic SDS-addedhydrogel is comparatively higher than that of non-ionic OP-addedsample. This behavior is attributed to the electrostatic repulsion ofthe O SO3

− groups of SDS molecules with the polyanionic chainin the crosslinking procedure, which is favorable to the synergisticimprovement of the porosity of the network and increase theswelling capacity of the hydrogel.

3.4. Effect of the dosage of surfactant on the swelling capacity

Fig. 4 shows the effect of the dosage of surfactant on the equilib-

rium swelling capacity of the SDS-added SM hydrogel. The swellingcapacity of the hydrogel enhanced from 610 g/g to 948 g/g in dis-tilled water and from 68 g/g to 94 g/g in 0.9 wt.% NaCl solution.The swelling capacity greatly increased with increasing the dosage

Page 5

X.-N. Shi et al. / Colloids and Surfaces B:

FS

otcaegedCt

sl

Fs

ig. 4. Effect of the SDS dosage on the equilibrium swelling of the GG-g-P(NaA-co-t)/APT hydrogel.

f SDS to 1.8 mmol/L, but decreased with the further increase ofhe amount of SDS. The incorporation of moderate amount of SDSontributes to improve the swelling properties of the hydrogel,nd the reasons have been discussed in the above section. How-ver, the addition of SDS at higher concentration resulted in theelling process at higher temperature [25], this meant the reactionfficiency decreased at this reaction temperature (70 ◦C), the three-imensional network of the hydrogel cannot be formed efficiently.onsequently, the swelling capacity of the sample decreased when

he concentration of SDS was beyond the optimum concentration.

By contrast, the feed amount of DTAB and OP on the equilibriumwelling capacity of the hydrogel were also investigated. The equi-ibrium swelling capacity of DM hydrogel always falls in the case of

ig. 5. Swelling kinetic curves (a and b) and t/Qt vs. t graphs (a′ , b′) of GG-g-P(NaA-co-St)/olution.

Biointerfaces 88 (2011) 279– 286 283

increasing the dosage of DTAB. For OP-added samples, with increas-ing the feed amount of OP, the equilibrium swelling capacity ofOM hydrogel slightly increased and then decreased. The maximumabsorbency was obtained (641 g/g in distilled water and 75 g/g in0.9 wt.% NaCl solution) at 2.4 mmol/L OP.

3.5. Effect of the type of surfactant on the swelling kinetics

Besides swelling capacity, the swelling rate of a hydrogelmaterial is vital for its various technological applications, suchas in biomedical, pharmaceutical, environmental, and agricul-tural engineering. The swelling rate of a hydrogel depends on itsstructure, composition, and the external environment. In this sec-tion, the dynamic swelling behaviors of different surfactant-addedhydrogels in distilled water and 5 mmol/L NaCl solution were inves-tigated (Fig. 5). Initially, the swelling rate sharply increases andthen begins to level off after 900 s until the equilibrium swelling isachieved. The swelling kinetic behavior can be evaluated with thefollowing second-order equation (2):

dQ

dt= ks(Q∞ − Q )2 (2)

where Q∞ and ks are the theoretical equilibrium swelling capacityand the swelling rate constant, respectively. The integration of theabove equation over the limits Q = Q0 at t = t0 and Q = Qt at t = t gavethe following second-order kinetics equation (3) [36]:

t

Qt= A + Bt (3)

where B = 1/Q∞ is the inverse of the theoretical maximum swelling,A = 1/ksQ 2∞ is the reciprocal of the initial swelling rate of the hydro-gel, and kis = ksQ 2∞ is the initial swelling rate. Graphs of t/Qt versust give perfect straight lines with good linear correlation coefficient

APT hydrogels with added different surfactant in distilled water and 5 mmol/L NaCl

Page 6

284 X.-N. Shi et al. / Colloids and Surfaces B: Biointerfaces 88 (2011) 279– 286

Table 1Swelling kinetic parameters of GG-g-P(NaA-co-St)/APT hydrogels added different surfactants in distilled water and 5 mmol/L NaCl solution.

SAP Qeq (g/g)a Qeq (g/g)b Q∞ (g/g)a Q∞ (g/g)b kis (g/g/s)a kis (g/g/s)b ks (×10−5, g/g/s)a Ks (×10−5, g/g/s)b Ra,b

M 608 229 617 230 8.1394 5.6427 2.1361 10.6284 1SM 859 254 870 256 13.5648 6.1854 1.7940 9.4564 1DM 551 230 559 232 7.2648 5.2587 2.3277 9.8141 1OM 644 235 649 237 10.9016 5.8931 2.5854 10.4947 1

(gksbttaipsccfh

3h

ins

Fa

a In distilled water.b In 5 mmol/L NaCl solution.

R > 0.999; Fig. 5a′ and b′), indicating that the swelling of the hydro-els obeys the Schott’s theoretical swelling model. The swellinginetic parameters such as ks, Q∞ and kis can be calculated by thelope and intercept of lines and are presented in Table 1. It cane seen that the initial swelling rate kis of the hydrogel in dis-illed water and in 5 mmol/L NaCl solution varied with altering theype of surfactants. The incorporation of proper amount of SDSnd OP in the preparation process of hydrogel gives rise to themprovement of the initial swelling rate through the creation oforosity structure in the hydrogel network. In the case of cationicurfactant DTAB, the attractive forces between anionic polymerichains and cationic surfactant molecules lead to the physicalrosslinking in the hydrogel networks, which reduced the networkree space and slowed the diffusion of water molecules into theydrogel.

.6. The salt dependence and responsiveness of the swelling of theydrogels

It is well known that swelling capacity of “anionic” hydrogelss significantly affected by the concentration and valence of exter-al electrolytes. In current study, the effects of electrolytes on thewelling behaviors of the SM hydrogel were examined using NaCl,

ig. 6. Salt dependence of the different surfactant-added hydrogels (a) and responsivenelternatively changed: (b) distilled water – 15 mmol/L NaCl, (c) distilled water – 15 mmo

CaCl2 and AlCl3 solutions and the swelling curves are shown inFig. 6a. It can be clearly observed that the equilibrium swelling val-ues were rapidly decreased in each cation solution with increasingthe concentration of the cations to 15 mmol/L, and then the swellingcapacity tends to a constant value. In addition, it can be foundthat the aqueous Ca2+ and Al3+ media had lower swelling capacitythan the Na+ media. Equilibrium swelling values of the sample SMin different cationic solution is in the order: NaCl > CaCl2 > AlCl3.These results may be attributed to a “charge screening effect” ofthe additional cations. The charge screening effect increases withincreasing the concentration of cations, it not only decreased theanion–anion electrostatic repulsion among graft polymer chainsbut the osmotic pressure between hydrogel networks and exter-nal aqueous solution, thus the swelling capacity was diminished.In addition, multivalent cations can complex with the carboxylategroups in the hydrogels to form an addition chemical crosslinking,which lead to the deswelling or contraction of the hydrogel. Butthe complexing capability of multivalent cations with carboxylategroups is different. For example, the logarithm of formation con-

stant of EDTA with Ca2+ and Al3+ is 10.69 and 16.30, respectively[37]. Hence, the Al3+ ion has stronger complexation capability thanthe Ca2+ ion, and Al3+ ion decreased the swelling capacity to a greatdegree.

ss behavior of the SDS-added hydrogel when the ionic aqueous swelling media isl/L CaCl2, (d) 15 mmol/L NaCl – 15 mmol/L CaCl2.

Page 7

ces B: Biointerfaces 88 (2011) 279– 286 285

StttaitbihanahcbtwpcasttcsbsC

3h

sas4kidodo

˘

wsrmgdattibtenrccf

X.-N. Shi et al. / Colloids and Surfa

In a series of evaluation, the swelling–deswelling cycles of theM sample in distilled water and 15 mmol/L NaCl, CaCl2 solu-ions were also tested as an example. As illustrated in Fig. 6b–d,he swelling–deswelling behavior is quite repeatable between dis-illed water and NaCl media as well as between distilled waternd CaCl2 media. The salt-dependent swelling reversibility of theonic hydrogels in NaCl solution has been reported [38,39]. Buthe starch-acrylic superabsorbent hydrogel particles that had onceeen immersed in the multivalent cations solution will not re-swell

n water again as described previously [40]. The similar behavioras also been reported by Feng and Guo [41] in the case of poly(vinyllcohol)/poly(acrylic acid) hydrogel fibers. This nonreversible phe-omenon has been explained by the difference in complexingbility of multivalent cations with the carboxylate groups. Whenydrogel was swelled in multivalent cation solutions, the ionicrosslinking mainly occurs at surface of particle and makes it rub-ery and very hard. In the case of the hydrogel SM, because ofhe synchronic effect of St and APT in its three-dimensional net-ork of the hydrogel [32], the steric hindrances of the hydrophobichenyl groups and rigid APT restrain the formation of stable ionicrosslinks and the caboxylate–Ca2+–carboxylate bridges are weaknd the hydrogel network can re-swell in distilled water. However,ome Na+ and Ca2+ containing the swollen gels that are transferredo distilled water, causing the increase of ion concentration in dis-illed water and decrease the osmotic pressure [42], so the swellingapacity decreased gradually in the following cycles. Based on theimilar reason, the SM hydrogel shows a good ion-exchange capa-ility, i.e., it can be repeatedly swelled and deswelled when thewelling media was alternatively changed between the Na+ anda2+ solutions with a same molarity (Fig. 6d).

.7. The pH dependence and swelling responsiveness of theydrogels

The equilibrium swelling of the samples (M, SM, DM, OM) weretudied at various pHs ranged from 2 to 13. As depicted in Fig. 7a,t the low pH values (<3), the equilibrium swelling capacity for allamples is low. As increasing the pH value of external solution to–9, the equilibrium swelling capacity rapidly increased and almosteep constant in the pH range from 4 to 9. Then, the swelling capac-ty drastically decreased when the pH value beyond 9. The swellingependence of the hydrogel on pH value was influenced by thesmotic pressure. Donnan equilibrium theory contributes to theetermination of osmotic pressure (˘ ion), which reveals the extentf swelling, as shown in the following Eq. (4) [43]:

ion = RT∑

i(cgel

i− cext

i ) (4)

here ci is the mobile ion concentration of species and super-cripts gel and ext represent the gel and external solution phases,espectively. At the low pH value, the COO− groups on the poly-er chains can be easily protonated and transformed into COOH

roups, which restrict the ionization of hydrophilic groups andecrease the amount of mobile ions in the hydrogel network. As

result, the osmotic pressure as well as the repulsion interac-ion among negatively charged polymer chains was reduced andhe swelling capacity was decreased. An inverse behavior occursn basic solution (pH > 10), the concentration of the Na+ ions inoth gel network and external solution was changed by the addi-ion of NaOH solution. However, the concentration of mobile ion inxternal solution increases more quickly than that in the hydrogeletwork, which results in the decrease of swelling pressure. As a

esult, the expansion of the network is reduced and the swellingapacity was decreased. Similar results have been reported in thease of other hydrogel systems [12,15]. In addition, it can be noticedrom Fig. 7a that the equilibrium swelling capacities of all the

Fig. 7. pH dependence of the different surfactant-added hydrogels (a) and pH-sensitive on–off switching behavior of the SDS-added hydrogel (b).

hydrogels in the range of pH 4–9 are almost equal to that in distilledwater. This can be attributed to the fact that some of carboxylategroups are ionized and the ionization degree of the carboxylategroups keeps almost constant, which induces a similar osmoticpressure between the hydrogel network and the external solutionas well as the electrostatic repulsion among the COO− groups.

The pH-dependent swelling reversibility of the SM hydrogel wasinvestigated in 1 mmol/L of phosphate buffer solution with pHs2.1 and 7.4 (Fig. 7b). In pH 7.4 media, the hydrogel was rapidlyswelled due to the ionization of COOH groups and anion–anionrepulsive electrostatic forces among COO− groups. When theswollen hydrogel was immersed in pH 2.1 solution, it rapidlydeswells and the gel network collapsed because of the protonationof carboxylate groups in acidic media. As illustrated in Fig. 7b, theswelling capacity and swelling–deswelling rate of the SM hydro-gel have no obvious reduction even after five swelling–deswellingcycles, which exhibited an excellent reversibility. Such on–off(swelling–deswelling) switchable swelling behavior of the hydro-gels makes them as a potential candidate for controlled drugdelivery system.

4. Conclusion

Fast-swelling porous GG-g-P(NaA-co-St)/APT superabsorbenthydrogel was prepared by a radical graft copolymerization reac-tion using a self-assembled micelle of surfactants as a templating

Page 8

2 ces B:

ppastrannrhtwcpabb2hi

A

Sn(Ce

R

[

[

[

[

[

[

[

[

[

[

[

[

[

[

[

[

[

[

[

[

[

[

[

[

[

[

[[

[

[

[

[

86 X.-N. Shi et al. / Colloids and Surfa

orogen. The action mechanism of the surfactants in the gellingrocess was confirmed by FTIR spectroscopy, and it only acts a micelle template for the formation of pores and could beimultaneously removed from the final hydrogel network withhe methanol/water (8:1, v/v) washing process. The experimentalesults proved that using self-assembled anionic surfactant SDS as

micelle template in the gelling process of the anionic polymericetwork could efficiently expand the mesh size of the hydrogeletwork and enhance the swelling capacity and initial swellingate. In saline solution, the equilibrium swelling capacity of theydrogel dramatically depends on the types and concentration ofhe cations. The influence of the external pH on swelling behaviorsas also studied; results indicate that the hydrogels exhibit almost

onstant and relatively higher swelling capacity in the ranges ofH 4–9, the ionic relationship account for this swelling extent. Inddition, the reproducible on–off switching behavior was observedetween distilled water and 15 mmol/L NaCl or CaCl2 solution,etween 15 mmol/L NaCl and CaCl2 solution, and between pH.1 and 7.4 solutions. The pH- and salt-sensitive properties of theydrogel make it not only suitable for water absorption material

n many environment fields but also for drug delivery system.

cknowledgments

The authors thank for jointly supporting by the National Naturalcience Foundation of China (No. 20877077), the Science and Tech-ology Support Project of Jiangsu Provincial Sci. & Tech. DepartmentNo. BY2010012), and the Open Fund of the Key Laboratory ofhemistry of Northwestern Plant Resources, China Academy of Sci-nces (No. CNPR2010kfkt01).

eferences

[1] K. Ofokansi, G. Winter, G. Fricker, C. Coester, Matrix-loaded biodegradablegelatin nanoparticles as new approach to improve drug loading and delivery ,Eur. J. Pharm. Biopharm. 76 (2010) 1.

[2] N. Bhattarai, J. Gunn, M.Q. Zhang, Chitosan-based hydrogels for controlled,localized drug delivery , Adv. Drug Delivery Rev. 62 (2010) 83.

[3] Y. Liang, W.S. Liua, B.Q. Han, C.Z. Yang, Q. Ma, F.L. Song, Q.Q. Bi, An in situformed biodegradable hydrogel for reconstruction of the corneal endothelium, Colloids Surf. B: Biointerfaces 82 (2011) 1.

[4] A. Li, A.Q. Wang, Synthesis and properties of clay-based superabsorbent com-posite , Eur. Polym. J. 41 (2005) 1630.

[5] H.J. van der Linden, S. Herber, W. Olthuis, P. Bergveld, Stimulus-sensitive hydro-gels and their applications in chemical (micro)analysis , Analyst 128 (2003)325.

[6] G.Q. Jiang, C. Liu, X.L. Liu, Q.R. Chen, G.H. Zhang, M. Yang, F.Q. Liu, Network struc-ture and compositional effects on tensile mechanical properties of hydrophobicassociation hydrogels with high mechanical strength , Polymer 51 (2010) 1507.

[7] R. Meena, M. Chhatbar, K. Prasad, A.K. Siddhanta, Development of a robusthydrogel system based on agar and sodium alginate blend , Polym. Int. 57 (2008)329.

[8] M.Z. Liu, R. Liang, F.L. Zhan, Z. Liu, A.Z. Niu, Preparation of superabsorbent slowrelease nitrogen fertilizer by inverse suspension polymerization , Polym. Int.56 (2007) 729.

[9] H.A. Abd El-Rehim, E.-S.A. Hegazy, H.L. Abd El-Mohdy, Radiation synthesis ofhydrogels to enhance sandy soils water retention and increase plant perfor-mance , J. Appl. Polym. Sci. 93 (2004) 1360.

10] V. Singh, P. Kumari, S. Pandey, T. Narayan, Removal of chromium (VI) usingpoly(methylacrylate) functionalized guar gum , Bioresour. Technol. 100 (2009)1977.

11] Y. Zheng, A.Q. Wang, Evaluation of ammonium removal using a chitosan-g-poly(acrylic acid)/rectorite hydrogel composite , J. Hazard. Mater. 171 (2009)671.

12] R. Tankhiwale, S.K. Bajpai, Graft copolymerization onto cellulose-based filterpaper and its further development as silver nanoparticles loaded antibacterialfood-packaging material , Colloids Surf. B: Biointerfaces 69 (2009) 164.

13] K. Kabiri, H. Omidian, S.A. Hashemi, M.J. Zohuriaan-Mehr, Synthesis of

fast-swelling superabsorbent hydrogels: effect of crosslinker type and concen-tration on porosity and absorption rate , Eur. Polym. J. 39 (2003) 1341.

14] A. Pourjavadi, M. Kurdtabar, Effect of different bases and neutralization stepson porosity and properties of collagen-based hydrogels , Polym. Int. 59(2010) 36.

[

[

Biointerfaces 88 (2011) 279– 286

15] W. Li, J.L. Wang, L.Z. Zou, S.Q. Zhu, Synthesis and characteristic of the thermo-and pH-sensitive hydrogel and microporous hydrogel induced by the NP-10aqueous two-phase system , Eur. Polym. J. 44 (2008) 3688.

16] J. Kuang, K.Y. Yuk, K.M. Huh, Polysaccharide-based superporous hydrogels withfast swelling and superabsorbent properties , Carbohydr. Polym. 83 (2011)284.

17] W.F. Lee, R.J. Wum, Superabsorbent polymeric materials. 1. Swelling behav-iors of crosslinked poly(sodium acrylated-co-hydroxyethyl methacrylate) inaqueous salt solution , J. Appl. Polym. Sci. 62 (1996) 1099.

18] J. Gotoh, Y. Nakatani, S. Sakohara, Novel synthesis of thermosensitive poroushydrogels , J. Appl. Polym. Sci. 69 (1998) 895.

19] K. Whang, C.H. Thomas, K.E. Healy, G. Nuber, A novel method to fabricatebioabsorbable scaffolds , Polymer 36 (1995) 837.

20] Y.M. Mohan, P.S.K. Murthy, K.M. Raju, Preparation and swelling behaviorof macroporous poly(acrylamide-co-sodium methacrylate) superabsorbenthydrogels , J. Appl. Polym. Sci. 101 (2006) 3202.

21] K. Kabiri, M.J. Zohuriaan-Mehr, Porous superabsorbent hydrogel composites:synthesis, morphology and swelling rate , Macromol. Mater. Eng. 289 (2004)653.

22] K. Kabiri, H.M. Zohuriaan-Mehr, Novel approach to highly porous superab-sorbent hydrogels: synergistic effect of porogens on porosity and swelling rate, Polym. Int. 52 (2003) 1158.

23] M. Antonietti, R.A. Caruso, C.G. Göltner, M.C. Weissenberger, Morphology vari-ation of porous polymer gels by polymerization in lyotropic surfactant phases, Macromolecular 32 (1999) 1383.

24] S. Partap, A. Muthutantri, T.U. Rehman, G.R. Davis, J.A. Darr, Preparation andcharacterisation of controlled porosity alginate hydrogels made via a simulta-neous micelle templating and internal gelation process , J. Mater. Sci. 42 (2007)3502.

25] J.C. Su, S.Q. Liu, S.C. Joshi, Y.C. Lam, Effect of SDS on the gelation of hydrox-ypropylmethylcellulose hydrogels , J. Therm. Anal. Calorim. 93 (2008) 495.

26] H. Evertsson, S. Nilsson, Microstructures formed in aqueous solutions of ahydrophobically modified nonionic cellulose derivative and sodium dode-cyl sulfate: a fluorescence probe investigation , Carbohydr. Polym. 40 (1999)293.

27] A.-L. Kjoniksen, K.D. Knudsen, B. Nystrom, Phase separation and structuralproperties of semidilute aqueous mixtures of ethyl(hydroxyethyl)cellulose andan ionic surfactant , Eur. Polym. J. 41 (2005) 1954.

28] S. Chatterjee, T. Chatterjee, S.H. Woo, A new type of chitosan hydrogel sorbentgenerated by anionic surfactant gelation , Bioresour. Technol. 101 (2010) 3853.

29] S. Chatterjee, D.S. Lee, M.W. Lee, S.H. Woo, Enhanced molar sorption ratio fornaphthalene through the impregnation of surfactant into chitosan hydrogelbeads , Bioresour. Technol. 101 (2010) 4315.

30] S. Chatterjee, D.S. Lee, M.W. Lee, S.H. Woo, Congo red adsorption from aque-ous solutions by using chitosan hydrogel beads impregnated with nonionic oranionic surfactant , Bioresour. Technol. 100 (2009) 3862.

31] M. Tizzotti, A. Charlot, E. Fleury, M. Stenzel, J. Bernard, Modification of polysac-charides through controlled/living radical polymerization grafting-towards thegeneration of high performance hybrids , Macromol. Rapid Commun. 31 (2010)1751.

32] W.B. Wang, Y.R. Kang, A.Q. Wang, Synthesis, characterization and swellingproperties of guar gum-g-poly(sodium acrylate-co-styrene)/muscovite super-absorbent composites , Sci. Technol. Adv. Mater. 11 (2010) 025006.

33] W.B. Wang, A.Q. Wang, Nanocomposite of carboxymethyl cellulose and atta-pulgite as a novel pH-sensitive superabsorbent: synthesis, characterization andproperties , Carbohydr. Polym. 82 (2010) 83.

34] J. Chen, K. Park, Synthesis of fast-swelling, superporous sucrose hydrogels ,Carbohydr. Polym. 41 (2000) 259.

35] J.P. Zhang, H. Chen, P. Li, A.Q. Wang, Study on superabsorbent composite, 14preparation of poly(acrylic acid)/organo-attapulgite composite hydrogels andswelling behaviors in aqueous electrolyte solution , Macromol. Mater. Eng. 291(2006) 1529.

36] H. Schott, Swelling kinetics of polymers , J. Macromol. Sci. B 31 (1992) 1.37] J.P. Zhang, R.F. Liu, A. Li, A.Q. Wang, Preparation, swelling behaviors and appli-

cation of polyacrylamide/attapulgite superabsorbent composites , Polym. Adv.Technol. 17 (2006) 12.

38] G.R. Bardajee, A. Pourjavadi, R. Soleyman, N. Sheikh, Irradiation mediated syn-thesis of a superabsorbent hydrogel network based on polyacrylamide graftedonto salep , Nucl. Instrum. Methods B 266 (2008) 3932.

39] G.R. Mahdavinia, A. Pourjavadi, H. Hosseinzadeh, M.J. Zohuriaan, Modifiedchitosan 4. Superabsorbent hydrogels from poly(acrylic acid-co-acrylamide)grafted chitosan with salt- and pH-responsiveness properties , Eur. Polym. J. 40(2004) 1399.

40] M.J. Zohuriaan-Mehr, A. Pourjavadi, Superabsorbent hydrogels from starch-g-PAN: effect of some reaction variables on swelling behavior , J. Polym. Mater.20 (2003) 113.

41] J.Q. Feng, L.X. Guo, Swelling/deswelling behavior of thermally induced PVA/PAAhydrogel fiber in aqueous salt solutions , J. Polym. Mater. 19 (2002) 103.

42] F. Horkay, I. Tasaki, P.J. Basser, Osmotic swelling of polyacrylate hydrogels inphysiological salt solutions , Biomacromolecules 1 (2000) 84.

43] F. Rosa, J. Bordado, M. Casquilho, Dynamic and equilibrium swelling of a sulfonicacid superabsorbent copolymer in salt solutions , J. Polym. Sci. Part B: Polym.Phys. 42 (2004) 505.