Differences in Binding of a Cationic Surfactant to Cross-Linked

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Differences in Binding of a Cationic Surfactant to Cross-Linked Sodium Poly(Acrylate) and Sodium Poly(Styrene Sulfonate) Studied by Raman Spectroscopy Per Johan Ra˚smark,† Martin Andersson,‡ Jan Lindgren,§ and Christer Elvingson*,† Department of Physical Chemistry, Uppsala University, Box 579, S-751 23 Uppsala, Sweden, Department of Pharmacy, Uppsala University, Box 580, S-751 23 Uppsala, Sweden, and Department of Materials Chemistry, Uppsala University, Box 538, S-751 21 Uppsala, Sweden Received December 17, 2004 Raman spectroscopy has been used to investigate the structure of gel-surfactant complexes. Crosslinked sodium poly(acrylate) and sodium poly(styrene sulfonate) were immersed in solutions of the cationic surfactant dodecyl trimethylammonium bromide. During the deswelling process, two distinct regions could be observed for both types of gels. Looking at the Raman spectra, however, for the poly(styrene sulfonate), the surfactant could be found throughout the gel particle, whereas for poly(acrylate), essentially all the surfactant was bound in a surface layer.

1. Introduction Polyelectrolyte gel systems are important in such various types of products and applications as diapers,1 artificial muscles,2 and drug delivery.3 Such charged networks can show large swelling effects in water, mainly due to the osmotic pressure of the counterions in the gel matrix. In general, this swelling is affected by salt concentration, pH, temperature, or by changing the solvent.4-6 The swelling/deswelling can often be described as a phase transition with a large volume change at a critical point determined by, e.g., temperature or solvent composition. Recently, the behavior of polyelectrolyte networks in surfactant solutions has also attracted much attention.7-10 In these systems, the original counterions are replaced by charged micelles, rendering a very strong collapse of the swollen network. For some polyelectrolyte gels, one has also observed the formation of a dense surface phase, usually referred to as a skin, surrounding a swollen core, due to a phase separation between the inner and outer parts of the gel.11,12 This is observed when starting either from a dry or swollen

gel.13 Another feature which has been observed for sodium poly(acrylate) by small-angle X-ray scattering is the ordering of micellar aggregates of cationic surfactants in the surface region showing either a cubic or hexagonal packing, depending on surfactant chain length, and thus on the structure of the micellar aggregate.14 Another tool for studying the structure of polymer systems and effects of a change in environment is Raman spectroscopy.15 This technique has, for example, been used to investigate the structure of water in poly(acrylic acid) solutions and to determine the structure of the corresponding polymer chains.16-19 Raman spectroscopy has also been used, for example, to study the distribution of surfactant in latex films20,21 and intermolecular interactions in gel electrolytes.22 In the present paper, we have used Raman spectroscopy aided by visual inspection to determine the distribution of the cationic surfactant, dodecyl trimethylammonium bromide (DoTAB), in spherical gel beads of sodium poly(acrylate) (PA) and sodium poly(styrene sulfonate) (PSS). 2. Experimental Section

* Author to whom correspondence should be addressed. E-mail: [email protected]. † Department of Physical Chemistry. ‡ Department of Pharmacy. § Department of Materials Chemistry. (1) Aliouche, D.; Ait-Amar, H.; Lahfati, K. Chem. Eng. J. 2001, 81, 317. (2) Otero, T. F.; Boyano, I.; Corte´s, M. T.; Va´zquez, G. Electrochim. Acta 2004, 49, 3719. (3) Matsumoto, A.; Yoshida, R.; Kataoka, K. Biomacromolecules 2004, 5, 1038. (4) Yoshida, N.; Theis, C. J. Colloid Interface Sci. 1967, 24, 29. (5) Tanaka, T.; Fillmore, D.; Sun, S.-T.; Nishio, I.; Swislow, G.; Shah, A. Phys. Rev. Lett. 1980, 45, 1636. (6) Tanaka, T.; Ishiwata, S.; Ishimoto, C. Phys. Rev. Lett. 1977, 38, 771. (7) Khokhlov, A. R.; Kramarenko, E. Y.; Makhaeva, E. E.; Starodubtzev, S. G. Macromol. Theory Simul. 1992, 1, 105. (8) Hansson, P. Langmuir 1998, 14, 2269. (9) Hansson, P. Langmuir 2001, 17, 4167. (10) Sasaki, S.; Fujimoto, D.; Maeda, H. Polym. Gels Networks 1995, 3, 145. (11) Khandurina, Y. V.; Rogacheva, V. B.; Zezin, A. B.; Kabanov, V. A. Polym. Sci. 1994, 36, 184. (12) Hansson, P.; Schneider, S.; Lindman, B. Prog. Colloid Polym. Sci. 2000, 115, 342.

2.1. Sample Preparation. 2.1.1. Preparation of Sodium Poly(styrene sulfonate) Gel Particles. Sodium styrene sulfonate (Aldrich), N,N′-methylene-bis-acrylamide (Sigma), ammonium persulfate (Sigma), N,N,N′,N′-tetramethylethylene diamine (TEMED) (Sigma), DoTAB (Aldrich), and Paraffin oil (VWR (13) Hansson, P.; Schneider, S.; Lindman, B. J. Phys. Chem. B 2002, 106, 9777. (14) Hansson, P. Langmuir 1998, 14, 4059. (15) Grasselli, J. G.; Snavely, M. K.; Bulkin, B. J. Phys. Rep. 1980, 65, 231. (16) Maeda, Y.; Ide, M.; Kitano, H. J. Mol. Liq. 1999, 80, 149. (17) Maeda, Y.; Tsukida, N.; Kitano, H.; Terada, T.; Yamanaka, J. J. Phys. Chem. 1993, 97, 13903. (18) Tsukida, N.; Muranaka, H.; Ide, M.; Maeda, Y.; Kitano, H. J. Phys. Chem. B 1997, 101, 6676. (19) Walczak, W. J.; Hoagland, D. A.; Hsu, S. L. Macromolecules 1992, 25, 7317. (20) Belaroui, F.; Grohens, Y.; Boyer, H.; Holl, Y. Polymer 2000, 41, 7641. (21) Belaroui, F.; Hirn, M. P.; Grohens, Y.; Marie, P.; Holl, Y. J. Colloid Interface Sci. 2003, 261, 336. (22) Adebahr, J.; Gavelin, P.; Ostrovskii, D.; Torell, L. M.; Wesslen, B. J. Mol. Struct. 1999, 482, 487.

10.1021/la0468693 CCC: $30.25 © 2005 American Chemical Society Published on Web 02/18/2005

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international) were used as received. Highly purified water (Millipore) was used throughout. To produce small gel particles of PSS (∼1.0 cm), the following proportions were used sodium styrene sulfonate 4 g; N,N′methylene-bis-acrylamide 0.26 g; ammonium persulfate 0.2 g; H2O (Milli-Q) 18.0 g; TEMED 0.09 g. First, all components (except TEMED) were dissolved under a nitrogen gas atmosphere at room temperature (20 °C), using a magnetic stirrer, for 15 min. After all components were completely dissolved, TEMED was added. One minute after the addition of TEMED, 2.0 cm3 of the reaction mixture was carefully added to a flask containing 70 cm3 degassed paraffin oil set to a temperature of 70 °C. The flask contained a magnetic stirrer, and the speed of rotation was 700 rpm. After 30 min, 50 cm3 of water was added to the paraffin oil, and the stirring ended. Most gel material rapidly enters the water phase and the gel particles were collected from the flask. The washing and removal of all paraffin oil from the gel/water phase without destroying the swelling gel beads (initially appearing to be a viscous fluid), was partially achieved with aid of a sieve of 300 µm mesh size. 2.1.2. Preparation of Sodium Poly(acrylate) Gel Particles. The poly(acrylate) gel was synthesized using acrylic acid (Aldrich) as monomer with N,N′-methylene bis-acrylamide (Sigma) as cross linker and the same accelerator and initiator as above. All ingredients, apart from the initiator, were mixed under nitrogen atmosphere in a Milli-Q water solution containing 2 M NaOH and 0.35 M NaCl. The amounts of reactants were 2.5 g of acrylic acid, 0.015 g of TEMED, 0.012 g of ammonium persulfate, 20.0 g of water containing NaOH and NaCl, and 0.05 g of N,N′methylene-bis-acrylamide. After the addition of the initiator, 5 cm3 of the reaction mixture was carefully injected to a flask with degassed paraffin oil under constant stirring. The rest of the procedure was identical to the one described above for the poly(styrene sulfonate) gel particles. A large poly(acrylate) gel in a DoTAB solution was provided by Per Hansson, Uppsala University. This particular gel has been in a DoTAB solution for several years and has not undergone any visible changes after the initial skin formation. 2.2. Visual Studies of PA and PSS Gel Particles. The gel beads used, both in the photographs presented in the next section and for the Raman measurement, were all washed in Milli-Q water to remove excess salt and possible unreacted material. The washing process extended over several weeks during which the water (400 cm3) was changed on a weekly basis. The shape of the globules changed during washing as they expanded. What initially appeared to be spherically symmetric particles showed deformations probably due to the initial heterogeneity in the polymer network formed during synthesis. The degree of deformation from a spherical shape differed, and for the following experiments, care was taken to choose particles of a regular shape. For the visual inspection of the PSS gel particles, gel globules with a diameter of ∼1.0 cm were placed in glass containers with 5 cm3 of 3.0 mM DoTAB solution, and photographs were taken at regular intervals during one week. For the PA gel, the particles collapsed even when immersed in a small volume of 1.0 mM DoTAB, and for the Raman measurement, a larger gel particle with a surface layer of measurable thickness was used. 2.3. Raman Spectroscopy. A Renishaw micro Raman system 2000 was used, equipped with a 783 nm red diode laser and a 514 nm argon ion laser to obtain Raman spectra from various parts of the gel particles. For the pure PSS gel and DoTAB, as well as for the PSS-DoTAB complexes, all measurements were done using the red laser. For the PA gels, however, we did not see any distinct peaks with the red laser. Using the argon laser instead, we could observe a big fluorescent peak on which features from the DoTAB molecules could be observed. The laser beam was focused using either a 50× objective for the pure components or a 63× water immersion objective for measurements on both the pure gel particles, as well as the gel-surfactant complexes. The spectra from different parts of the gel beads could be obtained by adjusting the focus of the laser beam (the focal volume). Focusing on a transparent object in a water solution is aided by the optical display of an octagon when a surface or interface is in focus. Raman spectra could be obtained from the gel at several locations in the sphere while moving the focus

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Figure 1. Measured depth profile for the focal volume of the laser beam. The solid line is the Lorentz fit of data. laterally inward toward the gel center, ensuring no contribution from the surface phase to the measurements on the particle core (and vice versa). For the macroscopic PA gel, measurements were both conducted as above and also by initially measuring at the surface of the gel and then gradually raising the microscope stage to scan from the surface layer downward. Measurements of the total intensity made sure that any significant changes in the spectra were not due to increased scattering with the depth of the probing volume. The intensities for the depth profile in Figure 1 are obtained from the intensities of the Si band at 520 cm-1 when stepwise changing the focus from 40 µm below to 40 µm above a Silicon plate surface. The data matched a Lorentz curve with a calculated halfwidth of 10.5 µm. The diameter of the PSS gel was approximately 1 mm, with the core diameter being half of that; thus, it was possible to measure the phases individually. To aid the interpretation of the spectra of the gel-DoTAB particles, additional spectra were also obtained from the monomer sodium styrene sulfonate and the cross-linker N,N′-methylene-bisacrylamide. Two spectra were also recorded for DoTAB: one for the salt crystals and another obtained from a concentrated water solution. Spectra were also obtained from a washed and water swollen gel bead of pure PSS and PA, respectively.

3. Results and Discussion 3.1. Visual Inspection. When a swollen PA or PSS gel is put into a solution of an oppositely charged surfactant like DoTAB, a decrease in volume will occur. The actual deswelling is almost visible to the unaided eye at suitable concentrations of surfactant, and the difference in volume between the initial and final state can be 2 orders of magnitude or more. The progress of the PA gel from a fully swollen bead to its final state can be seen in the series of pictures in the upper part of Figure 2. The change in volume is not the only visible reaction of the polymer gel to the DoTAB solution. As can be seen from the photographs, the turbidity of the particle also appears to increase as there is a thin but dense surface layer forming. This skin, or surface layer, is too thin for unambiguous measurements using the resolution of the microscope in the Raman measurements. Thus, for the PA gel, a larger gel particle (diameter 3 cm) with a thicker surface layer has been used. This macroscopic piece of gel had been immersed in a somewhat more concentrated DoTAB solution for a very long time (see above), but apart from a more developed outer skin, it showed no other differences from the smaller gel particles. From earlier studies, it is also known that it is even possible to, after an incision, peel off the skin and retain the core as a responsive network.13

Raman Spectroscopy Studies of Cation-Polymer Binding

Figure 2. The upper part shows a series of photographs of a PA gel bead during shrinking in DoTAB solution. The time for the photographs are from left to right: 1 min, 8 min, 5.5 h, 8.5 h, and 4 days The two pictures below show a PSS gel (left) and a PA gel (right) in DoTAB solution after (top) 15 and 18 min, respectively, and (bottom) 10 and 4 days.

For the PSS gel, a qualitatively different picture emerged. The deswollen gel is comprised of a turbid inner region and an outer region seemingly similar to the original gel bead. Clearly, this indicates the formation of two different structures in the gel, induced by the surfactant, also for a PSS gel. If an excess amount of surfactant is added, the turbid inner region will diminish or disappear entirely leaving the PSS particle clear and homogeneous, although deswelled.23 The difference between the PA and PSS gels with regard to the DoTAB concentration needed to induce a given degree of deswelling is much lower for the PA gel. This is in line with earlier measurements on solutions of the corresponding polyelectrolyte-surfactant systems24,25 and also seems to reflect the difference in the phase diagrams for the corresponding polyelectrolytesurfactant solutions.26,27 From the visual observations, one can thus conclude that for the PSS gel, the surfactant reaches the center of the polymer network, as the center becomes turbid, and thus that it is present throughout the gel since it would be unlikely for the outer phase to be free from surfactant. The presence of surfactant in the outer phase for the PA gel is also obvious by the drastic change in elastic properties of the surface layer, but one cannot by visual studies determine if the surfactant is present in the inner phase. The fact that, for macroscopic gels, the inner phase after removal of the outer skin will continue to swell indicates that the amount of surfactant is limited. This is also in line with earlier fluorescence measurements showing the absence of micellar aggregates in the core region.13 3.2. Raman Spectroscopy. For the Raman spectroscopic measurements, we first obtained the spectra for the pure PA and PSS gel without surfactant, and the two spectra are shown in Figures 3 and 4. For the pure PSS (23) Andersson, M.; Ra˚smark, P. J.; Elvingson, C.; Hansson, P. Langmuir, submitted for publication. (24) Kogej, K.; Sˇ kerjanc, J. Langmuir 1999, 15, 4251. (25) Kogej, K.; Evmenenko, G.; Theunissen, E.; Berghmans, H.; Reynaers, H. Langmuir 2001, 17, 3175. (26) Hansson, P.; Almgren, M. Langmuir 1994, 10, 2115. (27) Laatikainen, M.; Markkanen, I.; Tiihonen, J.; Paatero, E. Fluid Phase Equilib. 2002, 201, 381.

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Figure 3. Raman measurements of the two main components in the polystyrene sulfonate gels, together with a water-swollen, cross-linked network. From the top: a water-swelled poly(styrene sulfonate) gel; sodium styrene sulfonate (monomer); N,N′-methylene-bis-acrylamide (cross-linker). Relative total intensities between the spectra cannot be read from this image.

Figure 4. Raman measurement of a pure poly(acrylate) gel particle in water. To the left the O-H stretching of water is the dominant feature in the spectrum, while at shorter wavenumbers, the fluorescence from the polymer material is seen.

gel, it is possible to identify peaks corresponding both to the monomer and the cross linker. The prominent peak at 1130 cm-1 for the gel (1137 cm-1 for the monomer) corresponds to the in-plane stretching of the aromatic ring with contributions from the sulfonate side group.28 The energies for the anti-symmetric and symmetric stretching vibrations of the -SO3- group vary with the type of counterion present. For Na+, applicable to the present systems, the values have previously been established to 1188 cm-1 (anti-symmetric stretch) and 1042 cm-1 (symmetric stretch).28 These values correlate well with peaks found in the spectra for the gel and the monomer. In the gel spectrum, these peaks are found at 1197 cm-1 and 1044 cm-1 respectively. In the monomer spectrum, for the anti-symmetric stretch, double peaks can be expected, and they are observed.28 Less pronounced is, however, the expected peak around 1042 cm-1. (28) Zundel, G. Hydration and intermolecular interaction: infrared investigations with polyelectrolyte membranes; Academic Press: New York, 1969.

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Figure 5. Raman spectrum of pure DoTAB crystals (lower curve) together with the spectrum of DoTAB in a concentrated water solution (upper curve) using an excitation wavelength of 783 nm. Notice, for example, the difference between the peak(s) around 1440 cm-1. The split peaks in the lower curve is an effect of DoTAB being in its crystal form.

Figure 6. Raman spectrum of pure DoTAB crystals (lower curve) together with the spectrum of DoTAB in a concentrated water solution (upper curve) using a argon laser (514 nm). Notice the assembly of prominent C-H stretching peaks around 3000 cm-1. The peaks at low wavenumbers should be compared with Figure 5.

One can also observe the prominent peak at 1630 cm-1 for the monomer and cross-linker corresponding to the stretching of the carbon double bond.28 This has, to a large extent, been removed from the spectrum of the gel due to the breaking of the bond during polymerization and the removal of unreacted monomer in the washing of the gel. For the PA gel (Figure 4) where the argon laser was used (the red laser did not give any distinguishable features), the spectrum does still not show any distinct peaks except for water. This is in agreement with earlier investigations on the structure of water in PA gels and the conformation of the polymer chain. The broad peak at about 3400 cm-1 corresponds to the O-H stretching in water,16-18 but the spectral feature at shorter wavenumbers is fluorescence from the polymer material. For comparison, we also measured the Raman spectra for a concentrated water solution of DoTAB, as well as the crystal form of the surfactant both at 783 (Figure 5) and 514 nm (Figure 6). The reason to include a concentrated solution was to see if there were any changes between the crystal form and the surfactant in an environment more

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Figure 7. Raman spectra of a poly(styrene sulfonate) gel bead in a DoTAB solution. The upper curve shows the spectrum from the surface of the more transparent “shell” area, and the lower shows that from the center of the turbid “nucleus”. Positions of the marked peaks are identical between the curves. Relative total intensities for the two spectra cannot be read from this image.

similar to the one in the gel network. The broad peak at 1442 cm-1 in the upper spectrum of Figure 5 should correspond to the split peaks marked 1441 and 1465 cm-1 (and possibly also the peak at 1397 cm-1 in the crystal form). These represent bending vibrations in the hydrocarbon chain.28 Thus, in the gel, as well as in the DoTABwater spectrum, the two (or three) separate peaks merge into a single broad peak. Finally, one can note the peak marked 758 cm-1, which is a CH2 vibration, typical for aliphatic chains. In Figure 6, the details described above are still visible but the prominent features are the collection of C-H stretching peaks around 3000 cm-1 from the DoTAB molecule and in the upper spectrum the O-H stretching of water is also visible. In Figure 7 is shown the Raman spectrum from the surface and the center of the PSS gel in a DoTAB solution. The intensities for the two spectra differ considerably, and they have been rescaled to facilitate a comparison of the position of the different peaks. The change in intensity is an effect of the different swelling and thus different concentrations in the surface and core volumes. The two spectra have very similar features in terms of the position of the prominent peaks and the general appearance. One can note the two peaks at 1601 and 1452 cm-1, originating from the gel and the surfactant, respectively. Also the peaks at 792 and 761 cm-1, are features of the gel matrix and surfactant, which can be seen by comparison with Figures 3 and 5. In Figure 8 is shown a more detailed comparison for wavenumbers in the range between 1700 and 1400 cm-1. The overlap is almost perfect after the spectra have been rescaled. If there would be a difference in the concentration ratio between the polymer and the surfactant in the two different regions, one would expect the peaks of the two spectra to differ in relative intensity. Thus, it is reasonable to believe that the relative concentrations are in fact the same throughout the gel and that the difference between the two observed phases in PSS represents a difference in the packing of the aggregates and a change in the structure of the polymer network. This is also in agreement with previously determined phase diagrams of linear PSS-DoTAB. In this

Raman Spectroscopy Studies of Cation-Polymer Binding

Figure 8. A magnified part of the spectrum between 1400 and 1700 cm-1 in Figure 7, where the two spectra have been scaled and placed on top of each other. The dotted line marks the spectrum obtained from the surface of the gel globule, and the solid line marks the spectrum obtained from the inside of the nucleus.

case, one obtains a concentrated phase in equlibrium with a more dilute phase but with an equimolar ratio PSSDoTAB in both phases.26 Because of the thin outer layer found for PA gels in DoTAB (see Figure 2) and to improve the signal/noise ratio, the measurements of the spectrum for the PA gel with DoTAB was made using a larger gel bead, which had been equilibrated in DoTAB solution over long time. One could then anticipate that even though the diffusion of DoTAB could be slow in the PA gel, it should had time to equilibrate. The measurements were conducted as a series of measurements from the surface and toward the center in steps of 40 µm and a final measurement in the bulk of the core. The result of the measurements can be seen in Figure 9. For the uppermost spectrum, corresponding to the surface, there are clearly visible peaks that do not appear in the spectrum for the pure PA gel. The peak at 1447 cm-1 should correspond to the 1442 cm-1 peak seen in the water solution of DoTAB in Figure 5, and the collection of peaks at 3000 cm-1 can also be attributed to DoTAB by comparison with Figure 6. The following spectrum, corresponding to the situation 40 µm below the surface, hardly shows any remains of the DoTAB peaks. The following spectra taken even further below the surface do not show any features that can be associated with the surfactant. One can also notice that the intensity from the polymer network also decreases relative to the intensity of the water. This corresponds to a reduction of

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Figure 9. Raman spectra for a macroscopic PA gel in DoTAB solution. The uppermost spectrum is measured at the surface, and each of the following spectra represent going down 40 µm into the particle compared to the preceding spectrum, with the exception for the last spectrum, which is measured at the center of the gel bead.

the polymer concentration in accordance with the observation that the outer part of the particle has a more dense structure. Comparing Figures 4 and 9, one can see that the position of the maximum of the broad peak curve is shifted for the gel with surfactant and the gel without surfactant but is the same for the two gels in contact with DoTAB, even though there are still no traces of DoTAB in the interior of the PA gel. 4. Conclusion We have measured Raman spectra for poly(acrylate) and poly(styrene sulfonate) gels in water and in a cationic surfactant solution. For the PA gel, it is known that a skin is formed around the gel particles when immersed in a DoTAB solution, which is also observed in the present study. Looking at the Raman spectra at the surface and in the gel core, DoTAB is only seen in the surface layer, not in the swollen core of the particle. For the PSS particles, however, the surfactant migrates into the whole gel volume, although one can, also in this case, visually observe two regions in the gel during the deswelling process. Furthermore, the relative concentration of surfactant and polymer is the same in both phases for the PSS gel. Acknowledgment. C.E. acknowledges support from Uppsala University and Ingegerd Berghs stiftelse. LA0468693