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

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

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Journal of Colloid and Interface Science www.elsevier.com/locate/jcis

Thermally sensitive reversible microgels formed by poly(N-Isopropylacrylamide) charged chains: A Hofmeister effect study 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, France Biocolloid and Fluid Physics Group, Department of Applied Physics, University of Granada, Av. Fuentenueva S/N, 18071 Granada, Spain University of Lyon, F-69622 Lyon, France d University of Lyon-1, Villeurbanne, CNRS, (UMR 5007), LAGEP-CPE-308G, 43 bd. du 11 Nov. 1918, F-69622 Villeurbanne, France b c

a r t i c l e

i n f o

Article history: Received 4 February 2014 Accepted 10 April 2014 Available online 18 April 2014 Keywords: Microgels Hofmeister effects Electrokinetic behavior

a b s t r a c t In this study, we present a new method to obtain anionic and cationic stable colloidal nanogels from PNIPAM charged chains. The stability of the particles formed by inter-chain aggregation stems from the charged chemical groups attached at the sides of PNIPAM polymer chains. The particle formation is fully reversible—that is, it is possible to change from stable polymer solutions to stable colloidal dispersions and vice versa simply by varying temperature. In addition, we also demonstrate that the polymer LCST (lower critical solution temperature), the final particle size and the electrokinetic behavior of the particles formed are highly dependent on the electrolyte nature and salt concentration. These latter results are related to Hofmeister effects. The analysis of these results provides more insights about the origin of this ionic specificity, confirming that the interaction of ions with interfaces is dominated by the chaotropic/kosmotropic character of the ions and the hydrophobic/hydrophilic character of the surface in solution. Ó 2014 Elsevier Inc. All rights reserved.

1. Introduction Poly(N-Isopropylacrylamide) (hereafter called PNIPAM) has been extensively studied in the last two decades [1–18]. The great interest in this polymer is due to its extraordinary properties of solvency, which are highly dependent on the solvent characteristics such as salt concentration, pH, and especially, temperature [19–22]. It is well known that single polymer chains of PNIPAM dissolved in water undergo a sharp collapse transition from a highly hydrated extended coil into a compact globule when temperature is increased over a critical point, usually called lower critical solution temperature or LCST, which is around 31–34 °C for PNIPAM [23]. This peculiar behavior is due to the presence of both hydrophilic (amide groups) and hydrophobic (isopropyl groups) moieties in the NIPAM molecule. At room temperature, water behaves as a good solvent through hydrogen bonding with the amide groups. Upon heating, the water–amide hydrogen bonds get increasingly disrupted by thermal energy, causing the water to act as a poorer solvent. Above the LSCT, the monomer–monomer interactions become stronger than the monomer–solvent ⇑ Corresponding author. Fax: +34 958 243214. E-mail address: [email protected] (D. Bastos-González). http://dx.doi.org/10.1016/j.jcis.2014.04.020 0021-9797/Ó 2014 Elsevier Inc. All rights reserved.

interactions, leading to a polymer-chain contraction as the number of monomer–monomer contacts increases [24]. For uncharged PNIPAM chains, increasing the temperature over the LSCT usually leads to a phase separation between PNIPAM and water. In this work, we show that a completely different scenario arises when attaching charged groups at the ends of the PNIPAM chains. These charged groups provide certain amphiphilic character to the PNIPAM chains, which aggregate into configurations where the charged end-groups are exposed toward the aqueous continuous phase. This leads to the formation of particles with a certain surface charge density, which is eventually capable of stabilizing the growing of the particles and preventing complete phase separation. We show that the particles formed with this method are reversible; that is, simply by varying temperature, it is possible to transform a stable polymeric solution into a monodisperse colloidal suspension and vice versa. This novel PNIPAM-based system offers an interesting arena to study ion-specific effects or Hofmeister effects. It is widely known that different ions can specifically modify a broad range of interfacial phenomena from surface tensions to colloidal stability by means of ion accumulation or exclusion from the interfaces that cannot be explained simply by considering electrostatic interactions [25–29]. Regardless of the property studied, and with very

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2 2 CO2 > F > CH3 COO > Cl > Br > I > NO3 3 > SO4 > PO4 H 

> ClO4 > SCN The species on the left are referred to as kosmotropes/structuremakers/strongly hydrated/salting-out ions while those on the right are called chaotropes/structure-breakers/weakly hydrated/saltingin ions. Cl- is usually considered to be an indifferent ion or a reference point in the Hofmeister series. The terms kosmotropic and chaotropic have been traditionally associated with the effect of ions on the structure of water, a point which is controversial [30]. Nevertheless, these terms are customarily used when Hofmeister effects are studied, referring to the degree of hydration of the ions [6,31]. We will also use in this study the terms chaotrope or kosmotropic, meaning exclusively poorly hydrated or highly hydrate ions, respectively. Several mechanisms have been proposed in the last few years to explain Hofmeister effects. Collins attributes this origin to pairing between ions and charged interfacial groups with similar solvation-free energies [32]. This mechanism is known as the law of matching water affinities. On the other hand, some experimental works support the contention that ionic specificity effects depend crucially on the nature of the interfaces involved. These works show that the relative position of the ions in the Hofmeister series can be altered or even completely inverted as a function of the degree of hydrophobic/hydrophilic character of the surfaces. The mechanism underlying Hofmeister effects seems to be related to the structure of the water around both the ions and the surfaces [29,33]. A recent paper showing experimental and simulation results has been demonstrated that the specific interaction of ions with surfaces is dominated by solvation thermodynamics, i.e. the kosmotropic/chaotropic character of the ions and the hydrophobic/hydrophilic of the surfaces [34]. In addition, Schwierz el al. have demonstrated, using solvent molecular dynamic simulation, that Hofmeister ordering for halide anions can be altered depending on the charge and polarity of the surface [35,36]. Finally, a recent theoretical approach predicts Hofmeister effects observed in surface tensions and colloidal stability based on the polarizability and size of the ions [31,37]. In this paper, we use a new type of charged PNIPAM chains to investigate the mechanisms mentioned above. The paper has been divided into two parts. The first one deals with the solubility–insolubility of the PNIPAM chains as a function of temperature in presence of different salts (Na2SO4, NaCl, NaNO3, and NaSCN). A theoretical model proposed by Cremer and coworkers has been applied to our results in order to elucidate the molecular-level mechanism for the influence of Hofmeister ions on the hydrophobic collapse of PNIPAM chains [6,38,39]. The second part of the paper examines the properties of the stable colloidal particles that appear when PNIPAM chains collapse and aggregate. Average size, colloidal stability, and electrophoretic mobility data have been analyzed in order to gain information on the mechanisms controlling Hofmeister effects. The analysis of the data in this PNIPAM chain-colloidal particle ‘‘reversible’’ system has enabled us to confirm that Hofmeister effects depend strongly on the hydration degree of the interfaces and the ions interacting with them.

2,20 -azobis(2-amidinopropane) dihydrochloride (V50) from Wako, was recrystallized from a 50/50 (v/v) acetone/water mixture and potassium persulfate (KPS) from Prolabo, was used as received. NaCl, NaNO3, NaSCN, and Na2SO4, salts were of analytical grade and 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 prepared using batch radical polymerization of NIPAM (1.38 g), and KPS (33 mg) or V50 (33 mg), which were used as an anionic initiator and a cationic initiator, respectively, as adapted from the following Refs. [9,10,18,40]. It is important to note that the charge of our PNIPAM chains comes exclusively from the initiator, and therefore, our polymers are composed by a neutral backbone chain and two charged side groups. This structure is essentially different from the one resulting when a charged co-monomer is used in the synthesis, which leads to a charge that is homogeneously distributed along the polymer chain [9,41]. Polymerizations were performed in 50 ml of deionized water contained in a 100-ml reactor for 12 h at 70 °C under constant stirring (200 rpm) and nitrogen stream. The resulting polymers were used as such after dilution in deionized water. The polymer solution used was intentionally very polydisperse in terms of molecular weight, as largely reported in the case of conventional radical polymerization [42,43]. Indeed, our goal was to obtain particles, through PNIPAM self-association, similar to those particles produced by conventional synthesis methods, which are made of very polydispersed PNIPAM subchains connected by crosslinking points [10,11,18,40]. All the experiments were performed at a constant polymer concentration equal to 0.05 mg/ml. 2.3. Characterization The lower critical-solution temperature (LCST) of the polymer solutions was determined by using an UnikonxL spectrophotometer equipped with a thermosystem from Serbolabo Technologies (France). Solubility was evaluated by monitoring changes in the optical density (O.D.) of the samples. The LCST values were taken as the initial break points of these O.D. vs. temperature curves (as shown in Fig. 1). The diameter and the electrophoretic mobility of the nanoparticles were investigated by means of dynamic light

1,0 0,8

Optical Density (a.u.)

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

0,6 0,4 0,2 0,0

2. Experimental section 2.1. Materials N-Isopropylacrylamide (NIPAM from Kodak) was purified by recrystallization from a 60/40 (v/v) toluene/pentane mixture.

24

26

28

30

32

34

36

38

40

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

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scattering (DLS) measurements using a Zetasizer 3000 HS from Malvern Instruments (Malvern UK). The particle size, determined by the cumulant analysis of the measured intensity versus time correlation function, resulted in the translational diffusion coefficient (D) distribution G(D), from which the hydrodynamic (Rh) size distribution f(Rh) could be calculated by using the Stokes Einstein equation:

D¼

kT 3pgDh

ð1Þ

36

32 30

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

28 26 24 22

3. Results and discussion

20

3.1. Charged-PNIPAM polymer solutions The first part of the present work deals with polymeric solutions. Attention will be paid especially to how the electrolytes modify the solvency properties of PNIPAM in aqueous solutions. In this case, polymer–polymer, polymer–water, polymer–ion, and water–ion interactions must be considered. Although the temperature is the major factor governing the solubility of PNIPAM, the presence of salts also has effects. Fig. 1 shows the phase transition undergone by the cationic PNIPAM chains with temperature at different NaCl concentrations. The presence of salt shifts the LCST to lower values, this effect becoming more pronounced as the ionic strength increases. This feature, common to all the electrolytes studied, has been ascribed to the ability of anions to disrupt polymer–water hydrogen bonds [44–46]. Similar patterns were also observed with the anionic sample (figure not shown). It bears noting, however, that cationic and anionic pools in pure water showed a slight divergence in the LCST values (33 °C and 34 °C respectively), which probably come from differences in the initiators used. The KPS employed in the synthesis of the anionic pool gives rise to sulfate groups, more hydrophilic than those resulting from the cationic initiator (AEMH), which holds apolar sites as well as ionic groups. Therefore, KPS intensifies the global hydrophilic character of PNIPAM and shifts the LCST to a slightly higher temperature compared with that observed with AEMH.10 Nevertheless, it will be shown below that this difference vanishes as the salt effect increases. The specific effects of anions were then analyzed. The LCST of PNIPAM aqueous solutions as a function of salt concentration and anion type are plotted in Fig. 2a for the anionic sample, and 2b for the cationic one. Great differences between the different anions were found. For a given salt concentration (i.e. 0.2 M), the LCST values order the anions according to the Hofmeister series,  2 SCN > NO 3 > Cl > SO4 , so that species with more accentuated kosmotropic nature, that is more hydrated anions, provide lower LCST values. In other words, the destabilizing effect of salt increases with the kosmotropic character of the anion used. These results can be explained according to a microscopic mechanism based on ion-induced changes in water structure. Hydration of

0,0

0,2

0,4

0,6

0,8

1,0

0,8

1,0

Salt Concentration (mM) 36

(b)

34 32 30

LCST (ºC)

Four anions with different Hofmeister characters were chosen for this study, namely SO2 as kosmotropic, Cl as indifferent, 4  + NO and SCN as chaotropes. Na is considered an indifferent cat3 ion in the Hofmeister series. For this reason, sodium salts were used (NaSCN, NaNO3, NaCl, Na2SO4). As all of these share the same cation, the differences observed in our experiments must be caused by the specific effects produced by the anions. Moreover, as the study includes both positive and negative PNIPAM, it was feasible to analyze differences when the anions acted either as counter- or as co-ions.

(a)

34

LCST (ºC)

302

28 26 24 22 20

0,0

0,2

0,4

0,6

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

electrolytes in solution induces PNIPAM–H2O hydrogen bonding disruption, and the consequent desolvation of the carbonyl and amide groups of the polymer. As a result, the dehydrated chains collapse and aggregate through intra- and interchain hydrophobic interactions, which become dominant above the LCST. The more hydrated the anion, the higher the competition for the molecules hydrating PNIPAM, the disruption of the amide–water hydrogen bonds being more effective for kosmotropic anions than for chaotropic ones. Our results are consistent with those reported by other authors working with PNIPAM microgels, or neutral PNIPAM macromolecules [38,47,48]. To elucidate whether the mechanism proposed by Zhang et al. to explain the hydrophobic collapse of PNIPAM also works in our charged chains, we have applied their theory to our experimental results [6,38,39]. This theory considers different molecular mechanisms of interaction between the PNIPAM and the anions depending on their kosmotropic or chaotropic character. Fig. 2a,b shows a linear dependence of the PNIPAM LCST on salt concentration. The linearity is very clear for the kosmotropic anion (sulfate) and progressively disappears as the ion nature becomes more chaotropic. Zhang et al., working with a non-ionic PNIPAM system, modeled the changes in the LCST (DT) caused by adding salt by the following equation:

DT ¼ c½M þ

Bmax k½M ¼ DT linear þ DT Langmuir 1 þ k½M

isotherm

ð2Þ

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DT ¼ c½M þ

Bmax k½Meb½M 1 þ k½Meb½M

ð3Þ

The exponential term (eb[M]) accounts for electrostatic interactions between a charged surface and ions in solution. The authors used this last equation to evaluate the cloud-point temperature of positive lysozyme solutions. In this case, parameter b is related to the surface potential of this protein, and then, Bmax and b are both measures of the effectiveness for a specific anion to associate with the positive charges located on the protein surface. The dashed lines in Fig. 2a and b result from fitting Eq. (3) to the experimental data, using c, Bmax, k and b as fitting parameters. Table 1 summarizes the values of the parameters obtained from this fitting. The best fit was achieved when b was almost zero, for positive and negative PNIPAM chains. This means our PNIPAM charged chains behave more similar to neutral PNIPAM chains than positively charged lysozyme. This can be explained considering that the PNIPAM chains that we use differ very much from what might be considered as a ‘‘charged particle’’, as is the case of positive lysozyme. Indeed, below the LCTS, our PNIPAM chains are uncharged linear polymers with two punctual charges attached at the extreme of the chains, and thus they are essentially different from ‘‘charged particles’’ with a given surface potential. In addition, the values calculated for both anionic and cationic particles are very similar, confirming that the type of charge present in these PNIPAM systems does not influence the ionic specificity observed and validates the use of Eq. (2) to study salt effects in the LCST of our systems. As can be seen, in Table 1 the linear term is dominant for the kosmotropic anion whereas the non-linear term becomes important as the anions are more chaotropic. From these data we also find that Cl shows certain chaotropic character. In our case, all the electrolytes analyzed except the SO2 4 are chaotropes. Therefore, the linear term (c[M]) would be related to changes in the interfacial tension at the PNIPAM/water interface while the Langmuir isotherm would be the result of specific ion binding to the amide group of PNIPAM. Both terms thus have opposite effects. An increase in the interfacial tension (Dc) entails a lower solubility of the polymer that will tend to reduce its interfacial area by collapsing. On the contrary, ion adsorption will promote polymer

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

expansion due to electrostatic repulsions. This explains the opposite sign of the fitting parameters shown in Table 1. Since at a hydrophobic/water interface the interfacial tension increases 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 chaotropic ions, do find a direct proportionality between them. The same linear correlation was revealed when plotting the c values from our experimental data vs. the corresponding r values [38], as shown in Fig. 3. It is worthwhile recalling that this interpretation for the linear term works only for chaotropic and null ions, but not for kosmotropes (i.e. SO2 4 ). Hence, although only three experimental points are available with each sample (those corre sponding to SCN, NO 3 and Cl ), they suggest the linear dependence noted by Zhang et al. with non-charged PNIPAM. As mentioned above, the results do not depend on the PNIPAM chains charge, since no significant differences were detected between anionic and cationic particles. Hence, the analysis of the experimental and theoretical results clearly suggests that the charged groups are not involved in the PNIPAM solubility changes when the salt is added. The following results also point out in the same direction. 4. Microparticles formed by self-assembly of PNIPAM Solvency properties of charged PNIPAM macromolecules have allowed us to make comparisons with previous works related with uncharged PNIPAM macromolecules. However, the interest in working with charged polymers is due mainly to their ability to form stable colloidal systems beyond the LCST. These particles may enable us to analyze ion effects from another standpoint. 4.1. Particle formation The particle formation resulting from increasing the temperature was analyzed by monitoring the hydrodynamic diameter (Dh) evolution. Measurements were made by progressively heating the sample in the presence of a constant salt concentration (10 mM). Fig. 4 shows these data for both PNIPAM samples. Anionic particles will first be analyzed. All the curves in Fig. 4a exhibit the same trend: at low temperatures the average hydrodynamic

0 -10

SCN-

-20

c (ºC/mol)

The equation includes a linear term and a Langmuir isotherm contribution. In this expression [M] is the molar concentration of salt. The Langmuir term contains two constants: Bmax represents the maximum LCST increase at saturation ion binding while k is the binding constant of the anion to the polymer. The constant c appearing in the linear term has different interpretations depending on the kosmotropic/chaotropic character of the ion. According to Zhang’s model, c is directly related to the hydration entropy of the ion for kosmotropic ions, while it is related to changes in the interfacial tension (Dc) at the water/hydrophobic interface for chaotropes. Additionally, these authors proposed an extension to this equation, which incorporates electrostatic interactions of charged systems.

NO3-

Cl-

-30 -40 -50

SO42-

-60

k (M1)

Anion

c (°C/mol)

Bmax (°C)

Panionic

Pcationic

Panionic

Pcationic

Panionic

Pcationic

SO2 4 Cl NO 3 SCN

62.4 12.8 8.6 4.0

63.8 12.3 7.8 4.1

– 0.6 2.3 3.0

– 0.5 2.3 3.6

– 1.1 3.0 4.2

– 1.3 3.2 4.3

-70 0,0

0,5

1,0

1,5

2,0

2,5

3,0

σ (μNm2/mol) 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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1500

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

(a)

1250

Dh (nm)

1000 750

SCN < NO3 < Cl 6 SO2 4

500

which coincides exactly with the Hofmeister series. It is worth remarking that the hydrodynamic diameter determined in salt-free solutions is always smaller than in the presence of any salt. In solutions of extremely low ionic strength, the repulsive electric barrier between particles – resulting from the polymer charged groups – is relatively strong and thus interchain aggregates stabilize even at low surface-charge densities. As a result, the aggregation process rapidly stops and stable nanospheres with small diameters result. The presence of salt in solution reduces this repulsive electric barrier, basically because of the screening provoked by counter-ions on the surface charge of the particles. Hence, particles continue growing until they reach a stable configuration at which potential barrier is strong enough to prevent aggregation. Results can be readily understood through an ion-accumulation mechanism on hydrophobic surfaces. More chaotropic anions will tend to accumulate more strongly as the surface becomes more hydrophobic. Certainly, the screening caused by the counter-ions, Na+ ions for the anionic system, can be partially balanced if any specific adsorption of anions occurs. In this case, particles with different diameters would form, owing to the different nature of the anions employed (even if all of them have the same valence). Particularly, it would be expected that if the anion accumulation increases, the final particle size will decrease. The exclusion of the kosmotropic anion SO2 from the interface enhances the counter-ion screening and 4 leads to lower repulsive interparticle potentials. However, on the basis of this mechanism, we would expect clearer differences between Cl and SO2 4 . We will show next that these differences become more evident when mobility measurements are taken. Results with the cationic PNIPAM particles (Fig. 4b) also support this accumulation mechanism. The positive PNIPAM showed a pattern similar to the anionic one with the difference that colloidal stable particles formed only with the SCN- and the salt-free solutions. In this case, anions acted as counter-ions, so the screening of the surface charge by the different anions was enhanced producing a destabilizing effect. For this reason, no stable particles formed 2 with Cl and NO also 3 anions, and the double valence of SO4 increased the effectiveness of the surface screening, reflected in a more destabilizing effect. However, the formation of stable particles with the SCN could be explained if the accumulation of this chaotropic anion had been high enough to cause an inversion of the surface-charge density of cationic PNIPAM particles. This hypothesis was confirmed by measuring the electrophoretic mobility of the particles, as will be shown below.



250 0

25

30

35

40

45

40

45

Temperature (ºC) 2500

(b)

2000

Dh (nm)

1500

1000

500

0

25

30

35

Temperature (ºC) Fig. 4. Diameter of (a) anionic and (b) cationic PNIPAM particles formed by increasing the temperature in: (h) no salt added, ( ) 10 mM NaSCN, ( ) 10 mM NaNO3, ( ) 10 mM NaCl, and ( ) 10 mM Na2SO4 solutions.

diameter keeps constant until the phase transition temperature is reached (33–34 °C). The low light intensity scattered by the system in this region reflects the existence of single PNIPAM macromolecules. Water behaves as a good solvent and the polymer presents a well-hydrated coil configuration within this temperature range. It bears noting that, in this case, diameter values must be considered simply as indicative, since this region deals only with nonspherical coil-conformations. Then the average diameter begins to increase, and at a certain temperature Dh stops growing, whereupon the clusters achieve their maximum size, which is about 200 nm in a salt-free solution. The stabilization of the system at high temperatures has its origin in the repulsive electrostatic forces. The PNIPAM chains have charged groups at their ends coming from the initiator used in the synthesis. Hence, the resulting precursor particles have a certain surface charge, which is enhanced as the interchain association occurs. It is this charge that prevents further interaggregate association. Increasing temperature over this critical temperature of stabilization provokes a slight decrease in the particle size; see maximum in Fig. 4a, due to the contraction of PNIPAM with temperature, which makes particles become more and more compact. At higher temperatures, the particle size eventually reaches a plateau that corresponds to the state where the particle is fully compact. In the steady state, monodisperse kinetically stable colloidal particles are obtained instead of a complete phase separation.

4.2. Electrophoretic mobility Valuable insights into the mechanisms responsible for particle formation can be gained from electrophoretic mobility (le) measurements, particularly phenomena involving ion accumulation, since le is an indirect measurement of the charge state close to the particle surface. The influence of the ionic strength on le was first studied. For this task, NaCl, frequently considered as a reference salt, was chosen. Fig. 5 displays the temperature dependence of le for both anionic and cationic PNIPAM samples at two NaCl concentrations. Although curves present a shape similar to that typically found with PNIPAM microgels [5,7,10], their origin is

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0,0

5 4

-0,5

2

-1,0

μe (10-8 m2 V -1 s-1)

μe (10 -8 m2 V -1 s-1)

3

1 0 -1 -2

-1,5 -2,0 -2,5

-3 -4 -5

(a)

-3,0

25

30

35

40

20

45

25

Temperature (ºC)

30

35

40

45

Temperature (ºC) 2,0

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

(b)

completely different. With regard to microgels, electrophoretic mobility rises with temperature owing to an increase in the surface-charge density and a reduction in friction forces when particles shrink. The case of PNIPAM polymer solutions is more complex. At temperatures below the LCST, le is around zero, essentially because there are no particles in solution but slightly charged polymers with a random coil configuration. Beyond the LCST, rather spherical aggregates start to form. This self-assembly structure presents a non-negligible surface charge, which strengthens as particles grow, as reflected by the increase in electrophoretic mobility with temperature. As commonly observed in colloidal systems, higher salt concentrations imply lower le values caused by a compression of the electrical double layer [49]. On the other hand, Fig. 6 shows the results for the different salts at a 10-mM concentration. These data evidence the ionic specificity in these systems, and the accumulation mechanism proposed to explain differences in particles size. Regarding the anionic sample (see Fig. 6a), more chaotropic anions increase the negative electrophoretic mobility of the nanospheres as temperature rises in comparison with Cl-, indicating the effectiveness of the ionaccumulation process in each case. This anion accumulation  2 becomes more important in the sequence SCN > NO 3 > Cl > SO4 , which confirms the results for size and evidences that SO2 4 is more excluded than Cl from PNIPAM interface on becoming more hydrophobic. The same conclusion can be drawn from curves corresponding 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 the particles. Although le values other than zero were found with Cl and SO2 4 anions, their values were not high enough to stabilize the aggregates. Note that in the case of SO2 4 , despite its double valence, and acting as a counter-ion, le values were similar to those found with Cl. This feature indicates the exclusion of the SO2 from the positive particle surface. More significant is the 4 observed charge inversion with the most chaotropic anion, SCN, which indicates a very strong accumulation of this poorly hydrated anion, confirming the aforementioned size results. This important inversion of charge can explain why stable particles resulted from this anion (see Fig. 4b). It is also important to highlight that this charge inversion occurs at only 10 mM of salt concentration. We previously observed charge inversion with SCN and PNIPAM microgel particles46 and more recently with polystyrene particles [34]. These latter results showed that inversion occurs when the surface is hydrophobic but not with a hydrophilic one, confirming

μe (10 -8 m2 V -1 s-1)

1,5 1,0 0,5 0,0 -0,5 -1,0 -1,5 -2,0

25

30

35

40

45

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

that to explain ionic specific effects is essential to consider both the nature of the ions together with the hydrophobic/hydrophilic nature of intervening surfaces. For PNIPAM microgels the inversion also occurs at low salt concentrations, indicating that these systems are more sensitive to charge inversion, and hence to ionic specificity, than hard particles. This is probably because the soft PNIPAM surface allows the anions to penetrate more deeply, increasing the local concentration of adsorbed anions in this interface in comparison with hard surfaces. Finally, it is usually accepted that Hofmeister effects are approximately additive over all species in solution [25]. This feature can be readily tested by measuring the electrophoretic mobility to our PNIPAM systems. Fig. 7 compares the mobility for anionic and cationic samples as a function of temperature at  2 10 mM concentration of SCN, SO2 4 , and SCN + SO4 (5 mM each). These two anions exhibit opposite trends in the accumulation behavior. For the negative PNIPAM (see Fig. 7a), anions act as co-ions and an intermediate effect in the mobility of the two salts together is observed. However, for the cationic particles (see Fig. 7b) when anions act as counter-ions, the accumulation of SCN is more significant than the effect of SO2 4 and we can even detect a slight but clear charge inversion. These results reinforce the importance of hydrophobic forces in these kinds of systems and hence their relation with Hofmeister effects.

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0,0

(a)

μe (10-8 m2 V-1 s-1)

-0,5 -1,0 -1,5 -2,0 -2,5 -3,0 25

2,0

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35

40

45

35

40

45

Temperature (ºC)

(b)

1,5

μe (10-8 m2 V-1 s-1)

1,0 0,5 0,0 -0,5 -1,0 -1,5 -2,0

25

30

Temperature (ºC)

concentration. The same trend is found regardless the type of charge present on the PNIPAM chains, which indicates that the main interaction of anions with PNIPAM occurs along the backbone of the chains. The second part of the paper concerns particle-formation processes. The final particle size is highly dependent on the specific nature of the ion. These results are supported by electrophoretic measurements and reveal the existence of specific ion accumulation processes at the PNIPAM/water interface [46]. This accumulation does not depend on the sign of the surface charge of the PNIPAM particles but rather is directly related to the hydration degree of the anions and the hydrophobic character of the PNIPAM surface. The high sensitivity of these PNIPAM systems to ionic specificity is further reflected in the inversion of charge observed with the most chaotropic anion, SCN, when acting as a counterion at just 10 mM, and the corresponding formation of stable particles. The results of this part can be reasonably explained by considering that poorly hydrated ions accumulate on hydrophobic interfaces. In summary, when PNIPAM is well hydrated, i.e. when it exhibits hydrophilic character, highly hydrated or kosmotropic anions interact more strongly with it. On the contrary, when PNIPAM becomes hydrophobic, poorly hydrated or chaotropic anions interact more strongly with it. This occurs independently of the type of charge, positive or negative, of PNIPAM, which excludes the ion-pairing mechanism as the main interaction between the ions and the interface. These results reinforce the statement that the interaction of ions with interfaces is dominated by both the chaotropic/kosmotropic character of the ions and the hydrophobic/hydrophilic character of the surfaces [29,33,34], and they reveal the importance of considering the interaction with the solvent in water-mediated interfaces of extremely importance such as biological and technological processes. Acknowledgments

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

The authors wish to thank the financial support granted by the projects MAT 2012-3670-C04-02, MAT2010-20370 (European FEDER support included, MICINN, Spain), and CTS-6270 (Junta de Andalucía, Spain).

5. Conclusions

References

The synthesis and properties of a new system based on the selfassociation of PNIPAM charged chains have been reported. These PNIPAM particles have the advantage of being highly versatile: it is possible to turn a polymer solution into a stable colloidal dispersion simply by tuning the temperature, this process being reversible. In addition, the characteristics of the final particles can be controlled by salt addition. In the first part of the paper, we investigated specific ionic effects on the solubility of charged PNIPAM macromolecules. The results 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 PNIPAM macromolecules appropriately fits our results [6,38]. From both cationic and anionic PNIPAM systems, very similar parameters were found, confirming that the ion paring from the law of matching water affinities does not account for the ionic specificity observed. However, our results can be explained considering that at temperatures lower than LCST, the carbonyl and amide groups of PNIPAM are well hydrated. Salt addition provokes a competition for water molecules between these two groups and the added ions. Highly hydrated anions interact more strongly with water than do poorly hydrated ones and cause a more effective dehydration of PNIPAM, resulting in lower values of the LCST for a given salt

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