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Interaction of Sodium Dodecyl Sulfate with Polyethyleneimine: Surfactant-Induced Polymer Solution Colloid Dispersion Transition Ro´bert Me´sza´ros,*,†,‡ Laurie Thompson,† Martin Bos,† Imre Varga,‡ and Tibor Gila´nyi‡ Unilever Research Port Sunlight, Quarry Road East, Bebington CH63 3JW, UK, and Department of Colloid Chemistry, Lora´ nd Eo¨ tvo¨ s University, P.O. Box 32, Budapest 112, Hungary H-1518 Received September 27, 2002. In Final Form: November 13, 2002 The binding isotherm of sodium dodecyl sulfate (SDS) on a hyperbranched polyethyleneimine (PEI) was determined by an equilibrium dialysis method. Dynamic light scattering, electrophoretic mobility, and coagulation kinetics measurements were also performed in order to monitor the changes in the charged nature and size of PEI/SDS complexes. The experimental binding isotherm shows that the SDS interacts with PEI in two different ways. In a first binding process, the dodecyl sulfate ions bind in monomer form to the protonated amine groups, which is accompanied by an increase of the pH. A quantitative model is presented to describe the relation between the surfactant binding and the pH change. Above a critical amount of the bound surfactant, the PEI/SDS complex molecules collapse and precipitate. After the collapse of the polyelectrolyte/surfactant molecules, the SDS adsorbs on the surface layer of the collapsed particles (causing a charge reversal). This means that the interaction of the SDS with PEI can be divided into different characteristic SDS concentration ranges. At low surfactant concentrations, the system is a thermodynamically stable solution of the polymer/surfactant complex molecules. Above this critical concentration, the system is an unstable colloid dispersion of the complex particles. At even higher surfactant concentrations, the system may be a kinetically stable dispersion of the PEI/SDS particles, depending on the method of preparation. It can be concluded that the observed mechanism of PEI-SDS interaction is different from the general characteristics of the oppositely charged linear polyelectrolytes and surfactants, where the precipitated complex dissolves in the excess surfactant due to a collective (micelle-like) polymersurfactant interaction.
Introduction The polymer/surfactant systems are essentially important since macromolecules and surface-active agents are usually the main components of cosmetic, detergent, and other industrial applications. Therefore, an intensive effort has been made to characterize the nature of these interactions as well as their impact on phase separation, rheological and interfacial properties with special relevance to the various commercial applications.1-4 Among these mixtures the oppositely charged polyelectrolyte/surfactant systems attracted special attention.2 While the nonionic polymer-surfactant interactions are quite well-described in terms of a cooperative binding process of the surfactant, the situation in the oppositely charged systems is more complex. There is a growing trend to consider these interactions as basically cooperative in nature and as a sort of surface charge neutralization of micelles via the oppositely charged flexible polymers, supported by theoretical simulations and experiments.2,3 On the other hand, there is also evidence of noncooperative * Corresponding author. E-mail:
[email protected]. † Unilever Research Port Sunlight. ‡ Lora ´ nd Eo¨tvo¨s University. (1) Goddard, E. D. Interactions of Surfactants with Polymers and Proteins; Goddard, E. D., Ananthapadmanabhan, K. P., Eds.; CRC Press: Boca Raton, FL, 1993; Chapter 4. (2) Wei, Y.-C.; Hudson, S. M. J. Macromol. Sci. Rev. Macromol. Chem. Phys. 1995, C35, 15. (3) Hansson, P.; Lindman, B. Curr. Opin. Colloid Interface Sci. 1996, 1, 604-613. (4) Zana, R. In Polymer-Surfactant Systems; Kwak, J. C. T., Ed.; Surfactant Science Series; Marcel Dekker: New York, 1998; Chapter 10.
surfactant binding5,6 and also a specific binding mechanism involving cooperative and noncooperative steps as well.7 The key factors determining the cooperativity of the binding of an ionic surfactant to oppositely charged polyelectrolytes are not clearly understood yet but might include the chemical nature, rigidity, charge density, and molecular architecture of the polymer and also the chemical nature of the different part of the surfactant molecules.1 Another interesting feature of the polymer-surfactant interactions is the changes in polymer conformation as a consequence of surfactant binding. The fluorescence investigations of Chandar et al.8 and Chu et al.9 revealed the contraction of oppositely charged polymer/surfactant complexes for the aqueous mixtures of poly(acrylic acid) and alkyltrimethylammonium bromides before precipitation. Xia et al.10 studied the interaction of poly(dimethyldiallylammonium chloride) with sodium dodecyl sulfate (SDS) by light scattering. They found that intrapolymer complex formation occurs at low polymer concentration, and in the limit of excess surfactant concentration, strong chain expansion occurs due to the repulsion between the bound micelles. Similar results were observed by Herslof (5) Ohbu, K.; Hiraishi, O.; Kashiwa, I. J. Am. Oil Chem. Soc. 1982, 59, 108. (6) Hajakawa, K.; Kwak, J. C. T. J. Phys. Chem. 1982, 86, 3866. (7) Li, Y.; Ghoreishi, S. M.; Bloor, D. M.; Holzwarth, J. F.; WynJones, E. Langmuir 2001, 16, 3093-3100. (8) Chandar, P.; Somasundaran, P.; Turro, N. J. Macromolecules 1988, 21, 950-953. (9) Chu, D. Y. Thomas, J. K. J. Am. Chem. Soc. 1986, 108, 6270. (10) Xia, J.; Zhang, H.; Rigsbee, D. R.; Dubin, P. L.; Shaikh, T. Macromolecules 1993, 26, 2759-2766.
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et al.11 for sodium hylauronate/tetradecyltrimethylammonium mixtures by viscosity measurements. A minimum of viscosity as a function of surfactant concentration was found, which was interpreted by an initial contraction followed up by an expansion of the polymer coil as a consequence of the intense micellar binding.11 While the linear polyelectrolyte/surfactant systems are widely reported, only a limited amount of studies has been available for hyperbranched or similar types of polymers (star polymers, dendrimers). The crucial role of molecular architecture was clearly demonstrated by the recent studies on dendrimer/surfactant systems.12-16 Depending on the number of the consecutive generations, supramolecular surfactant/macromolecule structures, including interpolymer and intrapolymer structures, have been reported. Among the branched polymers, polyethylenimines (PEI) have aroused special interest because of their intensive usage as ingredients in a diversity of commercial applications.17 The interaction of PEI and its ethoxylated forms with SDS has been investigated in some detail. The studies of PEI/SDS systems indicate peculiar features including unusual pH and conductivity changes. The increase in pH of the PEI solution with increasing SDS concentration was qualitatively interpreted via the concept of ion exchange reactions18,19 and in terms of specific amine group-SDS interaction.20,21 On the basis of their dynamic NMR investigations, Winnik et al. interpreted the unusual conductivity increase of SDS in the presence of polyethyleneimine as a consequence of special ion transport processes within the polyamine/surfactant complex.21 Li and co-workers have reported a combined electromotive force and isotherm calorimetric study of the system and revealed a remarkable affinity of the SDS to the polymer.22 The appearance of precipitation was explained by 1:1 charge neutralization, whereas the resolubilization at higher surfactant concentration was interpreted by repulsive micellar interaction.22 It was suggested by the same authors that the SDS/PEI complexation at high pH was driven by the cooperative interaction of the surfactant via the uncharged nitrogen atoms of amine groups. Conversely, Winnik et al.23 using fluorescent-labeled PEI samples have suggested the dominance of hydrophobic interactions in PEI/SDS systems in common with other authors.24,25 Li et al. also presented a detailed neutron (11) Herslof, A.; Sundelof, L. O.; Edsman, K. J. Phys. Chem. 1992, 96, 2345-2348. (12) Caminati, G.; Turro, N. J.; Tomalia, D. A. J. Am. Chem. Soc. 1990, 112, 8515-8522. (13) Gopidas, K. R.; Leheny, A. R. Caminati, G.; Turro, N. J.; Tomalia, D. A. J. Am. Chem. Soc. 1991, 113, 7335-7342. (14) Ghoreishi, S. M.; Li, Y.; Holzwarth, J. F.; Khoshdel, E.; Warr, J.; Bloor, D. M.; Wyn-Jones, E. Langmuir 1999, 15, 1938-1944. (15) Li, Y.; Msmillan, C. A.; Bloor, D. M.; Penfold, J.; Warr, J.; Holzwarth, J. F.; Wyn-Jones, E. Langmuir 2000, 16, 7999-8004. (16) Ottaviani, M. F.; Andechaga, P.; Turro, N. J.; Tomalia, D. A. J. Phys. Chem. B 1997, 101, 6057-6065. (17) BASF literature on LUPASOL: Horn, D.; Linhart, F. In: Paper Chemistry; Roberts, J., Ed.; Blackie Academic & Professional: Glasgow and London, 1996; p 64-82. (18) Van den Berg, J. W. A.; Staverman, A. J. Recl. Trav. Chim. 1972, 91, 1151. (19) Bronich, T. K.; Cherry, T.; Vinogradov, S. V.; Eisenberg, A.; Kabanov, V. A.; Kabanov, A. V. Langmuir 1998, 14, 6101-6106. (20) Bystryak, S. M.; Winnik, M. A.; Siddiqui, J. Langmuir 1999, 15, 3748-3751. (21) Winnik, M. A.; Bystryak, S. M.; Chassenieux, V.; Strashko, V.; Macdonald, P. M.; Siddiqui, J. Langmuir 2000, 16, 4495-4510. (22) Li, Y.; Ghoreishi, S. M.; Bloor, D. M.; Holzwarth, J. F.; WynJones, E. Langmuir 2000, 16, 3093-3100. (23) Winnik, M. A.; Bystryak, S. M.; Siddiqui, J. Macromolecules 1999, 32, 624-632. (24) Yui, T. S. T. I.; Abilov, Zh. A.; Pal’mer, V. G.; Musabekov, K. B. Issled. Ravnovesnykh Sist. 1982, 78.
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scattering study of ethoxylated PEI with SDS, which showed the cooperative and noncooperative binding characteristics of the anionic surfactant, depending on the pH and the surfactant concentration.7 It can be concluded that the general features of the PEI-SDS interaction are still not clearly understood and need further investigations. One of the most important thermodynamic aspects of a polyelectrolyte/surfactant system is the binding isotherm of the surfactant. The isotherm provides basic information about the way binding occurs (e.g. monomer binding or micelle-like collective interaction). Binding isotherms can rarely be found in the literature, usually because of the difficulties and drawbacks of their correct experimental determination.26 The aim of our work is the determination of the binding isotherm of the SDS on PEI in the high pH range. The binding characteristics will be further exploited in order to quantitatively describe the peculiar pH changes of this system. Furthermore, dynamic light scattering and electrophoretic measurements will be presented to monitor the unique features of the conformational changes of the hyperbranched PEI due to the binding of the anionic surfactant. Finally, an attempt will be made to interrelate the binding characteristics and conformational transition with the phase behavior of the PEI/SDS system. Experimental Section Materials. Polyethyleneimine (PEI), with a mean molecular weight of 750 000 g/mol, was purchased from BASF in the form of a 33 wt % aqueous solution. The supplier’s polymer solution was purified by mixed-bed cation/anion exchange and dialysis. The PEIs are hyperbranched polymers with a large number of amine groups. Their structure is simple in the sense that each nitrogen atom is joined to each other via an ethylene group linkage and they contain the primary, secondary, and tertiary amine groups in a 1:2:1 ratio.17 The polymer does not contain excessive amount of impurities (
