Interactions of Hydrophobically Modified Polyelectrolytes with

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Interactions of Hydrophobically Modified Polyelectrolytes with Nonionic Surfactants Puspendu Deo and P. Somasundaran* NSF IUCR Center for Advanced Studies in Novel Surfactants, Langmuir Center for Colloid and Interfaces, Columbia University, New York, New York 10027 Received December 11, 2004. In Final Form: February 28, 2005 Interactions of surfactants with hydrophobically modified polyelectrolytes in aqueous solutions are important in several applications such as detergency, cosmetics, food, and paints. Complexes formed in these systems raise some fundamental questions about the polymer-surfactant interactions that control their behavior. In this work, the interactions of a nonionic surfactant, penta-ethyleneglycol mono n-dodecyl ether (C12EO5), with a hydrophobically modified anionic polymer, poly(maleic acid/octyl vinyl ether) (PMAOVE), in aqueous solutions were studied using surface tension, viscosity, electron paramagnetic resonance (EPR) spectroscopy, light scattering, and fluorescence spectroscopic techniques. When the nonionic surfactant C12EO5 was added to aqueous solutions of the anionic polymer PMAOVE, it was incorporated into the hydrophobic nanodomains of PMAOVE far below the the critical micelle concentration (cmc) of the surfactant. Two inflection points were observed corresponding to the critical complexation concentration (formation of mixed micelles composed of C12EO5 and the octyl chains of PMAOVE) and the saturation concentration (saturation of the polymer with C12EO5 molecules). Above the saturation concentration, the coexistence of pure C12EO5 micelles and mixed micelles of PMAOVE and C12EO5 was observed. Such a coexistence of complexes has major implications in their performance in colloidal processes.

Introduction Hydrophobically modified polyelectrolytes (HMPs) composed of hydrophilic and hydrophobic segments show a tendency to undergo self-organization in aqueous environments to form well-defined assemblies which exhibit special rheological properties.1-5 Solution interactions of surfactants with such polyelectrolytes have attracted substantial interest because of their wide application in several industrial fields.6,7 These interactions are considered to be the result of complexes formed between the surfactant and the polymer due to electrostatic and hydrophobic forces. These complexes, besides having important practical applications in detergency, cosmetics, food, and paints, also raise some fundamental questions about the mechanism of polymer-surfactant interactions that control their behavior.6-10 Previous studies on polymer-surfactant systems have been devoted mainly to those with attractive interactions that are either weak (e.g., between nonionic and anionic polymers and anionic surfactants) or strong (between oppositely charged polyelectrolytes and surfactants).6-8,11 * Corresponding author. E-mail: [email protected]. (1) Qiu, Q.; Lou, A.; Somasundaran, P.; Pethica, B. A. Langmuir 2002, 18, 5921. (2) Hsu, J.-L.; Strauss, U. P. J. Phys. Chem. 1987, 91, 6238-6241. (3) Barbieri, B. W.; Strauss, U. P. Macromolecules 1985, 18, 411. (4) Zdanowicz, V. S.; Strauss, U. P. Macromolecules 1993, 26, 4770. (5) Bokias, G.; Hourdet, D.; Iliopoulos, I. Macromolecules 2000, 33, 2929. (6) Goddard, E. D.; Ananthapadmanabhan, K. P. Interactions of Surfactants with Polymers and Proteins; CRC Press: Boca Raton, FL, 1993. (7) Kwak, J. C. T. Polymer-Surfactant Systems; Marcel Dekker: New York, 1998. (8) Deo, P.; Jockusch, S.; Ottaviani, M. F.; Moscatelli, A.; Turro, N. J.; Somasundaran, P. Langmuir 2003, 19, 10747. (9) Sen, S.; Sukul, D.; Dutta, P.; Bhattacharyya, K. J. Phys. Chem. B 2002, 106, 3763. (10) Nakagaki, M.; Handa, T. In Structure/Performance Relationships in Surfactants; Rosen, M. J., Ed.; ACS Symposium Series 253; American Chemical Society: Washington, DC, 1984; p 73. (11) Hayakawa, K.; Kwak, J. C. T. J. Phys. Chem. 1982, 86, 3866.

Although it had been recognized a long time ago, the role of attractive hydrophobic interactions between HMP and surfactants has been studied systematically only recently.12-21 In the present study, we explored the interactions between a hydrophobically modified copolymer, poly(maleic acid/octyl vinyl ether) (PMAOVE) (Chart 1), and a nonionic surfactant, penta-ethyleneglycol mono n-dodecyl ether (C12EO5). To study these attractive hydrophobic interactions, we used a multitechnique approach, involving measurements of surface tension, viscosity, electron paramagnetic resonance (EPR), light scattering, and fluorescence. Experimental Section Materials. Polymers. The hydrophobically modified polymer PMAOVE, provided by International Specialty Products, Inc., was synthesized using free-radical polymerization of a 1:1 mole ratio of maleic anhydride and octyl vinyl ether in toluene with Vazo-69 (azo bis-valeryl nitrile) as initiator. The products were purified twice by first dissolving them in acetone (5 wt %) followed by precipitation with an excess of tert-butyl alcohol (40 times in volume). Residual solvent was removed at 50 °C under vacuum to a constant mass. The anhydride moiety of the polymer was then hydrolyzed in triple distilled water to make an approximately 5 wt % solution. The solution was stirred at 500 rpm at 70 °C for about 12 h and then freeze-dried. As determined by gel (12) Piculell, L.; Guillemet, F.; Thuresson, K.; Shubin, V.; Ericsson, O. Adv. Colloid Interface Sci. 1996, 63, 1. (13) Philippova, O. E.; Hourdet, D.; Audebert, R.; Khokhlov, A. R. Macromolecules 1996, 29, 2822-2830. (14) Bromberg, L.; Temchenko, M.; Colby, R. H. Langmuir 2000, 16, 2609-2614. (15) Colby, R. H.; Plucktaveesak, N.; Bromberg, L. Langmuir 2001, 17, 2937-2941. (16) Maltesh, C.; Somasundaran, P. Colloids Surf. 1992, 69, 167. (17) Loyen, K.; Iliopoulos, I.; Audebert, R.; Olsson, U. Langmuir 1995, 11, 1053-1056. (18) Hansson, P.; Lindman, B. Curr. Opin. Colloid Interface Sci. 1996, 1, 604. (19) Iliopoulos, I. Curr. Opin. Colloid Interface Sci. 1998, 3, 493. (20) Tan, H.; Tam, K. C.; Jenkins, R. D. Langmuir 2000, 16, 56005606. (21) Iliopoulos, I.; Wang, T. K.; Audebert, R. Langmuir 1991, 7, 617.

10.1021/la046957n CCC: $30.25 © 2005 American Chemical Society Published on Web 03/31/2005

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Chart 1. Structures of the Polymer Poly(maleic acid/octyl vinyl ether) (PMAOVE), the Surfactant Penta-ethyleneglycol Mono n-Dodecyl Ether (C12EO5), the EPR Probe 5-Doxyl Stearic Acid (5-DSA), and the Fluorescence Probe Pyrene

permeation chromatography, the weight-average molecular weight (Mw) was 160 000 Da with a polydispersity index of 1.23. Probes and Additives. The spin probe 5-doxyl stearic acid (5DSA) (Aldrich, 99+% pure) (Chart 1), pyrene (Aldrich), and pentaethyleneglycol mono n-dodecyl ether (C12EO5) (Nikko Chemicals Co., Tokyo, Japan, 99+% pure), NaCl (Fluka, 99.5% pure), and solutions of 0.1 N hydrochloric acid (Fisher Scientific) and 0.1 N NaOH (Fisher Scientific) were used as received. Triple distilled water was used in all experiments. Methods. Surface Tension Measurements. The surface tension of surfactants and the polymer-surfactant solutions was measured with the Wilhelmy plate technique using a sandblasted platinum plate as the sensor. The pull exerted on the sensor was determined using a Beckman microbalance (model LM 600). The entire assembly was kept in a draft-free plastic cage at a temperature of 25 ( 0.05 °C. For each measurement, the sensor was in contact with the solutions for 30 min to allow equilibration. Viscosity Measurements. A calibrated capillary viscometer (Canon Instrument) was used for measuring the relative viscosity based on that of the triple distilled water at 25 ( 0.05 °C. The viscometer was cleaned with chromic acid and triple distilled water and thoroughly dried with acetone before the measurements. The efflux time for triple distilled water was checked before every measurement for reproducibility. EPR Measurements. ESR spectra were recorded using a Bruker EMX spectrometer operating at X band (9.5 GHz). The concentration of the probe molecule (5-DSA) used in all the studies was 10-4 M. For ESR measurements, the desired portions of 10-5 M 5-DSA in chloroform were added to a glass vial and the solvent was evaporated. Then, the aqueous polymer and polymersurfactant solutions of desired concentrations and volumes were added to the vials under stirring. ESR spectra of the above solutions were recorded 24 h after the sample preparation using Pyrex capillary tubes (∼1 mm inner diameter) as sample containers. Fluorescence Measurements. A concentrated stock solution of pyrene was prepared in acetone, and then, required amounts of this stock solution were transferred to glass vials to have a final pyrene concentration of 0.05 mM. After evaporation of acetone, the sample solutions were added to the vials, which were subsequently wrapped with aluminum foil, and then shaken at 250 rpm overnight. Fluorescence spectra were recorded on a SPEX Fluorolog3 2.2 spectrofluorometer (Jobin Yvon, Inc.). The excitation wavelength used was 335 nm, and the emission was monitored between 350 and 600 nm using sample cells of 10 mm path length.

Dynamic Light Scattering. Dynamic light scattering was run using a Brookhaven research grade system with a BI-900AT correlator and a BI-200SM goniometer with adjustable angles of detection from 15 to 155°. A water cooled Lexel argon laser light source was used at a wavelength of 488 Å. The samples were temperature controlled to (1 °C, and Decalin, a refractive index matching liquid, was used in the sample holder to reduce light bending at the glass interfaces. To minimize the dust effect encountered frequently in light scattering measurement, the sample solutions were filtered through a 0.2 µm Nalgene membrane prior to use. The filtration process was found to produce no detectable effect on the polymer concentration, and due to the relatively large size, it is believed not to change the conformation of the polymer or the polymersurfactant complexes. The sample, contained in a 20 mL glass vial, was then placed in the sample holder in a thermostated water bath. Depending upon the difference between the required temperature and room temperature, at least 30 min is required to equilibrate the system. All of the measurements are done in the dynamic mode (at 90° detection); that is, the instrument measures the diffusion coefficient of the molecules and back calculates to give the effective diameter assuming spherical shape.

Results and Discussions Surface tension measurements are frequently used to study surfactant micellization. Figure 1b shows the surface

Figure 1. Surface tension of aqueous solutions in the absence (b) and in the presence (a) of 0.1% (wt/wt) PMAOVE as a function of C12EO5 concentration. SD ) (2%.

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Figure 2. Relative viscosity of aqueous solutions in the absence (b) and in the presence (a) of 0.1% (wt/wt) PMAOVE as a function of C12EO5 concentration. SD ) (2%.

Deo and Somasundaran

Figure 3. EPR spectra of 5-doxyl stearic acid (0.1 mM) in (a) triple distilled water, (b) C12EO5 micelles ([C12EO5] ) 1 mM), and (c) 0.1% (wt/wt) PMAOVE; full lines are experimental spectra, and dashed lines are simulated spectra.

tension of aqueous C12EO5 solutions as a function of C12EO5 concentration. In agreement with the published values,22 a critical micelle concentration (cmc) of 0.06 mM was observed. As can be seen (Figure 1a), 0.1% PMAOVE lowered the surface tension of the solution from 72 to 53 mN/m. The addition of C12EO5 to the PMAOVE solution caused interesting changes in the surface tension of the mixture. The addition of C12EO5 up to 0.0075 mM to the PMAOVE solution caused a decrease in the surface tension. With the addition of more C12EO5, the surface tension steadily rose to reach a maximum at 0.075 mM C12EO5 (Figure 1a) and then decreased again. The increase in surface tension of the PMAOVE solution between 0.0075 and 0.075 mM C12EO5 can be explained by taking into account possible interactions between C12EO5 and PMAOVE. C12EO5 tends to be incorporated23,24 into the existing hydrophobic nanodomains of PMAOVE at 0.0075 mM, which leads to the depletion of both polymers and surfactants from the air/solution interface. Further addition of C12EO5 above 0.075 mM brings some of the C12EO5 to the air/solution interface, and the surface tension decreases rapidly. As Figure 1a shows, the concentration of free C12EO5 micelle formation is 1 mM. Above the cmc, the surface tension is identical to that of C12EO5 solutions without polymer, indicating that C12EO5 has completely displaced the polymer from the interface at high concentrations of C12EO5 and the interface is occupied under these conditions mainly by surfactant molecules. Figure 2 shows the dependence of the viscosity on the concentration of C12EO5 in the presence and absence of PMAOVE. As expected, in the absence of PMAOVE, the viscosity increases slightly above the cmc (0.06 mM) due to the formation of micelles (Figure 2b). The presence of 0.1% PMAOVE caused the viscosity to increase at a C12EO5 concentration of ∼0.01 mM (Figure 2a), indicating the incorporation of C12EO5 into the hydrophobic nanodomains of PMAOVE, and this leads to the swelling of those nanodomains. Upon further increase in the C12EO5 concentration, the viscosity continued to increase, suggesting incorporation of more C12EO5 molecules into the hydrophobic nanodomains of PMAOVE. These results are in accord with that expected from an increase in the

effective volume occupied by PMAOVE-C12EO5 mixed micelles, compared to that of the pure PMAOVE. EPR spectroscopy has been demonstrated to be a powerful technique to probe microenvironments of supramolecular structures involving surfactants and macromolecules.25,26 We selected 5-doxyl stearic acid (5-DSA) as the spin probe, because of its structural similarity with C12EO5 (Chart 1). Figure 3 shows the EPR spectra of 5-DSA in water, C12EO5 micelles, and PMAOVE. The EPR spectrum of 5-DSA in water solution consists of a sharp three-peak signal characteristic of free probe molecules in fast motion in a polar environment (Figure 3a). In the presence of C12EO5 micelles, a broadened EPR spectrum is observed (Figure 3b), which is consistent with partial hindered rotational mobility of the probe molecule in the C12EO5 micelles.27 In the presence of PMAOVE, a significantly different EPR spectrum was observed (Figure 3c). The spectrum is consistent with the probe in slow-motion condition, causing a partial resolution of the anisotropic components of the magnetic tensors. Because of the significant differences in spectra a and c, it can be concluded that 5-DSA interacts strongly with PMAOVE. Only a negligible fraction (∼1-2%) of 5-DSA remains in the bulk solution in fast-motion condition, as demonstrated by the small sharp peaks indicated by the arrows (Figure 3c). An increase in PMAOVE concentration decreases the fraction of free 5-DSA. The addition of C12EO5 to PMAOVE solutions containing the spin probe 5-DSA causes significant changes in the EPR spectra (Figure 4). The addition of C12EO5 up to 0.0075 mM showed experimentally indistinguishable EPR spectra (Figure 3c and Figure 4a). A change in the EPR spectrum was observed at a concentration of 0.01 mM C12EO5 (Figure 4b). A significant change in the EPR spectrum was observed at a concentration of 1 mM C12EO5. Further changes in the EPR spectrum were observed at a C12EO5 concentration between 1 and 10 mM (Figure 4d, e, and f).

(22) Mukerjee, P.; Mysels, K. J. Critical Micelle Concentrations of Aqueous Surfactant Systems; National Standard Reference Data System-National Bureau of Standards: Washington, DC, 1971. (23) Bromberg, L.; Temchenko, M.; Colby, R. H. Langmuir 2000, 16, 2609. (24) Colby, R. H.; Plucktaveesak, N.; Bromberg, L. Langmuir 2001, 17, 2937.

(25) Ottaviani, M. F.; Turro, N. J.; Jockusch, S.; Tomalia, D. A. J. Phys. Chem. 1996, 100, 13675. (26) Wasserman, A. M.; Kasaikin, V. A.; Zakharova, Y. A.; Aliev, I. I.; Baranovsky, V. Y.; Doseva, V.; Yasina, L. L. Spectrochim. Acta, Part A 2002, 58, 1241-1255. (27) Ottaviani, M. F.; Daddi, R.; Brustolon, M.; Turro, N. J.; Tomalia, D. A. Appl. Magn. Reson. 1997, 13, 347-363.

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Figure 4. EPR spectra of 5-doxyl stearic acid (0.1 mM) in the aqueous solutions of the complex of PMAOVE (0.1%, wt/wt) with C12EO5 at C12EO5 concentrations of 0.0075 mM (a), 0.01 mM (b), 0.75 mM (c), 1 mM (d), 2.5 mM (e), and 10 mM (f).

Figure 5. Rotational correlation time of the spin probe (5doxyl stearic acid; 0.1 mM) as a function of C12EO5 concentration of aqueous PMAOVE (0.1%, wt/wt) solutions. SD ) (2%.

The mobility parameter, rotational correlation time, of the spin probe was determined using the program of Schneider, Freed, and Budil.28,29 Figure 5 illustrates the change in rotational correlation time of 5-DSA in 0.1% PMAOVE as a function of C12EO5 concentration. Interestingly, two inflection points are observed, one at a C12EO5 concentration of 0.01 mM and the other one at 1 mM (Figure 5). The first inflection point (0.01 mM) was also observed in surface tension (Figure 1a) and viscosity measurements (Figure 2a). This C12EO5 concentration (0.01 mM) was assigned as the critical incorporation concentration, above which mixed micelles of hydrophobic chains of PMAOVE and C12EO5 molecules are formed. Figure 5 shows a second transition of the rotational correlation time (1 mM < [C12EO5] < 10 mM) at approximately 1 mM C12EO5. This C12EO5 concentration is proposed to correspond to the saturation of PMAOVE with C12EO5. Upon further addition of C12EO5, the rotational mobility of the spin probe remains constant, implying the (28) Schneider, D. J.; Freed, J. H. Biological Magnetic Resonance Spin Labeling; Plenum Press: New York, 1989; Vol. 8. (29) Budil, D. E.; Lee, S.; Saxena, S.; Freed, J. H. J. Magn. Reson., Ser. A 1996, 120, 155.

Figure 6. Hyperfine splitting constant (AN) of 5-doxyl stearic acid (0.1 mM) as a function of C12EO5 concentration of aqueous solutions in the absence (b) and in the presence (a) of PMAOVE (0.1%, wt/wt). SD ) (2%.

coexistence of pure C12EO5 micelles with mixed micelles of PMAOVE with C12EO5. The hyperfine-coupling constant (AN), which is a measure of the polarity of the medium in which the radical resides, is given by30-34

1 AN ) (A| + 2A⊥) 3 where A| is the time-averaged electron-nuclear hyperfine tensor (parallel) and 2A⊥ is the time-averaged electronnuclear hyperfine tensor (perpendicular). The variation of the hyperfine-coupling constant (AN) of 5-DSA in PMAOVE-C12EO5 complexes is illustrated in Figure 6 as a function of C12EO5 concentration. Free 5-DSA in water shows a high hyperfine-coupling constant (30) Stout, G.; Engberts, J. B. F. N. J. Org. Chem. 1974, 39, 3800. (31) Knauer, B. R.; Napier, J. J. J. Am. Chem. Soc. 1976, 98, 4395. (32) Abe, T.; Tero-Kubota, S.; Ikegami, Y. J. Phys. Chem. 1982, 86, 1358. (33) Janzen, E. G. Top. Stereochem. 1971, 6, 117. (34) Yamagata, Y.; Senna, M. Langmuir 2000, 16, 6136.

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of AN ) 15.8 G, because of the highly polar environment.35 The addition of C12EO5 (Figure 6b) does not change AN up to a C12EO5 concentration of 0.06 mM, the critical micelle concentration. Above 0.06 mM C12EO5, micelles are formed and 5-DSA is incorporated into the C12EO5 micelles, and this causes a decrease in the hyperfine-coupling constant to 15.1, because of the decreased polarity of the micelles (Figure 6b). A much lower hyperfine-coupling constant (AN ) 12.3G) was observed for 5-DSA located in hydrophobic domains of PMAOVE, indicating a more nonpolar environment.36-38 The addition of C12EO5 to 5-DSAPMAOVE systems causes a change in AN, and again, two inflection points are observed, at 0.01 and 1 mM (Figure 6a). The two inflection points are consistent with the results from the rotational correlation time (Figure 5), surface tension (Figure 1a), and viscosity (Figure 2a) measurements. The first inflection point (0.01 mM C12EO5) was assigned to the critical incorporation concentration. At a C12EO5 concentration of 0.01-1 mM, a hyperfinecoupling constant of 13.5 G was observed (Figure 6a), indicating the transfer of 5-DSA from a nonpolar environment (hydrophobic domains of PMAOVE) to an environment with increased polarity, such as mixed micelles of hydrophobic chains of PMAOVE and C12EO5. After saturation of PMAOVE with C12EO5 (second inflection point, approximately 1 mM, Figure 6a), AN reaches a value similar to that for C12EO5 micelles (Figure 6b), supporting the hypothesis that C12EO5 micelles without any PMAOVE form in this region. Another useful technique to study the micropolarity of surfactant containing systems is fluorescence spectroscopy using pyrene as the probe.39-42 The vibrational fine structure of the pyrene fluorescence depends strongly on the polarity of the environment. The ratio between the intensities of the third (I3) and first (I1) fluorescence peaks of pyrene is commonly used as the polarity probe.39,40 For C12EO5 systems only (Figure 7b), below the cmc, a low I3/I1 value was observed (I3/I1 ) 0.56), which is in agreement with the location of the pyrene molecules in the polar water solution. A sharp increase in the I3/I1 value was observed at the cmc of C12EO5 (0.06 mM), and above it, the I3/I1 value remained constant within the tested C12EO5 concentration range.42 Pyrene (0.05 mM) in PMAOVE solutions (without C12EO5) showed a I3/I1 value of 1.07, which is characteristic of a nonpolar environment similar to that of iso-propyl ether and p-xylene (I3/I1 ) 1.07 and 1.05, respectively).43 This indicates that the water insoluble pyrene is located in the hydrophobic microdomains of PMAOVE, and this observation is in agreement with the low hyperfine-coupling constant (AN ) 12.3 G) of the 5-DSA probe. The addition of small amounts of C12EO5 (