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Association of Naphthalene-Labeled Poly(acrylic acid) and Interaction with Cationic Surfactants. Fluorescence Studies Karin Schille´n,* Dan F. Anghel,† Maria da Grac¸ a Miguel,‡ and Bjo¨rn Lindman§ Physical Chemistry 1, Center for Chemistry and Chemical Engineering, Lund University, P.O. Box 124, S-221 00 Lund, Sweden Received July 5, 2000. In Final Form: September 12, 2000 Water-soluble poly(acrylic acid) has been covalently labeled with a fluorescent hydrophobic chromophore, naphthalene (Np), randomly attached onto the polymer backbone with an amount of 3 mol %. The polymer, which is a new type of hydrophobically modified polymer denoted PAAMeNp-34, was investigated using steady-state fluorescence spectroscopy in aqueous solutions of different pH and in methanol. The fluorescence emission spectra of PAAMeNp-34 in water exhibit both Np monomer emission (with intensity IM) and Np excimer emission (with intensity IE). The excimer emission is mainly due to the association of Np groups, preformed in their ground electronic state as a result of the hydrophobic interaction. For a PAAMeNp-34 aqueous solution, the intensity ratio, IE/IM, decreases in the pH range where the electrostatic repulsive forces overcome the hydrophobic interactions between the Np groups and the polymer chain expands because of the intrapolymer repulsion between the negatively charged carboxylate groups. In methanol, the excimer emission is low because hydrophobic interactions are insignificant in this solvent. The interaction between PAAMeNp-34 and cationic surfactants of different alkyl chain length (dodecyl-, tetradecyl-, and hexadecyltrimethylammonium chloride) was also studied in dilute aqueous solutions at pH 3.0 and pH 6.8. The addition of surfactants perturbs the Np-Np interactions because of polymer-surfactant associations. This causes a detectable change in the fluorescence emission, which is followed with increasing surfactant concentration. From the onset of the change, the force that dominates the interaction between the polymer and the surfactants at different pH can be examined. At low pH, PAAMeNp-34 is uncharged and hydrophobic forces dominate the polymer-surfactant interaction. The photophysical properties of the system therefore show a clear dependence on the hydrophobicity (or chain length) of the surfactants. On the other hand, at pH 6.8, where the polymer is negatively charged, almost no or very little difference between the three surfactants is observed at the onset of fluorescence change, which indicates that electrostatic forces dominate the interaction at the lowest surfactant concentrations.
Introduction Water-soluble polymers that have been chemically modified by attaching hydrophobic groups, for example alkyl or perfluoroalkyl chains, onto the polymer backbone are denoted hydrophobically modified (HM) polymers. HMpolymers are self-organizing polymers and are of great industrial importance because they may be used as solution thickeners in a variety of industrial fluids. Their unique solution properties in water are due to association of the hydrophobic groups; see, for example, refs 1-3 and the references therein. The grafting amount of the highly hydrophobic groups is usually 1-3 mol %. This structural change results in a solution behavior totally different from that of the parent polymer. For instance, it has been observed that the aqueous solutions of HM-poly(sodium acrylate), in the semidilute regime, have higher viscosities and exhibit reversible shear sensitivity as compared to the solutions of the unmodified material.4 Addition of salt * To whom correspondence should be addressed. E-mail:
[email protected]. † Permanent address: Institute of Physical Chemistry, Department of Colloids, Spl. Independentei 202, 77208 Bucharest, Romania. ‡ Permanent address: Departamento de Quı´mica, Universidade de Coimbra, 3049 Coimbra, Portugal. § E-mail:
[email protected]. (1) Winnik, M. A.; Yekta, A. Curr. Opin. Colloid Interface Sci. 1997, 2, 424. (2) Bromberg, L. E.; Barr, D. P. Macromolecules 1999, 32, 3649. (3) Candau, F.; Selb, J. Adv. Colloid Interface Sci. 1999, 79, 149.
produces a strong viscosity increase and may even induce gel formation.4,5 HM-polymer/surfactant mixtures display a wide range of applications from enhanced oil recovery to drug delivery systems.6 To investigate the different interaction forces that may exist in these systems, several studies have been performed by using various experimental techniques, for example, viscosimetry or rheology,7-15 light scattering techniques,11,16 nuclear magnetic resonance,17-20 electron spin resonance,10,21 microcalorimetry,22-24 cryogenic trans(4) Wang, T. K.; Iliopoulos, I.; Audebert, R. Polym. Bull. 1988, 20, 577. (5) Wang, T. K.; Iliopoulos, I.; Audebert, R. In Water-Soluble Polymer: Synthesis, Solution Properties, and Applications; Shalaby, S. W., McCormick, C. L., Butler, G. B., Eds.; American Chemical Society: Washington, DC, 1991; Vol. 467, p 218. (6) Glass, J. E. Hydrophilic Polymers-Performance with Environmental Acceptance; American Chemical Society: Washington, DC, 1996; Vol. 248. (7) Biggs, S.; Selb, J.; Candau, F. Langmuir 1992, 8, 838. (8) Chang, Y.; Lochhead, R. Y.; McCormick, C. L. Macromolecules 1994, 27, 2145. (9) Magny, B.; Iliopoulos, I.; Zana, R.; Audebert, R. Langmuir 1994, 10, 3180. (10) Senan, C.; Meadows, J.; Shone, P. T.; Williams, P. A. Langmuir 1994, 10, 2471. (11) Nystro¨m, B.; Thuresson, K.; Lindman, B. Langmuir 1995, 11, 1994. (12) Guillemet, F.; Piculell, L. J. Phys. Chem. 1995, 99, 9201. (13) Ka¨stner, U.; Hoffman, H.; Do¨nges, R.; Ehrler, R. Colloids Surf., A 1996, 112, 209. (14) Kjøniksen, A.-L.; Nystro¨m, B.; Lindman, B. Macromolecules 1998, 31, 1852. (15) Nilsson, S.; Thuresson, K.; Hansson, P.; Lindman, B. J. Phys. Chem. B 1998, 102, 7099.
10.1021/la000943+ CCC: $19.00 © 2000 American Chemical Society Published on Web 11/22/2000
Naphthalene-Labeled Poly(acrylic acid)
mission electron microscopy,22,25 potentiometry,26,27 and photophysical techniques.7,9,15,19,24,26,28 More general reviews on this subject may be found in refs 1, 3, and 29-31. Many of these investigations have been focused on the relationship between microstructure and macroscopic properties of the polymer system and also on the effect of salts, surfactants, pH, and/or temperature on the hydrophobic associates that exist in the system. As an example to illustrate the latter, the addition of an oppositely charged surfactant to a water solution of a HM polyelectrolyte causes a stronger increase of the solution viscosity than the addition of a nonionic or a similarly charged surfactant.9,32 In all cases, a viscosity maximum is noticed within a narrow surfactant concentration range. This phenomenon has also been shown to exist in mixed water solutions of uncharged HM polymers and ionic surfactants.7,15 The surfactant molecules interact with the hydrophobic groups of the polymers, forming micellar aggregates that contain several hydrophobes also from different polymer chains. This microscopic cross-linking picture is then reflected as a maximum in the viscosity curve. At high surfactant concentration, the viscosity decreases. The polymer-surfactant micellar aggregates, now in equilibrium with free surfactant micelles, contain only one or a few polymer intrapolymeric hydrophobes per aggregate, and the cross-linking effect is therefore lost. There are two kinds of fluorescence methods that may be applied in the investigations of polymer-surfactant and polymer-polymer systems: fluorescence probing and fluorescent labeling experiments. Fluorescence probing experiments have been utilized in several studies in which a free fluorescent chromophore (a probe) is added to the solution. They have been successfully employed for determination of aggregation numbers of surfactant micelles, polymer-surfactant aggregates, and polymerbound surfactant micelles; see, for example, refs 30 and 33-40 and the references therein. Other properties that (16) Thuresson, K.; Nystro¨m, B.; Wang, G.; Lindman, B. Langmuir 1995, 11, 3730. (17) Magny, B.; Lafuma, F.; Iliopoulos, I. Polymer 1992, 33, 3151. (18) Effing, J. J.; McLennan, I. J.; van Os, N. M.; Kwak, J. C. T. J. Phys. Chem. 1994, 98, 12397. (19) Thuresson, K.; So¨derman, O.; Hansson, P.; Wang, G. J. Phys. Chem. 1996, 100, 4909. (20) Petit, F.; Iliopoulos, I.; Audebert, R. Polymer 1998, 39, 751. (21) Wang, Y.; Lu, D.; Long, C.; Han, B.; Yan, H.; Kwak, J. C. T. Langmuir 1998, 14, 2050. (22) Kevelam, J.; van Breemen, J. F. L.; Blokzijl, W.; Engberts, J. B. F. N. Langmuir 1996, 12, 4709. (23) Wang, Y.; Han, B.; Yan, H.; Kwak, J. C. T. Langmuir 1997, 13, 3119. (24) Faes, H.; De Schryver, F. D.; Sein, A.-M.; Bijma, K.; Kevelam, J.; Engberts, J. B. F. N. Macromolecules 1996, 29, 3875. (25) Kamenka, N.; Kaplun, A.; Talmon, Y.; Zana, R. Langmuir 1994, 10, 2960. (26) Anthony, O.; Zana, R. Langmuir 1996, 12, 3590. (27) Harrison, I. M.; Candau, F.; Zana, R. Colloid Polym. Sci. 1999, 277, 48. (28) Winnik, F. M.; Ringsdorf, H.; Venzmer, J. Langmuir 1991, 7, 905. (29) Hansson, P.; Lindman, B. Curr. Opin. Colloid Interface Sci. 1996, 1, 604. (30) Winnik, F. M.; Regismond, S. T. A. Colloids Surf., A 1996, 118, 1. (31) Iliopoulos, I. Curr. Opin. Colloid Interface Sci. 1998, 3, 493. (32) Magny, B.; Iliopoulos, I.; Audebert, R.; Piculell, L.; Lindman, B. Prog. Colloid Polym. Sci. 1992, 89, 118. (33) Turro, N. J.; Yekta, A. J. Am. Chem. Soc. 1978, 100, 5951. (34) van Stam, J.; Depaemelaere, S.; De Schryver, F. C. J. Chem. Educ. 1998, 75, 93. (35) Winnik, F. M. In Interactions of Surfactants with Polymers and Proteins; Goddard, E. D., Ananthapadmanabhan, K. P., Eds.; CRC Press: Boca Raton, FL, 1993; p 367. (36) Lindman, B.; Thalberg, K. In Interactions of Surfactants with Polymers and Proteins; Goddard, E. D., Ananthapadmanabhan, K. P., Eds.; CRC Press: Boca Raton, FL, 1993; p 203.
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may be determined using fluorescent probes are the critical aggregation concentration (CAC) and the microenvironment (“local polarity” and “microviscosity”) within the polymer-surfactant aggregates. The main difficulty is to determine the location of the probe in the system and to what extent it may disturb the system. The other approach is to label the polymer by covalently attaching a fluorescent chromophore. This has been used in the investigation of the core-corona interface of block copolymer micelles of poly(isoprene) and poly(methyl methacrylate) in acetonitrile using time-resolved fluorescence energy transfer spectroscopy.41-44 The diblock copolymers were labeled at the block junction with a single fluorescent dye, either a donor chromophore or an acceptor chromophore. The study demonstrated for the first time that the fluorescence energy transfer technique can be utilized to determine the core radius and the aggregation number of block copolymer micelles under certain conditions.44 Chromophore-labeling of polymers has also been utilized in fluorescence studies of the cyclization rates of end-labeled polymers, such as poly(ethylene glycol) and polystyrene, in different organic solvents; see, for example, refs 45-47 and the references therein. The properties of hydrophobic polymers in organic solvents are not disturbed by the introduction of fluorescent labels, especially if one attaches only a single chromophore per macromolecule. However, if several chromophores are attached and distributed nonrandomly during the labeling reaction, then intrapolymer aggregates of chromophores may be observed.48 That apart, as will be discussed below, for water-soluble polymers the labels may act as hydrophobic substituents, which in turn will induce formation of ground-state chromophore aggregates and radically change the polymer characteristics. Water-soluble polymers labeled with hydrophobic chromophores are in fact a new class of HM polymers. These chromophore-labeled polymers may help in understanding, in molecular terms, the properties of HM polymers from photophysical measurements. For studies of the most industrially relevant polymers, the preferable way of introducing the chromophore is by chemical postmodification. This approach was used to attach chromophores to various cellulose ethers, such as (hydroxypropyl)cellulose, tylose, methylcellulose, and hydrophobically modified (hydroxypropyl)cellulose.49-51 Alternative labeling techniques are summarized in refs 30, 35, and 52-54. (37) Winnik, F. M.; Regismond, S. T. A. In Polymer-Surfactant Systems; Kwak, J. K. T., Ed.; Marcel Dekker: New York, 1998; p 267. (38) Hansson, P.; Almgren, M. J. Phys. Chem. 1996, 100, 9038. (39) Fundin, J.; Hansson, P.; Brown, W.; Lidegran, I. Macromolecules 1997, 30, 1118. (40) Noda, T.; Hashidzume, A.; Morishima, Y. Macromolecules 2000, 33, 3694. (41) Schille´n, K.; Yekta, A.; Ni, S.; Winnik, M. A. Macromolecules 1998, 31, 210. (42) Schille´n, K.; Yekta, A.; Ni, S.; Winnik, M. A. Prog. Colloid Polym. Sci. 1999, 112, 45. (43) Farinha, J. P. S.; Schille´n, K.; Winnik, M. A. J. Phys. Chem. B 1999, 103, 2487. (44) Schille´n, K.; Yekta, A.; Ni, S.; Farinha, J. P. S.; Winnik, M. A. J. Phys. Chem. B 1999, 103, 9090. (45) Lee, S.; Winnik, M. A. Macromolecules 1997, 30, 2633. (46) Lee, S.; Duhamel, J. Macromolecules 1998, 31, 9193. (47) Reis e Sousa, A. T.; Castanheira, E. M. S.; Fedorov, A.; Martinho, J. M. G. J. Phys. Chem. A 1998, 102, 6406. (48) Moffitt, M.; Farinha, J. P. S.; Winnik, M. A.; Rohr, U.; Mu¨llen, K. Macromolecules 1999, 32, 4895. (49) Winnik, F. M.; Winnik, M. A.; Tazuke, S.; Ober, C. K. Macromolecules 1987, 20, 38. (50) Winnik, F. M. In Hydrophilic Polymers: Performance with Environmental Acceptance; Glass, J. E., Ed.; American Chemical Society: Washington, DC, 1996; Vol. 248, p 409. (51) Nishikawa, K.; Yekta, A.; Pham, H. H.; Winnik, M. A. Langmuir 1998, 14, 7119.
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Photophysical experiments on chromophore-labeled polyelectrolytes may report on phenomena from the perspective of the polymer, for example, the polymer solution properties upon variation of pH and ionic strength and the interaction of the charged polymer with added substances, such as surfactants55-61 or a second polymer.62 Fluorescence studies of the interaction of chromophorelabeled uncharged polymers and surfactants in water have earlier been reported in the literature; see, for example, refs 30, 51, and 63-65. In a recent study, the photophysical properties of poly(acrylic acid) (PAA), randomly labeled with pyrene (Py) or naphthalene or both in aqueous and organic solvents were investigated by steady-state and time-resolved fluorescence spectroscopy.66 The emphasis was put on the pH-induced conformational changes of the polymer chain. At the writing of this report, an additional fluorescence study of perfluorinated high molecular weight PAA (1.25 × 106 g/mol) labeled with pyrene has been published.67 Also in that study the focus was set on the polymer conformational changes influenced by pH and the hydrophobic perfluoro-octylester side chains. Both the changes in conformation of Py-labeled PAA and Py-labeled poly(sodium acrylate) and the interaction with sodium dodecyl sulfate (SDS) and dodecyltrimethylammonium chloride, respectively, in aqueous solution have been studied in two earlier steady-state fluorescence investigations.55,56 In this paper, we report the results from steady-state fluorescence measurements on naphthalene-labeled poly(acrylic acid), denoted PAAMeNp-34, in different solvents and on different PAAMeNp-34/surfactant aqueous solutions. The fluorescence emission spectra display both Np monomer emission and Np excimer emission. First, the photophysical properties of the polymer in pure water, in methanol, and in methanol-water mixtures are described. We examine the balance between the hydrophobic and the electrostatic interactions in the PAAMeNp-34/water system as a function of pH and of salt concentration. The effect of cationic surfactants upon the aqueous solution behavior of PAAMeNp-34 is thereafter investigated at different pH. The surfactants studied are alkyltrimethylammonium chlorides with a hydrocarbon chain length varying from 12 to 16 carbons, C12TAC, C14TAC, and C16TAC, respectively. The changes in the steady-state fluorescence properties allow us to draw conclusions on (52) Ezzell, S. A.; McCormick, C. L. Macromolecules 1992, 25, 1881. (53) Winnik, F. M. Chem. Rev. 1993, 93, 587. (54) Branham, K. D.; Shafer, G. S.; Hoyle, C. E.; McCormick, C. L. Macromolecules 1995, 28, 6175. (55) Maltesh, C.; Somasundaran, P. Colloids Surf. 1992, 69, 167. (56) Magny, B. Polyelectrolytes Associatifs: Methodes de Synthese, Comportement en Milieu dilue et Semi-Dilue. Ph.D. Thesis, Universite´ Pierre et Marie Curie, Paris, 1992. (57) Kramer, M. C.; Welch, C. G.; Steger, J. R.; McCormick, C. L. Macromolecules 1995, 28, 5248. (58) Yoshida, K.; Morishima, Y.; Dubin, P. L.; Mizusaki, M. Macromolecules 1997, 30, 6208. (59) Winnik, M. A.; Bystryak, S. M.; Siddiqui, J. Macromolecules 1999, 32, 624. (60) Liaw, D.-J.; Huang, C.-C.; Sang, H.-C.; Kang, E.-T. Langmuir 1998, 14, 3195. (61) Mizusaki, M.; Morishima, Y.; Dubin, P. L. J. Phys. Chem. 1998, 102, 1908. (62) Turro, N. J.; Arora, K. S. Polymer 1986, 27, 783. (63) Hu, Y.-Z.; Zhao, C.-L.; Winnik, M. A.; Sundararajan, P. R. Langmuir 1990, 6, 880. (64) Quina, F.; Abuin, E.; Lissi, E. Macromolecules 1990, 23, 5173. (65) Winnik, F. M.; Ringsdorf, H.; Venzmer, J. Langmuir 1991, 7, 912. (66) Anghel, D. F.; Alderson, V.; Winnik, F. M.; Misuzaki, M.; Morishima, Y. Polymer 1998, 39, 3035. (67) Pokhrel, M. R.; Bossmann, S. H. J. Phys. Chem. B 2000, 104, 2215.
Schille´ n et al. Chart 1
what type of force is dominating the interaction between the polymer and the surfactants at different pH. It is well-known that an aqueous system of a polymer and a surfactant of opposite charge phase-separate associatively into one dilute phase and one concentrated phase, rich in both polymer and surfactant. The tendency for phase separation depends on both the surfactant hydrophobic chain length and the polymer molecular weight.36 The same applies to the PAAMeNp-34/surfactant systems at higher surfactant concentrations and especially at high pH. To eliminate any ambiguous results, no fluorescence measurements were performed in these turbid two-phase regions. Only the results from the measurements in the one-phase regions are presented here. Experimental Section Materials and Sample Preparation. Poly(acrylic acid) (PAA) with a nominal molecular weight of 150 000 was purchased from Wako Chemicals as a 25% water solution. The solid PAA used in the synthesis was obtained by freeze-drying this solution. The polymer was labeled randomly with a fluorescent chromophore (naphthalene, Np) by PAA amidation. The synthesis, together with the purification procedures, has been described elsewhere.66 Reagent-grade chemicals were used in the synthesis. 1-Naphthylmethylamine was a Fluka product. 1,3-Dicyclohexylcarbodiimide and anhydrous 1-methylpyrolidone were obtained from Aldrich Chemicals. The dialysis membranes with a molecular weight cutoff of 12000-14000 were supplied by Spectrum Medical Industries. The amount of naphthalene grafted on the PAA polymer was determined both by ultraviolet (UV) absorption measurements and by 1H nuclear magnetic resonance (1H NMR) measurements. The UV absorption measurements were performed by using a methanol solution of 1-naphthylmethylamine as a model compound solution. The 1H NMR spectra were recorded in deuterium oxide. The Np content obtained was [Np]UV ) 4.1 × 10-4 mol/g and [Np]NMR ) 4.5 × 10-4 mol/g from UV and 1H NMR measurements, respectively. This corresponds to ca. 1 naphthalene group per 34 acrylic acid monomer units. The polymer is referred to as PAAMeNp-34 and is shown in Chart 1. Dodecyltrimethylammonium chloride (C12TAC) and hexadecyltrimethylmmonium chloride (C16TAC) were purchased from Acros Organics, and tetradecyltrimethylammonium chloride (C14TAC) was purchased from Tokyo Kasei TCI. They were used as received. Sodium chloride (NaCl) was received from Sigma Chemicals Co. The water used in the solutions was purified with a Millipore Milli-RO4 purification system. Aqueous stock solutions of PAAMeNp-34 with a polymer concentration of 0.6 g/L (or 600 ppm) and of different pH were prepared by dissolving the polymer in water at room temperature under magnetic stirring for 24 h. The surfactant solutions of various concentrations were prepared from dilution of stock aqueous solutions of C12TAC (1 M), C14TAC (0.1 M), and C16TAC (0.1 M). The pH levels of the polymer and the surfactant stock solutions were adjusted to pH 3 and pH 7 by the addition of aqueous HCl (5 M) or NaOH (5 M), respectively. For each surfactant, a series of polymer-surfactant solutions were thereafter prepared with a constant polymer concentration of 0.05 g/L (50 ppm) and a varying surfactant
Naphthalene-Labeled Poly(acrylic acid) concentration. This was achieved by mixing aliquots of polymer stock solution with appropriate volumes of surfactant solution and adding water of the corresponding pH to a final volume of 3 mL. After the polymer-surfactant solutions were mixed, the pH was checked and adjusted not to differ more than (0.1 units (average values, 〈pH〉 ) 3.0 and 6.8). Prior to the steady-state fluorescence measurements, the prepared polymer-surfactant solutions were allowed to equilibrate for 2 h. Ultraviolet-Visible Absorption Spectroscopy. Ultraviolet (UV) absorption measurements were carried out on a PerkinElmer Lambda 14 UV-vis double-beam spectrophotometer. Absorption spectra of PAAMeNp-34 solutions and PAAMeNp34-surfactant solutions were measured in a 1 cm quartz cell from which the background was subtracted by using the pure solvent (water at the corresponding pH). A naphthalene excitation wavelength (λex) of 290 nm was chosen to be used in the steadystate fluorescence measurements described below. (The UV absorption maximum of naphthalene is at 278 nm.) For all solutions investigated, the absorbance at 290 nm was always
