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Polymer/Surfactant Complexes at the Water/Air Interface: A Surface Tension and X-ray Reflectivity Study Cosima Stubenrauch,*,† Pierre-Antoine Albouy,‡ Regine v. Klitzing,§ and Dominique Langevin‡ Physikalische Chemie 1, Universita¨ t zu Ko¨ ln, Luxemburgerstrasse 116, 50939 Ko¨ ln, Germany, Laboratoire de Physique des Solides, Baˆ t. 510, Universite´ Paris Sud, 91405 Orsay, France, and Iwan-N.-Stranski Institut, Sekr. ER1, TU-Berlin, Strasse des 17. Juni 112, 10623 Berlin, Germany Received September 28, 1999. In Final Form: December 6, 1999 The water/air interface of dilute mixed solutions of dodecyl trimethylammonium bromide and different non-surface-active anionic polyelectrolytes has been investigated by measuring the surface tensions and the X-ray reflectivities. A strong synergistic lowering of the surface tension is found for all surfactant/ polyelectrolyte mixtures. This decrease is caused by the formation of polyelectrolyte/surfactant complexes at the surface. It has been detected that these complexes form a relatively dense surface layer. When the surfactant and/or polyelectrolyte concentration is varied, the thickness and density of these surface layers remain nearly constant for a given polyelectrolyte. However, although all the different polyelectrolytes adsorb in a flat configuration, the properties of the adsorbed layers depend slightly on the nature of the polyelectrolyte.
1. Introduction The interaction between surfactants and polymers is an active field of research in colloid science.1 For many industrial applications mixtures of polymers and surfactants are used. In water-based formulations such as paints and drilling muds, polyelectrolytes are of particular interest because they are generally water soluble and they have interesting rheological properties. Polyelectrolyte solutions are less well understood than neutral polymer solutions, although recent work has enhanced the current knowledge.2 These polymers form more extended structures, with effective persistence lengths much larger than those of neutral polymers. When two polyelectrolytes of opposite charge are mixed, the two polyions associate, thus releasing the counterions and increasing the entropy of the solution.3 It was observed that the behavior of polyelectrolyte-surfactant solutions is similar to the behavior of polyelectrolyte-polymer solutions: no association when the surfactant or the polymer is nonionic or when the two species have the same charge, and strong association for opposite charges. In the case of surfactantpolymer mixtures, one has to keep in mind that the size and the shape of the surfactant aggregates can vary. In mixtures of surfactants with both neutral polymers4 and polyelectrolytes, the complexation in the bulk phase occurs at a critical aggregation concentration (cac) that is much smaller than the critical micelle concentration (cmc) of the pure surfactant. In the polymer-surfactant complexes the micelles are bound to the polymer chains; their aggregation number is sometimes different from that of the pure micelles, depending on the nature of the polyelectrolyte.5 †
Universita¨t zu Ko¨ln. Universite´ Paris Sud. § Iwan-N.-Stranski Institut. ‡
(1) Goddard, E. D.; Ananthapadmanabhan, K. P. Interactions of Surfactants with Polymers and Proteins; CRC Press: Boca Raton, FL, 1993. (2) Barrat, J. L.; Joanny, J. F. Adv. Chem. Phys. 1996, 94, 1. (3) Lindman, B. Adv. Colloid Interface Sci. 1992, 41, 149. (4) Cabane, B.; Duplessix, R. J. Phys. 1982, 43, 1529.
Polymers and surfactants also form complexes at the liquid-solid, liquid-liquid, and liquid-air interfaces. These complexes are important for practical applications such as colloidal stabilization, wettability, and adhesion. Most of the studies that have focused on the complexation of surfactant and polyelectrolyte at the liquid-air interface, i.e., at the surface, address the problem of polyelectrolyte complexation with insoluble monolayers.6-8 The subject of the present paper is the surfactant-polyelectrolyte complexation at the free surface of an aqueous surfactant solution, i.e., the complexation between a polyelectrolyte and a soluble surfactant. This case is frequent in practice, but difficult to analyze: contrary to insoluble monolayers, the amount of surfactant adsorbed at the surface is not known and has to be either measured or inferred from thermodynamic models. In our study we have investigated the complexation between the cationic surfactant dodecyl trimethylammonium bromide (C12TAB) and the following model polymers: polyacrylamide sulfonate (PAMPS), polystyrene sulfonate (PSS), Xanthan, and DNA. PAMPS and PSS are rather flexible polymers with intrinsic persistence lengths of about 10 Å.2 PAMPS is water soluble regardless the degree of sulfonation x (defined as the molar ratio of sulfonated monomers to neutral monomers), whereas PSS is water soluble only if x > 0.3.9 This is because the PSS backbone is more hydrophobic, and as we will see later, it leads to significant differences in the surface behavior. Xanthan and DNA are more rigid: the intrinsic persistence length of Xanthan is between 50 and 1400 Å depending on its configuration in the solution (coil, simple or double helix),10 and for the (5) Hansson, P.; Almgren, M. Langmuir 1994, 10, 2115; J. Phys. Chem. 1995, 99, 16694. (6) Babak, V. G.; Anchipolovskii, M. A.; Vikhoreva, G. A.; Lukina, I. G. Colloid J. 1996, 2, 145 (translated from Russian). (7) de Meijere, K.; Brezesinski, G.; Mo¨hwald, H. Macromolecules 1997, 30, 2337. (8) Sundaran, S.; Stebe, K. J. Langmuir 1997, 13, 1729. Sundaran, S.; Ferry, K. J.; Vollhart, D.; Stebe, K. J. Langmuir 1998, 14, 1208. (9) Essafi, W.; Lafuma, F.; Williams, C. E. J. Phys. II 1995, 5, 1269. Essafi, W. Thesis, Universite´ Paris VI, 1996. (10) Milas, M.; Rinaudo, M.; Duplessix, R.; Borsali, R.; Lindner, P. Macromolecules 1995, 28, 3119.
10.1021/la991277j CCC: $19.00 © 2000 American Chemical Society Published on Web 03/04/2000
Polymer/Surfactant Complexes
double-helix conformation of the DNA an intrinsic persistence length of 500 Å is reported.2,11 Despite these differences, the semidilute solutions of the four polymers have similar properties: the mesh size of the networks is comparable for comparable polymer concentrations.9,10 This is due to the fact that the mesh size is more related to the total than to the intrinsic persistence length. As the total persistence length is the sum of the intrinsic and the electrostatic one, it contains an important or even dominating electrostatic contribution.2 However, the polymer configuration in the bulk and in the surface aggregates is very different: at the surface the polymer has to stretch in order to complex with the ions of the surfactant monolayer and to take a configuration ressembling that of half a bilayer, whereas the bulk aggregates are made of coils decorated by surfactant micelles.4 The stretching of the polymer chain is expected to be easier for the rigid polyelectrolytes. Note that it is not only the configuration of the polymer which is different in the bulk and in the surface aggregates. The composition of the mixed aggregates differs also: the surface aggregates start to form well below the cac. The aim of the present work is to investigate the influence of the polymer rigidity, i.e., of its persistence length, on the properties and structure of the mixed surface complexes. This work is also motivated by the observation that mixed solutions of flexible polyelectrolytes and oppositely charged surfactants can form stable thin liquid films under conditions where a film made from the pure surfactant solution is completely unstable. Indeed, the chosen surfactant C12TAB does not form stable films at low surfactant concentrations.12 It has been shown with the so-called “thin film balance, TFB”, that the addition of the anionic polyelectrolytes PAMPS and PSS leads to the formation of stable thin liquid films.13,14 In contrast to PAMPS and PSS the mixtures of C12TAB with Xanthan and DNA do not form stable films. (Note that mixtures of nonionic surfactants with Xanthan are able to form stable films14b.) The reason for this completely different behavior is unknown. Because foam film stability is closely related to the properties of the surface layers, a study of the mixed monolayers was expected to bring some explanations to the problem. In this paper, we present an investigation of the properties of mixed surface complexes consisting of C12TAB and one of the four polymers mentioned above. For that purpose we have carried out surface tension and X-ray reflectivity measurements. It will be shown that all polyelectrolytes coadsorb with the cationic surfactant at the surface where they form highly surface-active complexes. The thickness and density of these complexes have been investigated. We will compare the results for the four different polyelectrolyte/surfactant systems and discuss the differences. 2. Experimental Section 2.1. Materials. Dodecyl trimethylammonium bromide (C12TAB) was purchased from Aldrich and recrystallized twice before use. Polystyrene sulfonate (PSS) was obtained from Aldrich. It has a molar mass of 7 × 104 g/mol and a degree of charge of almost 100%. The polyelectrolyte denoted PAMPS, synthesized by SNF Floerger, is a statistical copolymer of neutral acrylamide (11) van der Maarel, J. R. C.; Groot, L. C. A.; Mandel, M.; Jesse, W.; Jannink, G.; Rodriguez, V. J. Phys. II 1992, 2, 109. (12) Bergeron, V. Langmuir 1997, 13, 3474. (13) Asnacios, A.; Espert, A.; Colin, A.; Langevin, D. Phys. Rev. Lett. 1997, 78, 4974. (14) (a) v. Klitzing, R.; Espert, A.; Asnacios, A.; Hellweg, T.; Colin, A.; Langevin, D. Colloids Surf., A 1999, 149, 131. (b) v. Klitzing, R.; Espert, A.; Colin, A.; Langevin, D. Colloids Surf., A., in press.
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Figure 1. Monomer units of (a) PAMPS, (b) PSS, and (c) Xanthan and, (d) one nucleotide of a single DNA-strand. monomers and charged acrylamido methyl propane sulfonate monomers. The polymer used in this study has a molar mass of about 5 × 105 g/mol and a fraction of charged monomers of 25 mol %. Xanthan is a polysaccharide. Its exact molar mass is not known, but it is lower than 107 g/mol, which was the mass of the polymer before purification (see below). We did not pay much attention to the exact value of the molecular weights of the polymers because the structure of polymers at surfaces is in general independent of their weight. We have verified this independency for the mixed C12TAB-PAMPS surface complexes.15 Xanthan solutions were made without added salt so that the polymer is in its disordered conformation, i.e., a random coil with an intrinsic persistence length of 50 Å. The polyelectrolytes PSS, PAMPS, and Xanthan were purified by ultrafiltration and freeze-dried before use. The PAMPS and Xanthan samples are a gift of J. F. Argillier (Institut Franc¸ ais du Pe´trole). For the DNA surface tension measurements, we used DNA fragments with a narrow polydispersity (denoted DNAshort). The lengths of these fragments are between 130 and 600 base pairs (bp) with 50% of them having 146 ( 7 bp. The DNA solutions have been extensively dialyzed against a solution of 2 mM NaCl before use. These solutions have been prepared by the group of F. Livolant in the laboratory. The DNA for the X-ray reflectivity measurements was purchased from Sigma (D-1501, Lot87H7840, Sodium Salt, TypeI, “highly polymerized” from calf thymus). All DNA solutions have been prepared with 2 mM NaCl to ensure a double-helix conformation. The “highly polymerized” DNA from Sigma was used without further purification. The structures of all polymers are shown in Figure 1, and the properties of importance for the present paper are summarized in Table 1. 2.2. Methods. Surface Tensions. The surface tensions were measured with an open frame version of the Wilhelmy plate to avoid the wetting problems of the classical plate. The measurements were carried out in a Teflon trough placed in a Plexiglas box with an opening for the tensiometer. For mixed solutions at low surfactant concentrations and polyelectrolytes of large molecular weight (PAMPS and Xanthan), the approach to the equilibrium could take several hours and it was assumed arbitrarily that equilibrium had been reached when the surface (15) Asnacios, A.; Langevin, D.; Argillier, J.-F. Macromolecules 1996, 29, 7412; Eur. Phys. J. B. 1998, 5, 905.
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Table 1. Properties of the Polymers Used in the Present Work: Molar Mass of the Polymer Mpoly and the Monomer Unit Mmono, Degree of Polymerization N, Persistence Length l0, Charge Per Monomer F, Monomer Size a, Distance between Consecutive Charges A, Specific Volume vspe, Reduced Electron Density δ, and Electron Density G PAMPS Mpoly (g/mol) Mmono (g/mol) N l0b simple helix (Å) double helix (Å) “wormlike chain” (Å) F a (Å) A (Å) vspe (mL/g) δd (10-6) Fd (e-/Å3)
∼ 5 × 105 110 ∼4550
10 (ref 31) 0.25 2.5 10.0 0.77 (ref 24) 4.70 0.441
PSS
DNAa
Xanthan
70 000 206 340
