Studies on Interaction between Similarly Charged Polyelectrolyte

Langmuir 2003, 19, 9321-9327

9321

Studies on Interaction between Similarly Charged Polyelectrolyte: Fatty Acid System Anand Gole,† Sumant Phadtare,‡ Murali Sastry,‡ and Dominique Langevin*,† Laboratoire de Physique des Solides, Universite´ Paris Sud, 91405 Orsay, France, and Materials Chemistry Division, National Chemical Laboratory, Pune 411 008, India Received July 4, 2003. In Final Form: August 29, 2003 Interaction between an anionic polyelectrolyte, poly(acrylamido)methylpropanesulfonate (PAMPS), and anionic fatty acid, arachidic acid (AA), has been studied at the air-water interface. Although similarly charged, the polyelectrolyte complexes with the AA monolayer are possibly driven by interactions other than electrostatic, such as hydrophobic and hydrogen bonding. The absence of electrostatic interaction between the polymer and the fatty acid was demonstrated by performing the experiments in the presence of salt, at low pH, or with a neutral surface-active molecule such as octadecanol (ODO). The effect of polymer charge density on its interaction/degree of complexation with the monolayer has also been investigated. The degree of complexation depends on the net charge on the polymer and increases with the decrease in the charge. To further supplement the role of nonelectrostatic interactions, we have studied the interaction of PAMPS with an anionic surfactant, dioctyl sulfosuccinate (AOT), by surface tension measurements. At lower polymer concentrations, we observe the presence of a critical aggregation concentration (CAC), which is typical for interacting systems. The complexation of polymer with AA has been followed by surface pressure-area (π-A) isotherms and Brewster angle microscopy (BAM). Attempts to form Langmuir-Blodgett (LB) films of the complexes did not yield any lamellar layer by layer stacking, possibly due to the bulkiness of the polymer and the nature of the interactions used. Quartz crystal microgravimetry (QCM) and Fourier transform infrared spectroscopy (FTIR) have been used to characterize such films.

Introduction Currently there is a great deal of interest in studying the interactions between polyelectrolytes and lipid/surfactants for fundamental reasons as well as for the wide range of technological applications they offer.1 From a fundamental point of view, these studies would in particular throw light on membrane-protein interactions, wherein protein could be considered as a charged polymer. The membrane of plant and animal cells is typically composed of 40-50% lipids and 50-60% proteins. Proteins (or charged polymers) bound to this lipid membrane act as signal transducers, transmitting signals from the world outside the cell to its interior. Studies on protein/ polyelectrolyte-lipid interactions can thus be looked upon as a study directed toward understanding membrane mimicking.2 On the other hand, aqueous mixtures of polymers and lipid/surfactants have properties that are quite different from those of the individual components and could be exploited in various industrial processes such as enhanced oil recovery, emulsion polymerization, food stuffs, pharmaceuticals, cosmetics, detergents, coating fluids, inks, and paints.1 Literature abounds in studies on the interaction between polyelectrolytes and oppositely charged surfactants.1,3,5 Polyelectrolyte surfactant complexes are formed * To whom all correspondence should be addressed. Ph: +331-6915-5351. Fax: +33-1-6915-6086. E-mail: langevin@ lps.u-psud.fr. † Universite ´ Paris Sud. ‡ National Chemical Laboratory. (1) Goddard, E. D.; Ananthapadmanabhan, K. P. Interaction of Surfactants with Polymers and Proteins; CRC Press: Boca Raton, FL, 1993. (2) (a) Fendler, J. H. Membrane mimetic chemistry; Wiley-Interscience: New York, 1982. (b) Jost, P. C.; Griffith, O. H. Lipid-Protein Interactions; John Wiley & Sons: New York, 1982; Vol. 2. (3) Kwak, J. C. T. Polymer-Surfactant Systems, Surfactant Science Series; Marcel Dekker: New York, 1998.

in this case due to the electrostatic interaction between the ionizable polymer units and surfactant headgroups. The tail of the surfactant also interacts with the apolar regions of the polymer via the hydrophobic interactions. Furthermore, the association between polyelectrolytes and surfactants of the same sign can be expected to be feeble or absent mainly because of the unfavorable electrostatic repulsions between these species in aqueous solution. Nonetheless studies on such similarly charged systems also revealed complex formation due to hydrophobic interactions.4a-c The role of hydrophobic interactions is also predominant when the interaction of a charged entitiy (polymer or surfactant) with an uncharged entity (polymer or surfactant) is studied.4d-g Such studies have been predominantly performed with soluble systems. Our group has been actively involved in studying the interactions of oppositely charged polymers with surfactants at the airwater interface, in bulk, in thin films, and in foams.5 Herein we report a study of the interaction between similarly charged polyelectrolyte and insoluble fatty acid systems. Arachidic acid (AA) (anionic) was spread on the surface of solutions of anionic polyelectrolytes, poly(acrylamido)methylpropanesulfonate (PAMPS), with two (4) (a) Bakeev, K. N.; Ponomarenko, E. A.; Shishkanova, T. V.; Tirrell, D. A.; Zezin, A. B.; Kabanov, V. A. Macromolecules 1995, 28, 2886. (b) Smith, G. L.; McCormick, C. L. Langmuir 2001, 17, 1719. (c) McClements, D. J. J. Agric. Food Chem. 2000, 48, 5604. (d) Bromberg, L.; Temchenko, M.; Colby, R. H. Langmuir 2000, 16, 2609. (e) Ghoreishi, S. M.; Fox, G. A.; Bloor, D. M.; Holzwarth, J. F.; Wyn-Jones, E. Langmuir 1999, 15, 5474. (f) Goddard, E. D.; Leung, P. S. Langmuir 1992, 8, 1499. (g) Anghel, D. F.; Saito, S.; Baran, A.; Iovescu, A. Langmuir 1998, 14, 5342. (5) (a) Langevin, D. Adv. Colloid Interface Sci. 2001, 89-90, 467. (b) Asnacios, A.; Klitzing, R.; Langevin, D. Colloids Surf. A 2000, 167, 189. (c) Asnacios, A.; Langevin, D.; Argillier, J. F. Macromolecules 1996, 23, 7412. (d) Radlinska, E.; Gulik, T.; Lafuma, F.; Langevin, D.; Urbach, W.; Williams, C.; Ober, R. Phys. Rev. Lett. 1995, 74, 4237. (e) Asnacios, A.; Langevin, D.; Argillier, J. F. Eur. Phys. J. B. 1998, 5, 905. (f) Bergeron, V.; Asnacios, A.; Langevin, D. Langmuir 1996, 12, 1550.

10.1021/la0352063 CCC: $25.00 © 2003 American Chemical Society Published on Web 10/02/2003

9322

Langmuir, Vol. 19, No. 22, 2003

different fractions of charged monomers (10 and 25%). The complexation of PAMPS to the AA monolayer was studied as a function of time with surface pressure-area (π-A) isotherms and Brewster angle microscopy (BAM). A clear dependence of polymer charge density and polymer concentration on the rate and degree of complexation could be seen. The role of different possible interactions in the complexation process has been investigated by changing the pH, adding salt, and using a neutral fatty amphiphile, octadecanol (ODO). To further demonstrate the role of nonelectrostatic interactions between similarly charged systems, we have performed surface tension studies on mixtures of PAMPS with varying concentrations of water soluble anionic surfactant dioctyl sulfosuccinate (AOT). Therein we clearly observe three distinct regions in the surface tension curve: (a) synergistic lowering in surface tension as a function of increasing concentration of AOT, (b) a plateau region resembling the critical aggregation concentration (cac) and finally, (c) a fall to the critical micelle concentration plateau (cmc). The presence of cac clearly indicates the interaction between polymer and surfactant, thus confirming the role of nonelectrostatic interactions. Furthermore, attempts have also been made to form multilayer films of PAMPS-AA on Si(111) wafers and quartz crystal microgravimetry (QCM) crystals by the Langmuir-Blodgett (LB) technique. These films were further characterized by Fourier transform infrared spectroscopy (FTIR) and ellipsometry. Lack of lamellar growth of the films by the LB technique might be possibly due to the bulkiness of the polymer and weaker interactions used for film formation process. Presented below are the details of the investigation. Experimental Section Chemicals. Arachidic acid (AA) was obtained from Aldrich chemicals and used as received. A 1 mg/mL concentrated solution of AA in chloroform was prepared and used for all experiments. Anionic surfactant sodium dioctyl sulfosuccinate (AOT) was purchased from Sigma (99%) and used as received. The polymer, PAMPS (synthesized by SNF Floerger), is a statistical copolymer composed of neutral monomers of acrylamide (AM) and charged monomers of sodium (acrylamido)methylpropanesulfonate (AMPS). The polymer chemical structure has been characterized by titration via a bromidation reaction for the amide function and potentiometric titration for the sulfonate. The molecular weight and polydispersity of the polymer were measured by size exclusion chromatography (SEC) coupled with multiangle light scattering. The molecular weight of the polymer used in this case was 4 × 105 and two fractions of charged monomers were investigated: f ) 10 and 25 mol % (the number of charged monomers per macromolecule is x) fN, where N is the total number of monomers). These polymers will be referred to as PAMPS10% and PAMPS25%. To eliminate any traces of salt and low molecular weight impurities, the polymer solutions were passed through an ultrafiltration unit with a 20 000 cutoff membrane. Final concentrations were determined using a total carbon analyzer Shimatzu TOC 5050. After this purification, the polymer displays no surface activity at concentrations below 2000 ppm. Pure water was taken from a Millipore Milli-Q system. In our study we have used aqueous solutions containing 228 and 30 ppm polymer (1 ppm ) 10-6 g/cm3) concentrations. Langmuir-Blodgett (LB) Studies. A 25 µL aliquot of 1 mg/mL concentrated AA solution in chloroform was spread on the subphase of 30 or 228 ppm aqueous solution of PAMPS (pH ) 6.5) in a Nima 601 BAM Langmuir film balance trough equipped with a Wilhelmy plate pressure sensor. The trough was interfaced with a Brewster angle microscope (BAM), model mini-BAM, NFT-Nanofilm Technologie GmbH employing a 688 nm diode laser (30 mW) at an angle of incidence of 52-54°. The imaging system has a field of view of 4.8 × 6.4 mm2. Images are recorded using a standard CCD camera. Pressure-area (π-A) isotherms of the AA-polymer system were recorded immediately

Gole et al. after spreading of the AA Langmuir monolayer and at appropriate time intervals. Brewster angle images of the composite system were recorded during the compression cycles. Multilayer films of the composite were formed on a Nima 611 model of a Langmuir trough (equipped with a Wilhelmy plate pressure sensor), via the classical Langmuir-Blodgett technique6 at a surface pressure of 30 mN/m. The substrates used for deposition were Si(111) substrates for FTIR measurements and gold-coated AT-cut quartz crystals for QCM measurements. The second type of substrates are thin quartz resonators prepared by slicing through a quartz rod at an angle of approximately 30° with respect to the x-axis. Indeed, the converse piezoelectric effect being the basis of QCM, crystal symmetry should be such that strain induced in a piezoelectric material by an applied potential of one polarity will be equal and opposite in direction to that resulting from the opposite polarity.7 Surface Tension Measurements. Surface tensiometry was used to study the PAMPS-AOT similarly charged system. The concentration of PAMPS25% was fixed to 30 ppm, and the concentration of anionic surfactant AOT was varied to obtain a range of different solutions (0.01-5 mM AOT). Experiments were performed at room temperature (20 ( 1 °C). Measurements were carried out in a Teflon trough (6 cm diameter) housed in a Plexiglas box with an opening for the tensiometer. The surface tension was measured with an open-frame version of the Wilhelmy plate (to avoid the wetting problems of a classical plate). The rectangular (20 mm × 10 mm) open frame, made from a 0.19 mm diameter platinum wire, was attached to a force transducer (HBM Q11) mounted on a motor, allowing it to be drawn away from the surface at a controlled constant rate. For mixed solutions (PAMPS-AOT), at low concentrations of surfactant, the approach to the equilibrium could take more than 3 h, and we did not find any reliable method to get the equilibrium surface tension of such a system by extrapolation to infinite time. Thus, it was assumed arbitrarily that equilibrium had been reached when the surface tension variation was less than 0.01 mN/m over 10 min. The reproducibility, including long equilibration time and/ or contamination effects, was 0.5 mN/m for mixed solutions. Surface tensions measured on polymer-free solutions of surfactants were in good agreement with the literature values. QCM Studies. Thin films of the composite system were deposited onto gold-coated AT cut quartz crystals by the LB technique. The change in the frequency was recorded as a function of the number of immersion cycles, after thorough washing (in deionized water) and drying (in flowing nitrogen) of the crystals. The frequency counter used was an Edwards FTM5 instrument operating at a frequency stability and resolution of (1 Hz. For the 6 MHz crystal used in this investigation, this translates into a mass resolution of 12 ng/cm2. The frequency changes were converted to mass loading using the standard Sauerbrey formula.7 FTIR Spectroscopy Measurements. FTIR spectroscopy was used as a primary tool to characterize the LB films of the polymer-lipid composite system. These measurements were performed on Si(111) substrates in the diffuse reflectance mode at a resolution of 4 cm-1 on a Perkin-Elmer Spectrum One spectrometer. To obtain good signal-to-noise ratios, 256 scans of the LB films were taken in the range 400-4000 cm-1.

Results and Discussion The complex structures formed by self-assembly are governed, at a molecular level, by the combination of electrostatic, hydrophobic, hydrogen bonding, van der Waals, and steric interactions.8 Electrostatic interactions (6) Ulman, A. An Introduction to Ultrathin Organic Films: From Langmuir-Blodgett to Self-Assembly; Academic Press: San Diego, CA, 1991. (7) (a) Sauerbrey, G. Z. Phys. (Munich) 1959, 155, 206. (b)Buttry, D. A.; Ward, M. D. Chem. Rev. 1992, 92, 1356. (c) Wang, J.; Frostman, L. M.; Ward, M. D. J. Phys. Chem. 1992, 96, 5224. (8) (a) Philip, D.; Stoddart, J. F. Angew. Chem., Int. Ed. 1996, 35, 1155. (b) Whitesides, G. M. Sci. Am. 1995, Sept, 114. (c) Israelachvili, J. N. Intermolecular and Surface Forces; Academic Press: San Diego, CA, 1997. (d) Tanford, C. The Hydrophobic Effect: Formation of Micelles and Biological Membranes; Wiley-Interscience: New York, 1997. (e) Evans, D. F.; Wennerstrom, H. The Colloidal Domain; Wiley-VCH: New York, 1999.

Interactions between Similarly Charged Polyelectrolyte

Langmuir, Vol. 19, No. 22, 2003 9323

Figure 1. (A) Pressure-area (π-A) isotherms for 30 ppm PAMPS10% solutions (pH ) 6.5) and an AA monolayer as a function of time after spreading the AA monolayer: (curve a) pure AA spread on the surface of water; (curve b) immediately after spreading AA on PAMPS surface; (curve c) after t ) 2 h; (curve d) after t ) 4 h; (curve e) t ) 22 h after spreading the AA monolayer. (B) Pressure-area (π-A) isotherms for 228 ppm PAMPS10% (pH ) 6.5) and an AA monolayer as a function of time after spreading the AA monolayer: (curve a) immediately after spreading AA on PAMPS surface; (curve b) after t ) 2 h; (curve c) after t ) 4 h; (curve d) t ) 22 h after spreading the AA monolayer.

to a large extent drive the association of oppositely charged entities, for instance, surface-modified nanoparticles and surfactants,9 proteins and lipids,10 polymers and polymers, or polymers and surfactants.1,3,5,11 In these systems, however, interactions other than purely electrostatic play important roles, as evidenced, for instance, by the differences found when the surfactants are replaced by simple salts. Simplifying the problem further, in the cases where the polymer is uncharged or similarly charged to the lipid or surfactant, hydrophobic, hydrogen bonding, dipolar, van der Waals interactions, and steric interactions are expected to be predominant. Herein we demonstrate such a system wherein the polyelectrolyte and the lipid are similarly charged. At pH ) 6.5 (conditions used for the experiments) the carboxylic acid group of the fatty acid, AA, is ionized (pKa of AA ) 4.5).12 Furthermore, at this pH the polyelectrolyte is negatively charged. This would lead to a strong repulsive interaction between the fatty acid molecules at the interface and the polyelectrolyte in the bulk. But, as will be shown subsequently, the polyelectrolyte interacts with AA, resulting in the formation of complexes that can be transferred onto solid substrates. This suggests that interactions other than electrostatic could be responsible for such a process. The use of these interactions for immobilization onto 2D and 3D substrates has been shown previously by some of the researchers for nanoparticles13 and biomolecules.14 Figure 1A shows the pressure-area (π-A) isotherms as a function of time after spreading the arachidic acid (9) (a) Sastry, M.; Rao, M.; Ganesh, K. N Acc. Chem. Res. 2002, 35, 847. (b) Gole, A.; Sainkar, S. R.; Sastry, M. Chem. Mater. 2000, 12, 1234. (c) Caruso, F.; Lichtenfeld, H.; Giersig, M.; Mohwald, H. J. Am. Chem. Soc. 1998, 120, 8523. (d) Iler, R. K. J. Colloid Interface Sci. 1966, 21, 569. (e) Lvov, Y.; Ariga, K.; Ichinose, I.; Kunitake, T. J. Am. Chem. Soc. 1995, 117, 6117. (10) (a) Gole, A.; Dash, C.; Mandale, A. B.; Rao, M.; Sastry, M. Anal. Chem. 2000, 72, 4301. (b) Gole, A.; Dash, C.; Rao, M.; Sastry, M. Chem. Commun. 2000, 297. (c) Hamachi, I.; Honda, T.; Noda, S.; Kunitake, T. Chem. Lett. 1991, 1121. (d) Rusling, J. F. Acc. Chem. Res. 1998, 31, 363. (11) (a) Decher, G. Science 1997, 277, 1232. (b) Ko¨tz, J.; Kosmella, S.; Beitz, T. Prog. Polym. Sci. 2001, 26, 1199. (c) Abraham, T.; Giasson, S. Colloids Surf. A 2001, 180, 103. (12) Roser, S. J.; Lovell, M. R. J. Chem. Soc., Faraday. Trans. 1995, 91, 1783.

(AA) on the surface of a 30 ppm aqueous solution of PAMPS10% with pH 6.5. It can be clearly seen that there is an expansion of monolayer as a function of time. Specifically, the takeoff area at the beginning (t ) 0 h) after spreading of the monolayer, is ca. 41 Å2 (curve b, Figure 1A), whereas after a period of 22 h, the equilibrium takeoff area is ca. 84 Å2 (curve e, Figure 1A). This indicates some amount of complexation of the polymer with the AA monolayer. For comparison, the π-A isotherms of AA on the surface of water (pH ) 6.5) have also been recorded (curve a, Figure 1A). A control experiment was also performed wherein AA monolayer was spread on the surface of water (pH ) 6.5) and π-A isotherms were recorded as a function of time. No expansion of the monolayer was observed in this case (data not shown for brevity). Figure 1B shows similar π-A isotherms of AA on the surface of a 228 ppm solution of PAMPS10%. As in the case of 30 ppm PAMPS10% (Figure 1A), one can also see some amount of complexation of the polymer with the AA monolayer (Figure 1B, curves a-d for t ) 0 h to t ) 24 h). The equilibrium takeoff area of the AA monolayer is smaller, 65 Å2. This indicates a dependence of the interaction between PAMPS and AA on the concentration of the polymer. Furthermore, as a function of time, the degree of hysteresis of the isotherm cycle increases, and its slope decreases. We have done similar experiments for PAMPS25%. Figure 2 shows compression cycles for a 30 ppm solution. There is an expansion of the monolayer with time, as was observed for the PAMPS10% case. Figure 2B shows the complexation of a 228 ppm solution of PAMPS25% with an AA monolayer. One sees that the degree of expansion of the monolayer is larger for PAMPS10% than that PAMPS25% for a given polymer concentration (takeoff area of 60 and 55 Å2 for 30 or 228 ppm, respectively). The complexation could be explained as follows. At pH ) 6.5, the carboxylic acid groups of the AA monolayer, (13) (a) Malynych, S.; Luzinov, I.; Chumanov, G. J. Phys. Chem. B 2002, 106, 1280. (b) Sun, L.; Crooks, R. M. Langmuir 2002, 18, 8231. (14) (a) Gole, A.; Vyas, S.; Sainkar, S. R.; Lachke, A.; Sastry, M. Langmuir 2001, 17, 5964. (b) Schmitt, A.; Fernandez-Barbero, A.; Cabrerizo-Vilchez, M.; Hidalgo-Alvarez, R. Prog. Colloid Polym. Sci. 1997, 104, 144. (c) Elgersma, A. V.; Zsom, R. L. J.; Norde, W.; Lyklema, J. J. Colloid Interface Sci. 1990, 138, 145.

9324

Langmuir, Vol. 19, No. 22, 2003

Gole et al.

Figure 2. (A) Pressure-area (π-A) isotherms for 30 ppm PAMPS25% (pH ) 6.5) and an AA monolayer as a function of time after spreading the AA monolayer: (curve a) immediately after spreading AA on PAMPS surface; (curve b) after t ) 2 h; (curve c) t ) 24 h after spreading the AA monolayer. (B) Pressure-area (π-A) isotherms for 228 ppm PAMPS25% and an AA monolayer as a function of time after spreading the AA monolayer: (curve a) immediately after spreading AA on PAMPS surface; (curve b) after t ) 2 h; (curve c) t ) 24 h after spreading the AA. Scheme 1. AM/AMPS Copolymer Structure

and the sulfonate groups of the polymer (see Scheme 1) are fully ionized. This leads to electrostatic repulsion of the polymer and the fatty acid at the air-water interface. Thus no complexation and no expansion in the monolayer should be expected. We see, however, a significant amount of expansion in the monolayer (Figures 1 and 2). This suggests that other interactions such as hydrophobic and hydrogen bonding could be responsible for such a complexation.13,14 As mentioned earlier, nonelectrostatic interactions are important and are used for the formation of thin films of biomolecules and nanoparticles. To completely eliminate the possibility of any electrostatic (including dipolar or hydrogen bonding) interaction, we performed a set of experiments under high salt and low pH conditions. Figure 3A shows the π-A isotherms of complexation of AA monolayer with 30 ppm PAMPS10% as a function of time in the presence of 0.1 M NaCl. A clear and significant amount of expansion in the monolayer can still be seen and the equilibrium takeoff (curve d, Figure 3A) is roughly at ca. 85 Å2, which is similar to those without salt (Figure 1A). At such high salt concentrations (0.1 M), the electrostatic interactions are screened and differences in the curves should have been seen if these interactions were predominant in the absence of salt. Furthermore, however, the possibility of formation of sodium stearate could not be neglected. We have therefore also performed a study at a pH of 3 (inset of Figure 3A). At this pH the carboxylic acid groups of the monolayer are un-ionized12 and thus the role of electrostatic would be completely eliminated. As can be seen from the inset of Figure 3A, the isotherms are similar to those recorded at pH 6.5 (Figure 1A). These two experiments (experiment with salt and experiment at low pH)

rule out the electrostatics in the complexation process. A last strong evidence is supplied by an experiment performed with a neutral fatty alcohol (octadecanol, ODO) spread on the surface of 30 ppm PAMPS25% solution. The π-A isotherms (as shown in Figure 3B) clearly indicate that the polymer also interacts with the neutral alcohol. The equilibrium takeoff area in this case (72 Å2 after complexation for 24 h) is higher than that for AA with 30 ppm PAMPS25% (Figure 2A), where the equilibrium takeoff after 24 h complexation is ca. 60 Å2. Mo¨hwald and co-workers15a have studied the influence of polymer charge density on lipid-polyelectrolyte complexes at the air-water interface. In their case, they have used negatively charged phosphatidic acid (DPPA) and positively charged polyelectrolyte (PDADMAC) with variable charge density. The π-A isotherms also reveal an expansion that increases with decreasing polymer charge. The mean lateral compressibilities increase here (the slopes decrease) as the total charge on the polymer decreases. They attribute this effect to the increasing flexibility of the polymer as a result of a reduction of the number of charges. They mention that the adsorption of polyelectrolytes to lipids might not only be a result of electrostatic coupling but also of nonelectrostatic contributions to the adsorption energy.15 In our case, the monolayer and the polymer are similarly charged and the association is not electrostatic. The trend observed for decreasing charge densities is similar and might also be due to the fact that the polymer becomes increasingly flexible as the charge is reduced. We conjecture here that there could be a penetration of the polymer into the lipid monolayer, thus enhancing hydrophobic interactions (as shown in Scheme 2). As the charge increases (10-25%), the flexibility of the polymer reduces and less polymer penetrates into the monolayer, leading to a smaller expansion of the monolayer. Let us note that hydrophobic interactions need to be sufficiently strong to overcome the unfavorable electrostatic repulsion. It is probable (15) (a) deMeijere, K.; Brezesinski, G.; Pfohl, T.; Mohwald, H. J. Phys. Chem. B 1999, 103, 8888. (b) Antonietti, M.; Kaul, A.; Thunemann, A. Langmuir 1995, 11, 2633. (c) deMeijere, K.; Brezesinski, G.; Mohwald, H. Macromolecules 1997, 30, 2337. (d) Stuart, M. A. C.; Fleer, G. J.; Lyklema, J.; Norde, W.; Scheutjens, J. H. M. Adv. Colloid Interface Sci. 1991, 34, 477.

Interactions between Similarly Charged Polyelectrolyte

Langmuir, Vol. 19, No. 22, 2003 9325

Figure 3. (A) Pressure-area (π-A) isotherms for 30 ppm PAMPS10% (pH ) 6.5) and an AA monolayer as a function of time after spreading the AA monolayer. 0.1 M NaCl was added to the PAMPS solution. Key: (curve a) immediately after spreading AA on PAMPS surface; (curve b) after t ) 2 h; (curve c) after t ) 4 h; (curve d) t ) 22 h after spreading the AA. The inset shows the pressure-area (π-A) isotherms for 30 ppm PAMPS10% and an AA monolayer as a function of time after spreading the AA monolayer. In this case, the pH of the PAMPS solution was adjusted to 3 (by 0.1 M HCl) prior to experimentation. (B) Pressure-area (π-A) isotherms of complexation of 30 ppm PAMPS25% with ODO monolayer as a function of time after spreading the ODO monolayer. The pH of the solution was adjusted to 3 prior to the start of the experiment. Key: (curve a) immediately after spreading ODO on PAMPS surface; (curve b) after t ) 2 h; (curve c) t ) 22 h after spreading the ODO. Scheme 2. Possible Method of Interaction of the Polymer with the AA Lipid Monolayer at the Air-Water Interface

though that the counterions are partly condensed in the interfacial region in order to reduce the electrostatic repulsion. Optical characterization at the air-water interface has been performed using Brewster angle microscopy (BAM) measurements. Figure 4 shows BAM images for one such system (AA-30 ppm PAMPS10%, pH ) 6.5). The other systems showed similar behavior and are not shown. The data have been displayed for two time intervals (t ) 0 and t ) 22 h) for the same system and at roughly the same surface pressures for better comparison. It can be clearly seen from the images that the monolayers are inhomogeneous, even at very low pressures, although the pure AA monolayers are fully uniform (images not shown). This is another proof of the surface complexation. The reason for the inhomogeneity is unclear but resembles that observed in mixed monolayers of PAMPS with cationic surfactants.16 To generalize the nature of the interactions between similarly charged systems, we have tried to compare these results with similarly charged polymer-soluble surfactant (16) Jain, N.; Albouy, P. A.; Langevin, D. Langmuir, in press.

system. In one of our earlier studies on interactions of PAMPS with various surfactants, we did not observe any surface tension change in the case of AOT when the polymer was added. We have used, however, a large polymer concentration, 750 ppm, in these experiments.5c Solutions of 30 ppm PAMPS25%-anionic surfactant AOT were prepared with varying concentrations of surfactant. Figure 5 (circles) shows surface tension measurements on these mixed PAMPS-AOT systems (as explained in the Experimental Section). A lowering of surface tension at very low surfactant concentrations can be seen. As shown in Figure 5, circles, the surface tension isotherm exhibits two plateaus beginning at two characteristic break points. The first could be the critical aggregation concentration (cac), corresponding to the appearance of polymer-surfactant complexes in the bulk,5 whereas the second corresponds to the cmc of polymer-free AOT. For comparison, we have also shown the surface tension profile for a pure AOT system as a function of concentration (Figure 5, squares connected by line). We would like to point out that the only difference between these results and the previous ones (ref 5c) is the concentration of polymer used (750 ppm in prevous study as compared to 30 ppm of present study). We therefore find again an important role of the polymer concentration in this unusual complexation of two species having the same electrical charge. We do not understand at this stage the possible reason behind the behavior of PAMPS at lower and higher concentrations. It could be noted, however, that the overlap concentration C*, corresponding to the transition between dilute (isolated coils) and semidilute (network) regimes is around 100 ppm for the polymers used.17 It is not clear, though, why this could affect the adsorption regime. The opposite behavior is indeed found with cationic surfactants.5b We have further tried to form films of the PAMPS-AA system using the classical Langmuir-Blodgett (LB) technique. Figure 6A shows quartz crystal microgravimetry (QCM) data of layer by layer deposition for the 30 ppm PAMPS25%-AA system by the LB technique. As (17) Ritacco, H.; Langevin, D.; Kurlat, D. J. Phys. Chem., in press.

9326

Langmuir, Vol. 19, No. 22, 2003

Gole et al.

Figure 5. Surface tension data for 30 ppm PAMPS25%-AOT system as a function of AOT concentration (circles). The encircled plateau region could correspond to a cac region. The surface tension data for pure AOT as a function of concentration has also been shown for comparison (squares connected with line).

Figure 4. Brewster angle images at various surface pressures recorded for the 30 ppm PAMPS10%-AA system: (a, c, e, g) images taken at time t ) 0 for pressures corresponding to 0, 6, 17, and 30 mN/m, respectively; (b, d, f, h) images taken at time t ) 22 h for pressures corresponding to 0, 6, 17, and 30 mN/m, respectively.

shown in Figure 6A, there is a large amount of mass uptake after the second dip of the QCM crystal, but the subsequent dips do not show any significant amount of mass change. This indicates a nonuniform mass uptake profile as a function of the number of dips. This is perhaps due to the fact that the film formation is due to interactions such as hydrophobic interactions and also due to bulkiness of the polymer. Some of us in our earlier studies of salts/surfacemodified nanoparticles with oppositely charged monolayer have demonstrated a uniform, lamellar mass uptake/film formation by the LB technique. In those studies, strong attractive electrostatic interactions were predominant.9a,18 In the present case, the absence of attractive electrostatic interactions would be responsible for poor film formation. We had earlier observed a similar problem while attempting LB deposition of the enzyme pepsin with octadecylamine (ODA) monolayers although the charges were opposite (unpublished results). The bulkiness of the polymer could therefore also be responsible for a poor deposition. (18) (a) Gole, A.; Kaur, J.; Pavaskar, N.; Sastry, M. Langmuir 2001, 17, 8249. (b) Pal, S. Ph.D. Thesis, University of Pune, 1995

LB films of the 30 ppm PAMPS25%-AA system were deposited on Si(111) substrates and the films analyzed by Fourier transform infrared spectroscopy (FTIR). For comparison, FTIR analysis of drop dried pure AA LB films and PAMPS25% on Si(111) substrates was also performed. As shown in Figure 6B, a strong band at ca. 1700 cm-1 can be clearly seen for as deposited AA and PAMPS, respectively (band b, curves 1 and 2). This peak is assigned to the carbonyl stretching vibration.19a,b In the PAMPS-AA composite film, the carbonyl peak occurs at ca. 1743 cm-1 (curve 3, band a, Figure 6B). This feature might also be present in the as deposited PAMPS films (curve 2, Figure 6B) but is not clearly visible due to a large band around 1700 cm-1. The polymer has a few amide bands (C-N, N-H) in the region 1680-1500)18a,c and can be clearly seen in curve 2 for as deposited PAMPS. There is a shift in one of the amide bands when the polymer is complexed with AA, which can be clearly seen as a broad band centered at ca. 1648 cm-1 (curve 3, band c, Figure 6B). Hence, QCM and FTIR measurements indicate that PAMPS complexed with AA, could be deposited onto solid substrates. The method of deposition (LB technique) does not allow us to build thick films. For such systems, the alternative horizontal deposition method such as the Langmuir-Schaefer method20 would be more appropriate, as demonstrated in the case of proteins. Conclusions In the present paper we demonstrate the presence of nonelectrostatic interactions, possibly hydrophobic interactions, leading to surface complexation of similarly charged polymer-fatty acid systems. Scheme 2 shows a possible complexation mechanism. The polymer penetrates the fatty acid monolayer at the air-water interface. We do not completely understand this behavior at this moment. Further experiments such as the role of rigidity of the polymer backbone (the use of highly rigid polymer (19) (a) Bellamy, L. J. The Infrared Spectra of Complex Molecules; John Wiley & Sons, Inc.: New York, 1966. (b) Rabolt, J. F.; Burns, F. C.; Schlotter, N. E.; Swalen, J. D. J. Chem. Phys. 1983, 78. (c) Maeda, Y.; Higuchi, T.; Ikeda, I. Langmuir 2001, 17, 7535. (20) (a) Nicolini, C.; Erokhin, V.; Antolini, F.; Catasti, P.; Facci, P. Biochim. Biophys. Acta 1993, 1158, 273. (b) Chen, X.; Moser, C. C.; Pilloud, D. L.; Dutton, P. L. J. Phys. Chem. B 1998, 102, 6425.

Interactions between Similarly Charged Polyelectrolyte

Langmuir, Vol. 19, No. 22, 2003 9327

Figure 6. (A) QCM mass uptake kinetics as a function of number of immersion cycles performed for the 30 ppm PAMPS25%-AA system after allowing complete complexation for 24 h (see text for details). (B) FTIR spectra recorded on Si(111) substrate: as deposited (drop dried) AA film (curve 1); as deposited (drop dried) film of 30 ppm PAMPS25%-AA system (curve 2); 30 ppm PAMPS25%-AA system after five immersion cycles by the LB method (curve 3). See text for assignments.

xanthan) and its comparison with semiflexible polymer such as carboxymethyl cellulose (CMC) would be interesting. At the present stage, we simply report that nonelectrostatic interactions, possibly hydrophobic, are responsible for interaction between similarly charged polyelectrolyte and fatty lipid system. Furthermore, as the charge on the polymer increases, the degree of complexation reduces (compare Figures 1 and 2), as in systems where the complexation is electrostatic. This could be rationalized by the increasing rigidity of the polymer when its charge increases. The complexation also depends

on the concentration of the polymer used and weakens when the concentration increases. The mixed monolayer appears inhomogeneous as seen from BAM. A full explanation of this unexpected surface behavior is still lacking. Acknowledgment. We thank the Indo-French center (CEFIPRA in France and IFCPAR in India) for funding the work. LA0352063