ORIGINAL CONTRIBUTION

Effects of addition of anionic and cationic surfactants to poly(N-isopropylacrylamide) microgels with and without acrylicacid groups

Nodar Al-Manasir & Kaizheng Zhu &

Anna-Lena Kjøniksen & Kenneth D. Knudsen &

Bo Nyström

Received: 9 December 2011 /Revised: 2 February 2012 /Accepted: 4 February 2012 /Published online: 29 February 2012# Springer-Verlag 2012

Abstract The interaction of the anionic surfactant sodiumdodecyl sulfate (SDS) and the cationic surfactant hexadecyltrimethyl ammonium bromide with poly(N-isopropylacryla-mide) (PNIPAAM) microgels with and without poly(acrylicacid) (PAA) was investigated by means of dynamic lightscattering (DLS), zeta potential, and turbidimetry measure-ments. The DLS results show that the PNIPAAM microgelswith PAA will contract when an anionic or cationic surfac-tant is added to the suspension, while the PNIPAAM micro-gels without PAA expand in the presence of an ionicsurfactant. A collapse of the PNIPAAM microgels is ob-served when the temperature is increased. From the zetapotential measurements, it is observed that the charge den-sity of PNIPAAM microgels in the presence of an ionicsurfactant is significantly affected by temperature and theattachment of the negatively charged PAA groups. Theturbidity measurements clearly indicate that the interactionbetween PNIPAAM and SDS is more pronounced than thatof the cationic surfactant.

Keywords Temperature-responsive contraction .

Aggregation . Charged surfactant . PNIPAAM .Dynamiclight scattering . Turbidity . Zeta potential

Introduction

Stimuli-responsive microgels gels have attracted much at-tention for a long time because of their interesting featuresand potential applications [1–9]. The swelling/deswellingresponse of microgels and how to control this behavior isof great interest for biological, industrial, and theoreticalpurposes [10–14]. Microgels are sponge-like particles thatare physically or chemically cross-linked with a networkstructure that is swollen when dispersed in a suitable sol-vent. The microgel particles exhibit a reversible conforma-tional transition, from coil-to-globule state, in response toaltering the solvent quality, which may be affected by anumber of external stimuli including temperature [1–5,14–16], ionic strength [1, 2, 4], level of surfactant addition[5, 17–20], and pH [1, 2, 4, 5, 14, 15]. During the course ofthis transition, the microgel particle adopts a more compactconformation to minimize polymer–solvent interactions, onlyretuning to its original conformation when solvent conditionsbecome more favorable.

The poly(N-isopropylacrylamide) (PNIPAAM) is one ofthe most frequently studied temperature-sensitive microgels,with a lower critical solution temperature (LCST) at around32 °C [21, 22]. The swelling properties of the microgels canbe tuned by means of co-polymerization with neutral or elec-trically charged monomers and by adding ionic surfactants [5,17–20]. Several studies [5, 16–19] have shown that additionof an anionic surfactant to PNIPAAM-based microgels causedthe particle diameter to increase and the LCST is shifted to

N. Al-Manasir :K. Zhu :A.-L. Kjøniksen :B. Nyström (*)Department of Chemistry, University of Oslo,P.O. Box 1033, Blindern,N-0315 Oslo, Norwaye-mail: bo.nystrom@kjemi.uio.no

A.-L. KjøniksenDepartment of Pharmaceutics, School of Pharmacy,University of Oslo,P.O. Box 1068, Blindern,N-0316 Oslo, Norway

K. D. KnudsenDepartment of Physics, Institute for Energy Technology,P. O. Box 40,N-2027 Kjeller, Norway

Colloid Polym Sci (2012) 290:931–940DOI 10.1007/s00396-012-2607-0

higher temperatures. It has been reported [23] that the inter-action between PNIPAAM microgels and sodium dodecylsulfate (SDS) is a cooperative process. It was argued [23] thatthe surfactant binds to the microgel as small aggregates withdifferent aggregation number in two steps, which was attrib-uted to the inhomogeneous structure of the particles. Tam et al.[16] have found that both anionic and cationic surfactantsinteract with PNIPAAM microgels. Addition of the anionicSDS causes swelling of the microgels, whereas the cationicdodecyl trimethylammonium bromide had no effect on theswelling properties of the microgel particles, but it inducedcoagulation of the microgel at the LCST. Quite recently, theinteraction between cetyl-trimethylammonium bromide(CTAB) and PNIPAAM was reported [9] and it was conclud-ed that CTAB binds to the PNIPAAMmicrogels in two ways.At low surfactant concentration, CTAB binds to the microgelparticles as monomers and the particle size is constant. Abovethe critical aggregation concentration (cac), CTAB binds tothe gel particles in the form of aggregates in both the swollenand collapsed states of PNIPAAM. In spite of many papers onthe interaction between ionic surfactant and PNIPAAM, ourunderstanding of the intricate interplay between hydrophobicand electrostatic interactions when the temperature is changedis incomplete.

The aim of this work is to elucidate the complexrelationship between temperature-induced hydrophobicityand electrostatic interactions when the external stimuliare altered. This interplay is important to understand inorder to be able to preserve the stability of the particlesunder various conditions. For this purpose, we havesynthesized two PNIPAAM-based microgels with andwithout poly(acrylic acid) (PAA) cross-linked with N,N′-methylenebis(acrylamide) (BIS) by free radical precip-itation polymerization. The weakly charged PNIPAAMand the microgel decorated with acrylic acid groups arestudied by means of dynamic light scattering, zeta po-tential, and turbidity experiments in the presence ofSDS (charges of the same sign) or CTAB (oppositelycharged surfactant). We will show that a narrow con-traction zone of the microgels with PAA is found at lowsurfactant concentration (both with SDS and CTAB),and this minimum is more pronounced at high temper-atures, but no transition zone is observed for the micro-gels without acrylic acid groups. This is a novelfinding, and to the best of our knowledge, this featurehas not previously been reported. In this paper, wedemonstrate the delicate balance between temperature-induced hydrophobicity and electrostatic interactions byadding an anionic or cationic surfactant to negativelycharged microgels of different charge density. This in-terplay is imperative to be able to tailor-made microgelsfor industrial applications such as enhanced oil recoveryand controlled drug delivery.

Experimental

Materials

The surfactants SDS and CTAB were supplied by Fluka andthey were used without further purification. N-isopropyla-crylamide (NIPAAM, Acros) was recrystallized from atoluene/hexane mixture solvent and dried at room tempera-ture under vacuum prior to use. BIS, ammonium persulfate(APS), and acrylic acid (AA) were utilized in the prepara-tion of microgel samples, and all chemicals were purchasedfrom Sigma-Aldrich AS, Norway and used as received. Thewater employed in this investigation was purified with a Milli-pore Milli-Q system, and the resistivity was approximately18 MΩ cm.

Synthesis of microgels

Microgels composed of PNIPAAM and poly(N-isopropyla-crylamide-co-acrylic acid) (PNIPAAM-co-AA) were syn-thesized via surfactant-free, free radical precipitationpolymerization in water according to procedures reportedpreviously [3, 5–8]. APS was thermally decomposed at 70 °C to initiate the polymerization of NIPAAM, BIS (cross-linker), with or without AA. The monomer mixtures werecomprised of 98% NIPAAM and 2% BIS for PNIPAAMmicrogel, 92% NIPAAM, 6% acrylic acid and 2% BIS forPNIPAAM-co-AA microgel (70 mM total monomer con-centration). The synthesized thermoresponsive PNIPAAMmicrogel and thermo- and pH-responsive PNIPAAM-co-AAmicrogel have been well-documented previously [5].

Surfactant solutions of different concentrations were pre-pared by dissolving SDS or CTAB in Millipore water. Aconstant amount of microgel was suspended in these surfac-tant solutions to obtain the desired microgel concentration of0.01 wt.%. The pH of the suspensions was measured to beca 6 for all the samples. All samples were prepared byweighing the components and then stirring the solutionsfor 1 day at room temperature to ensure that the solutionsbecame homogeneous. Both the turbidity and dynamic lightscattering measurements were carried out in the presence ofa heating rate of 0.2 °C/min. The temperature-inducedchanges of the microgel suspensions were found to bereversible for all the systems, except for the samples wherestrong flocculation and sedimentation were observed.

Turbidimetry

The transmittances of 0.01 wt.% suspensions of PNIPAAMand PNIPAAM–co-PAA microgels in the presence of vari-ous amounts of surfactants were measured with a tempera-ture controlled Helios Gamma (Thermo Spectronic,Cambridge, UK) spectrophotometer at a wavelength of

932 Colloid Polym Sci (2012) 290:931–940

500 nm. The apparatus is equipped with a temperature unit(Peltier plate) that gives a good temperature control over anextended time. The heating rate of the spectrophotometerwas controlled by a PC that was interfaced to the apparatusand equipped with homemade software that gives the pos-sibility of performing temperature scans with user-definedprotocols.

The turbidity τ of the samples can be determined from thefollowing relationship: τ0(−1/L) ln (It/I0) where L is thelight path length of the cell (1 cm), It is the transmitted lightintensity, and I0 is the incident light intensity. The resultsfrom the spectrophotometer measurements are presented interms of turbidity.

Dynamic light scattering

The dynamic light scattering (DLS) experiments were con-ducted with the aid of an ALV/CGS-8 F multidetectorversion compact goniometer system, with eight fiber-optical detection units, from ALV-GmbH., Langen,Germany. The beam from a Uniphase cylindrical 22 mWHeNe laser, operating at a wavelength of 632.8 nm withvertically polarized light, was focused on the sample cell(10-mm NMR tubes, Wilmad Glass Co., of highest quality)through a temperature-controlled cylindrical quartz contain-er (with two plane-parallel windows), vat (the temperatureconstancy being controlled to within ±0.01 °C with a heat-ing/cooling circulator), which is filled with a refractiveindex matching liquid (cis-decalin). In order to avoid dust,the samples were filtered in an atmosphere of filtered airthrough a 5 μm filter (Millipore) directly into precleanedNMR tubes. The correlation function data were recordedcontinuously with an accumulation time of 2 min.

In light scattering experiments, we probe on a lengthscale of q−1, where q is the wave vector defined as q04πn sin(θ/2)/l. Here, l is the wavelength of the incident lightin a vacuum, θ is the scattering angle, and n is the refractiveindex of the medium.

In these dilute suspensions of microgels, the scatteredfield can be assumed to exhibit Gaussian statistics and theexperimentally recorded intensity autocorrelation functiong2(q,t) is directly linked to the theoretically amenable first-order electric field autocorrelation g1(q,t) through the Sie-gert relationship [24] g2(q,t)01+B|g1(q,t)|2, where B (≤1) isan instrumental parameter.

For suspensions containing particles of different sizes,the non-exponential behavior of the autocorrelation functioncan be portrayed by using a Kohlrausch–Williams–Watts[25, 26] stretched exponential function. This procedure hasbeen reported [27–30] to be powerful in the analysis ofcorrelation functions obtained from various colloid systems.This approach was also successful to describe the correla-tion function data in this work and the algorithm employed

in the analysis of the present correlation function data can becast in the following form

g1ðtÞ ¼ exp � t=tfeð Þbh i

ð1Þ

where, τfe is some effective relaxation time and β (0<β≤1) isa measure of the width of the distribution of relaxation times.The high values of β (β≥0.9) observed for the present micro-gel systems at all conditions suggest that the particles arenearly monodisperse. The mean relaxation time is given by

t f ¼ ðtfe=bÞΓ ð1=bÞ ð2Þwhere, Γ(1/β) is the gamma function.

The correlation functions were analyzed by using a non-linear fitting algorithm (a modified Levenberg–Marquardtmethod) to attain best-fit values of the parameters τfe and βappearing on the right-hand side of Eq. 1. A fit was consid-ered to be satisfactory if there are no systematic deviationsin the plot of the residuals of the fitted curve and the valuesof the residuals are small. Since the relaxation mode alwaysis diffusive (τf~q

−2) for these systems, the apparent hydro-dynamic radii Rh of the microgels can be calculated by usingthe Stokes–Einstein relationship

Rh ¼ kBT

6pηDð3Þ

where, kB is the Boltzmann constant, T is the absolutetemperature, η is the viscosity of the solvent, and D (D01/τf q

2) is the mutual diffusion coefficient. The suspensionsbecome rather turbid at elevated temperatures; this may leadto problems with multiple scattering. Measurements werenot conducted under these conditions.

Zeta potential

A Malvern Zetasizer 2000 from Malvern Instruments wasused to measure the electrophoretic mobility of the PNIPAAMmicrogels as a function of the surfactant concentration atdifferent temperatures. The values of the electrophoretic mo-bility were converted to zeta potential values. Measurementswere carried out at a microgel concentration of 0.01 wt.%.

Results and discussion

DLS, zeta potential, and turbidity measurements have beencarried out to study the effects of adding anionic or cationicsurfactant to PNIPAAM microgels and to compare themwith the corresponding microgels equipped with negativelycharged PAA groups. Before we consider the effect of addedsurfactant, it is instructive to investigate the differencesbetween PNIPAAM and PNIPAAM–co-PAA microgels inwater.

Colloid Polym Sci (2012) 290:931–940 933

The effect of temperature on the size of 0.01 wt.% PNI-PAAM microgels with and without PAA is illustrated inFig. 1a. For PNIPAAM microgels, the hydrodynamic radiusdecreases from 450 to 200 nm when the temperature isincreased from 25 °C to 35 °C, indicating that the particlesare strongly compressed around the LCST (cf. Fig. 1c) dueto disruption of hydrogen bonds between the –CONHgroups and water molecules at elevated temperatures, caus-ing an enhanced hydrophobicity of the particles. It is wellknown from the literature that when the temperature israised above the LCST (ca. 32 °C), the PNIPAAM particlesin a dilute suspension collapse in a narrow temperaturerange, forming compact particles with a low water content[19]. This is in agreement with the behavior of the purePNIPAAM microgels studied here. In the case of the PNI-PAAM microgel with attached acrylic acid groups, Rh

exhibits a gradual, nearly linear, decrease in size over thewhole temperature range. The PAA groups attached to thePNIPAAM microgels generate an augmented charge densityof the microgels, thereby increasing the LCST (see Fig. 1c).There is a competition between the hydrophobic interactionscaused by isopropyl groups and the electrostatic interactionscaused by PAA groups. By increasing the temperature, thehydrophobic interactions become stronger causing a con-traction of the particles, while at the same time the electro-static repulsion between the PAA groups will counteract thecollapse of the microgels. This explains the less pronounceddrop of Rh with increasing temperature for the PNIPAAM–

co-PAA system as compared with the PNIPAAM microgels.In addition, the PAA groups are suggested to act as“defects” in the PNIPAAM microgels causing a wideningof the collapse region of the microgels [31–34].

The zeta potential of the PNIPAAM microgels is plottedas a function of temperature in Fig. 1b. The zeta potential ofPNIPAAMmicrogels is about −5 mVat 25 °C, and when thetemperature is increased above the LCST, a higher negativevalue is observed (−30 mV). The negative charges originatefrom the sulfate groups of the initiator. By raising thetemperature above the LCST, the PNIPAAM microgelbecomes very compact, causing more sulfate groups to beexpelled from the hydrophobic core of the microgel par-ticles, thereby increasing the charge density of the micro-gels. For the PNIPAAM–co-PAA system, the high value ofthe zeta potential of about −30 mVat 25 °C is mainly causedby the COOH groups, which are now attached to the PNI-PAAM microgel. The slightly increasing absolute values ofthe zeta potential as the temperature is raised originate fromcharged groups that are pressed out onto the surface of themicrogels as they gradually contract.

Before the turbidity results are presented and discussed, itmay be instructive to look into factors that can influence theturbidity values. The common factors that increase the tur-bidity values are higher particle concentration, larger parti-cle sizes, and presence of aggregates. Hence, the formationof large-scale association complexes in the solution willcause higher turbidity values. The other factor that affects

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934 Colloid Polym Sci (2012) 290:931–940

the turbidity values is the differences in refractive index be-tween the particles and the solvent as the particles collapse atelevated temperatures [5, 35]. Accordingly, compression ofthe particles into more compact structures acts towards anincrease in the turbidity of the samples.

Figure 1c shows the temperature dependence of the turbid-ity for 0.01 wt.% PNIPAAM microgels without and withPAA. At 25 °C, the PNIPAAM particles are in a swollen state,but when the temperature is raised, the PNIPAAM microgelsbecome more hydrophobic. This leads to a collapse of theparticles, which results in higher turbidity values [5]. Eventhough the PAA containing microgels are larger than the purePNIPAAM microgels at most temperatures (see Fig. 1a), theturbidity values of the PNIPAAM–co-PAAmicrogel are lowerthan those of the PNIPAAM microgels without PAA over thewhole temperature range. This indicates that the particles withPAA are less compressed than the particles without PAA, aneffect that is attributed to the electrostatic repulsions of thePAA groups [5]. In agreement with a previous study [36],copolymerization of NIPAAM with a hydrophilic monomershifts the transition temperature of the resultant copolymer inaqueous solution to a higher temperature. That paper demon-strates a clear effect on the LCSTof longer hydrophilic chains.The temperature at which the first deviation of the scatteredintensity from the baseline occurs is indicated in Fig. 1c. It canbe seen that the value of the transition temperature increasesfrom about 28 °C to 39 °C when PAA is attached to thePNIPAAM microgels. A similar behavior has been reportedfor copolymerization of N-isopropylacrylamide with N,N-dimethylacrylamide in dilute aqueous solution [32].

PNIPAAM microgels with CTAB or SDS

The hydrodynamic radii of the 0.01 wt.% PNIPAAM micro-gels were determined by DLS at various temperatures atdifferent concentrations of CTAB and SDS. The surfactantsbind to the PNIPAAM microgels as single molecules atconcentrations below the cac, which usually is lower thanthe critical micelle concentration (cmc), and at concentra-tions above cac the surfactant binds in the form of aggre-gates, in both the swollen and collapsed state of thePNIPAAM microgels [17, 34]. It is well-known from theliterature that in polymer–surfactant mixtures, CTAB hasmuch lower cmc and cac values than those of SDS [19].The results obtained from the DLS measurements shown inFig. 2a, clearly demonstrate that Rh of PNIPAAM microgelsincreases when the concentration of the cationic surfactantCTAB is raised, due to formation of polymer/surfactantaggregates even at low CTAB concentrations. Since thesurfactant is oppositely charged compared to the weaklycharged PNIPAAM microgels, CTAB will be adsorbed ontothe microgels through attractive electrostatic forces, andhydrophobic interactions between the microgels and the

surfactant may also play a role at elevated temperatures. Inaddition, the electrostatic repulsion forces between the ionicsurfactant molecules inside the microgel particles may leadto swelling of the particles; the increase of Rh with increas-ing level of surfactant addition is ascribed to this effect (cf.Fig. 2a), as well as the shift of the broader transition regionof Rh to higher temperatures. It should be mentioned that itwas not possible to determine the apparent Rh for the PNI-PAAM microgel with 0.05 mm CTAB because of the for-mation of aggregates and an incipient phase separation, evenat fairly low temperatures. The reason for this is attributed tocharge neutralization of the microgels at this level of CTABaddition. The particles are no longer charge stabilized and theenhanced hydrophobicity at elevated temperatures facilitatesflocculation.

The effect of SDS addition to the PNIPAAM microgels isshown in Fig. 2b. At the highest SDS concentration (6 mm),the transition zone of Rh is significantly shifted towardhigher temperatures and the transition region is broader.This indicates that a large amount of SDS is adsorbed tothe microgels, resulting in an augmented charge density ofthe particles, which promotes a shift of the transition regionto higher temperatures because the hydrophobic associativeforces are counteracted by the repulsive Columbic forces. Itshould be noted that since SDS is negatively charged thecharge density increases as SDS is adsorbed to the microgelsand the binding is probably mediated through hydrophobicinteractions between the surfactant and the polymer. At low

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Colloid Polym Sci (2012) 290:931–940 935

concentrations, the surfactants will adsorb to the microgelsin its monomeric form [17, 34]. At this stage, (≤2 mm SDSor 0.25 mm CTAB), there is almost no change in the Rh ofthe particles compared to the microgels without any surfac-tant. However, even at these low surfactant concentrations,there is a broadening of the transition zone in the presence ofsurfactant. This wider transition region is even more evidentat higher surfactant concentrations; this suggests that thecharge density of the microgels is increased and it has beenargued that the bound surfactant molecules act as "defects"in the PNIPAAM network [31–34] .

Figure 3a shows that by adding a small amount of thecationic CTAB to PNIPAAM microgels, which are weaklynegatively charged, a charge reversal occurs. Since a reversingof the charge only occur if the surfactant specifically binds tothe particles, this clearly indicates that CTAB is bond to themicrogels even at very low concentrations [9]. At highersurfactant concentrations, the charge density of PNIPAAMmicrogels further increases to more positive values. The zetapotential of 0.05mmCTAB could not be measured because ofpronounced aggregation when the system is close to chargeneutralization. It is interesting to note that at high levels ofCTAB addition, the zeta potential is virtually independent oftemperature. This behavior reflects that at high CTAB con-centrations, the contribution from the extra charges that arepressed out when the microgels are contracted is negligible.On the other hand, at a low surfactant concentration this effectis significant at elevated temperatures.

Figure 3b reveals that at temperatures well below theLCST, small amounts of added SDS cause the zeta potentialof the particles to approach zero. This suggests that at lowSDS concentration and low temperatures, little amount ofsurfactant is bond to the surfaces of the PNIPAAM micro-gels and it seems that the charges onto the particle surfacesare screened by the counter ions of the surfactant. Aninspection of the data at high temperatures discloses highernegative values of the zeta potential in the presence of smalland moderate levels of SDS addition than for the PNIPAAMmicrogels in the absence of added surfactant. The conjectureis that some charges from the surfactant in the interior of themicrogels are pressed out in connection with the compres-sion of the microgels at high temperatures. As the concen-tration of SDS is increased, the zeta potential becomes morenegative due to binding of the negatively charged surfactantto the microgels. It should be noticed that at the highest SDSconcentration (12 mm), the zeta potential is practically in-dependent of temperature over the considered interval. Thisis attributed to the same effect as discussed above for thePNIPAAM microgels/CTAB systems, namely that at highcharge densities the contribution of the charges pressed outupon contraction of the microgels is insignificant for thetotal charge density.

Figure 4 displays the turbidity changes for 0.01 wt.% ofPNIPAAM microgels in the presence of different concen-trations of CTAB or SDS. The turbidity measurementsclearly indicate that the interaction between PNIPAAM

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936 Colloid Polym Sci (2012) 290:931–940

and SDS is more pronounced than for CTAB at the highertwo surfactant concentrations. This is also reflected in the Rh

values (Fig. 2), which show that addition of more than 2 mmCTAB only has a moderate effect on Rh, whereas a pro-nounced effect is observed for the corresponding concen-trations of SDS. This suggest that while more CTAB isinteracting with the microgels at low surfactant concentra-tions (due to its lower cac value), the interaction between thePNIPAAM microgels and SDS is much stronger at highsurfactant concentrations [19]. The transition zones for PNI-PAAM microgels with moderate and high amount of CTABare located at approximately the same temperature, suggest-ing that the amount of CTAB absorbed on the microgels isarrested at fairly low levels of CTAB addition; therefore, thelocation of the transition zone is not affected by furthersurfactant addition At a very low CTAB concentration(0.05 mm), the transition zone is shifted towards a lowertemperature, and a macroscopic phase separation occurs athigher temperatures. As mentioned earlier, this effect is dueto charge neutralization by oppositely charged surfactantmolecules, which promotes the formation of aggregates.This type of behavior has been reported previously foraqueous solutions of PNIPAAM nanogels with oppositelycharged surfactants [9]. For the PNIPAAM microgels withSDS, the transition temperature is shifted towards highertemperatures when the concentration of SDS is increased(Fig. 4b). This trend suggests that a higher SDS concentra-tion promotes more surfactant to bind to the microgels; thisbehavior is consistent with the DLS results (see Fig. 2b). Asa result, a higher temperature is needed to induce aggrega-tion through hydrophobic associations. At these conditions,the PNIPAAM microgels have high negative values of thezeta potential (Fig. 3b) and the contraction due to enhancedhydrophobicity at elevated temperatures is suppressed bythe repulsive electrostatic forces caused by the SDS adsorp-tion to the particles.

PNIPAAM–co-PAA microgels with CTAB and SDS

According to the Derjaguin and Landau, Verwey and Over-beek theory, the colloid stability of a highly electrostaticallystabilized suspension is reduced when the surface chargedensity is decreased and when the ionic strength of themedium is increased [37, 38]. The apparent hydrodynamicradii of 0.01 wt.% of the negatively charged PNIPAAM–co-PAA microgels with different amounts of the positivelycharged CTAB and the negatively charged SDS are dis-played in Fig. 5. Again, a temperature-induced compressionof the microgels is observed. In this case, the attachment ofPAA groups to the microgels yields high charge density ofthe microgels (see Fig. 6a). Since CTAB is oppositelycharged compared to the PNIPAAM–co-PAA microgels,initial addition of surfactant will lead to neutralization of

the charges on the microgel surfaces and the microgels willshrink. However, after charge reversal at higher levels ofsurfactant addition, the particles will be positively chargedbut the swelling is much less pronounced than for the micro-gels with attached acrylic acid groups. This indicates thatthe adsorbed surfactant is less efficient to generate swellingthan the acrylic acid groups. This finding is also observedfor other charged hydrogels which contain hydrophobicgroups when they are mixed with oppositely charged sur-factants [39].

When a surfactant (cationic or anionic) is added, the ionicstrength of the suspension will gradually increase, therebyscreening the electrostatic repulsion between the chargedPAA groups, causing the particles to shrink. However, sincethe addition of CTAB or SDS to a suspension of PNI-PAAM–co-PAA particles has a similar contraction effecton the microgels, it is likely that both the positive end ofthe CTAB molecules as well as positive counter ions to SDSwill interact strongly with the negatively charged PAAgroups on the particle surfaces. The impact of this effect isfurther accentuated in the results presented below.

In general, the most important force between the chargedmicrogel and an oppositely charged surfactant is the elec-trostatic attraction, which is reinforced by a cooperativeaggregation of bond surfactant molecules [40, 41]. In addi-tion, the tails of the surfactants may associate with thePNIPAAM parts of the microgels by hydrophobic interac-tion with the isopropyl groups. Adding more CTAB will

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Colloid Polym Sci (2012) 290:931–940 937

lead to higher positive values of the zeta potential of themicrogels (see Figure 6a). The zeta potential results forPNIPAAM–co-PAA microgels with CTAB clearly demon-strate that at surfactant concentrations of 1 mm and above,the zeta potential is only weakly dependent of temperature.This finding supports the hypothesis that the chargespressed out by compression of the microgels at highersurfactant concentrations have little impact on the totalcharge density.

Addition of SDS will increase the ionic strength of thesolvent; and as mentioned above, the counterions will playan important role in the partial screening of the internalcharges of the microgels. As a result, Rh decreases(Figure 5b) upon addition of SDS at low temperatures, andthe zeta potential of the microparticles become less negative(Figure 6b). The most compact particles are obtained at1 mm SDS; this surfactant concentration also produces thelowest absolute value of the zeta potential. At high SDSconcentrations, a significant amount of the surfactant bindsto the microgels through hydrophobic interactions, and thezeta potentials assume larger negative values. This causes areswelling of the microgels, and higher values of Rh atmoderate temperatures.

Figure 7 displays the turbidity changes for 0.01 wt.% ofPNIPAAM–co-PAA microgels in the presence of differentconcentrations of CTAB or SDS. It is evident that the micro-gels with PAA need a higher concentration of CTAB to

exhibit charge neutralization than the PNIPAAM microgelwithout PAA, as a result of the higher zeta potential of thePNIPAAM–co-PAA microgels. At 0.25 mm CTAB concen-tration, the PNIPAAM–co-PAA microgels exhibit chargeneutralization (see Fig. 6a), which causes phase separation,while the pure PNIPAAM microgels were neutralized al-ready at 0.05 mm CTAB. Figure 7a also shows that theturbidity of PNIPAAM–co-PAA with different levels ofCTAB addition is higher than for the PNIPAAM–co-PAAmicrogels without added surfactant due to the formation ofmore compact particles. This observation is also confirmedby DLS measurements (Fig. 5a). The transition zone of theturbidity of 0.01 wt.% PNIPAAM–co-PAA microgels withdifferent levels of SDS addition is shifted toward lower orhigher values depending on the surfactant concentration(Fig. 7b). At low SDS concentrations, the zeta potentialassumes less negative values than for the correspondingsystem without SDS (cf. Fig. 6b); this behavior suggeststhat charges from acrylic acid groups in the microgels arescreened and the surfactant acts as a salt. This is expected toshift the transition temperature to a lower value. At moder-ate and high SDS concentrations, the transition zone isdisplaced toward higher temperatures because of augmenta-tion of the charge density as more SDS is attached to themicrogels by hydrophobic interactions. Solubilization ofsome hydrophobic groups may also take place at high levelsof SDS addition. This leads to higher charge density and to a

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938 Colloid Polym Sci (2012) 290:931–940

decreased hydrophobicity, thereby shifting the transition to ahigher temperature.

Summary and conclusions

The results obtained from DLS, zeta potential, and turbiditymeasurements show that PNIPAAM microgels copolymer-ized with the anionic PAA exhibit notably different propertiesfrom the corresponding microgels without attached acrylicacid groups. The LCST values of the PNIPAAM–co-PAAmicrogels are shifted toward higher temperatures than thoseof the corresponding PNIPAAM microgels because of thehigher charge density of the former microgels. The overallbehavior of Rh of PNIPAAM and PNIPAAM–co-PAA micro-gels at various levels of CTAB or SDS addition at differenttemperatures can be summarized in the following way(Fig. 8). For the PNIPAAM microgels, Rh increases by in-creasing the concentration of the surfactant (for both surfac-tants) at all temperatures. The increase of Rh with surfactantaddition is caused by swelling of the microgels due to therepulsive electrostatic interactions between surfactant moie-ties bound to the microgels. This effect is more pronouncedfor SDS than CTAB. For the PNIPAAM microgels withCTAB, Rh is leveling out at concentrations above 2 mm, whilein the presence of SDS Rh increases continuously over theconsidered surfactant range considered here.

For PNIPAAM–co-PAA, on the other hand, the generalpicture is that Rh goes through a minimum when the surfac-tant concentration (both for SDS and CTAB) is increased. Itis interesting to note that the general pattern of behavior isvery similar upon addition of both SDS and CTAB. The

surfactants increase the ionic strength of the suspension, andat low surfactant concentrations, this will screen the electro-static repulsions between the PAA groups. Since this effectis manifested in the presence of both SDS and CTAB(Fig 8c,d) and only for microgels with attached PAA groups,the features are attributed to electrostatic interactions. Inview of this, our hypothesis is that counterions contributeto the screening of the repulsive forces inside the microgelsat low surfactant concentrations. For both surfactants, theminimum of the hydrodynamic radius seems to be locatedaround 1 mm. The minimum is more pronounced as thetemperature is raised because of stronger hydrophobic com-pression the PNIPAAM particles at high temperatures. Inaddition, CTAB will bind to the oppositely charged microgelsby electrostatic interactions, causing a charge neutralization ofthe microgels.

At higher surfactant concentrations, there is a rechargingof the microgels due to binding of more surfactant mole-cules to the microgels; generating more charges in theinterior of the microgels. This causes a reswelling of themicrogels and correspondingly higher values of Rh. Bothwith SDS and CTAB, the apparent hydrodynamic radius ofthe microgels decreases by increasing the temperature due todisruption of the hydrogen bonds between PNIPAAM andwater, which causes an enhancement of the hydrophobicityof the microgels.

In conclusion, we have demonstrated how the addition ofcontrolled amounts of ionic surfactants to PNIPAAMmicrogelscan be used to drastically modify the temperature response ofsuch systems, and moreover how the incorporation of chargedgroups (PAA) into these microgels opens up the possibility tofurther tune this behavior.

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Colloid Polym Sci (2012) 290:931–940 939

Acknowledgments We gratefully acknowledge the financial supportfrom the Norwegian Research Council for projects 177665/V50 and190403.

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