Loose Complexation of Weakly Charged Microemulsion Droplets and

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J. Phys. Chem. B 2006, 110, 6415-6422

6415

Loose Complexation of Weakly Charged Microemulsion Droplets and a Polyelectrolyte E. Buhler,*,†,‡ J. Appell,† and G. Porte† Laboratoire Colloı¨des, Verres, Nanomate´ riaux (LCVN), UMR 5587 CNRS-UniVersite´ Montpellier II, CC26, 34095 Montpellier Cedex 05, France, and Laboratoire Spectrome´ trie Physique (LSP), UMR 5588 CNRS-UniVersite´ Joseph Fourier de Grenoble, BP87, 38402 St Martin d’He` res, France ReceiVed: December 2, 2005; In Final Form: January 31, 2006

The effect of polyelectrolyte addition on the properties of an oil-in-water (O/W) microemulsion of weakly charged spherical micelles is studied. The 81 Å radius O/W droplets in this system can be charged by the partial substitution of the nonionic surfactant by a cationic surfactant. The effect of the addition of poly(acrylic acid) (PAA), which is a charged pH-dependent polyelectrolyte, on the interactions between charged or noncharged droplets has been investigated using SANS. We have characterized the phase behavior of this pH-smart system as a function of the microemulsion and the polyelectrolyte concentration and the number of charges per droplet at three pH values: pH ) 2, 4.5, and 12. In particular, an associative phase separation due to the bridging of the droplets by the neutral PAA chains through H-bonds is observed with extremely low PAA addition at low pH. At the opposite, an addition of PAA at pH ) 4.5 generates a strong repulsive contribution between neutral droplets. Electrostatic bonds between charged droplets and PAA, controlled by the number of charges per droplet, are responsible for a pH drift and then for an associative phase separation similar to that observed at low pH. Finally, at high pH, the creation of electrostatic bonds between fully charged PAA and charged droplets liberates sufficiently counterions in solution at high droplet charge density to screen the electrostatic interactions and to allow an associative phase separation.

1. Introduction The control of interactions between charged colloids by the addition of small amounts of an oppositely charged polyelectrolyte is widely used in industries (water treatment, paper industry, smart and stimuli responsive materials, etc.). Complexes formed by polyelectrolytes and a spherical colloidal particle of opposite sign such as proteins,1 charged surfactant micelles,2-7 or charged vesicles have been experimentally8-12 and theoretically13-15 extensively studied recently. Not only the nature and the structure of the complex but also the interactions between the spherical colloidal particles depend on many parameters: the charge and the radius of the sphere, the linear charge density and the flexibility of the polyelectrolyte, and the ionic strength and the pH of the solution. For these kinds of studies, an oil-in-water (O/W) microemulsion of weakly charged spherical micelles can be used. Indeed, microemulsions are thermodynamically stable isotropic multicomponent fluids, normally composed of water, oil, and surfactant. The surfactants assemble as dividing surfaces between oil and water domains. In many respects, the properties of nanometer-sized O/W spherical droplets resemble those of small colloidal particles. Consequently, by introducing a small amount of ionic surfactants among the otherwise neutral surfactants, a microemulsion can be weakly charged at will.16-20 These surface charges allow for a control of the phase behavior and of the structure of the system by the addition of an oppositely charged polyelectrolyte. Indeed, by the addition of polyelectrolytes, it is possible to obtain systems for which the phase behavior (phase separation vs * To whom correspondence should be addressed. E-mail: [email protected]; [email protected]; porte@ gdpc.univ-montp2.fr. † CNRS-Universite ´ Montpellier II. ‡ CNRS-Universite ´ Joseph Fourier de Grenoble.

redispersion) is extremely sensitive to small pH variations. For example, the sensitivity of the conformation of the polyelectrolyte poly(acrylic acid) (PAA), which bears charges in aqueous solutions at high pH, to the physicochemical environment (pH, ionic strength, etc.) can be used to design smart microemulsions. Work by other groups mainly concerns complexes formed by O/W droplets and neutral polymers such as amphiphilic graft copolymers,21 block copolymers, triblock copolymers,22-24 and hydrophobically modified hydrosoluble polymers, that is, telechelic polymers.25-30 In particular, in a previous study,25 experimental SANS and phase behavior results are discussed in terms of decoration and/or bridging of O/W droplets by the hydrophobically modified water-soluble poly(ethylene oxide) (PEO). Under appropriate conditions, the telechelic polymers can generate reversible networks as a result of the association of the two hydrophobic ends with the droplets and the formation of bridges between droplets. In this study, we planned to characterize the interactions between positively charged O/W microemulsion spherical droplets in the presence of the polyacid PAA of high molecular weight. For this, we use a microemulsion of decane droplets stabilized in water by a monolayer of the nonionic surfactants Triton X-100 and Triton X-35. The oil-in-water droplets in this system can be weakly charged by replacing a small fraction (0.01-0.03 in weight) of the nonionic surfactant with the cationic surfactant CPCl. Under these experimental conditions, the microemulsion droplets are unperturbed. To this microemulsion we add a water-soluble polymer, PAA, which is neutral and with a coil conformation at low pH (∼2) and which becomes progressively charged and swells upon pH increase; at high pH (∼12), it is totally charged and stretched. By mixing the polyanion PAA with the oppositely charged droplets, we expect the polyelectrolyte chains and the charged droplets will bind

10.1021/jp057019e CCC: $33.50 © 2006 American Chemical Society Published on Web 03/03/2006

6416 J. Phys. Chem. B, Vol. 110, No. 12, 2006 electrostatically. With the neutral droplets, we will argue that hydrogen bonds are formed between the PAA and the droplets’ coronas at low pH, that is, at a pH where PAA is neutral or weakly charged. The mixed PAA/charged or neutral microemulsion system presents similarities with the telechelic PEO/ microemulsion system25-30 as PAA chains can also decorate and/or link the droplets. However, there are a number of different features: (i) bonds between the droplets and the PAA are possible all along the PAA chain, (ii) these bonds can be electrostatic bonds and/or hydrogen bonds depending on the pH conditions, and (iii) the PAA charge and conformation change with pH. It is then possible to control the interactions introduced by the PAA between the charged or noncharged droplets, and thus the phase behavior of the system, simply by varying the pH of the solution. To study the structure of these mixed systems, small angle neutron scattering (SANS) appears as the most appropriate experimental technique since (i) the scattering is dominated by the droplets signal, (ii) the correlations and the interactions introduced by the polyelectrolyte addition between the droplets can be determined in the available q range, and (iii) the shape of the droplets can be checked and controlled very easily using a classical Porod analysis of the form factor at large scattering wave vectors. In the present paper, we report on the phase behavior study and on the scattering experiments performed on neutral or charged O/W microemulsions-PAA mixed systems as a function of the relevant parameters, namely, the microemulsion weight fraction, φ, the polyelectrolyte concentration, ψ, and δ, the fraction of charge bearing by surfactants (CPCl), at three pH values: pH ) 2, 4.5, and 12. In section 2 of the paper, we describe the materials and experimental techniques used in this study. In section 3, the phase behavior and the SANS patterns of the samples are described and the results are then qualitatively discussed in terms of the effective interactions introduced by the PAA chains. 2. Materials and Methods 2.1. Sample Characteristics. Triton X35 (TX35, M ) 338 g/mol) and Triton X100 (TX100, M ) 624 g/mol) nonionic surfactants from Sigma Chemicals and decane (H3C(CH2)8CH3) from Fluka were used as received. The cetylpyridinium chloride ([H3C(CH2)15]C5H5N+, Cl-) (CPCl, M ) 339.5 g/mol) surfactant from Fluka has been purified by successive recrystallization in water and in acetone in our laboratory. Poly(acrylic acid) (PAA), which is a pH-dependent charged polymer, of numberaverage molecular weight Mn ) 240 K and polydispersity 1.05 is obtained from Fluka (monomer mass equal to 72 g/mol). To fully characterize the PAA, we have performed a pH-titration measurement. This measurement allowed us to determine a pKa of 6.1 and showed that 99.8% of the monomers are acidic groups COOH. Indeed, PAA is a water-soluble polymer, which bears charges in aqueous solutions at high pH. At low pH values, that is, below 3, the acidic groups of the PAA are undissociated and therefore the polymer exists as a compact coil characterized by a radius of gyration of 60 Å (determined using SANS experiments). When the pH solution is increased, the acidic groups of the PAA chain become progressively ionized. This generates some repulsion between the adjacent groups, and as a result, the polymer coil expands. Above a pH value of ∼89, all the acidic groups on the PAA are ionized and the polymer is in its most expanded state.31 In all experiments, the concentration of PAA is low; it is below the overlap concentration evaluated for a 240 K polymer.31

Buhler et al. To prepare the microemulsion, we proceeded as follows: The microemulsion32 is a thermodynamically stable dispersion in water of oil (decane) droplets surrounded by a surfactant film. We seek conditions where the microemulsion is below, but close to, the line of emulsification failure. The line of emulsification failure is the limit above which the microemulsion droplets are saturated with oil and coexist with excess oil. On this line, the microemulsion droplets have a radius corresponding to the spontaneous curvature of the surfactant film.33 Under such conditions, it is well established that the droplets of the microemulsion are spheres of a well-defined radius34 and that they can be diluted over a large concentration range.35,36 The spontaneous radius of curvature of the surfactant film, composed of surfactant TX100 and cosurfactant TX35, is adjusted by varying its composition: The weight ratio of TX35 to TX100 is set equal to 0.5 in water and to 0.48 in D2O. The constant oil-to-surfactant ratio, that is, the weight ratio of decane to TX100 + TX35 is, in what follows, equal to 0.7 in H2O and to 0.76 in D2O. The spherical droplets were made weakly charged by replacing a small fraction of the nonionic surfactants TX100 and TX35 with the cationic surfactant CPCl. δ, the weight ratio of ionic to neutral surfactant, ranges from 0 to 3 wt %, retaining the droplet size. δ ) 1% corresponds approximately to 22 CPCl molecules per decane droplet, itself surrounded by approximately 1550 surfactant molecules. SANS experiments were performed in D2O, whereas the phase diagram was studied in H2O. Within these experimental conditions, the droplets of the microemulsion are spherical with a narrow distribution of size and adopt a mean radius of 81 ( 2 Å,30 easily determined using a classical Porod analysis of the SANS data (cf part 2.4.) and can be diluted over a range of ∼1-20 wt %. The microemulsion droplets plus the PAA polymer systems were prepared by weight using initial solutions of polymer and droplets at the desired pH. In the following, φ (the droplets concentration), ψ (the PAA concentration), and δ (the fraction of charge bearing by surfactants) represent weight fractions. 2.2. Observation of the Phase Behavior of the Samples. The samples prepared as described above are thoroughly shaken to ensure homogenization and then kept at the temperature of observation, here T ) 20 °C, for several days before visual examination. When a phase separation is observed, the samples are rehomogenized and set back to rest for a couple of days to confirm the observations. 2.3. Small Angle Neutron Scattering Measurements. SANS experiments were carried out on the spectrometer D11 at the Laue Langevin Institut at Grenoble (ILL, France) and on the spectrometer PACE at the Le´on Brillouin Laboratory at Saclay (LLB, Orphe´e reactor, France). The chosen incident wavelength λ depends on the set of experiments, as follows. For a given wavelength, the range of the amplitude of the transfer wave vector q was selected by changing the detector distance D. The scattering wave vector q is given by

q)

4π θ sin λ 2

(1)

where θ is the scattering angle. Three sets of sample-to-detector distances and wavelengths were chosen at ILL (D ) 2 m, λ ) 6 ( 0.5 Å; D ) 8 m, λ ) 6 ( 0.5 Å; and D ) 28 m, λ ) 6 ( 0.5 Å) so that the following full q range was available: 1.62 × 10-3 e q (Å-1) e 1.86 × 10-1. Two sets only of sample-todetector distances and wavelengths were chosen at LLB (D ) 2 m, λ ) 6 ( 0.5 Å; and D ) 4.7 m, λ ) 10 ( 1 Å) so that the following full q range was available: 4.1 × 10-3 e q (Å-1) e

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Figure 1. Scattering length density profile. The best fit is for R1 ) 66 Å, R2 ) 72 Å, and R3 ) Rm ) 81 Å with a standard deviation of 15 Å (the two coronae retain a constant width).

1.77 × 10-1. Measured intensities were callibrated to absolute values (cm-1) using the normalization by the attenuated direct beam classical method. Standard procedures to correct the data for the transmission, detector efficiency, and backgrounds (solvent, empty cell, electronic, and neutronic backgrounds) were performed. 2.4. Characterization of the O/W Droplets with Charges and/or in the Presence of PAA. The scattered intensity of a spherical colloids dispersion with a distribution of size is given by the following relation37

I(q) ) I0(q)SM(q)

(2a)

I0(q) ) φvVm-1〈A2〉

(2b)

and

In eq 2, φv is the volume fraction, and Vm and 〈A2〉 represent, respectively, the volume and the squared scattered amplitude of the colloids (O/W droplets) averaged over the size distribution. SM(q), the measurable structure factor, reflects the interactions between the droplets; it is equal to 1 at large q values or for very dilute samples where interactions can be neglected. The best simulation is obtained for spherical droplets of outer radius R3 but with three successive scattering length densities corresponding roughly to an inner sphere of decane, a corona containing the aliphatic chains of surfactant, and an outer corona containing the polar heads, which are short PEO chains. The scattering length density profile is sketched in Figure 1. The averaged squared amplitude is calculated from the amplitude for spherical concentric shells38 and with a Schulz-Zimm distribution for the size of the droplets.39 To simulate correctly the experimental spectra, the model spectra are convoluted with the instrumental response function taking into account the actual distribution of the neutron wavelength and the angular definition.40 The first important question is to know whether the microemulsion droplets remain identical or not with a partial substitution of the nonionic surfactant by the cationic surfactant CPCl and/or with an addition of PAA in the solution. We expect that adding PAA to the solution and/or electrostatic charges to the droplets does not modify the droplets. This can be checked by comparing the SANS patterns in the Porod representation for the bare microemulsion and for the samples with charged droplets and/or PAA added. Such a comparison is made in Figure 2 for φ ) 11% samples with δ ) 0% and ψ ) 0%, δ ) 1% and ψ ) 0%, δ ) 0% and ψ ) 0.55%, δ ) 1% and ψ ) 0.55%, and for samples with φ ) 2%; δ ) 0, 0.25, 0.5, and 1%; and ψ ) 0%, respectively. The form factor oscillations, damped by the size distribution, are amplified in the q4 × I(q)

Figure 2. SANS patterns in the Porod representation illustrating the superimposition of the form factor in the high q range for (a) φ ) 11%, ψ ) 0%, δ ) 0% (O); ψ ) 0%, δ ) 1% (4); ψ ) 0.55%, δ ) 0% (+); and ψ ) 0.55%, δ ) 1% (0); and for (b) φ ) 2%, ψ ) 0%, δ ) 0 (4); 0.25 (O); 0.5 (0); and 1% (3). The solid lines are the spectra computed as explained in the text with Rm ) R3 ) 81 Å and ∆R ) 15 Å.

representation. The patterns are perfectly superimposed in the large q range (when SM(q) ∼ 1), indicating no change in the droplets. Similar observations have been made for all samples with 2 < φ < 20%, 0 < δ < 5%, and 0 < ψ < 0.55%. The oscillations are reasonably well reproduced with I0(q) calculated as described above with the scattering length density profile given in Figure 1, ∆R ) 15 Å and Vm ) (2.5 ( 0.3) × 10-18 cm3. The microemulsions are well represented by a suspension of spheres with a small size polydispersity (∆R/Rm ∼ 0.18). The analysis of Figure 2 provides then strong evidence that the droplets remain identical, that is, spherical with an external mean radius equal to R3 ) Rm ) 81 Å. Having established that the droplets remain identical under our experimental conditions, we can now examine in the following parts the interactions introduced between the droplets by the presence of the polyelectrolyte PAA chains for various pH conditions. To do so, we have two kinds of experimental evidence, namely, the phase behavior of our system and the evolution of the SANS patterns in the low q range as a function of the relevant parameters (φ, ψ, δ, and pH). 3. Results and Discussion This section deals with the phase behavior and the SANS experiments performed on noncharged and on charged O/W microemulsions in the presence of PAA chains at T ) 20 °C. We will describe successively the interactions introduced by the PAA chains at three pH values (pH ) 2 (neutral PAA), pH ) 4.5 (partially charged PAA), and pH ) 12 (fully charged

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Figure 3. Phase behavior of the microemulsion upon the addition of PAA at pH ) 2 and T ) 20 °C.

PAA)) for the two situations: first neutral O/W droplets and second charged O/W droplets. 3.1. Effect of PAA Addition on the Interactions Between Neutral Droplets. In this part, the O/W droplets are neutral and the behavior of the mixed system is driven by the pH-dependent PAA chain conformation. 3.1.1. Evidences of Bridging at Low pH. We have determined the partial phase diagram of mixtures as a function of the noncharged O/W droplets and PAA weight fractions at pH ) 2. In Figure 3, we have sketched the phase evolution of the samples with the ratio ψ/φ (vertical axis) and with the droplet weight fraction, φ (horizontal axis). For extremely low PAA addition, the solutions are monophasic and transparent over the whole range of microemulsion concentration. When the PAA concentration is increased, one observes for low droplet concentrations a phase separation: a dilute phase coexists with a more concentrated and viscous phase, while, at higher droplet concentrations, the samples remain clear monophasic solutions. The shape of the phase diagram reported in Figure 3 is reminiscent of that observed in previous studies30,41 performed on similar associative mixed systems. The phase separation observed in Figure 3 is an associative phase separation42 brought about by an effective attractive interaction between the droplets. This interaction must be due to the bridging of the droplets by the PAA chains. How do the PAA chains, neutral at this low pH, bind to the neutral droplets? It is known31,43-45 that the ethylene oxide of poly(ethylene oxide) (PEO) and the undissociated carboxylic acidic group, COOH, of the PAA (active site) bind through hydrogen bonds at low pH. We assume that here, similarly, H-bonds occur between the PAA chain and the ether oxygen of the droplets’ coronas. One PAA chain can then bind onto one droplet (i.e., decoration) or onto two or more droplets (i.e., bridging). As mentioned above, the shape of the phase diagram is consistent with a “sticky-droplets phase separation” and SANS results indicate that the concentrated upper phase contains the major part of the droplets and of the PAA, while the lower fluid phase is a very dilute solution of both droplets and PAA. The number of combinations for the possible H-bonds is much larger in the concentrated phase: this phase separation has a purely entropic origin.46-48 The critical point associated with it is approximately at φ ∼ 10% and ψ/φ ) 0.00075. It is noteworthy that the phase separation is observed for minute quantities of PAA (1 PAA chain for 50-200 droplets depending on φ). Figure 4 displays the variation of the scattered intensity with q for PAA concentrations equal to 0, 0.002, 0.006,

Figure 4. Variation of the scattered intensity, I, with q for PAA concentration ψ ) 0 (b), 0.002 (O), 0.006 (2), 0.008 (0), and 0.01% (1) at φ ) 11% and pH ) 2.

0.008, and 0.01% (i.e., just before the phase separation) at φ ) 11% and pH ) 2. A very steep increase of the attractive interaction is observed just before the phase separation. As a matter of fact, it is well established that poly(ethylene oxide) (PEO) and PAA form an interpolymer aggregate through hydrogen bonding in an aqueous medium at low pH.31,43-45 In particular, it has been shown that a definite number of hydrogen binding sites are necessary for a stable interpolymer aggregate to be formed and that cooperative interaction among sites plays an important role in aggregate formation.49-52 Our observations seem to indicate a similar cooperative interaction between the PAA and the EO of the droplet’s corona. Despite the large number of hydrogen-bonded active sites on a chain, the occurrence of this strong attractive interaction between the droplets leading to the phase separation produced by adding a very small amount of PAA remains to be understood. 3.1.2. Behavior at Intermediate pH. We now focus on the behavior of the mixed system at pH ) 4.5. At pH ) 4.5, some of the acidic groups of the PAA chain are ionized and the expected degree of dissociation is 1% (roughly 26 charges per polymer chain). The PAA conformation is then a “stretched chain of electrostatic blobs”, a concept introduced53 to describe the behavior of a weakly charged flexible polyelectrolyte. An electrostatic blob is a chain subunit within which the Coulombic repulsion is only a weak perturbation (less than kT) and is not sufficient to deform the chain. At a length scale greater than the blob size (equal to ∼135 Å at pH ) 4.5), the Coulombic interaction stretches the PAA chain and the string of these blobs (6 per PAA chain) assumes a rodlike conformation. At this pH, the samples of the mixed system remain clear monophasic solutions for large PAA concentration below the ratio ψ/φ ) 0.2. In Figure 5a, the SANS scattering patterns of PAA/neutral droplets systems, with a constant ratio ψ/φ ) 0.05, and with increasing the weight fraction, φ, are displayed. Surprisingly, for this series of samples, a very well-defined sharp correlation peak is observed. Moreover, the maximum of the correlation peak clearly moves to larger q values as φ increases, as expected for an ordering due to a regular repulsive interaction between droplets. Furthermore, the evolution with φ of the characteristic distance between the droplets, d ) 2π/qmax, where qmax is the maximum of the peak, plotted in Figure 5b, is well described by a scaling law characterized by an exponent equal

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Figure 6. Evolution of the SANS patterns with φ for bare microemulsions at φ ) 1.5 (]), 3 (4), 7.25 (O), and 15% (0) at T ) 20 °C and pH ) 4.5.

Figure 5. (a) SANS patterns for noncharged microemulsions as a function of the droplets weight fraction, φ ) 2 (3), 4 (2), 8 (O), and 11% (b) at a fixed ratio ψ/φ ) 0.05 and pH ) 4.5. (b) Average distance, d, between droplets (as deduced from qmax, the position of the maximum of the correlation peak of curve a) as a function of φ. The line represents the fit of the data, d ∼ φ-1/3.

to -1/3, d ∼ φ-1/3, as expected for the simple dilution of a 3D periodic lattice of droplets. The repulsive interaction between droplets is much more pronounced than that observed in the bare microemulsion20 as can be seen upon inspection of the SANS patterns for the bare microemulsions presented in Figure 6. Contrary to the situation shown in Figure 5, where PAA is added, no repulsive correlation peak is observed for the lowest droplet concentrations. The origin of this repulsive interaction is most probably an electrostatic repulsion between the droplets. The droplets, initially neutral, bind to the PAA through hydrogen bonds (as in the previous situation at the lower pH), with most of the PAA sites still being undissociated, but the PAA chain is now weakly charged. The decorated droplets are thus charged and a repulsive electrostatic interaction sets in. Nevertheless, it is difficult to understand the structure factor observed under the experimental condition ψ/φ ) 0.05, where each group of four droplets (on the average) must share one PAA chain. This puzzling point remains to be explained by further investigations. A phase separation takes place at higher PAA content in the system, that is, for ratio ψ/φ larger than 0.2. This phase separation is different in nature from that observed at pH ) 2. The solution becomes turbid and a phase separation occurs with a thin oily phase floating above a clear solution. Note that, for ψ/φ ) 0.2, the ratio between PAA chains and droplets is equal to ∼1.1 (in other words, 3600 acrylic acid monomers are available on average for one droplet). As described above, the

Figure 7. Comparison of the SANS pattern of a φ ) 11% microemulsion in the presence of ψ ) 0.55% PAA at pH ) 12 (b) and of a bare microemulsion at the same concentration (φ ) 11% and ψ ) 0% (0)).

microemulsion has been prepared just below the emulsification failure limit; the surfactant film then has its optimum radius of curvature. When decorated with large amounts of PAA, this optimum radius probably decreases and the system adjusts to a larger number of smaller droplets with the same (or almost the same) total area of the surfactant film, then the total volume of decane that can be enclosed in the droplets decreases and a small part of it is rejected at the top of the sample. A similar emulsification failure has been studied for a microemulsion decorated by polyethylene chains anchored to the droplet by an aliphatic end chain.54 3.1.3. Presence of Two Distinct Populations at High pH. At pH ) 12, all the acidic groups on PAA are ionized and the polymer is fully charged. As a result, attractive interactions between neutral droplets and PAA through hydrogen bonds are not possible anymore. Consequently, the system is composed of a mixture of two independent and noninteracting populations, namely, fully charged PAA and neutral drops. This can be checked in Figure 7, where the SANS pattern of a neutral microemulsion in the presence of PAA at pH ) 12 is compared with that of a bare microemulsion at the same concentration.

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Figure 8. Phase evolution of the samples at pH ) 4.5 with δ, the weight ratio of ionic to neutral surfactants (plotted as the vertical axis), and with the microemulsion weight fraction, φ (plotted as the horizontal axis), at a fixed temperature T ) 20 °C and fixed ratio ψ/φ ) 0.05. δ ) 1% represents 22 CPCl per droplet; see text.

The scattering patterns, dominated by the droplet signal, are nearly identical. 3.2. Effect of PAA Addition on the Interactions Between Charged Droplets. We stated above that neutral or weakly charged PAA (at low or intermediate pH) binds to the droplets via hydrogen bonds between the undissociated carboxylic groups and the EOs of the droplet corona. In the presence of charged droplets, a new possibility opens up, namely, an electrostatic bond between a carboxylic group and an ionic surfactant CPCl

COOH + CP+Cl- f CP+COO- + H+ + ClSuch a bond is readily formed on each ionic surfactant as the right-hand side of this equilibrium is favored by the gain in entropy due to the release of the two small ions H+ and Cl-. The situation is different at high pH when the PAA chain is totally charged; it will be described below. 3.2.1. Effect of PAA Addition at Low pH. At this low pH, the neutral PAA binds to the droplets mainly through H-bonds but also through the electrostatic bonds between the ionic surfactant and a carboxylic group as sketched above. Thus, the charges on the droplets are neutralized and the Coulombic repulsive interaction, which exists between charged droplets when they are alone in solution, is suppressed. The situation is thus similar to that described for PAA and neutral droplets at pH ) 2; we indeed observe a similar phase behavior (cf Figure 3) upon charge addition on the droplets (δ ranging from 1 to 3%). 3.2.2. Associative Phase Separation at Intermediate pH. The phase evolution of the samples with the number of charges per droplet is displayed in Figure 8. The weight ratio of PAA to droplets is fixed and equal to ψ/φ ) 0.05 (∼1 PAA chain for 4 droplets). The vertical axis is δ, the weight fraction of ionic to neutral surfactants. For low charges addition (δ < 1%), the solutions are monophasic and transparent over the whole range of droplet concentration. When the number of charges per droplet is increased, one observes an associative phase separation similar to the one described above: a clear dilute phase coexists with a more concentrated and viscous phase

Buhler et al.

Figure 9. Evolution with δ, the charge density, of the SANS patterns for φ ) 11%, ψ ) 0.55%, and pH ) 4.5: δ ) 0 (b), 0.5 (O), 0.9 (2), 1 (3), and 1.14% (9, at the vicinity of the phase separation).

containing almost all droplets and PAA (as indicated by the large q values SANS patterns in both phases). In our experimental conditions, depletion effects are negligible as shown by previous SANS experiments performed under a mixed PEO/microemulsion system with similar polymer and droplets concentrations.25-30 Also, in our case, we observe an associative phase separation. Contrary to a phase separation caused by depletion effects, a dilute phase coexists with a more concentrated and viscous phase containing almost all droplets and large molecular weight PAA. Indeed, at δ ) 0, the situation is that described for neutral droplets with an effective repulsion between the droplets induced by the small number of charges on PAA (cf section 3.1.2.). At δ ) 0, the PAA binds to the droplets by hydrogen bonds, but when δ increases, electrostatic bonds between the ionic surfactants and the carboxylic groups become possible as described above. The repulsive interaction is progressively wiped out and replaced by an attractive interaction leading to the observed phase separation. These electrostatic bonds participate, as the hydrogen bonds, to the binding of the droplets to PAA, but their major contribution is the release of protons, thus the pH decreases. A rough estimate of the pH shift induced in the φ ) 11% sample at δ ) 1.2% (phase separation) yields pH ∼ 2.5. Following this decrease of pH, PAA is driven progressively back to its uncharged state as the ratio of CPCl to TX increases. This, of course, greatly influences the effective interaction induced by PAA between the droplets: the system switches back to a situation similar to that observed at low pH with an associative phase separation. Note, however, that for low O/W droplet concentration, the phase separation appears for larger δ. Indeed, to obtain the desired pH drift, the number of electrostatic bonds per droplet must be larger. In Figure 9, the variation of the scattered intensity with q is displayed for droplet charge densities, δ ) 0, 0.5, 0.9, 1, and 1.14% at fixed microemulsion and PAA concentration (φ ) 11% and ψ ) 0.55%). It corroborates the progressive change of an effective repulsive interaction to an attractive interaction between droplets. For low droplet charge densities, we retrieve the signature of the repulsive interaction between the droplets that is, the depletion of the intensity at low q values and the presence of the correlation peak. When the charge density, δ, increases,

Microemulsion Droplets and a Polyelectrolyte

Figure 10. Phase diagram of mixtures of charged O/W droplets and PAA for pH ) 12 as a function of the charge density, δ, and of the microemulsion weight fraction, φ, at a fixed ratio ψ/φ ) 0.05 and temperature T ) 20 °C. δ ) 1% represents 22 charged CPCl surfactants per droplet.

an important rise of the scattered intensity at low q values corresponding to an attractive interaction between charged droplets is observed. The rise of the scattered intensity at low wave vectors is maximum in the vicinity of the phase separation appearing for δ ∼ 1.2%. 3.2.3. Attractive Interactions at High pH. At pH ) 12, all the acidic groups on the PAA chains are ionized and the polymer is fully charged and in its most expanded state due to the electrostatic repulsions between the neighboring charged monomers. Hydrogen bonds between droplets and PAA are not possible anymore, but the charges on the droplet allow for bonds of electrostatic origin. The partial phase diagram of mixtures of charged O/W droplets and PAA has been determined for pH ) 12 as a function of δ, the weight fraction of ionic to neutral surfactants, and of the microemulsion weight fraction, φ, at a fixed ratio ψ/φ ) 0.05 (i.e., 1 PAA chain for 4 droplets). The determination was again made visually, and Figure 10 summarizes the observations. For low charge densities, δ, the solutions are monophasic and transparent over the whole range of microemulsion concentrations, whereas these are biphasic at higher charge densities. Upon increasing δ, one again observes a phase separation between a concentrated and a dilute phase. Also, for low droplet concentrations, the phase separation appears for larger δ values. Furthermore, it is important to mention that the phase separation appears for large values of δ, that is, for much larger values than that observed at pH ) 4.5 (see Figure 8). As stated above, at this pH, the PAA and the droplets can only bind through an electrostatic interaction. At pH ) 12, the fully charged polyanion PAA is surrounded with a cloud of counterions Na+ coming from the NaOH excess, while the charged droplets are surrounded by a diffuse cloud of Clcounterions. The polymer chains and the charged droplets bind through electrostatic interaction between the dissociated acrylic acidic groups and the CP+ ionic groups; this liberates the Na+ and Cl- ions in solution. This bond is here again favored by the resulting entropy gain for the small ions

CP+Cl- + COO-Na+ f CP+COO- + Na+ + ClThe number of released ions Na+ and Cl- is identical to that of

J. Phys. Chem. B, Vol. 110, No. 12, 2006 6421

Figure 11. Variation of the scattered intensity, I, with q for droplet charge density: δ ) 0 (b), 1 (O), and 1.5% (2) at φ ) 11%, ψ ) 0.55%, and pH ) 12.

the electrostatic bonds, that is, approximately to the total number of ionic surfactants. Also, each electrostatic bond neutralizes a charge on the droplets. For sufficiently high charge densities, δ, the concentration of released counterions is sufficient to screen the repulsive electrostatic interactions between the remaining charges on the PAA chains. For example, for φ ) 11%, the Debye screening length is equal to 14 Å at the phase separation. Such an electrostatic screening allows the collapse of the system, and this leads to an associative phase separation. One should note that the concentration of released counterions Cl- is constant with the droplet weight fraction, φ, at the phase separation and is approximately equal to 10-2 M. Figure 11 displays the variation of the scattered intensity with q for droplet charge density equal to 0, 1, and 1.5% at φ ) 11% and ψ ) 0.55% (ψ/φ ) 0.05). This series of experiments is realized below the phase separation. For δ ) 0%, the SANS pattern is identical to that of the bare microemulsion at the same concentration: as noted previously, there is no interaction between the neutral droplets and the charged PAA (see Figure 7). The inspection of the patterns in the low q range shows that the addition of charges in the droplets produces first an increase of the repulsive interaction as compared with the neutral microemulsion (δ ) 1%, decrease of I(qf0)) and then, in a second step, an increase of the attractive interaction (δ ) 1.5%, important increase of I(qf0)) as the electrostatic screening progressively sets in (at this concentration, the phase separation takes place at roughly δ ) 3%). 4. Conclusion In this preliminary investigation of the mixed system PAA plus O/W microemulsion droplets, we found evidence that the PAA chains and the charged droplets bind through electrostatic and/or hydrogen bonds, while neutral droplets bind only through hydrogen bonds. Decoration and/or bridging of the droplets are then possible. A wealth of situations is encountered, strongly dependent on the droplet charge and on the pH. We suggest some explanations for the observed facts, but further work is clearly necessary, in particular to better understand the two most striking features: first, the minute quantity of PAA sufficient to induce the associative phase separation at low pH and, second, the strong ordering of the neutral droplets at intermediate pH even when 1 PAA is shared (on average) by 4 droplets.

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