Langmuir 2007, 23, 4237-4247
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Effect of Mixing on the Formation of Complexes of Hyperbranched Cationic Polyelectrolytes and Anionic Surfactants Ama´lia Mezei, Ro´bert Me´sza´ros,* Imre Varga, and Tibor Gila´nyi Laboratory of Interfaces and Nanosized Systems, Institute of Chemistry, Eo¨tVo¨s Lora´ nd UniVersity, 1117 Budapest, Pa´ zma´ ny Pe´ ter se´ ta´ ny 1/A, Hungary ReceiVed December 6, 2006. In Final Form: January 23, 2007 The effect of different mixing protocols on the charged nature and size distribution of the aqueous complexes of hyperbranched poly(ethylene imine) (PEI) and sodium dodecyl sulfate (SDS) was investigated by electrophoretic mobility and dynamic light scattering measurements at different pH values, polyelectrolyte concentrations, and ionic strengths. It was found that at large excess of the surfactant a colloidal dispersion of individual PEI/SDS nanoparticles forms via an extremely rapid mixing of the components by means of a stop-flow apparatus. However, the application of a less efficient mixing method under the same experimental conditions might result in large clusters of the individual PEI/SDS particles as well as in a more extended precipitation regime compared with the results of stop-flow mixing protocol. The study revealed that the larger the charge density and concentration of the PEI, the more pronounced the effect of mixing becomes. It can be concluded that an efficient way to avoid precipitation in the solutions of oppositely charged polyelectrolytes and surfactants might be provided by extending the range of kinetically stable colloidal dispersion of polyelectrolyte/surfactant nanoparticles via the application of appropriate mixing protocols.
Introduction The oppositely charged polyelectrolytes and surfactants constitute the main ingredients in a variety of drug delivery and home and personal care applications. Such systems also show many interesting features from a scientific point of view. The combined practical and fundamental interest initiated intensive research in the last two decades, and the most important findings are summarized in a number of reviews.1-4 The oppositely charged amphiphiles and macromolecules also have important relevance in biological systems. For instance, in gene transfection the reversible collapse and swelling of DNA molecules are required. This can be achieved by the subsequent complexation of DNA with cationic and anionic surfactants.5 One of the most common observations in the solutions of polyelectrolytes and oppositely charged surfactants is that at certain compositions phase separation takes place. The coacervate or precipitate is usually (except in the presence of high supporting electrolyte concentration) enriched in both components, whereas the solution phase contains only a small amount of them.6 Since in the efficiency and long-term stability of products the phase properties play a crucial role, intensive experimental and theoretical efforts have been made to characterize the features of phase separation and thermodynamic stability in these systems.6-15 * Corresponding author. E-mail:
[email protected]. (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, 604613. (4) Kogej, K.; Skerjanz, J. In Surfactant Science Series; Radeva, T., Ed.; Marcell Dekker, Inc.: New York, 2001; Vol. 99, p 793. (5) Dias, R. S.; Lindman, B.; Miguel, M. G. J. Phys. Chem. B 2002, 106, 12608-12612. (6) Bergfeldt, K.; Piculell, L.; Linse, P. J. Phys. Chem. 1996, 100, 36803687. (7) Thalberg, K.; Lindman, B.; Karlstrom, G. J. Phys. Chem. 1991, 95, 3370. (8) Thalberg, K.; Lindman, B.; Bergfeldt, K. Langmuir 1991, 7, 2893-2898. (9) Ilekti, P.; Piculell, L.; Tournilhac, F.; Cabane, B. J. Phys. Chem. B 1998, 102, 344-351. (10) Ranganathan, S.; Kwak, J. C. T. Langmuir 1996, 12, 1381-1390.
In the dilute corner of the experimental phase diagrams, the general trends at a fixed polyelectrolyte concentration can be summarized as follows.7-10 At low surfactant concentrations, the solution is a transparent one-phase system. By increasing surfactant concentration, the solution becomes turbid, and phase separation occurs. At even larger surfactant concentrations, a one-phase region or redissolution of the polyelectrolyte/surfactant complexes might be achieved. The general behavior emanating from the theoretical and simulation studies on phase stability is qualitatively similar to these findings. Namely, at an intermediate polyelectrolyte-to-surfactant ratio, the charges of the polyelectrolytes are compensated by the bound surfactant molecules, which results in phase separation. At higher or lower polyelectrolyte-to-surfactant ratio, stable one-phase systems form due to the significant net charge of the polyelectrolyte/surfactant complexes.11-15 The majority of the experimental and theoretical studies refer to linear polyelectrolyte/surfactant systems, and the role of polyelectrolyte architecture has not been explored yet in great detail. In the present paper, we focus on the aqueous mixtures of hyperbranched poly(ethyleneimine) (PEI) and sodium dodecyl sulfate (SDS). The solution and surface behavior of the PEI/SDS system has been investigated by a variety of techniques. These studies revealed peculiar features which were found to significantly depend on the pH, ionic strength, and structure of the PEI.16-28 (11) Hansson, P. Langmuir 2001, 17, 4167-4180. (12) Allen, R. J.; Warren, P. B. Langmuir 2004, 20, 1997-2009. (13) Nguyen, T. T.; Shklovski, B. I. J. Chem. Phys. 2001, 114, 5905-5916. (14) Schiessel, H.; Bruinsma, R. F.; Gelbart, W. M. J. Chem. Phys. 2001, 115, 7245-7252. (15) Skepo¨, M.; Linse, P. Macromolecules 2003, 36, 508-519. (16) Van den Berg, J. W. A.; Staverman, A. Recl. TraV. Chim. 1972, 91, 1151. (17) Bystryak, S. M.; Winnik, M. A.; Siddiqui, J. Langmuir 1999, 15, 37483751. (18) Winnik, M. A.; Bystryak, S. M.; Siddiqui, J. Macromolecules 1999, 32, 624-632. (19) Winnik, M. A.; Bystryak, S. M.; Chassenieux, C.; Strashko, V.; Macdonald, P. M.; Siddiqui, J. Langmuir 2000, 16, 4495-4510. (20) Bastardo, L.; Garamus, V. M.; Bergstro¨m, M.; Claesson, P. M. J. Phys. Chem. B 2005, 109, 167-174.
10.1021/la0635294 CCC: $37.00 © 2007 American Chemical Society Published on Web 03/03/2007
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At low charge density of the PEI (high pH), a new interpretation for the phase properties of PEI/SDS solutions was given recently.24 According to this approach, the system is described as a thermodynamically stable solution of the solvated PEI/SDS complex molecules only at low surfactant concentrations. Above a critical surfactant concentration, the system is considered an unstable colloidal dispersion of the collapsed PEI/SDS particles, which results in precipitation. At even higher surfactant concentrations, the surfactant binding takes place as an adsorption of the surfactant molecules on the surface of the neutral PEI/ SDS particles. In this latter case, the system might become an electrostatically stabilized dispersion of the polyelectrolyte/ surfactant nanoparticles. It was also demonstrated in the same work24 that at high surfactant concentrations kinetically stable colloidal dispersion can be prepared only by applying appropriate mixing procedures. Otherwise, the system precipitated, e.g., the polyelectrolyte/surfactant particles irreversibly coagulated. For aqueous mixtures of SDS and various cationic linear polyelectrolytes, the state of the system was found to strongly depend on the order of addition of the components and the way the solution is mixed.29-31 Furthermore, the observed differences were found to persist over very long periods of time. Thus, the mixtures of polyelectrolytes and oppositely charged surfactants are prone to be trapped in long-lived nonequilibrium states. Therefore, the study and control of the solution preparation are of paramount importance both in fundamental research and in technical applications utilizing such mixtures. Up to now, it is still not clear how the applied experimental conditions, for instance, the concentration and charge density of the polyelectrolytes, as well as the ionic strength of the medium, may affect the state of the system during the application of different mixing procedures. In this paper, we present a systematic investigation of the effect of different mixing protocols on the charged nature and size distribution of the PEI/SDS complexes at different pH, polyelectrolyte concentrations, and ionic strengths by means of electrophoretic mobility and dynamic light scattering measurements. The general validity of the previously given interpretation for the PEI/SDS phase properties at high pH will also be tested, and potential practical applications of the results will be suggested. Experimental Section Materials. The 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 investigated PEI is a hyperbranched polymer containing the primary, secondary, and tertiary amine groups in a 1:2:1 ratio. The polymer (21) Li, Y.; Ghoreishi, S. M.; Warr, J.; Bloor, D. M.; Holzwarth, J. F.; WynJones, E. Langmuir 2000, 16, 3093-3100. (22) Li, Y; Xu, R; Couderc, S.; Bloor, D. M.; Warr, J.; Penfold, J.; Holzwarth, J. F.; Wyn-Jones, E Langmuir 2001, 17, 5657. (23) Zhou, S.; Burger, C.; Chu, B. J. Phys. Chem. B 2004, 108, 10819. (24) Me´sza´ros, R.; Thompson, L.; Bos, M.; Varga, I.; Gila´nyi, T. Langmuir 2003, 19, 609-615. (25) Wang, H.; Wang, Y.; Yan, H.; Zhang, J.; Thomas, R. K. Langmuir 2006, 22, 1526-1533. (26) Me´sza´ros, R.; Thompson, L.; Varga, I.; Gila´nyi, T. Langmuir 2003, 19, 9977-9980. (27) Penfold, J.; Tucker, I.; Thomas, R. K.; Zhang, J. Langmuir 2005, 21, 10061-10073. (28) Penfold, J.; Tucker, I.; Thomas, R. K.; Taylor, D. J. F.; Zhang, J.; Bell, C. Langmuir 2006, 22, 8840-8849. (29) Naderi, A.; Claesson, P. M.; Bergstrom, M.; Dedinaite, A. Colloids Surf., A 2005, 253, 83-93. (30) Naderi, A.; Claesson, P. M. J. Dispersion Sci. Technol. 2005, 26, 329340. (31) Naderi, A.; Claesson, P. M. Langmuir 2006, 22, 7639-7645.
Mezei et al. does not contain an excessive amount of impurities (
