Reconciling Low- and High-Salt Solution Behavior of Sulfobetaine

Jun 19, 2007 - These polymers bear both positive ammonium, and negative sulfonate charges on each monomer and as a result present an upper critical so...
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J. Phys. Chem. B 2007, 111, 7767-7777

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Reconciling Low- and High-Salt Solution Behavior of Sulfobetaine Polyzwitterions Pascaline Mary,† Denis D. Bendejacq,* Marie-Pierre Labeau,‡ and Patrick Dupuis Rhodia - Centre de Recherche et de Technologie d’AuberVilliers, 52 rue de la Haie Coq, 93308 AuberVilliers, France ReceiVed: March 12, 2007; In Final Form: May 11, 2007

We investigate the water-solubility upon salt additions, of homogeneous families of sulfobetaine-based polyzwitterions. These polymers bear both positive ammonium, and negative sulfonate charges on each monomer and as a result present an upper critical solution temperature (UCST) in the 0-100 °C temperature range. Two chemistries are investigated, with either a carboxylate-carrying function (SPE) or an amidocarrying one (SPP). In agreement with the literature published on pSPEs, we find that an addition of simple salts improves the water-solubility of pSPEs, as well as that of pSPPs, yet only once a threshold concentration of added salt has been reached in the solution. We verify using scaling arguments that the onset of solubility promotion, corresponds exactly to the complete screening of the attraction between positive and negative charges inside a polyzwitterionic coil. On the contrary, for salt concentrations smaller than the threshold concentration, we observe that an addition of salt can be adverse to the solubility of polyzwitterions, depending on the degree of polymerization, the type of salt, and the type of zwitterionic motive. Thanks to zeta-potential measurements and systematic variations of these three parameters, we demonstrate, in agreement with theoretical prediction, that this molecular weight-dependent enhanced solubility at small salt concentrations is due to charge asymmetry resulting from partial hydrolysis, combined with specific interactions between salts and zwitterion constituents, evidencing the complexity of the solution behavior of these macromolecules. We thereby reconcile the different behaviors in the domains of low- and high-salinity.

1. Introduction Polyampholytes are macromolecular entities which bear, at the same time, positive and negative charges on distinct monomers. Because they seemed to share common traits with biological molecules like proteins or DNA and may offer new applicative properties, these polymers have been widely investigated for the past decade, both theoretically1-7and experimentally.8,9 The pioneering work of Higgs and Joanny,1 Kantor et al.,2 and Dobrynin and Rubinstein3 in solution established the fundamentals of the physics of polyampholytes as model macromolecular objects bearing charges of both algebraic signs, with or without charge asymmetry. Naturally, their solution behavior in water was closely looked into,1,2 before refinements were introduced to include polymers with microstructural peculiarities, imposing a certain distribution of quenched positive and negative charges.1,3 Alternating and random polyampholytes, for instance, were then predicted to behave differently,1 while the extent of charge asymmetry was shown to control the behavior in solution of the chain considered.3 Later on, polymers with weak acidic or basic motives and, thus, possible charge annealing along the backbones were considered.4,5 Both refinements in fact evidenced the crucial importance of the chemical microstructure of the polymer at a much smaller scale and, thus, that of the charges distributions in the physics of the solution behavior of polyampholytes. Due to the specificities resulting from the ampholytic nature of these polymers, their interactions with surfactants and, most particularly, their adsorption on charged surfaces and at interfaces, were rapidly investigated10,11 * Corresponding author. [email protected]. † Current address. Ecole Supe ´ rieure de Physique et Chimie Industrielles (ESPCI), 10 rue Vauquelin, 75005 Paris. ‡ Current address. Bristol Research and Technological Centre, 350 George Patterson Blvd. Bristol, Pennsylvania 19007.

and followed by several teams over the past decade, as recently condensed in a thorough review, with numerous experimental investigations.12 However, many of the theoretical predictions remain unverified and even challenged by recent simulations.13 Because the challenge is imposed on chemists who must synthesize appropriate polymers, well-defined in terms of local microstructure, charge (a)symmetry and, molecular weight, many questions regarding polyampholytes remain open. Polyzwitterions, which constitute a subclass of polyampholytes, have attracted a lot of attention at an experimental level over the past decade:14-21 while polyampholytes are made of monomers bearing either positive or negative charges not necessarily in equal molar amounts, neat polyzwitterions bear both charges on the same monomeric unit and are thus virtually neutral. Finally, provided the monomers used bear quenched positive and negative charges, i.e., charges whose existence does not depend on the pH or salt concentration, the problems of charge annealing, chain microstructure, and oVerall net charge of the macromolecule, all become irrelevant. Polyzwitterions can thus be considered as true model macromolecules, where positive and negative charges are in equal number and at a fixed distance from each other as they are born by the same side function. They may be more appropriate to verify the predictions made by theorists. In water, sulfobetaine-based polyzwitterions, which we will focus on in the present paper, present very specific properties in temperature and especially in the presence of salts, which makes them particularly attractive from a fundamental viewpoint, as well as a practical one. They have been reported to be thermosensitive and to present critical solution temperatures, denoted Tc, dependent on both the polymer volume fraction and the chain degree of polymerization: above Tc, the solutions are transparent, while they are rather whitish and opaque or simply turbid, or even macroscopi-

10.1021/jp071995b CCC: $37.00 © 2007 American Chemical Society Published on Web 06/19/2007

7768 J. Phys. Chem. B, Vol. 111, No. 27, 2007 cally biphasic, under. Their phase diagrams in Tc vs φp, usually present a dumb-bell shape which resembles that predicted by the Flory theory,22 characteristic of such polymers presenting an upper critical solution temperature (UCST) for a precise polymer concentration in water. Huglin et al. have verified several predictions derived from the Flory approach, for a class of poly(sulfobetaine)s made of propylsulfonate dimethylammonium ethyl methacrylate (polySPEs), of well-known, controlled degrees of polymerization obtained by preparative chromatography on a single polydisperse specimen. The physics of poly(sulfobetaine)s in solution is now largely accepted as roughly following Flory’s formalism. According to the latter, the phase diagrams Tc vs φp of a polymer in a solvent essentially depend on the solvent quality, closely linked to the Flory-Huggins interaction parameter defined as kBTχms ) ms-1/2[mm + ss], where ms, mm, and ss are the monomer-solvent, monomermonomer, and solvent-solvent contact energies, respectively. For most like-charged polymers, the tuning of the solvent quality of water via an addition of salts is often used to modify the solution behavior, as it potentially affects both the monomersolvent and the monomer-monomer contact energies. As a result, literature can be found on the impact of the addition of salts, on the critical temperature and solubility in water of polyzwitterions, and poly(sulfobetaine)s, mostly pSPEs, in particular. The most commonly reported effect is the promotion of their solubility in water, the so-called “anti-polyelectrolyte” effect.1,9,14,18,19,23,24 This phenomenon refers to how salts may decrease the transition temperature, hence effectively making polyampholytes more soluble. Its mechanism is generally understood as a Coulomb screening effect. Added salts decrease the Debye length lD )1/κD ≡ [4πlBcs]-1/2, where lB is the Bjerrum length in pure water and cS is the concentration in added salt (expressed in inverse cubic nanometers). This length represents the range of the electrostatic Coulombic interaction, as it is the typical distance over which two charges sense each other. For most polyelectrolytes with a hydrophobic backbone, whether weak or strong, solubility in water is only due to the presence of charges of the same algebraic sign present along the backbone, and electrostatic repulsion favors a stretching of the chains.25 Increasing the amount of salts generally results in a screening of the repulsion and eventually provokes the collapse, and sometimes the precipitation, of the backbone. Quite symmetrically, the insolubility in water of polyzwitterions is understood as resulting from the Coulombic attraction between ammonium and sulfonate groups of opposite charges, present along the polymer backbone. This attraction may be inter- or intramolecular, meaning an attraction between ammonium and sulfonate of the same or distinct monomers or polymer chains. Added salts screen this attraction: upon salt addition, a polyzwitterion coil will thus swell with water. This feature resembles what is known for any conventional polymer dispersed in a solvent as the Flory-Huggins interaction parameter becomes increasingly negative (that is, its absolute value becomes larger). A macroscopic consequence of this is finally the decrease of the critical temperature Tc ∝ 1/|χ|. By analogy, a decrease upon salt addition of the critical temperature of poly(sulfobetaine)s in water is interpreted as a promotion of solubility. We stress, however, that this phenomenon has been observed only when large enough amounts of salts had been added, while the domain of small salt additions was rarely, if ever, cautiously investigated. Very few studies actually report on the effect of small salt additions: quite remarkably, a study15 even reports that a small addition of salt is in fact adverse to

Mary et al. solubilization, as the critical temperature first increases upon salt addition, before the usual decrease is observed as solubility promotion occurs. Such contradictory behaviors, for now, remain unexplained. The object of the present article is thus 2-fold, that is, to elucidate and reconcile the effects of small and large salt additions on the solubility of sulfobetaines polymers. For this, we construct the phase diagrams at a controlled ionic force, of a series of two different polyzwitterionic families based on ethylmethacrylate- or propylmethacrylamido-bearing zwitterions, of controlled degrees of polymerization ranging from a few hundreds to several thousands. Our strategy is to elucidate the problems of small or large salt additions, by systematically comparing the behaviors of the two sulfobetaines chemistries, in the presence of different monovalent salts from the Hoffmeister series, with which poly(sulfobetaine)s are known to have specific interactions. We first report on the domain of large salt additions where specific interactions are negligible: we will quantitatively confirm that promotion of solubility is due indeed to an osmotic effect in polyzwitterions, as recently modeled by Georgiev et al.26 A simple, yet accurate mean-field model, similar to that developped for polyampholyte by Higgs and Joanny, will prove sufficient to explain the experimental data. In a second part, we report on the domain of small salt additions, where charge asymmetry combined with the well-known specific interactions between zwitterion and salt electrolytes leads to a molecular-weight-dependent reduction of the anti-polyelectrolyte effect (that is, an increase of critical temperature and, thus, a decrease of solubility) consistent with the prediction of Dobrynin and Rubinstein. Zeta-potentiometry will prove crucial to demonstrate this feature. 2. Materials and Methods 2.1. Polymer Synthesis and Characterization. Two poly(sulfobetaine) families were investigated, whose monomeric units are structurally slightly different; their chemical formulas are indicated in Table 1. The pSPE family denotes polymers made of propylsulfonate dimethylammonium ethylmethacrylate, and pSPP, those made of propylsulfonate dimethylammonium propylmethacrylamide. All polymers were synthesized using radical polymerization techniques described in other references.14 In Table 1, we indicate the number-average molar weight Mn of the polymer synthesized as measured by multiangle laser scattering (MALLS) and the corresponding numberaverage degree of polymerization N, ranging in all cases from a one hundred to a few thousands. For the sake of clarity, the polymers will now be referred to via their acronym (i.e., pSPE or pSPP) and their number average degree of polymerization as a subscript. The polymers are obtained at the end of the synthesis, as water solutions of large dry extract (approximately 50 wt %) and of large conductivity due to the presence of salts resulting mainly from the decomposition of the peroxodisulfate ammonium initiator. The polymers were thus dialyzed in Spectra Por membranes (cutoff 7500 g/mol) against deionized water during several days, until the conductivity of the external water reservoir reached a few tens of microsiemens per centimeter. To reach small molar weights, large quantities of initiator are necessary in the synthesis step, as often in any conventional radical polymerization process. Consequently, the smaller the molar weight targeted, the larger the conductivity and the longer the dialysis step. If undialyzed, the conductivity of 1 wt % solutions of pSPEs is relatively large, of several tens of millisiemens per centimeter. The dialyzed solutions were then freeze-dried, and the polymers were collected in a powdered form. The absolute molar weights of the polymers were

Solution Behavior of Polyzwitterions

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TABLE 1: Specimen Characteristicsa type

chemical name

pSPE

propylsulfonate dimethylammonium ethylmethacrylate

pSPP

propylsulfonate dimethylammonium propylmethacrylamide

topological formula

ref name

Mw (g/mol)

Ip

N

% hydrolysis

SPE177 SPE429 SPE895 SPE1611 SPE6442 SPP178 SPP445 SPP531 SPP1404 SPP2363 SPP3425

49 500 120 000 250 000 450 000 1 800 000 52 000 130 000 155 000 410 000 690 000 1 000 000

2.3 1.8 2.3 3.0 2.9 3.2 4.8 4.5 3.8 3.0 5.7

177 429 895 1611 6442 178 445 531 1404 2363 3425

15% 11% 12% 15% 12% 8% 8% 8% 8% 8% 8%

a Type, chemical names, topological formulas, absolute average molecular weights, and index of polydispersity as measured by MALLS and degrees of polymerization so computed of the different families of poly(sulfobetaine)s. The last column gives the percentage of hydrolysis of zwitterionic motives into acrylic acid, as measured by 1H NMR.

measured using chromatography, with as the mobile phase, a 1M NH4NO3 water solution added with 100 ppm NaN3, necessary to obtain water solubility by screening the electrostatic attraction between the positive and negative charges of the sulfobetaine motives, as will be fully demonstrated in the present paper. The experiments were carried out at a debit 1 mL/min in three 30 cm Shodex 806 M-HQ columns. A refractive index detector WATERS 410, and a multi-angle laser scattering (MALLS) detector Mini Dawn or Wyatt He-laser 633 nm, were used. The concentration of the polymer solutions injected was 0.6 wt % in the mobile phase, with a 100 µL-injection loop. The number- and weight-average molecular weights Mn and Mw, as well as the polydispersity index Ip ) Mw/Mn of order 1.8 to 5.7 (most of them being in the range 2.0-3.0). Nuclear magnetic resonnance (NMR) was also used to measure the degree of hydrolysis of all the polymers from the pSPE family: as hydrolysis naturally occurs during the synthesis itself, the zwitterionic motives then transform irreversably into acrylic acid functions. Table 1 shows that the extent of hydrolysis was approximately 11-15% for pSPEs and a constant 8% for pSPPs. 2.2. Critical Temperature, Theta Temperature, and Phase Diagrams. A temperature-monitored Jackson turbidimeter was set up: we measured the temperature of a polymer solution placed on a heating and stirring plate thanks to a thermometer, and at the same time, the intensity transmitted through this sample shone with a white light source, using as a detector, a semiconducting silicium-germanium photodiode of large detection spectrum ranging from 200 to 1100 nm. The percentage of transmitted intensity Itr was then plotted as a function of the temperature T. Although the critical temperature Tc may be defined in various ways (onset of turbidity upon heating or upon cooling), we chose to use the temperature where the transmitted light reaches 50%, as often done in the literature. We verified that for pure poly(sulfobetaine)s solutions such as those used here, the intensities measured upon cooling or heating were approximately the same, suggesting that the transition is perfectly reversible, as expected from thermodynamic considerations. The polymer volume fraction was computed according to

φp )

1 1 + dp/dS(1/wp - 1)

(1)

where wp is the polymer weight fraction and dp and dS are the densities of the polymer and of the solvent (water), respectively. For pSPE, we used the value dpSPE ) 1.395 g/cm3, as computed from the value VSPE ) 0.332 nm3 of the specific volume of a SPE motive, as found in the literature.18,19 Note that the density

used for pSPP is the same, for the lack of experimental data. The temperature-dependence of the density of water on the range 0-100 °C is tabulated,27 and taken into account in the computation of the polymer volume fraction, while the density of pSPE is considered nearly constant with temperature, mainly for a lack of precise data, although it is generally the case with amorphous polymers. Finally, the theta temperature Tθ, defined as the critical temperature of a solution of a polymer of infinite molar weight, was determined using two different methods: (i) Cornet and Ballegooigen28 proposed a semiempirical method which consists of plotting 1/Tc vs log φp, which does not require the molar weight of the polymer used, but only its phase diagram in the dilute region; this plot produces a straight line, whose intercept with the y-axis (i.e., for log φp ) 0) gives the reciprocal theta temperature 1/Tθ. (ii) Schultz-Flory’s method29 relies on plotting the reciprocal value of the critical temperature at the UCST, 1/T/c vs X ) 1/N1/2+ 1/2N should give a straight line whose intercept is, as before, 1/Tθ. In Figure 1a, we show the Cornet and Ballegooigen plot for the dialyzed pSPE6442 of increasing concentration φp in water, and in Figure 1b, the Flory-Schultz plot for series of dialyzed pSPEs of increasing degrees of polymerization, all at critical concentrations φ/p associated with the UCST is shown. In neither case is the error on Tc represented as the latter is negligeable compared to 1/Tc. The two methods respectively give intercepts 103/Tθ ) 2.565 K-1 and 103/Tθ ) 2.51 K-1, therefore leading to values Tθ ) 117 and 125 °C, respectively, which are rather close to each other and validate our MALLS determination of the average degree of polymerization. The Flory-Schultz method has the advantage of being more precise, because binodal and spinodal curves meet at the UCST where our definition of the transition temperature is the most accurate. However, the Cornet and Ballegooigen method necessitates only one polymer and does not require establishing the whole phase diagram. 2.3. Zeta-Potential Measurements. The zeta-potential ζ was measured by the technique of micro-electrophoresis, a technique which makes it possible to study electrostatic interactions and surface chemistry in suspensions. One then determines the value of the zeta-potential via the measurement of electrophoretic mobility U connected to ζ by the relation ζ ) {3ηUf(ΚR)}/ {2}, where η is the viscosity of the liquid,  is its permittivity, and R is the radius of the particle; f(ΚR) is a variable factor from 2/3 to 1 according to whether the radius R of the particle is large or small compared to the thickness of the double layer. 3. Results and Discussion Let us first make a few general comments on the phase behavior of our poly(sulfobetaine)s. From Flory’s theory, a

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Figure 2. Critical temperature Tc vs the weight fraction φp of dialyzed pSPEs in order of increasing degree of polymerization NSPE as indicated.

Figure 1. (a) Cornet and Ballegooigen plot obtained on the extensively dialyzed pSPE1611 and pSPE6442. (b) Flory-Schultz plot for the family of dialyzed pSPEs of Table 1.

polymer presenting an upper critical solution temperature (UCST) denoted T/c , is expected to show a curve Tc vs φp with a maximum at T/c , located at a critical polymer volume fraction φ/p. The dependence of T/c with the degree of polymerization N is predicted to be

1/Tc/ ) 1/Tθ(1 + (1/ψ1)(1/xN + 1/2N))

(2)

where Ψ1 is the entropy of mixing parameter (∆S1 ∝ Ψ1) and Tθ is the so-called theta temperature, that is, the upper critical solution temperature of a polymer of infinite degree of polymerization. Equation 2 predicts that plotting the reciprocal upper critical solution temperature 1/T/c as a function of X ) 1/N1/2 + 1/ N should give a straight line. On the other hand, the position 2 φ/p of the UCST is predicted to obey:

φ/c ) 1/(1 + N1/2)

(3)

These relations were precisaly verified by Huglin et al.19,20 In Figure 2, we show our own phase diagrams of dialyzed pSPEs of increasing degrees of polymerization NSPE. The results agree with those found in the literature and are consistent with the phase separation curve as predicted by Flory: (i) the curves have a dumb-bell shape with a maximum Tc/ found for a critical concentration φ/p on the order of 0.01-0.02; (ii) at any concentration, the critical temperature is an increasing function of NSPE (for instance, at φp ) 0.002, Tc increases from 6 to 75 °C, as NSPE increases from 177 to 6442). Within experimental uncertainties, the critical concentration φ/p corresponding to the UCST seems independent of the degree of polymerization. This is not consistent with Flory’s eq 3 verified by Huglin et al. for pSPEs and which our polymers obviously deviate from: a factor

of approximately 5 should be expected between the positions of the largest and smallest UCSTs, according to eq 3. The experimental setup reveals that the variation in transmitted light is the most abrupt when the degree of polymerization of the polymer, or its concentration in water, is the largest. Considering our definition of the critical temperature, that is, 50% of transmitted light reached upon cooling or heating and the fact that cooling or heating are never slow enough to ensure that thermodynamic equilibrium was reached at any given temperature, we necessarily work within the metastable region between binodal and spinodal curves, which define the phase separation between polymer-rich and polymer-poor phases. According to Flory, the width ∆Tbs of this region is a direct function of the polymer concentration and degree of polymerization. Binodal and spinodal curves meet at the UCST, where ∆Tbs ) 0 by definition, while they increasingly depart from each other as φp becomes increasingly smaller or larger than φ/p, or as the degree of polymerization becomes larger. Therefore, the definition of the critical concentration φ/p corresponding to the UCST is theoretically the least subject to uncertainties resulting from experimental setup and definitions. The fact that we do not see the expected variation in the position of the UCST with the degree of polymerization must be sought for somewhere else. The present polymers have a quite large polydispersity in the degree of polymerization, as opposed to those of Huglin et al. for well-defined polyzwitterions of the same chemistry19,20 obtained by preparative chromatography. However, polydispersity is not likely to be the cause for the absence of shift. Indeed, it is quite extensive for all polymers, but always of the same order of magnitude. We have no explanation for the moment for this descrepancy. Figure 3 shows an example phase diagram of salt-free pSPP3425 in the form Tc vs wp, where wp is the weight fraction of pSPP. We observe that the shape is essentially the same as that obtained with pSPEs and that the position of the UCST is found between 1 and 2 wt %, very much like with pSPEs. This strongly suggests that the physics of phase behavior of the pSPE and pSPP chemistries are similar. When we superimpose the data obtained for salt-free pSPE429, that is, a polymer nearly 1 order of magnitude smaller than pSPP3425, it is in fact very close: therefore, a pSPP dispersion has a critical temperature comparable to that of a pSPE, if the degree of polymerization is increased by almost 1 order of magnitude. The insert in Figure 3 shows the corresponding plot 103/Tc vs log φp for salt-free pSPP3425: we, thereby, verify that a straight line can satisfac-

Solution Behavior of Polyzwitterions

Figure 3. Phase diagram of the salt-free pSPP3425 superimposed with the data from Figure 2 for the salt-free pSPE429. The lines are guides for the eye. (inset) Associated Cornet and Ballegooigen plots 1/Tc vs log(φp). The straight lines are linear fits to the experimental data.

torily fit the experimental data according to the Cornet and Ballegooigen description. However, the intercept 103/Tθ ) 2.76 K-1 gives a theta temperature (i.e., the UCST at infinite degree of polymerization) of 89 ( 5 °C for a SPP-based polymer, while that of a SPE-based polymer is 117 ( 5 °C as determined by the same method from the data of the extensively dialyzed pSPE6442 shown in Figure 2. Therefore, a difference of nearly 30 °C is observed between the theta temperatures of polymers based on these two chemistries, confirming that a pSPP will be systematically more soluble than a pSPE of an equivalent degree of polymerization. Note that, in the same way, the critical temperatures of pSPPs are always 20-30 °C smaller that those of pSPEs of comparable degrees of polymerization. This property is a direct consequence of the replacement of the carrying ethylcarbonyl group in SPE, by the propylamido group in SPP. Although it is spatially exterior to the zwitterionic ammonium-sulfonate couple, whose electrostatic attraction controls the solubility in water as will be detailed in the following section, the carrying function obviously affects the electronic environment of the ammonium-sulfonate couple, which greatly affects the whole phase diagram of the corresponding polymer. We use this property to our advantage, to investigate how polyzwitterions react to additions of simple salts. 3.1. Effects of Large Amounts of Salt. In Figure 4, we show, as examples, the effect of adding NaCl in a controlled manner, to the salt-free SPE6442 and SPP3425, at three different small polymer weight fractions in water, i.e., 0.0006, 0.0012, and 0.003. The results demonstrate for both zwitterionic chemistries and regardless of the polymer concentration used as long as it is small enough, the existence of two different domains separated from each other by a crossover concentration denoted c/S, on the order of 0.001 mol/L: a first domain (i) (i.e., cS e c/S) where a small addition of salt results in an increase of the critical temperature and thus in a loss of solubility, as described in ref 2, and a second domain (ii) (i.e., cS g c/S) where a large addition of salt results in a decrease of the critical temperature and thus in the gain of solubility commonly reported. Let us now detail the characteristic features of these two regimes. For a large enough addition of salt (cS g c/S), Figure 5 shows the effect of adding NaCl in a controlled manner, to the different salt-free pSPEs, at a constant concentration 0.3 wt % in water. Very much like SPE6442, the other pSPEs show a decrease of

J. Phys. Chem. B, Vol. 111, No. 27, 2007 7771

Figure 4. Critical temperature Tc as a function of added NaCl concentration cS at different polymer weight fractions wp ) 0.0006 (empty diamonds), 0.0012 (crosses), and 0.003 (filled circles) for SPE6442 and SPP3425.

the critical temperature as cS is increased. The anti-polyelectrolyte effect, often interpreted as the consequence of screening, is indeed a general feature observed and reported on. However, our data show an additional feature: that the crossover concentration c/S depends on NSPE, since c/S decreases from 0.0162 to 0.0033 mol/L as the degree of polymerization NSPE increases from 177 to 6442. The classical anti-polyelectrolyte effect in polyzwitterions thus depends on the degree of polymerization. This first result comes as a surprise, as the phenomenon is supposed to result solely from the salt-induced screening of the attractive interaction between the negative sulfonate and positive quaternized ammonium sites. Let us try to elucidate this feature. At cS ) c/S, the Debye length, defined macroscopically, reads lD ) [4πlB(Tc)c/S]-1/2 where lB(Tc) ) e2/4π(Tc)kBTc is the Bjerrum length at temperature Tc. The temperature dependence of the dielectric permittivity of water  is tabulated27 and allows the computation of lB(Tc): as shown in Table 2, lB increases by more than 10% over the range of critical temperatures 8.5-89.5 °C of the pSPE family. On the other hand, in a mean-field manner, the average microscopic distance l( between ammonium and sulfonate charges confined inside a polyzwitterionic coil schematically represented in Figure 5b may be defined as 4π/3(l(/2)3 = 1/c( with

c( ≡

2 NSPE 2 NSPE 1-3β = ) (3/2πa3)NSPE Vcoil 4π/3R 3

(4)

g

as the concentration of charges confined inside the coil. The quantity Rg ∝ aNSPEβ is the usual de Gennes scaling relation giving the radius of gyration Rg of the coil formed by a polymer of degree of polymerization NSPE in a solvent, while β is the power exponent associated with the solvent quality for the polymer chain (i.e., 3/5 for a chain in good solvent, 1/2 for a chain in theta conditions, and 1/3 for a collapsed chain in nonsolvent conditions) and a ) 3xVSPE ) 3x0.332 ) 0.69 nm is the characteristic size of an SPE monomer. The very definition of our measure of the critical temperature, i.e., the transition from good to bad solvent, implies that β should be equal to 1/2, as Huglin et al. already verified for their own

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Figure 6. (a) Distance l( between opposite charges vs the Debye length lD at the critical temperature Tc of the different pSPEs investigated. The line is a linear fit to the experimental data, of slope +1.0 (correlation coefficient R2 ) 0.969). (b) Crossover NaCl concentration c/S (in moles per liter) vs the degree of polymerization of the family of pSPEs. The line is a power law fit to the experimental data, of exponent +0.44 (correlation coefficient R2 ) 0.996). Figure 5. (a) Tc (in kelvin) vs cS (in moles per liter) in doublelogarithmic scale: effect of the NaCl concentration on the critical temperature of pSPEs of increasing degrees of polymerization, at a constant polymer weight fraction of 0.3 wt %. (b) Schematic representation of a coil added with electrolytes and of the distance l(.

TABLE 2: Physical Data at the Crossover Salt Concentrationa polymer

NSPE

Tc (°C)

Tc (K)

(Tc)

lB(Tc)

SPE6442 SPE1611 SPE895 SPE429 SPE177

6442 1611 895 429 177

84.5 63 51 33 8.5

357.65 336.15 324.15 306.15 281.65

59.99 66.10 69.78 75.72 84.74

0.775 0.748 0.735 0.717 0.697

a Dielectric permittivity and corresponding Bjerrum length (in nanometers) at the measured critical temperature Tc, of the pSPEs investigated.

pSPEs. Within this assumption, whose validity will be fully verified a posteriori, we finally derive the following:

l( ) (4a3NSPE1/2)1/3

(5)

We can now compute both the average microscopic distance l( between ammonium and sulfonate sites confined inside a polyzwitterion coil and the Debye length lD imposed by the amount of added salt. In Figure 6a, we plot l( as a function of lD at critical temperature Tc of the different pSPEs investigated: we observe that there exists a remarkable correlation between the two. This unmistakably demonstrates that the

crossover concentration c/S associated with the onset of solubility promotion corresponds exactly to a Debye length equal to the average distance between charges inside the coil. From a purely osmotic point of view, the thermodynamic requirement of equality of the electrolytes chemical potentials inside and outside the coil ensures that, in the absence of a specific attractive interaction between the zwitterionic charges and either ion of the added NaCl salt (that is, under the assumption that ions Na+ and Cl- indifferently enter the polymer coil to become the counterions of each ammonium and sulfatonate sites), the osmotic penetration of electrolytes inside the coil is forbidden as long as it would imply an electrolytes concentration inside the coil larger than that of the external reservoir. Consequently, the crossover concentration c/S should be exactly equal to the concentration c( of confined zwitterionic charges. From eq 4, we therefore expect c/S = (3/2πa3)NSPE1-3β. We thereby predict a power law dependency c/S ∝ NSPEγ with γ ) 1 - 3β. The exponent γ ) 1 - 3β is necessarily negative, since β g 1/3 by definition. In Figure 6b, we plotted c/S vs NSPE: we found a power law dependence with γ ) -0.44, from which we extract β ) 0.48, in close agreement with β ) 1/2 in a theta solvent. In the domain where φp is small enough, the Cornet and Ballegooigen relation reads as follows: 1/Tc ∝ -log φp. Given φp ∝ Rg3 ∝ N3b and c/S ∝ N(1-3β), we compute φp ) c/3β/(1-3β) which implies -log φp ) S -3β/(1-3β) log c/S and, thus, -log φp ∝ -log cS. Therefore, plotting Tc data vs -log cS makes sense to determine the position c/S of the onset of solubilization. Our approach is similar to that developped by Higgs and Joanny on polyampholytes, whose

Solution Behavior of Polyzwitterions

J. Phys. Chem. B, Vol. 111, No. 27, 2007 7773

Figure 8. Theta temperature Tθ vs NaCl concentration cS for the pSPE (open circles) and pSPP (filled circles) families. (inset) Entropy of mixing parameter Ψ1 vs NaCl concentration cS for the pSPE family, as extracted from the slopes and intercepts of the straight lines in part a.

Figure 7. (a) Flory-Schultz plots 1/Tc vs X ) 1/NSPE1/2 + 1/2NSPE, for the pSPE family, at different concentrations cS of added NaCl salt: 0.003, 0.01, 0.03, and 0.05 mol/L, from top to bottom. (b) FlorySchultz plots 1/Tc vs X ) 1/NSPE1/2 + 1/2NSPE at cS ) 0.05 mol/L, for our pSPE family at a polymer weight fraction φp ) 0.02 (filled symbols), and comparison to the published data (from Huglin et al.) at φp ) φ/p.

positive and negative charges are born by distinct monomer species. The approach also seems to capture the physics of polyzwitterions. In Figure 7a, we plot the reciprocal critical temperature of pSPE solutions as a function of X ) 1/NSPE1/2+ 1/2NSPE and in the presence of different weight fractions of added NaCl salt, i.e., cS ) 0.003, 0.01, 0.03, and 0.05 mol/L. In each case, we observe linear dependencies between 1/Tc and X, in agreement with the Flory-Schultz description. Nevertheless, these measures were all taken at a polymer weight fraction φp ) 0.003 instead of the critical concentration c/S ≈ 0.01-0.02 associated with the UCST, Tc*, which Flory and Schultz actually refer to. One might thus expect deviations to occur from the actual values of theta temperatures as a result of this approximation: in Figure 7b, we thus plot 1/Tc vs X plots for pSPE solutions and superimpose data from Huglin et al. measured in identical conditions, i.e., at φp ) 0.02 ≈ φ/p and cS ) 2cNaCl ) 0.05 mol/ L. From the slope and intercepts of linear fits to this experimental data at cS ) 0.05 mol/L, we respectively obtain for our pSPE family, Tθ ≡ Tc(X f +∞) ) 310.5 K and an entropy of

mixing parameter Ψ1 ) 0.647, while values Tθ ) 307.7 K and Ψ1 ) 0.639 K-1 are deduced from the pSPE data from Huglin et al. We find a difference of 2.8 K between the two values of Tθ, while those of Ψ1 differ from each other by less than 1.2%. We believe that this small discrepancy between the pSPE family of the present work and that of ref 4 may be attributed to a slight error in the determination of the degree of polymerization in either work, although it is difficult to say which one. The values being rather close, we trust that our polymers and degrees of polymerization are reliable enough and proceed with our quantitative study. From the experimental data in Figure 7a, we now extract Tθ and Ψ1 at the different salt concentrations investigated. The resulting values of Tθ and Ψ1 are represented in Figure 8 and its inset, respectively. The theta temperature is found to decrease from 365.1 to 298.3 K (that is, from 91.9 to 25.2 °C) as cS is increased from 0.003 to 0.05 mol/L. With over 1 order of magnitude of increase in the salt concentration, the theta temperature of pSPEs therefore dramatically decreases by almost 70 °C. Meanwhile, the entropy of mixing parameter is found to increase from 0.258 to 0.738 K-1, effectively demonstrating that an addition of salt actually promotes the solubility of pSPEs via an apparent increase of the entropy of mixing of these polymers in water, and not via an enthalpic effect. We have thus demonstrated that the promotion of solubility of pSPEs in water by an addition of salt has little to do with a change in the polymer/solvent enthalpic cost of contact usually associated with the existence of a UCST in “classical” polymers. We now comment on the impact of the functional zwitterionic group, as we pursue this quantitative study, going from pSPEs to pSPPs. Flory-Schultz plots similar to those shown in Figure 1b for pSPEs were also obtained for pSPPs, and the values of Tθ and Ψ1 were so extracted. In Figure 8 and its inset, we have also superimposed the results obtained for pSPPs, to those of pSPEs. On the whole salt concentration range investigated, we observe that TpSPP is 28 to 16 K smaller than TpSPE as cS is θ θ increased from 0.003 to 0.05 mol/L, thereby confirming the result obtained with dialyzed pSPPs and pSPEs (i.e., virtually salt-free, cS ) 0): pSPPs have critical temperatures 20-30 K smaller than those of pSPEs of comparable degrees of polymerization. Meanwhile, the apparent entropy of mixing parameter

7774 J. Phys. Chem. B, Vol. 111, No. 27, 2007

Mary et al.

Figure 9. (a and b) Critical temperature Tc and zeta-potential of SPE6442 at a polymer weight fraction of 0.003, as a function of the concentration cS of different salts from the Hoffmeister series. (c and d) Critical temperature Tc and zeta-potential, as a function of the concentration cS of added NaCl salt (moles per liter) measured on solutions of SPE1611 and SPP1404 of similar degrees of polymerization, both at a constant polymer concentration of 0.3 wt %. In all figures, the lines are guides for the human eye.

ΨpSPP starts as much larger than ΨpSPE in the smallest salt 1 1 concentration range and remains so on the whole range of salt concentrations investigated, yet eventually converges to almost the same value, when enough salt has been added: at cS ) 0.05 ) 0.74 K-1 while ΨpSPP mol/L, ΨpSPE ) 0.76 K-1. This 1 1 suggests that, although the type of zwitterionic motive impacts the solubility of a polyzwitterion in water when the NaCl concentration is small, solubility eventually becomes independent of the motive when a sufficient amount of NaCl has been added: polyzwitterions with different chemistries such as pSPEs and pSPPs, then, become effectively equivalent. However, this screening phenomenon should only depend on the overall electrolyte concentration, but not on the nature of the added salt itself: yet, the solubilization of poly(sulfobetaine)s is wellknown to depend on the nature of the added salt, as numerous early papers report.14,15,19-21 This will be investigated via the Hoffmeister series in the next section, dedicated to the domain of small additions of different salts, combined with the two chemistries of the SPE and SPP zwitterionic motives. 3.2. Effects of Small Amounts of Salt. For a small enough additions of salt (cS e c/S), the problem seems completely different. In Figure 9a and b, we respectively plot the critical temperature Tc and the corresponding zeta potential of SPE6442 at constant polymer weight fraction wp ) 0.003, as a function of the concentration cS of different salts from the Hoffmeister series. Let us note first that, in the high salt regime, for a given salt concentration, the critical temperature depends directly on the type of salt added: as reported on in the earliest papers dedicated to polyzwitterions, all sodium-based salts result in a decrease of Tc (the salt is said to be a critical temperature depressant), yet iodine is more depressant than bromine, which

is more depressant than chlorine. This phenomenon was long ago interpreted as resulting from preferential interactions between the zwitterionic motives along the backbone and their associated ion of opposite charge as coming from the added salt. In other terms, the sulfonated sites can have a stronger interaction with the added cation than the quaternized ammonium sites do with the anions and vice versa, which we only confirm here. However, our findings show again an additional characteristic in the low salt domain: while the evolution of Tc is monotonic for NaI and NaBr, showing a progressive decrease as cS is increased, the evolution of Tc with NaCl shows first a large increase from 75 to 88 °C, before the usual decrease and subsequent promotion of solubility. Such an increase has already been reported on15 yet remained unexplained. Moreover, the existence of the increase depends on the molecular weight of the poly(sulfobetaine)s: Figure 5 clearly shows that only SPE6642 displays an increase in Tc with added NaCl, while all other degrees of polymerization do not. The same was found for the SPP chemistry. The zeta-potential data provides additional information. Let us first comment on the values of the zeta-potentials of the saltfree pSPEs and pSPPs. From Figure 9b, we observe that SPE6442 has an apparent negative zeta-potential of -12.7 mV at the smallest salt concentration investigated, that is, at the end of the dialysis, on the order of 10-4 mol/L. This is in fact a general feature, since all salt-free pSPEs were found to present a negative zeta-potential. On the other hand, all pSPPs were found to present a positive zeta-potential on the order of +15 mV (see, for instance, SPP1404 in Figure 9d, which presents an apparent positive zeta-potential of +13.0 mV). In the case of pSPEs, the apparent negative zeta potential may be consistent

Solution Behavior of Polyzwitterions with a partial hydrolysis of 10% of the SPE motives into acrylic acid motives, known to occur during the synthesis step itself and quantified by NMR. The pSPEs of the present study thus have an anionic net charge of at most 10%, due to the dissociation of the AA sites. At natural pH 6-7, the dissociation of AA is on the order of 50%, and therefore, the anionic net charge of pSPEs should not in fact exceed 5%. It appears that this slight deviation from a perfect zero-net charge pSPE does not significantly impact the overall behavior in water. On the contrary, the cationic net charge of pSPPs is surprising. Indeed, in spite of a constant 8% hydrolysis of the SPP motive into acrylic acid ones, these polymers still bear a net positive charge according to zeta-potentiometry, implying that negative sulfonate sites are absent. As the latter follows from the quaternization of the tertiary amine with propane sultone, one could suggest a nontotal quaternization of the tertiary amine. However, 1H NMR gave no reliable measurable proof of a nontotal quaternization, within the margin of error of the technique, on the order of less than 1%. It seems unlikely that the cationic charge of all pSPPs could be due to a very small fraction of unquaternized amines. The cationic net charge remains unexplained for now. Let us now proceed with the evolution of the zeta-potential of pSPEs and pSPPs upon an addition of different salts. From Figure 9b, we observe that the evolution of ζ(pSPE) is constant with cNaBr, while it increases with cNaCl and decreases with cNaI. Therefore, NaBr seems the most “neutral” salt for pSPEs: anion Br- interacts with -(CH3)2N+- as much as cation Na+ does with -SO3-. In other terms, these electrolytes may indifferently enter a pSPE coil. Consequently, NaBr does not impact the apparent zeta-potential. On the contrary, NaCl and NaI do impact the zeta-potential and, quite remarkably, in opposite ways: an increase of ζ with cNaCl implies that Na+ interacts more strongly with -SO3- than Cl- does with -(CH3)2N+-. The preferential complexation of a pSPE with Na+ results in an increase of the zeta potential, as the Na+ positive charges compensate for the net anionic charge of partially hydrolyzed pSPEs. According to Nesterenko and Haddad,30 who investigated zwitterionic ion-exchanging resins of similar chemistry, a sulfonate group -SO3- interacts more preferably with a quaternized one -(CH3)2N+- than a chlorine anion Cl- does. This is consistent with a promoted entry of Na+ at the expense of that of Cl- anions, which shall be schematically represented by the following:

On the other hand, a further decrease toward negative values, of ζ with cNaI implies that Na+ interacts less with -SO3-, than I- does with -(CH3)2N+-. Indeed, a preferential complexation of pSPE with I- results in a decrease of the zeta potential, as the I- negative charges add to the net anionic charge of pSPEs. We now understand an increase of Tc upon small salt addition, by a zeta-potential approaching zero. Indeed, for poly(sulfobetaine)s bearing a finite (i.e., nonzero) net charge, the very existence of charges contributes to swell and solubilize, at least partially, the polymer coil and thus reduces its apparent critical temperature. For the electrolytes that contribute to increase the absolute value of ζ, solubility is immediately promoted. On the

J. Phys. Chem. B, Vol. 111, No. 27, 2007 7775 contrary, for those which contribute to decreasing the absolute value of ζ, solubility is first inhibited and then promoted for larger salt additions. This can be summarized for pSPEs with the following rules between the anion/cation binding (interaction) energies

a classification widely accepted in most papers dedicated to pSPEs,14,15,19-21 although the present zeta-potentiometric study is the first quantitative explanation of the phenomenon. Note that, according to this classification, NaBr would have been a better choice than NaCl for our study of the salty domain (ii) (i.e., cS g c/S), although this could not be foreseen in the beginning: NaBr, as a neutral salt, never induces an increase in the Tc of pSPEs. As for pSPPs, in Figure 9d, we observe that ζ(SPP1404) decreases with cNaCl, while for a pSPE of a comparable degree of polymerization, ζ(SPE1611) increases with cNaCl, very much like what was found above for SPE6442. Therefore, while the carboxylate-based zwitterionic motive in pSPEs favors an association with Na+ cations, the amido-based zwitterionic motive in pSPPs favors an association with Clanions, which results in a decrease of the zeta-potential. This can be summarized for NaCl with the following rules between the anion/cation binding (interaction) energies:

The binding with Cl- of the ammonium site in the amido-based zwitterionic motive is thus stronger than that of the carboxylatebased one. In pSPEs, the electro-attractive oxygen of the carboxylate carrying function makes the ammonium site -(CH3)2N+- even more electro-positive and, thus, more attractive for anions, including the sulfonate group -SO3- itself. In pSPPs, the ammonium site is not as electro-positive not only because the amido carrying function itself is less electro-attractive but also because of the extra -CH2- spacer between the ammonium and the amido -NH- carrying function. Overall, the potential of the ammonium site for binding anions is simply less in pSPPs than in pSPEs. This also explains why pSPEs have critical temperatures larger than pSPPs. Quite reversably, Nesterenko and Haddad,30 comparing sulfobetaines to carboxybetaines, suggested that a carboxylate site -COO- is less complexating for a quaternized ammonium one -(CH3)2N+- than a sulfonate site -SO3- is. Finally, it is crucial to note that, unlike SPE1611, SPP1404 crosses the zero-zeta-potential line when cNaCl ) 0.003 mol/L (see Figure 9d), which precisely corresponds to the maximum reached in the critical temperature and, thus, to the onset of the decrease in Tc with cNaCl. The evolution of the zetapotential of SPP1404 upon an addition of the different salts of the Hoffmeister series is also investigated: ζ(SPP1404) always decreases as the salt concentration is increased, regardless of the salt, yet the zeta-potential decreases faster from NaCl to NaBr and NaI, suggesting a preferential entry of anions Cl-, Br-, and I-. As for pSPEs, the order Cl- < Br- < I- of interactions between anions and the quaternized ammonium site, is respected.

7776 J. Phys. Chem. B, Vol. 111, No. 27, 2007 One may then argue that only the specific interactions in the different couples, {-(CH3)2N+-}/{anions I-, Br-, or Cl-} and {-SO3-}/{Na+}, whether the zwitterionic motive is carried by carboxylate (in pSPEs) or amido (in pSPPs) functions, are at work in the low-salt regime. However, the increase in Tc and loss of solubility identified in the low-salt windows are also molecular-weight dependent, as they appear only for the largest degrees of polymerization which were characterized by zetapotentiometry. For the lower molecular weights, the effect was not observed, a fact which cannot be explained on the sole basis of specific interactions and favored entries. Let us go back to the predictions made on nonperfectly neutral polyampholytes, characterized by nonperfectly equal numbers of positive and negative charges N+ and N-, and which thus bear a net finite charge. Dobrynin and Rubinstein3 have predicted that depending on how large the charge asymmetry δ ≡ |N+ - N-|/(N+ + N-)1/2 is, polyampholytes can be expected to display different behaviors in temperature. In particular, for a large asymmetry, δ > 1, the authors expect polyampholytes to show a deviation from the classical transition from globuleto-swollen coil, which usually occurs for a small asymmetry, δ < 1, as temperature or salt concentration are increased. In between these two behaviors, the authors predict that polyampholytes with large enough charge asymmetry should display a polyelectrolytelike behavior, where solubility is notably promoted due to the repulsive contribution between uncompensated charges, which dominate over the attractive contribution between dipoles. Their prediction has been verified recently with welldefined polyampholyte terpolymers comprising neutral units, as well as positive and negative ones.31 However, no such experimental verification has been carried out in the case of pure zwitterion homopolymers, to the best of our knowledge. The recent work of Ali et al.32 and also that of Liaw et al.33 aimed only at comparing sulfobetaine-based homopolymers and their fully hydrolyzed polyelectrolyte version and did not include charge asymmetry. Their even more recent work34 filled the gap, by considering copolymers of zwitterion and electrolyte motives, which indeed showed a dual polyampholyte/polyelectrolyte behavior. The authors then showed that the type of behavior these copolymers will show in solution, depends on the fraction of electrolyte in the copolymer. In the same way, in the present case, acrylic acid functions resulting from partial hydrolysis along our poly(sulfobetaine)s can be regarded as uncompensated charges: the number of positive and negative charges is not exactly equal, and a charge asymmetry exists. In the case of pSPEs, for instance, an excess negative charge is present in salt-free conditions, as zetapotentiometry confirmed. Within this approach, let us now consider a family of polyampholytes of increasing degree of polymerization N, yet all bearing a constant percentage x of excess negative (or positive) charges resulting from a reaction of degradation, such as a hydrolysis reaction. The net excess charge thus reads |N+ - N-| ≡ xN, while the total number of charges reads N+ + N- ) (2 - x)N. We define a charge asymmetry δ ) xxN/(2 - x)1/2 which depends on the degree of polymerization N: only for degrees of polymerization larger than the threshold value Nc ) (2 - x)/x2 does charge asymmetry δ become larger than unity. Our understanding of the phenomenon is thus as follows. For polymers whose degree of polymerization exceeds Nc, hydrolysis leads to a large enough charge asymmetry that Coulombic repulsion between likecharged sites is not negligeable: the chain solubility is promoted compared to that of an ideal pure polyzwitterion with no charge asymmetry. Once NaCl is added to pSPEs, the preferential

Mary et al. interaction of sulfonate sites with Na+ counterions promotes their entry, which compensates for the negative charge of acrylic acid sites: charge asymmetry is reduced, as confirmed by zetapotentiometry, and the behavior of the chain shifts to that of a true polyampholyte, where Coulombic attraction between oppositely charged sites dominates. The macroscopic consequence is the increase in critical temperature we witness. On the contrary, in the case of pSPPs, the entry of Cl- is promoted and thus aggravates the charge asymmetry: the polyelectrolyte-like behavior and the associated increase in Tc should appear sooner in molecular weight. This is consistent with our experimental findings, since Figure 9c showed that SPE1611 did not show an increase, while SPP1404 already does. To see whether our scenario is valid, let us try to estimate Nc in our practical cases. Since acrylic acid is a weak acid, its dissociation depends on pH, acrylic acid concentration, and ionic force. If all acrylic acid functions were dissociated, the net charge cannot be greater than xN. However, considering that approximately half of the acrylic acid functions should be charged at a pH of 7, we take x/2 as a more reasonable estimate of the percentage of negative acrylic acid sites due to hydrolysis, leading to Nc ≈ 820 for a percentage of hydrolysis on the order of 10%. This value should be considered only as indicative of the order of magnitude, since Flory-like approaches always assume all motives to have the same reference volume, which is far from true here since acrylic acids are more than half the size of a zwitterionic monomer. For pSPEs, we found that specimen SPE1611 behaves classically, while SPE6442 showed the peculiar increase in Tc: the critical degree of polymerization where the increase in Tc at low salt concentration becomes visible lies in between 1611 and 6442. For pSPPs, it lies in between 531 and 1404. Both experimental values are of the same order of magnitude as the estimate Nc ≈ 820 of the critical degree of polymerization where charge asymmetry effects should become visible based on a 10% hydrolysis, although, for pSPPs, the process which leads to a positive net charge remains unknown for now. 4. Conclusions We provide insight into the physics that regulates the watersolubility of sulfobetaine-based polyzwitterions considered as model polyampholytes, both in the low- and high-salt regimes. Quite expectedly, the study of the impact of different salts on the critical temperatures of pSPEs and pSPPs suggests that poly(sulfobetaine)s-salts interactions and solution behaviors depend both on the nature of the salt and on that of the zwitterionic motive, via specific interactions between the two. The very existence of these specific interactions must not be overlooked: differences between the binding energies of the different ionic couples involved, {-(CH3)2N+-/-SO3-}, {Cl-/Na+}, {-(CH3)2N+-/Cl-}, and {-SO3-/Na+}, whether the zwitterionic motives are carried by a carboxylate in pSPEs, or an amido function in pSPPs induce a penetration of the added electrolytes inside the polymer coil not necessarily symmetrical, which dramatically impacts the solution behavior of the polymer as was fully illustrated. A simple osmotic-based, mean-field model for large enough salt additions necessarily fails at explaining the differences in behavior observed between pSPPs and pSPEs of comparable degrees of polymerization, simply because it relies on the very assumption that no specific, asymmetrical interactions exist between the zwitterionic positive and negative charges and their potential anionic and cationic counterions coming from the added salt. According to our description, the evolution of the critical temperature with salt

Solution Behavior of Polyzwitterions concentration should be independent of both the zwitterionic motive and the nature of salt. This, of course, is not the case for either one, as illustrated in numerous published salt studies. However, the additional zeta-potentiometric data and the studies in the low-salt regime helped elucidate these peculiarities: the charge asymmetries resulting from partial degradations such as hydrolysis, combined with the known selective interactions between the different ionic species, all contribute to affect the solubility and subsequent properties of these polymers. Acknowledgment. The authors are thankful to Rhodia for authorizing publication. D.D.B. greatly acknowledges fruitful discussions with Christophe Chassenieux, Universite´ du Maine, France. References and Notes (1) Higgs, P. G.; Joanny, J.-F. J. Chem. Phys. 1991, 94, 1543. Wittmer, J.; Johner, A.; Joanny, J.-F. Europhys. Lett. 1993, 24, 263. (2) Kantor, Y.; Li, H.; Kardar, M. Phys. ReV. Let. 1993, 69, 61. Kantor, Y.; Kardar, M.; Li, H. Phys. ReV. E 1994, 49, 1383. (3) Dobrynin, A. V.; Rubinstein, M. J. Phys. II 1995, 5, 677. (4) Borukhov, I.; Andelman, D.; Orland, H.; Eur. Phys. J. B 1998, 5, 869. (5) Moldakarimov, S.; Johner, A.; Joanny, J.-F. Eur. Phys. J. E 2003, 10, 303. (6) Diehl, A.; Barbosa, M. C.; Levin, Y. Phys. ReV. E 1996, 54, 6516. (7) Dobrynin, A. V.; Rubinstein, M. Prog. Polym. Sci. 2005, 30, 1049. (8) Ohlemacher, A.; Candau, F.; Munch, J. P.; Candau, S. J. J. Polym. Sci. Part B: Polym. Phys. 1996, 34, 2747. (9) Skouri, M.; Munch, J. P.; Candau, F.; Neyret, S.; Candau, S. J. Macromolecules 1994, 27, 69. (10) Joanny, J.-F. J. Phys. II 1994, 4, 1281. (11) Dobrynin, A. V.; Rubinstein, M.; Joanny, J.-F. Macromolecules 1997, 30, 4332. (12) Dobrynin, A. V.; Colby, R. H.; Rubinstein, M. J. Polym. Sci. Part B: Polym. Phys. 2004, 42, 3513.

J. Phys. Chem. B, Vol. 111, No. 27, 2007 7777 (13) Yamakov, V.; Milchev, A.; Limbach, H. J.; Du¨nweg, B.; Everaers, R. Phys. ReV. Lett. 2000, 85, 4305. (14) Schulz, D. N.; Peiffer, D. G.; Agarwal, P. K.; Larabee, J.; Kaladas, J. J.; Soni, L.; Handwerker, B.; Garner, R. T. Polymer 1986, 27, 1734. (15) Liaw, D-J.; Lee, W-F.; Whung, Y-C.; Lin, M-C. J. Appl. Polym. Sci. 1987, 34, 999. (16) (a) Liaw, D. J.; Huang, C. C.; Chou, Y. P. Eur. Polym. J. 1997, 33, 829-836. (b) Liaw, D. J.; Huang, C. C. Polymer 1997, 38, 63556372. (17) Salamone, J. C.; Volkson, W.; Oison, A. P.; Israel, S. C. Polymer 1978, 19, 1157. (18) Salamone, J. C.; Volksen, W.; Israel, S. C.; Olson, A. P.; Raia, D. C. Polymer 1977, 18, 1058. (19) Huglin, M. B.; Radwan, M. A. Polym. Int. 1991, 26, 97. (20) Huglin, M. B.; Radwan, M. A. Macromol. Chem. Phys. 1991, 192, 2433. (21) Lee, W.-F.; Tsai, C.-C. Polymer (Guildf.) 1995, 36, 357. (22) Flory, P. J. Principles of polymer chemistry; Cornell University Press: Ithaca, NY, 1953; Chapter 13, p 542. (23) Wielema, T. A.; Engberts, J. B. F. N. Eur. Polym. J. 1987, 23, 947. (24) Monroy Soto, V. M.; Galin, J. C. Polymer 1984, 25, 254. (25) Polyelectrolytes; Hara, M., Ed.; Marcel Dekker: New York, 1993. (26) Georgiev, G. S.; Kamenska, E. B.; Vassileva, E. D.; Kamenova, I. P.; Georgieva, V. T.; Iliev, S. B.; Ivanov, I. A. Biomacromolecules 2006, 7, 1329. (27) Handbook of physics and chemistry, 69th ed.; CRC Press Inc.: Boca Raton, FL, 1989. (28) Cornet, C. F.; Van Ballegooighen, H. Polymer 1966, 7, 293. (29) Schultz, A. R.; Flory, P. J. J. Am. Chem. Soc. 1952, 74, 4760. (30) Nesterenko, P. N.; Haddad, P. R. Anal. Sci. 2000, 16, 565. (31) Ezell, R. G.; Gorman, I.; Lokitz, B.; Treat, N.; McConaughy, S. D.; McCormick, C. L. J. Polym. Sci., Part A: Polym. Chem. 2006, 44, 4479. (32) Ali, S. A.; Mazumder, M. A. J.; Al-Muallem, H. A. J. Polym. Sci., Part A: Polym. Chem. 2003, 41, 172. (33) (a) Liaw, D. J.; Huang, C. C. Macromol. Chem. Phys. 2000, 201, 1101. (b) Liaw, D. J.; Huang, C. C.; Sang, H. C.; Kang, E. T. Polymer 2001, 42, 209. (34) Mazumder, M. A. J.; Umar, Y.; Ali, S. A. Polymer (Guildf.) 2004, 45, 125.