Introduction

The interaction of nonionic gels with charged surfac-tants was extensively studied in recent years [1–4]. Thisprocess was shown to be governed mainly by electro-static and hydrophobic forces. The electrostatic inter-actions prevail when a gel and a surfactant areoppositely charged, while in other cases (a gel and asurfactant of similar charges, uncharged gel and chargedsurfactant) hydrophobic interactions dominate. Theinteraction between surfactants and hydrogels is also of

great importance in understanding the fundaments ofhydrogel volume phase transition because of theamphoteric property of surfactants. Hydrogel can beionized upon binding of ionic surfactant molecules tothe polymeric network through hydrophobic interac-tions. The swelling ratios and the volume phase transi-tion temperatures of hydrogels were found to beremarkably enhanced, which was interpreted on thebasis of electrostatic repulsion due to binding of ionicsurfactants to polymer chains. The change in the tran-sition temperature was found to be influenced strongly

Tuncer Caykara

Melike Demiray

Olgun Guven

Effect of type and concentrationof surfactants on swelling behaviorof poly[N-[3-(dimethylamino)propyl]methacrylamide-co- N,N-methylene-bis(acrylamide)] hydrogels

Received: 19 December 2004Accepted: 15 June 2005Published online: 15 September 2005� Springer-Verlag 2005

Abstract A series of thermosensitivehydrogels were prepared from N-[3-(dimethylamino)propyl]methacryla-mide (DMAPMA) monomer byusing 11.6–17.8% (m/m) N,N-meth-ylenebis(acrylamide) (MBAAm) asthe crosslinker and comonomer inwater. A kinetic study of theabsorption determined the transportmechanism. The diffusion coeffi-cients of these hydrogels were cal-culated for the Fickian mechanism.It was shown that the swellingbehavior of the P(DMAPMA-co-MBAAm) hydrogels can be con-trolled by changing the amount ofMBAAm. The swelling equilibriumof the P(DMAPMA-co-MBAAm)hydrogels was also investigated as afunction of temperature in aqueoussolutions of the anionic surfactantsodium dodecyl sulfate (SDS) andthe cationic surfactant dodecyltrim-ethylammonium bromide (DTAB).In pure water, irrespective of the

amount of MBAAm, the P(DMAP-MA-co-MBAAm) hydrogels showeda discontinuous phase transitionbetween 30 and 40 �C. However, thetransition changed from discontinu-ous to continuous with the additionof surfactants, this is ascribed to theconversion of non-ionic P(DMAP-MA-co-MBAAm) hydrogel intopolyelectrolyte hydrogels due tobinding of surfactants through thehydrophobic interaction. Addition-ally, the amount of free SDS andDTAB ions was measured at differ-ent temperatures by a conducto-metric method, it was found that theelectric conductivity of theP(DMAPMA-co-MBAAm) – sur-factant systems depended stronglyon both the type and concentrationof surfactant solutions.

Keywords Hydrogel Æ Swelling ÆN-[3-(dimethylamino)propyl]meth-acrylamide Æ Surfactants

Colloid Polym Sci (2005) 284: 258–265DOI 10.1007/s00396-005-1365-7 ORIGINAL CONTRIBUTION

T. Caykara (&) Æ M. DemirayDepartment of Chemistry,Faculty of Science, Gazi University,06500 Besevler, Ankara, Turkey

O. GuvenDepartment of Chemistry,Hacettepe University,06532 Beytepe, Ankara, Turkey

by the addition of small amounts of ionic surfactants[5–7].

In this investigation, the following two types ofsurfactants which have dodecyl groups as the hydro-phobic chain have been used: the anionic surfactantCH3(CH2)11SO4Na (sodium dodecyl sulfate, SDS) andthe cationic surfactant CH3(CH2)11N(CH3)3Br (dode-cyltrimethylammonium bromide, DTAB).

In this study, poly(N-[3-(dimethylamino)pro-pyl]methacrylamide-co- N, N-methylenebis(acrylam-ide)) [P(DMAPMA-co-MBAAm)] hydrogels withdifferent levels of crosslinking were synthesized fromDMAPMA monomer and MBAAm comonomer orcrosslinker in water. The effects of ionic surfactants onthe equilibrium swelling ratio of P(DMAPMA-co-MBAAm) hydrogels were studied in water and inaqueous system as a function of surfactant type andtheir respective concentrations. The association of io-nic surfactants with the P(DMAPMA-co-MBAAm)chains was further investigated by conductometricmeasurements for aqueous solutions includingboth the surfactant and hydrogel.

Experimental procedure

Materials

The monomer N-[3-(dimethylamino)propyl]meth-acrylamide (DMAPMA), the crosslinker or comono-mer N,N-methylenebisacrylamide (MBAAm), theinitiator ammonium persulfate (APS), the acceleratorN,N,N¢,N¢-tetramethylethylenediamine (TEMED),surfactants SDS and DTAB. were purchased fromAldrich Chemical Co. and used as received. All aque-ous solutions were prepared using deionized water.

Hydrogel synthesis

P(DMAPMA-co-MBAAm) hydrogels were prepared bythe free-radical crosslinking polymerization of theDMAPMA in aqueous solution at 22 �C for 24 h inthe presence of predetermined concentrations of theMBAAm crosslinking agent or comonomer (see Scheme1). APS (0.056 M) and TEMED (0.32 M) were used asthe redox initiators in the gelation process. TheDMAPMA (1.0 mL), APS (1.0 mL) and MBAAm(0.12 g) were dissolved in distilled water (4 mL) and thesolution was purged with nitrogen gas for 10 min. Afterthe addition of TEMED (0.5 mL), the solution wasplaced in poly(vinylchloride) straws of 4 mm diametersand about 10 cm long. The poly(vinylchloride) strawswere sealed and immersed in a thermostated water bathat 22 �C, and the polymerization was conducted for24 h. Upon completion of the reaction, the hydrogelswere cut into specimens of approximately 10 mm inlength and immersed in large excess of water to wash outany unreacted monomers and the initiator. The hydrogelsamples were then dried at 50 �C under vacuum toconstant weight. The ratio between mass of driedcopolymer and calculated copolymer mass for 100%conversion for these samples was found in the range of1.10–1.18. An analysis of these values shows both thepresence of bound water and conversion of monomer topolymer to be close to 100%. The cross-linked N-substituted acrylamide hydrogels always contain about10–20 wt% of bound water, even after several months ofdrying under vacuum [8].

Determination of swelling kinetics

The swelling kinetics of the P(DMAPMA-co-MBAAm)hydrogels containing different MBAAm content were

Scheme 1

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measured gravimetrically. The dried samples wereplaced in distilled water at 22 �C and removed fromwater at regular time intervals. After the water on thesurfaces of the hydrogels was wiped off with moistenedfilter paper, the weights of the hydrogels were recorded.The swelling ratio was defined as follows:

SR ¼ mt � md

mdð1Þ

where md and mt are the masses of the dry and swollenhydrogels at time t, respectively.

Determination of equilibrium swelling ratio

For the temperature-response studies, hydrogels wereequilibrated in distilled water at temperatures rangingfrom 10 to 60 �C. The hydrogels were allowed to swell indistilled water for at least 24 h at each predeterminedtemperature, controlled up to ±0.1 �C in a constant-temperature water bath (Thermo Haake K10). Afterimmersion in distilled water at a predetermined tem-perature, the hydrogels were removed from the waterand blotted with wet filter paper for the removal ofexcess water on the hydrogel surface; they were thenweighed. After this weight measurement, the hydrogelswere re-equilibrated in distilled water at another prede-termined temperature, and their swollen weight wasdetermined. The average values of three measurementswere taken for each hydrogel, and the equilibriumswelling ratio was calculated as follows:

ESR ¼ ms � md

mdð2Þ

where ms is the mass of the swollen hydrogel, respec-tively.

Similarly, for the swelling measurements in aqueoussurfactant solutions, the P(DMAPMA-co-MBAAm)hydrogels were immersed in vials filled with aqueoussurfactant solutions (SDS or DTAB). Aqueous surfac-tant solutions were prepared by dilution of the 30 mMsurfactant stock solutions with various volumes of wa-ter. The volume of the surfactant solutions in the vialswas much larger than the hydrogel volume so that theconcentration of the solution was practically unchanged.The hydrogels were equilibrated in aqueous surfactantsolutions at temperatures ranging from 10 to 60 �C. Thehydrogels were allowed to swell in aqueous surfactantsolutions for at least 24 h at each predetermined tem-perature. After immersion in aqueous surfactant solu-tions at a predetermined temperature, the hydrogelswere removed from the aqueous surfactant solutions andblotted with wet filter paper for the removal of excesswater on the hydrogel surface; they were then weighed.After this weight measurement, the hydrogels were

re-equilibrated in aqueous surfactant solutions at an-other predetermined temperature, and their swollenweight.

Conductivity measurements

The conductivity of the P(DMAPMA-co-MBAAm)hydrogel and surfactant (DTAB or SDS) systems wasmeasured in the concentration range of 3.8–30 mMsurfactant solutions at different temperatures by using aconductometer (Model WPA CM 35). The temperaturewas controlled with water circulation using a thermo-static circulator (Thermo Haake K10).

Results and discussion

Effect of cross-linker content on swelling kinetic

When the hydrogels was immersed in water, they readilyswelled up to size depending on the degree of cross-linking. The water adsorption of P(DMAPMA-co-MBAAm) hydrogels containing different MBAAmcontent was determined gravimetrically as a function oftime. Figure 1 shows the swelling ratio–time curves forthe P(DMAPMA-co-MBAAm) hydrogels depending onthe crosslinker content. The data show that the swellingrate decreased with the increase in the crosslinker con-tent, as from 11.6 to 17.8%. The hydrogel with 11.6%MBAAm has about 6.3 swelling ratio within 96 min, or17.2 within 420 min, whereas the hydrogel with 17.8%MBAAm has about 3.2 and 6.5, respectively, within thesame time frames.

The swelling process is a complicated phenomenonand involves three successive steps [9–11]: (1) the diffu-sion of water molecules into the polymeric network, (2)

Fig. 1 Swelling kinetics of the P(DMAPMA-co-MBAAm) hydro-gels containing different MBAAm content. The crosslinker con-tents of the hydrogels are indicated as the insert

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the relaxation of the hydrated polymer chains, and (3)the expansion of the polymeric network into the sur-rounding aqueous solution. Before the swelling, thereexisted strong intermolecular and/or polymer–polymerinteractions, such as hydrogen bonds and hydrophobicinteractions, in the dried hydrogel samples, whichremained in a glassy state [9]. This suggests that a glassyinner core might exist in a dried hydrogel having ahigher crosslinking level because the high crosslinkingmay lead to strong such interactions. As a result, withincreasing amount of MBAAm, the swelling ratio of thehydrogels reduced due to dense three-dimensionalstructure formed at the high crosslinker concentration.

To determine the nature of water diffusion into thehydrogel, the following equation was used [12]:

Mt

M1¼ ktn ð3Þ

where Mt and M¥ represent the amount of waterabsorbed by the hydrogel at time t and at equilibrium, kis a constant characteristic of the system, n is an expo-nent which takes into account the mode of watertransport. A value of n=0.5 indicates a Fickian diffu-sion mechanism, while a value of 0.5 £ n £ 1 indicatesthat diffusion is anomalous or non-Fickian. The expo-nents n and k values were calculated from the slope andintercept of the plots of log Mt/M¥ versus log t for theP(DMAPMA-co-MBAAm) hydrogels (Fig. 2). Eq. 3 isvalid for the first 60% of normalized solvent absorbed[12]. From the intercept and the slope of the curves, thevalues of the kinetic constant, k, and diffusion exponent,n, are obtained. These results are shown in Table 1. Aslight variation of diffusion exponent with MBAAmcontent is observed, and its value higher than 0.50,indicating diffusion of water to the interior of all thehydrogels, follows an anomalous mechanism and revealsthe existence of certain coupling between moleculardiffusion and tension relaxation developed duringswelling of the hydrogels. The highly anomalousbehavior of these hydrogels is due to the regularity of thechain and strong interchain interactions via the forma-tion of hydrogen bonding (Fig. 3), leading to a compactstructure which would accentuate the anomalous aspectsof diffusion even for a molecule as small as water.

Diffusion coefficients are important penetrationparameters of some chemicals to polymeric systems.Using n and k values, the diffusion coefficient (D) ofwater in the hydrogel could be calculated using the fol-lowing equation [12]:

D ¼ k4

� �1=n

pr2 ð4Þ

where D is the diffusion coefficient and r is the radius ofthe hydrogel. The D values of the hydrogels are alsopresented in Table 1. The diffusion coefficients D slightly

Fig. 2 The plots of log Mt/M¥ versus log t for the P(DMAPMA-co-MBAAm) hydrogels containing different MBAAm content. Thecrosslinker contents of the hydrogels are indicated as the insert

Table 1 The parameters of diffusion of water into theP(DMAPMA-co-MBAAm) hydrogels

MBAAm % (m/m) k·100 n D·107/cm2 .s)1

11.6 1.09 0.72 2.5513.2 0.79 0.77 2.8514.8 1.74 0.67 2.8216.1 1.52 0.69 2.8817.8 2.09 0.65 2.87

Fig. 3 Proposed hydrogen-bonding interactions in the P(DMAP-MA-co-MBAAm) hydrogel

261

increase with an increase in MBAAm content in thehydrogel. Because, at a high crosslinking level, the porenumbers of the hydrogels with 16.1 and 17.3%MBAAmwere also increased and large numbers of channels wereavailable for the surrounding water to diffuse into, andthis slightly increased the diffusion coefficients for thehydrogels with 16.1 and 17.3% MBAAm.

Effect of cross-linker content on swelling equilibrium

Figure 4 illustrates the temperature dependence of theequilibrium swelling ratio of P(DMAPMA-co-MBAAm) hydrogels with different amount of cross-linker in water when the temperature increased from 10to 60 �C. The data show that all the P(DMAPMA-co-MBAAm) hydrogels, regardless of the amount of theMBAAm, have similar swelling behaviors as a functionof temperature, and the phase-transition temperaturesof these hydrogels are in the range of 30–40 �C. It isalso shown that all P(DMAPMA-co-MBAAm) hydro-gels exhibited a positive temperature-coefficient, whichswells at higher temperature and shrinks at lowertemperature. Under equilibrium swelling conditions, allP(DMAPMA-co-MBAAm) hydrogels show increasingswelling at higher temperatures, but they arewell belowtheir phase-transition temperatures because of theaggregation of the network chains. When the externaltemperature was increased from 10 �C toward phase-transition temperature, the volume or water contentinside P(DMAPMA-co-MBAAm) hydrogels firstdecreased slowly up to 30 �C and then increaseddrastically between 30 and 40 �C (at phase transitiontemperature range) and finally remained almost con-stant. Even though the phase-transition temperaturerange of the P(DMAPMA-co-MBAAm) hydrogels

were virtually not affected by the amount of MBAAmwithin the ranges studied in this work, the data inFig. 4 clearly show also that, the equilibrium swellingratios of these hydrogels decreased 67% at 10 �C and73% at 60 �C with increasing amount of MBAAmfrom 11.1 to 17.3%. It is believed that an increase inthe level of crosslinking agent would reduce the freevolume within the hydrogel network structure in whichwater would reside during swelling and would also ledto the reduction of pore size of the corresponding hy-drogels, which, in turn, reduced the water holdingcapacity because of the decreased pore volume.

Effects of surfactants on swelling equilibrium

The effect of each ionic surfactant on the swelling processof P(DMAPMA-co-MBAAm) hydrogel containing11.1% MBAAm, which has the highest equilibriumswelling ratio in water among the other hydrogels pre-pared and studied in this work was investigated in detail.Figures 5 and 6 show the temperature dependence of theequilibrium swelling ratio of P(DMAPMA-co-MBAAm)hydrogel with 11.1% MBAAm in the presence of twodifferent ionic surfactants with various concentrations.The overall effect observed is a reduction in the equilib-rium swelling ratio of the hydrogel in surfactant solutionsfor the whole temperature range investigated when thiswas compared with in pure water. The DTAB and SDSmolecules consists of a long aliphatic hydrocarbon chainand P(DMAPMA-co-MBAAm) has hydrophobic unitssuch as (CH3)2N(CH2)3 and CH2CCH3 as pendant groupand main chain, respectively. When DTAB or SDSmolecules diffuse into the gel network of the P(DMAP-MA-co-MBAAm), strong association should take place

Fig. 4 Equilibrium swelling ratios of the P(DMAPMA-co-MBAAm) hydrogels in water shown as a function of temperature.The crosslinker contents of the hydrogels are indicated as the insert

Fig. 5 Equilibrium swelling ratio of the P(DMAPMA-co-MBAAm) hydrogel with 11.1% MBAAm in DTAB solutionsshown as a function of temperature. The concentrations of DTABsolutions are indicated as the insert

262

through the hydrophobic interaction between thehydrophobic groups of the P(DMAPMA-co-MBAAm)and long-chain alkyl groups of the DTAB or SDS, in thiscase, the equilibrium swelling ratio of hydrogel decreases.

In the absence of surfactants, this hydrogel under-went a discontinuous phase transition (chain collapse) at30–40 �C temperature range with a swelling ratio changeat this temperature range defined by ratio SRswollen/SRcollapsed � 1.3. In solutions containing DTAB(Fig. 5), sharp discontinuity in the phase transition theP(DMAPMA-co-MBAAm) hydrogel was convertedinto a more smooth change while maintaining a break inthe curves corresponding to onset of transition. How-ever, in the SDS solutions (Fig. 6), the P(DMAPMA-co-MBAAm) hydrogel has almost the same swelling ratioirrespective of the concentration of SDS solutions andexhibited a negative temperature-coefficient withincreasing of temperature from 10 to 60 �C. Thisbehavior may be attributed to the differences in thecounterions, the ionizable groups and the bindingamounts of the surfactants. In the absence ofP(DMAPMA-co-MBAAm) hydrogel, pH of DTAB andSDS solutions was measured as 6.1 and 7.8, respectively.Therefore pH dependent protonation of amine groups isvery unlikely. On the other hand, the equilibriumswelling ratio of the hydrogels with binding surfactantmolecules depend on the types and nature of waterbinding sites such as – O – SO)

3 and –N+ (CH3)3 of thesurfactant molecules. Their high electric fields not onlypolarize, immobilize, and electrostrict nearest neighbormolecules, but they also induce additional order beyondthe first layer of water molecules. Ions like – O – SO)

3,however, can immobilize the water molecules of only the

first layer [13]. The higher attractive field that can be feltto several layers in the case of –N+ (CH3)3 probablycauses many more layers of water to be associated to thefirst layer, which is in the immediate vicinity of thepolymer in comparison to – O – SO)

3 group, and hencemore water uptake is seen in the hydrogel with thebinding of DTAB containing –N+ (CH3)3 groups.

The binding of surfactant molecules to hydrogels hasbeen studied by many groups [1, 14, 15]. Generally,increased binding of charged surfactants would increasethe charge density of hydrogels and thus lead toenhanced swelling ratio. The effect of SDS in inducingsome measure of polyelectrolyte behavior with chainexpansion has been demonstrated by viscometric mea-surements on aqueous solutions of the uncharged poly-mer, poly(N-vinyl-2-pyrrolidone) [14]. Binding of SDShas also been observed for thermosensitive crosslinkedhydrogels of modified cellulose ethers [15]. However,Shinde et al. [1] observed non-uniform swelling ofpoly(N-isopropylacrylamide) hydrogels in SDS solu-tions, but did not analyze surfactant binding or itsrelation to micellisation or aggregation. They haveattributed this to the presence of heterogeneous phasesas a result of uneven binding of surfactant molecules.But in this present study, our hydrogels are alwaystransparent and swelling is uniform throughout thebulk, so there is homogeneous interaction of SDS withour hydrogels. Therefore, we are unable to attribute thedecrease of the swelling ratio with increasing surfactantconcentration to heterogeneous distribution of surfac-tant molecules in the hydrogel. However, the observedswelling ratio changes can be attributed to the micellar-like hydrophobic domains, which act as physical cross-linkers to restrain the swelling of the hydrogel networks,arise from hydrophobic side groups dangling on thehydrogel networks (Fig. 9). This is in agreement with thelow swelling behavior of the P(DMAPMA-co-MBAAm)hydrogels observed in more concentrated surfactantsolutions. The intermolecular aggregation force ofpolymer segments may play a dominant role in theformation of the micellar-like hydrophobic domainsbecause of the disappearance of protonation effect onthe basic dimethylamino group in slightly basic SDSsolutions. In this case, in contrast to the DTAB solutionswith slightly acidic pH, the micellar-like hydrophobicdomains form only in the presence of lightly basic SDSsolution.

On the other hand, in this study, the high surfactantconcentrations (SDS and DTAB) were used. In this case,the equilibrium swelling ratio of the P(DMAPMA-co-MBAAm) hydrogel in the surfactant solutions wasappreciably decreased comparing to the values measuredin deionized water. This is due to decrement in theexpansion of the hydrogel network because of repulsiveforces of counter ions acting on the polymeric chainshielded by the binding ionic changes. Therefore, the

Fig. 6 Equilibrium swelling ratio of the P(DMAPMA-co-MBAAm) hydrogel with 11.1% MBAAm in SDS solutions shownas a function of temperature. The concentrations of SDS solutionsare indicated as the insert

263

difference in the osmotic pressure between the hydrogelnetwork and the external solution decreased with anincrease in the ionic strength of the surfactant concen-tration, as a result of which the hydrogel exhibited thetrends to shrink.

Conductivity measurements were used to determinethe free DTAB and SDS amounts in their aqueoussolutions in the presence of the P(DMAPMA-co-MBAAm) hydrogel. This hydrogel is expected to absorba portion of the DTAB and SDS molecules and since thepolymers within the hydrogel structure are non-ionic, theconductivity should be proportional to the amount offree ionic surfactant molecules. Figures 7 and 8 show thetemperature dependence of conductivity in the presenceof this hydrogel in the respective surfactant solutions.

The conductivity was linearly dependent with respect totemperature variation for the temperature range inves-tigated, except in the temperature range of 30–40 �C,where a significant sudden decrease in conductivity wasobserved, this did not change with the type of surfactant.This temperature range can be thus referred to as thebinding temperature [16, 17]. The magnitude of thedecrease in the free DTAB or SDS concentrations,indicated by the decrease in conductivity, was found toincrease with the surfactant concentration. The observedeffects of both the cationic surfactant DTAB and anionicsurfactant SDS on the P(DMAPMA-co-MBAAm) hy-drogel can thus be readily understood by assuming thatthe hydrogel have a partially ionic character uponbinding of DTAB or SDS molecules the polymer net-

Fig. 7 Temperature dependence of conductivity of DTAB solu-tions in the presence of the P(DMAPMA-co-MBAAm) hydrogelwith 11.1% MBAAm. The concentrations of DTAB solutions areindicated as the insert

Fig. 8 Temperature dependence of conductivity of SDS solutionsin presence of the P(DMAPMA-co-MBAAm) hydrogel with 11.1%MBAAm. The concentrations of SDS solutions are indicated as theinsert

Fig. 9 Schematic representa-tion of the adsorption via freemolecular and micelle (sphere)forms of the surfactants ontothe P(DMAPMA-co-MBAAm)hydrogel

264

work. It is reasonable to suppose that the DTAB or SDSbinds to the hydrogel within the gel phase thoughhydrophobic interactions. Because, it is also evident thatthe P(DMAPMA-co-MBAAm) hydrogel does not con-tain ionizable groups in their chemical structure; hence,there should not be any electrostatic repulsion forcesbetween the polymer chains. The strong hydrophobicinteractions between DTAB or SDS molecules and theP(DMAPMA-co-MBAAm) hydrogel enables the surf-actants to bind to polymer networks not only from freeDTAB or SDS molecules but also from micelles of these,though the destruction or breakdown of the later [13, 14](Fig. 9). The critical micelle concentrations (CMC) ofDTAB and SDS are 15.6 and 8.3 mM at room temper-ature, respectively [18]. It is known that surfactant mi-celle formation in the presence of polymer is observed atthe surfactant concentration beyond critical aggregationconcentration (CAC) that below CMC. At surfactant

concentrations beyond this characteristic concentration,the excess ‘free’ surfactant is in a dual equilibrium withmicelle formation and binding to hydrogel. Both caseswould result in a saturation value for amount of sur-factant bound to the hydrogel.

However, after 40 �C, the conductivity of the hydro-gel– surfactant systems increased again linearly withincreasing temperature. This conductivity behavior mustbe the result of the desorption of the DTAB and SDSmolecules at around the surface of hydrogel. Because, thehydrophobic interactions between the hydrogel and thesurfactant molecules having a low energy would beovercomewith increasing temperature and in this case, theconductivity of the hydrogel– surfactant systems increases.

Acknowledgements O.G. acknowledges the support of TUBA, theAcademy of Sciences of Turkey. This work was supported by StatePlanning Organization of Turkey (2003 K 120 470-31 DPT).

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