Journal of Colloid and Interface Science 426 (2014) 300–307

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Journal of Colloid and Interface Science

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Thermally sensitive reversible microgels formedby poly(N-Isopropylacrylamide) charged chains:A Hofmeister effect study

http://dx.doi.org/10.1016/j.jcis.2014.04.0200021-9797/� 2014 Elsevier Inc. All rights reserved.

⇑ Corresponding author. Fax: +34 958 243214.E-mail address: dbastos@ugr.es (D. Bastos-González).

Teresa López-León a, Juan L. Ortega-Vinuesa b, Delfi Bastos-González b,⇑, Abdelhamid Elaissari c,d

a EC2M, UMR Gulliver CNRS-ESPCI 7083 – 10 Rue Vauquelin, F-75231 Paris Cedex 05, Franceb Biocolloid and Fluid Physics Group, Department of Applied Physics, University of Granada, Av. Fuentenueva S/N, 18071 Granada, Spainc University of Lyon, F-69622 Lyon, Franced University of Lyon-1, Villeurbanne, CNRS, (UMR 5007), LAGEP-CPE-308G, 43 bd. du 11 Nov. 1918, F-69622 Villeurbanne, France

a r t i c l e i n f o a b s t r a c t

Article history:Received 4 February 2014Accepted 10 April 2014Available online 18 April 2014

Keywords:MicrogelsHofmeister effectsElectrokinetic behavior

In this study, we present a new method to obtain anionic and cationic stable colloidal nanogels fromPNIPAM charged chains. The stability of the particles formed by inter-chain aggregation stems fromthe charged chemical groups attached at the sides of PNIPAM polymer chains. The particle formationis fully reversible—that is, it is possible to change from stable polymer solutions to stable colloidal disper-sions and vice versa simply by varying temperature. In addition, we also demonstrate that the polymerLCST (lower critical solution temperature), the final particle size and the electrokinetic behavior of theparticles formed are highly dependent on the electrolyte nature and salt concentration. These latterresults are related to Hofmeister effects. The analysis of these results provides more insights about theorigin of this ionic specificity, confirming that the interaction of ions with interfaces is dominated bythe chaotropic/kosmotropic character of the ions and the hydrophobic/hydrophilic character of thesurface in solution.

� 2014 Elsevier Inc. All rights reserved.

1. Introduction interactions, leading to a polymer-chain contraction as the number

Poly(N-Isopropylacrylamide) (hereafter called PNIPAM) hasbeen extensively studied in the last two decades [1–18]. The greatinterest in this polymer is due to its extraordinary properties ofsolvency, which are highly dependent on the solvent characteris-tics such as salt concentration, pH, and especially, temperature[19–22]. It is well known that single polymer chains of PNIPAMdissolved in water undergo a sharp collapse transition from ahighly hydrated extended coil into a compact globule whentemperature is increased over a critical point, usually called lowercritical solution temperature or LCST, which is around 31–34 �C forPNIPAM [23]. This peculiar behavior is due to the presence of bothhydrophilic (amide groups) and hydrophobic (isopropyl groups)moieties in the NIPAM molecule. At room temperature, waterbehaves as a good solvent through hydrogen bonding with theamide groups. Upon heating, the water–amide hydrogen bondsget increasingly disrupted by thermal energy, causing the waterto act as a poorer solvent. Above the LSCT, the monomer–monomerinteractions become stronger than the monomer–solvent

of monomer–monomer contacts increases [24]. For unchargedPNIPAM chains, increasing the temperature over the LSCT usuallyleads to a phase separation between PNIPAM and water. In thiswork, we show that a completely different scenario arises whenattaching charged groups at the ends of the PNIPAM chains. Thesecharged groups provide certain amphiphilic character to thePNIPAM chains, which aggregate into configurations where thecharged end-groups are exposed toward the aqueous continuousphase. This leads to the formation of particles with a certainsurface charge density, which is eventually capable of stabilizingthe growing of the particles and preventing complete phase sepa-ration. We show that the particles formed with this method arereversible; that is, simply by varying temperature, it is possibleto transform a stable polymeric solution into a monodisperse col-loidal suspension and vice versa.

This novel PNIPAM-based system offers an interesting arena tostudy ion-specific effects or Hofmeister effects. It is widely knownthat different ions can specifically modify a broad range of interfa-cial phenomena from surface tensions to colloidal stability bymeans of ion accumulation or exclusion from the interfaces thatcannot be explained simply by considering electrostatic interac-tions [25–29]. Regardless of the property studied, and with very

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ical

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sity

(a.u

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T. López-León et al. / Journal of Colloid and Interface Science 426 (2014) 300–307 301

few exceptions, cations and anions consistently order themselvesin the same sequence, called the Hofmeister series [25,26]. In thecase of anions, a representative series would be:

CO2�3 > SO2�

4 > PO4H2� > F� > CH3COO� > Cl� > Br� > I� > NO�3> ClO�4 > SCN�

The species on the left are referred to as kosmotropes/structure-makers/strongly hydrated/salting-out ions while those on the rightare called chaotropes/structure-breakers/weakly hydrated/salting-in ions. Cl- is usually considered to be an indifferent ion or a refer-ence point in the Hofmeister series. The terms kosmotropic andchaotropic have been traditionally associated with the effect ofions on the structure of water, a point which is controversial[30]. Nevertheless, these terms are customarily used when Hof-meister effects are studied, referring to the degree of hydrationof the ions [6,31]. We will also use in this study the terms chao-trope or kosmotropic, meaning exclusively poorly hydrated orhighly hydrate ions, respectively. Several mechanisms have beenproposed in the last few years to explain Hofmeister effects. Collinsattributes this origin to pairing between ions and charged interfa-cial groups with similar solvation-free energies [32]. This mecha-nism is known as the law of matching water affinities. On theother hand, some experimental works support the contention thationic specificity effects depend crucially on the nature of the inter-faces involved. These works show that the relative position of theions in the Hofmeister series can be altered or even completelyinverted as a function of the degree of hydrophobic/hydrophiliccharacter of the surfaces. The mechanism underlying Hofmeistereffects seems to be related to the structure of the water aroundboth the ions and the surfaces [29,33]. A recent paper showingexperimental and simulation results has been demonstrated thatthe specific interaction of ions with surfaces is dominated by solva-tion thermodynamics, i.e. the kosmotropic/chaotropic character ofthe ions and the hydrophobic/hydrophilic of the surfaces [34]. Inaddition, Schwierz el al. have demonstrated, using solvent molecu-lar dynamic simulation, that Hofmeister ordering for halide anionscan be altered depending on the charge and polarity of the surface[35,36]. Finally, a recent theoretical approach predicts Hofmeistereffects observed in surface tensions and colloidal stability basedon the polarizability and size of the ions [31,37].

In this paper, we use a new type of charged PNIPAM chains toinvestigate the mechanisms mentioned above. The paper has beendivided into two parts. The first one deals with the solubility–insol-ubility of the PNIPAM chains as a function of temperature in pres-ence of different salts (Na2SO4, NaCl, NaNO3, and NaSCN). Atheoretical model proposed by Cremer and coworkers has beenapplied to our results in order to elucidate the molecular-levelmechanism for the influence of Hofmeister ions on the hydropho-bic collapse of PNIPAM chains [6,38,39]. The second part of thepaper examines the properties of the stable colloidal particles thatappear when PNIPAM chains collapse and aggregate. Average size,colloidal stability, and electrophoretic mobility data have beenanalyzed in order to gain information on the mechanisms control-ling Hofmeister effects. The analysis of the data in this PNIPAMchain-colloidal particle ‘‘reversible’’ system has enabled us to con-firm that Hofmeister effects depend strongly on the hydrationdegree of the interfaces and the ions interacting with them.

24 26 28 30 32 34 36 38 40

0,0

Temperature (ºC)

Fig. 1. Optical density of cationic PNIPAM solutions as a function of temperature inpresence of different NaCl concentrations. ( ) 10 mM; ( ) 50 mM; ( ) 100 mM;( ) 250 mM; and ( ) 500 mM.

2. Experimental section

2.1. Materials

N-Isopropylacrylamide (NIPAM from Kodak) was purified byrecrystallization from a 60/40 (v/v) toluene/pentane mixture.

2,20-azobis(2-amidinopropane) dihydrochloride (V50) from Wako,was recrystallized from a 50/50 (v/v) acetone/water mixture andpotassium persulfate (KPS) from Prolabo, was used as received.NaCl, NaNO3, NaSCN, and Na2SO4, salts were of analytical gradeand purchased from different firms: Merck, Sigma, and Scharlau.Water was of Milli-Q quality.

2.2. Synthesis of charged PNIPAM chains

Anionic and cationic poly(NIPAM) polymers were preparedusing batch radical polymerization of NIPAM (1.38 g), and KPS(33 mg) or V50 (33 mg), which were used as an anionic initiatorand a cationic initiator, respectively, as adapted from the followingRefs. [9,10,18,40]. It is important to note that the charge of ourPNIPAM chains comes exclusively from the initiator, and therefore,our polymers are composed by a neutral backbone chain and twocharged side groups. This structure is essentially different fromthe one resulting when a charged co-monomer is used in the syn-thesis, which leads to a charge that is homogeneously distributedalong the polymer chain [9,41].

Polymerizations were performed in 50 ml of deionized watercontained in a 100-ml reactor for 12 h at 70 �C under constant stir-ring (200 rpm) and nitrogen stream. The resulting polymers wereused as such after dilution in deionized water. The polymer solu-tion used was intentionally very polydisperse in terms of molecu-lar weight, as largely reported in the case of conventional radicalpolymerization [42,43]. Indeed, our goal was to obtain particles,through PNIPAM self-association, similar to those particles pro-duced by conventional synthesis methods, which are made of verypolydispersed PNIPAM subchains connected by crosslinking points[10,11,18,40]. All the experiments were performed at a constantpolymer concentration equal to 0.05 mg/ml.

2.3. Characterization

The lower critical-solution temperature (LCST) of the polymersolutions was determined by using an UnikonxL spectrophotome-ter equipped with a thermosystem from Serbolabo Technologies(France). Solubility was evaluated by monitoring changes in theoptical density (O.D.) of the samples. The LCST values were takenas the initial break points of these O.D. vs. temperature curves(as shown in Fig. 1). The diameter and the electrophoretic mobilityof the nanoparticles were investigated by means of dynamic light

24

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28

30

32

34

36(a)

LCST

(ºC

)

302 T. López-León et al. / Journal of Colloid and Interface Science 426 (2014) 300–307

scattering (DLS) measurements using a Zetasizer 3000 HS fromMalvern Instruments (Malvern UK). The particle size, determinedby the cumulant analysis of the measured intensity versus timecorrelation function, resulted in the translational diffusion coeffi-cient (D) distribution G(D), from which the hydrodynamic (Rh) sizedistribution f(Rh) could be calculated by using the Stokes Einsteinequation:

D ¼ kT3pgDh

ð1Þ

where kB, g and T are the Boltzman constant, the solution viscosity,and the absolute temperature, respectively.

0,0 0,2 0,4 0,6 0,8 1,020

22

Salt Concentration (mM)

0,0 0,2 0,4 0,6 0,8 1,020

22

24

26

28

30

32

34

36(b)

LCST

(ºC

)

Salt Concentration (M)

Fig. 2. LCST of the (a) anionic and (b) cationic PNIPAM solutions in presence ofdifferent salt concentrations. ( ) NaSCN; ( ) NaNO3; ( ) NaCl; and ( ) Na2SO4.

3. Results and discussion

Four anions with different Hofmeister characters were chosenfor this study, namely SO4

2� as kosmotropic, Cl� as indifferent,NO3� and SCN� as chaotropes. Na+ is considered an indifferent cat-

ion in the Hofmeister series. For this reason, sodium salts wereused (NaSCN, NaNO3, NaCl, Na2SO4). As all of these share the samecation, the differences observed in our experiments must be causedby the specific effects produced by the anions. Moreover, as thestudy includes both positive and negative PNIPAM, it was feasibleto analyze differences when the anions acted either as counter- oras co-ions.

3.1. Charged-PNIPAM polymer solutions

The first part of the present work deals with polymeric solu-tions. Attention will be paid especially to how the electrolytesmodify the solvency properties of PNIPAM in aqueous solutions.In this case, polymer–polymer, polymer–water, polymer–ion, andwater–ion interactions must be considered. Although the temper-ature is the major factor governing the solubility of PNIPAM, thepresence of salts also has effects. Fig. 1 shows the phase transitionundergone by the cationic PNIPAM chains with temperature at dif-ferent NaCl concentrations. The presence of salt shifts the LCST tolower values, this effect becoming more pronounced as the ionicstrength increases. This feature, common to all the electrolytesstudied, has been ascribed to the ability of anions to disrupt poly-mer–water hydrogen bonds [44–46]. Similar patterns were alsoobserved with the anionic sample (figure not shown). It bears not-ing, however, that cationic and anionic pools in pure water showeda slight divergence in the LCST values (33 �C and 34 �C respec-tively), which probably come from differences in the initiatorsused. The KPS employed in the synthesis of the anionic pool givesrise to sulfate groups, more hydrophilic than those resulting fromthe cationic initiator (AEMH), which holds apolar sites as well asionic groups. Therefore, KPS intensifies the global hydrophilic char-acter of PNIPAM and shifts the LCST to a slightly higher tempera-ture compared with that observed with AEMH.10 Nevertheless, itwill be shown below that this difference vanishes as the salt effectincreases.

The specific effects of anions were then analyzed. The LCST ofPNIPAM aqueous solutions as a function of salt concentrationand anion type are plotted in Fig. 2a for the anionic sample, and2b for the cationic one. Great differences between the differentanions were found. For a given salt concentration (i.e. 0.2 M), theLCST values order the anions according to the Hofmeister series,SCN� > NO3

� > Cl� > SO42�, so that species with more accentuated

kosmotropic nature, that is more hydrated anions, provide lowerLCST values. In other words, the destabilizing effect of saltincreases with the kosmotropic character of the anion used. Theseresults can be explained according to a microscopic mechanismbased on ion-induced changes in water structure. Hydration of

electrolytes in solution induces PNIPAM–H2O hydrogen bondingdisruption, and the consequent desolvation of the carbonyl andamide groups of the polymer. As a result, the dehydrated chainscollapse and aggregate through intra- and interchain hydrophobicinteractions, which become dominant above the LCST. The morehydrated the anion, the higher the competition for the moleculeshydrating PNIPAM, the disruption of the amide–water hydrogenbonds being more effective for kosmotropic anions than for chao-tropic ones. Our results are consistent with those reported by otherauthors working with PNIPAM microgels, or neutral PNIPAM mac-romolecules [38,47,48].

To elucidate whether the mechanism proposed by Zhang et al.to explain the hydrophobic collapse of PNIPAM also works in ourcharged chains, we have applied their theory to our experimentalresults [6,38,39]. This theory considers different molecular mecha-nisms of interaction between the PNIPAM and the anions depend-ing on their kosmotropic or chaotropic character. Fig. 2a,b shows alinear dependence of the PNIPAM LCST on salt concentration. Thelinearity is very clear for the kosmotropic anion (sulfate) and pro-gressively disappears as the ion nature becomes more chaotropic.Zhang et al., working with a non-ionic PNIPAM system, modeledthe changes in the LCST (DT) caused by adding salt by the followingequation:

DT ¼ c½M� þ Bmaxk½M�1þ k½M� ¼ DT linear þ DTLangmuir isotherm ð2Þ

T. López-León et al. / Journal of Colloid and Interface Science 426 (2014) 300–307 303

The equation includes a linear term and a Langmuir isothermcontribution. In this expression [M] is the molar concentration ofsalt. The Langmuir term contains two constants: Bmax representsthe maximum LCST increase at saturation ion binding while k isthe binding constant of the anion to the polymer. The constant cappearing in the linear term has different interpretations depend-ing on the kosmotropic/chaotropic character of the ion. Accordingto Zhang’s model, c is directly related to the hydration entropy ofthe ion for kosmotropic ions, while it is related to changes in theinterfacial tension (Dc) at the water/hydrophobic interface forchaotropes. Additionally, these authors proposed an extension tothis equation, which incorporates electrostatic interactions ofcharged systems.

-30

-20

-10

0

/mol

)

SCN-

NO3-

Cl-

DT ¼ c½M� þ Bmaxk½M�e�b½M�

1þ k½M�e�b½M� ð3Þ

The exponential term (e�b[M]) accounts for electrostatic interac-tions between a charged surface and ions in solution. The authorsused this last equation to evaluate the cloud-point temperature ofpositive lysozyme solutions. In this case, parameter b is related tothe surface potential of this protein, and then, Bmax and b are bothmeasures of the effectiveness for a specific anion to associate withthe positive charges located on the protein surface.

The dashed lines in Fig. 2a and b result from fitting Eq. (3) to theexperimental data, using c, Bmax, k and b as fitting parameters.Table 1 summarizes the values of the parameters obtained fromthis fitting. The best fit was achieved when b was almost zero,for positive and negative PNIPAM chains. This means our PNIPAMcharged chains behave more similar to neutral PNIPAM chains thanpositively charged lysozyme. This can be explained consideringthat the PNIPAM chains that we use differ very much from whatmight be considered as a ‘‘charged particle’’, as is the case of posi-tive lysozyme. Indeed, below the LCTS, our PNIPAM chains areuncharged linear polymers with two punctual charges attachedat the extreme of the chains, and thus they are essentially differentfrom ‘‘charged particles’’ with a given surface potential. In addition,the values calculated for both anionic and cationic particles arevery similar, confirming that the type of charge present in thesePNIPAM systems does not influence the ionic specificity observedand validates the use of Eq. (2) to study salt effects in the LCST ofour systems.

As can be seen, in Table 1 the linear term is dominant for thekosmotropic anion whereas the non-linear term becomes impor-tant as the anions are more chaotropic. From these data we alsofind that Cl� shows certain chaotropic character. In our case, allthe electrolytes analyzed except the SO4

2� are chaotropes. There-fore, the linear term (c[M]) would be related to changes in theinterfacial tension at the PNIPAM/water interface while the Lang-muir isotherm would be the result of specific ion binding to theamide group of PNIPAM. Both terms thus have opposite effects.An increase in the interfacial tension (Dc) entails a lower solubilityof the polymer that will tend to reduce its interfacial area by col-lapsing. On the contrary, ion adsorption will promote polymer

Table 1Fitted values for c, Bmax and k from the LCST data corresponding to the anionic andcationic PNIPAM sample.

Anion c (�C/mol) Bmax (�C) k (M�1)

Panionic Pcationic Panionic Pcationic Panionic Pcationic

SO42� �62.4 �63.8 – – – –

Cl� �12.8 �12.3 0.6 0.5 1.1 1.3NO3� �8.6 �7.8 2.3 2.3 3.0 3.2

SCN� �4.0 �4.1 3.0 3.6 4.2 4.3

expansion due to electrostatic repulsions. This explains the oppo-site sign of the fitting parameters shown in Table 1.

Since at a hydrophobic/water interface the interfacial tensionincreases proportionally to the salt concentration [25]:

Dc ¼ r½M� ð4Þ

Consequently, the c parameter and the r one (r = dDc/d[M])might be related. In fact, Zhang et al., when working with chao-tropic ions, do find a direct proportionality between them. Thesame linear correlation was revealed when plotting the c valuesfrom our experimental data vs. the corresponding r values [38],as shown in Fig. 3. It is worthwhile recalling that this interpreta-tion for the linear term works only for chaotropic and null ions,but not for kosmotropes (i.e. SO4

2�). Hence, although only threeexperimental points are available with each sample (those corre-sponding to SCN�, NO3

� and Cl�), they suggest the linear depen-dence noted by Zhang et al. with non-charged PNIPAM. Asmentioned above, the results do not depend on the PNIPAM chainscharge, since no significant differences were detected betweenanionic and cationic particles. Hence, the analysis of the experi-mental and theoretical results clearly suggests that the chargedgroups are not involved in the PNIPAM solubility changes whenthe salt is added. The following results also point out in the samedirection.

4. Microparticles formed by self-assembly of PNIPAM

Solvency properties of charged PNIPAM macromolecules haveallowed us to make comparisons with previous works related withuncharged PNIPAM macromolecules. However, the interest inworking with charged polymers is due mainly to their ability toform stable colloidal systems beyond the LCST. These particlesmay enable us to analyze ion effects from another standpoint.

4.1. Particle formation

The particle formation resulting from increasing the tempera-ture was analyzed by monitoring the hydrodynamic diameter(Dh) evolution. Measurements were made by progressively heatingthe sample in the presence of a constant salt concentration(10 mM). Fig. 4 shows these data for both PNIPAM samples. Anio-nic particles will first be analyzed. All the curves in Fig. 4a exhibitthe same trend: at low temperatures the average hydrodynamic

0,0 0,5 1,0 1,5 2,0 2,5 3,0-70

-60

-50

-40

SO42-

σ (μNm2/mol)

c (º

C

Fig. 3. The c parameter of Eq. (1) (and/or 2) vs. r parameter of Eq. (3) for anionic ( )and cationic PNIPAM ( ).

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250

500

750

1000

1250

1500(a)

D h (nm

)

Temperature (ºC)

25 30 35 40 450

500

1000

1500

2000

2500(b)

D h (nm

)

Temperature (ºC)

Fig. 4. Diameter of (a) anionic and (b) cationic PNIPAM particles formed byincreasing the temperature in: (h) no salt added, ( ) 10 mM NaSCN, ( ) 10 mMNaNO3, ( ) 10 mM NaCl, and ( ) 10 mM Na2SO4 solutions.

304 T. López-León et al. / Journal of Colloid and Interface Science 426 (2014) 300–307

diameter keeps constant until the phase transition temperature isreached (33–34 �C). The low light intensity scattered by the systemin this region reflects the existence of single PNIPAM macromole-cules. Water behaves as a good solvent and the polymer presentsa well-hydrated coil configuration within this temperature range.It bears noting that, in this case, diameter values must be consid-ered simply as indicative, since this region deals only with non-spherical coil-conformations. Then the average diameter beginsto increase, and at a certain temperature Dh stops growing, where-upon the clusters achieve their maximum size, which is about200 nm in a salt-free solution. The stabilization of the system athigh temperatures has its origin in the repulsive electrostaticforces. The PNIPAM chains have charged groups at their endscoming from the initiator used in the synthesis. Hence, the result-ing precursor particles have a certain surface charge, which isenhanced as the interchain association occurs. It is this charge thatprevents further interaggregate association. Increasing tempera-ture over this critical temperature of stabilization provokes a slightdecrease in the particle size; see maximum in Fig. 4a, due to thecontraction of PNIPAM with temperature, which makes particlesbecome more and more compact. At higher temperatures, the par-ticle size eventually reaches a plateau that corresponds to the statewhere the particle is fully compact. In the steady state, monodis-perse kinetically stable colloidal particles are obtained instead ofa complete phase separation.

Although all the curves appearing in Fig. 4a show the sametrend, clear differences between the different electrolytes areobserved. Ion specificity leads to large quantitative differences,even at relatively low salt concentrations (10 mM), especiallyregarding the final particle size. If ions are ranked according tothe Dh values found once particles reach kinetic stability, thefollowing series will result:

SCN� < NO�3 < Cl� 6 SO�24

which coincides exactly with the Hofmeister series. It is worthremarking that the hydrodynamic diameter determined in salt-freesolutions is always smaller than in the presence of any salt. In solu-tions of extremely low ionic strength, the repulsive electric barrierbetween particles – resulting from the polymer charged groups – isrelatively strong and thus interchain aggregates stabilize even atlow surface-charge densities. As a result, the aggregation processrapidly stops and stable nanospheres with small diameters result.The presence of salt in solution reduces this repulsive electric bar-rier, basically because of the screening provoked by counter-ionson the surface charge of the particles. Hence, particles continuegrowing until they reach a stable configuration at which potentialbarrier is strong enough to prevent aggregation. Results can bereadily understood through an ion-accumulation mechanism onhydrophobic surfaces. More chaotropic anions will tend to accumu-late more strongly as the surface becomes more hydrophobic. Cer-tainly, the screening caused by the counter-ions, Na+ ions for theanionic system, can be partially balanced if any specific adsorptionof anions occurs. In this case, particles with different diameterswould form, owing to the different nature of the anions employed(even if all of them have the same valence). Particularly, it wouldbe expected that if the anion accumulation increases, the finalparticle size will decrease. The exclusion of the kosmotropic anionSO4

2� from the interface enhances the counter-ion screening andleads to lower repulsive interparticle potentials. However, on thebasis of this mechanism, we would expect clearer differencesbetween Cl� and SO4

2�. We will show next that these differencesbecome more evident when mobility measurements are taken.

Results with the cationic PNIPAM particles (Fig. 4b) also supportthis accumulation mechanism. The positive PNIPAM showed a pat-tern similar to the anionic one with the difference that colloidalstable particles formed only with the SCN- and the salt-free solu-tions. In this case, anions acted as counter-ions, so the screeningof the surface charge by the different anions was enhanced produc-ing a destabilizing effect. For this reason, no stable particles formedwith Cl� and NO3

� anions, and the double valence of SO42� also

increased the effectiveness of the surface screening, reflected in amore destabilizing effect. However, the formation of stableparticles with the SCN� could be explained if the accumulation ofthis chaotropic anion had been high enough to cause an inversionof the surface-charge density of cationic PNIPAM particles. Thishypothesis was confirmed by measuring the electrophoreticmobility of the particles, as will be shown below.

4.2. Electrophoretic mobility

Valuable insights into the mechanisms responsible for particleformation can be gained from electrophoretic mobility (le) mea-surements, particularly phenomena involving ion accumulation,since le is an indirect measurement of the charge state close tothe particle surface. The influence of the ionic strength on le wasfirst studied. For this task, NaCl, frequently considered as a refer-ence salt, was chosen. Fig. 5 displays the temperature dependenceof le for both anionic and cationic PNIPAM samples at two NaClconcentrations. Although curves present a shape similar to thattypically found with PNIPAM microgels [5,7,10], their origin is

25 30 35 40 45-5

-4

-3

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-1

0

1

2

3

4

5

μ e (10

-8 m

2 V-1 s

-1)

Temperature (ºC)

Fig. 5. Electrophoretic mobility vs. temperature for the anionic ( ) and cationic ( )PNIPAM. Blank symbols: 1 mM NaCl. Solid symbols: 10 mM NaCl.

20 25 30 35 40 45

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-2,5

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-1,5

-1,0

-0,5

0,0(a)

μ e (10

-8 m

2 V-1 s

-1)

Temperature (ºC)

25 30 35 40 45-2,0

-1,5

-1,0

-0,5

0,0

0,5

1,0

1,5

2,0(b)

μ e (10

-8 m

2 V-1 s

-1)

Temperature (ºC)

Fig. 6. Electrophoretic mobility of (a) anionic and (b) cationic PNIPAM particles as afunction of temperature in saline solutions (10 mM). ( ) NaSCN; ( ) NaNO3; ( )NaCl; and ( ) Na2SO4.

T. López-León et al. / Journal of Colloid and Interface Science 426 (2014) 300–307 305

completely different. With regard to microgels, electrophoreticmobility rises with temperature owing to an increase in thesurface-charge density and a reduction in friction forces whenparticles shrink. The case of PNIPAM polymer solutions is morecomplex. At temperatures below the LCST, le is around zero, essen-tially because there are no particles in solution but slightly chargedpolymers with a random coil configuration. Beyond the LCST,rather spherical aggregates start to form. This self-assembly struc-ture presents a non-negligible surface charge, which strengthens asparticles grow, as reflected by the increase in electrophoreticmobility with temperature. As commonly observed in colloidalsystems, higher salt concentrations imply lower le values causedby a compression of the electrical double layer [49].

On the other hand, Fig. 6 shows the results for the different saltsat a 10-mM concentration. These data evidence the ionic specificityin these systems, and the accumulation mechanism proposed toexplain differences in particles size. Regarding the anionic sample(see Fig. 6a), more chaotropic anions increase the negativeelectrophoretic mobility of the nanospheres as temperature risesin comparison with Cl-, indicating the effectiveness of the ion-accumulation process in each case. This anion accumulationbecomes more important in the sequence SCN� > NO3

� > Cl� > SO42-

�, which confirms the results for size and evidences that SO42� is

more excluded than Cl� from PNIPAM interface on becoming morehydrophobic. The same conclusion can be drawn from curvescorresponding to the positive sample (see Fig. 6b). In this case,however, the accumulations of anions at the particle interface,now acting as counter-ions, involve a reduction in the le of theparticles. Although le values other than zero were found withCl� and SO4

2� anions, their values were not high enough to stabilizethe aggregates. Note that in the case of SO4

2�, despite its doublevalence, and acting as a counter-ion, le values were similar tothose found with Cl�. This feature indicates the exclusion of theSO4

2� from the positive particle surface. More significant is theobserved charge inversion with the most chaotropic anion, SCN�,which indicates a very strong accumulation of this poorly hydratedanion, confirming the aforementioned size results. This importantinversion of charge can explain why stable particles resulted fromthis anion (see Fig. 4b). It is also important to highlight that thischarge inversion occurs at only 10 mM of salt concentration. Wepreviously observed charge inversion with SCN� and PNIPAMmicrogel particles46 and more recently with polystyrene particles[34]. These latter results showed that inversion occurs when thesurface is hydrophobic but not with a hydrophilic one, confirming

that to explain ionic specific effects is essential to consider both thenature of the ions together with the hydrophobic/hydrophilicnature of intervening surfaces. For PNIPAM microgels the inversionalso occurs at low salt concentrations, indicating that these sys-tems are more sensitive to charge inversion, and hence to ionicspecificity, than hard particles. This is probably because the softPNIPAM surface allows the anions to penetrate more deeply,increasing the local concentration of adsorbed anions in this inter-face in comparison with hard surfaces.

Finally, it is usually accepted that Hofmeister effects areapproximately additive over all species in solution [25]. Thisfeature can be readily tested by measuring the electrophoreticmobility to our PNIPAM systems. Fig. 7 compares the mobilityfor anionic and cationic samples as a function of temperature at10 mM concentration of SCN�, SO4

2�, and SCN� + SO42� (5 mM each).

These two anions exhibit opposite trends in the accumulationbehavior. For the negative PNIPAM (see Fig. 7a), anions act asco-ions and an intermediate effect in the mobility of the two saltstogether is observed. However, for the cationic particles (seeFig. 7b) when anions act as counter-ions, the accumulation of SCN�

is more significant than the effect of SO42� and we can even detect a

slight but clear charge inversion. These results reinforce the impor-tance of hydrophobic forces in these kinds of systems and hencetheir relation with Hofmeister effects.

25 30 35 40 45

-3,0

-2,5

-2,0

-1,5

-1,0

-0,5

0,0(a)

Temperature (ºC)

μ e (10

-8 m

2 V-1 s

-1)

25 30 35 40 45-2,0

-1,5

-1,0

-0,5

0,0

0,5

1,0

1,5

2,0(b)

μ e (10

-8 m

2 V-1 s

-1)

Temperature (ºC)

Fig. 7. Electrophoretic mobility of (a) anionic and (b) cationic PNIPAM particles as afunction of temperature. ( ) 10 mM of Na2SO4, ( ) 10 mM of NaSCN; ( ) 5 mM ofNa2SO4 + 5 mM of NaSCN.

306 T. López-León et al. / Journal of Colloid and Interface Science 426 (2014) 300–307

5. Conclusions

The synthesis and properties of a new system based on the self-association of PNIPAM charged chains have been reported. ThesePNIPAM particles have the advantage of being highly versatile: itis possible to turn a polymer solution into a stable colloidal disper-sion simply by tuning the temperature, this process being revers-ible. In addition, the characteristics of the final particles can becontrolled by salt addition.

In the first part of the paper, we investigated specific ioniceffects on the solubility of charged PNIPAM macromolecules. Theresults were independent of the PNIPAM sign of charge—that is,they did not depend on whether anions act as co- or counter-ions.The empirical model proposed by Zhang et al. for neutral PNIPAMmacromolecules appropriately fits our results [6,38]. From bothcationic and anionic PNIPAM systems, very similar parameterswere found, confirming that the ion paring from the law of match-ing water affinities does not account for the ionic specificityobserved. However, our results can be explained considering thatat temperatures lower than LCST, the carbonyl and amide groupsof PNIPAM are well hydrated. Salt addition provokes a competitionfor water molecules between these two groups and the added ions.Highly hydrated anions interact more strongly with water than dopoorly hydrated ones and cause a more effective dehydration ofPNIPAM, resulting in lower values of the LCST for a given salt

concentration. The same trend is found regardless the type ofcharge present on the PNIPAM chains, which indicates that themain interaction of anions with PNIPAM occurs along the backboneof the chains.

The second part of the paper concerns particle-formation pro-cesses. The final particle size is highly dependent on the specificnature of the ion. These results are supported by electrophoreticmeasurements and reveal the existence of specific ion accumula-tion processes at the PNIPAM/water interface [46]. This accumula-tion does not depend on the sign of the surface charge of thePNIPAM particles but rather is directly related to the hydrationdegree of the anions and the hydrophobic character of the PNIPAMsurface. The high sensitivity of these PNIPAM systems to ionicspecificity is further reflected in the inversion of charge observedwith the most chaotropic anion, SCN�, when acting as a counter-ion at just 10 mM, and the corresponding formation of stableparticles. The results of this part can be reasonably explained byconsidering that poorly hydrated ions accumulate on hydrophobicinterfaces.

In summary, when PNIPAM is well hydrated, i.e. when itexhibits hydrophilic character, highly hydrated or kosmotropicanions interact more strongly with it. On the contrary, whenPNIPAM becomes hydrophobic, poorly hydrated or chaotropicanions interact more strongly with it. This occurs independentlyof the type of charge, positive or negative, of PNIPAM, whichexcludes the ion-pairing mechanism as the main interactionbetween the ions and the interface. These results reinforce thestatement that the interaction of ions with interfaces is dominatedby both the chaotropic/kosmotropic character of the ions and thehydrophobic/hydrophilic character of the surfaces [29,33,34], andthey reveal the importance of considering the interaction withthe solvent in water-mediated interfaces of extremely importancesuch as biological and technological processes.

Acknowledgments

The authors wish to thank the financial support granted by theprojects MAT 2012-3670-C04-02, MAT2010-20370 (EuropeanFEDER support included, MICINN, Spain), and CTS-6270 (Junta deAndalucía, Spain).

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