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Interactions of Sugar-Persubstituted Poly(Amidoamine) Dendrimers with Anionic Surfactants Munetaka Miyazaki, Kanjiro Torigoe, and Kunio Esumi* Department of Applied Chemistry and Institute of Colloid and Interface Science, Science University of Tokyo, Kagurazaka, Shinjuku-ku, Tokyo 162-8601, Japan Received March 23, 1999. In Final Form: October 12, 1999 The interactions of sugar-persubstituted poly(amidoamine) dendrimers (generation, 3.0 and 5.0) (sugar ball) with anionic surfactants such as sodium dodecyl sulfate (SDS) and lithium perfluorooctanesulfonate (LiFOS) were investigated at a constant concentration of the sugar ball. At low surfactant concentrations, aggregates of the anionic surfactants at the surface of the sugar ball are formed, showing a strong surface activity. The size of the mixed aggregates increases with the anionic surfactant concentration, and also depends on the generation of the sugar ball: the size for generation 3.0 is greater than that for generation 5.0. The association process is also affected by the kind of hydrophobic chain of the surfactant. These differences reflect solubilization behaviors of biphenyl and decafluorobiphenyl into the mixed systems of the sugar ball and the surfactant.
Introduction The interactions of polymers and surfactants in aqueous solutions have been intensively investigated using many techniques.1 The strength of interactions depends on the kinds of polymers and surfactants. Such mixed systems are of great importance to many industrial fields such as cosmetics, paints, detergents, foods, and formulations of drugs and pesticides. It has been shown1 that the binding of ionic surfactants to neutral polymers occurs mainly by hydrophobic interactions, whereas a combination of electrostatic and hydrophobic interactions is involved in the binding of oppositely charged polymers and surfactants. In addition, it has been reported2,3 using different hydrophobic chains of surfactants that the interactions of poly(vinyl pyrrolidone) with hydrocarbon surfactant are different from those of poly(vinyl pyrrolidone) with fluorocarbon surfactant. Until now, polymers used for such mixed systems have been limited to linear polymers. Recently, dendrimers, being highly branched polymers, have become the subject of extensive studies,4-7 because their functional groups and specific shape have unique properties compared to those of conventional linear polymers. For example, Caminati et al.8 studied the interactions of poly(amidoamine) dendrimers with surface carboxyl groups having different generations with dodecyltrimethylammonium bromide (DTAB) by the fluorescence probe method and found that the interactions of DTAB with the dendrimers are significantly affected by the generations of the dendrimers whose results are consistent with a change in morphology from an open surface structure for generation 0.5-3.5 to a closed, compact surface structure for generation 4.5-9.5. Aoi et (1) Kwak, J. C. T. Polymer-Surfactant Systems Marcel Dekker: New York, 1999. (2) Nojima, T.; Esumi, K.; Meguro, K. J. Am. Oil Chem. Soc. 1992, 69, 64. (3) Sesta, B.; Segre, A. L.; D’Aprano, A.; Proietti, N. J. Phys. Chem. B, 1997, 101, 198. (4) Tomalia, D. A.; Naylor, A. M.; Goddard, W. A. Angew. Chem., Int. Ed. Engl. 1990, 29, 138. (5) Frechet, J. M. J. Science 1994, 263, 1710. (6) Jansen, J. F. G. A.; Meijer, E. W. J. Am. Chem. Soc. 1995, 117, 4417. (7) Zeng, F.; Zimmerman, S. C. Chem. Rev. 1997, 97, 1681. (8) Caminati, G.; Turro, N. J.; Tomalia, D. A. J. Am. Chem. Soc. 1990, 112, 8515.
al.9 synthesized fully sugar-substituted globular dendrimers and studied the interaction of the dendrimers and lectin. Since sugar-substituted dendrimers are mimics of naturally occurring multiantennary oligosaccharides, they are of considerable interest in various fields such as biochemical and medical applications. For many applications, it is basically important to characterize the interactions of sugar-substituted dendrimers with surfactants. It is also interesting to compare how the interactions differ by the kind of hydrophobic chain of surfactant, such as hydrocarbon and fluorocarbon surfactants. The objectives of this work were to study the interactions between sugar-substituted dendrimers and anionic surfactants using several techniques, including surface tension, fluorescence probe method, and NMR. Experimental Section Materials. Sodium dodecyl sulfate (SDS) was obtained from Nakalai Tesque, Inc. and recrystallized several times from ethanol. Lithium perfluorooctanesulfonate (LiFOS) was obtained by anionic exchange from the corresponding potassium product supplied by Dainippon Ink and Chemical, Inc. and used after recrystallization from mixtures of hexane and 1-butanol. Biphenyl and decafluorobiphenyl were purchased from Tokyo Kasei Kogyo and Aldrich Chemical Co., respectively. The water used in this study was purified by passing it through a Milli-Q Plus System until its specific conductivity fell below 0.1 µScm-1. The other chemicals were of analytical grade. Synthesis of Poly(amidoamine) Dendrimer(PAMAM) and Sugar Ball. PAMAM dendrimers (generation of 3.0 and 5.0) were prepared by using ethylenediamine as an initiator core according to a previous paper.10 Sugar balls (SBn, n ) generation 3.0 and 5.0) were synthesized by the reaction of the amineterminated PAMAM dendrimers with an excess amount of aldonolactone.9 To obtain lactobionate, lactobionic acid was evaporated several times from methanol in vacuo at 50 °C. PAMAM dendrimer was dissolved in dry dimethyl sulfoxide under a nitrogen atmosphere. Then, an excess amount of lactobionate in dimethyl sulfoxide was added to the solution by a dropping funnel with stirring and the mixture was reacted at 40 °C for 9 h. When the solution was poured into a large amount of ethanol, precipitation occurred. The precipitate was purified by using a cellulose tube to remove unreacted lactobionate and finally white (9) Aoi, K.; Itoh, K.; Okada, M. Macromolecules 1995, 28, 5391. (10) Tomalia, D. A.; Baker, H.; Dewald, J.; Hall, M.; Kallos, G.; Martin, S.; Roeck, J.; Ryder, J.; Smith, P. Polym. J. 1985, 17, 117.
10.1021/la990347j CCC: $19.00 © 2000 American Chemical Society Published on Web 01/15/2000
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Figure 1. Schematic representation of SB3. powdery sugar balls were obtained. The purity of these samples was confirmed by 1H- and 13C NMR. In addition, gel permeation chromatography analysis suggested9 that each sugar ball consists of a single component. The numbers of sugar residues and the molecular weight are 32 and 17 765.8 for SB3 and 128 and 72 252.7 for SB5, respectively. The structure of sugar ball SB3 is shown in Figure 1. Measurements. The surface tension of aqueous solutions of surfactant and sugar ball-surfactant mixtures was measured with a Kruss K12 tensiometer by the Wilhelmy plate technique. Fluorescence spectra of 1-pyrene carboxaldehyde (PCA) were obtained using a fluorescence spectrophotometer (Hitachi, 65010S) where the concentration of PCA was 1 × 10-5 mol dm-3. The excitation wavelength was set at 356 nm. Solubilization measurements were carried out as follows. Surfactant and sugar ball-surfactant mixtures containing an excess amount of solubilizate were ultrasonicated for 20 min, and then shaken for 2 days. Then, after filtering the mixtures the solutions were diluted with methanol and their spectra were measured with a diode array spectrophotometer (HewlettPackard 8452 A). The concentration of solubilizate was determined from the calibration curve. 1H- and 19F NMR spectra were obtained with a JEOL JMN EX-400 spectrometer. The spin-lattice relaxation time, T1, was measured using a standard 180-τ-90 inversion recovery pulse sequence. Dynamic light scattering was carried out with DLS-700 (Otsuka Electronic Co. Ltd). The light source used was an argon ion laser (75 mW). The correlation function was fitted to the exponential decay curve as given in the following equations
g2(t) ) 1 + β{g1(t)}2 1
(1) 2
g (t) ) exp(-Γt) ) exp(-Daq t)
(2)
where g1(t) and g2(t) are the photon-intensity correlation function and the auto-correlation function, respectively, and β the coherent capacity, Γ the decay rate, Da the apparent diffusion coefficient, and q the scattering vector. The mutual diffusion coefficient, Dt, was obtained by extrapolating to q ) 0. In all experiments, the concentration of the sugar balls was fixed at 0.5 g dm-3. All measurements were performed at 25 °C.
Results and Discussion Figure 2a and b shows the surface tension vs logarithm of surfactant concentration for the SDS, SDS-SBn, LiFOS, and LiFOS-SBn systems. In the surfactant system alone, the surface tension decreased with concentration and had a break point which was taken as the critical micelle concentration (cmc). The cmc values of SDS and LiFOS were 8.0 and 6.3 mmol dm-3, respectively. In the case of the SDS-SBn and LiFOS-SBn systems, the surface tensions at low surfactant concentrations were considerably lower than those of the respective surfactant solutions (γcmc) at the cmc. In addition, the SDS-SB3 and LiFOSSB3 systems had lower surface tension values than those of the SDS-SB5 and LiFOS-SB5 systems. These results suggest a strong binding of the surfactant and SBn at low surfactant concentrations. Such binding has also been observed for mixtures of cationic surfactant and dendrimers with external anionic surfaces.8 With increasing surfactant concentration, the surface tensions for both systems approached those of the respective γcmc. These results show a similar trend to those of surfactant-linear polymer systems such as SDS-poly(ethylene oxide),11 SDS-poly(vinyl pyrrolidone),2,12, and LiFOS-poly(vinyl
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Figure 2. Surface tension vs logarithm of surfactant concentration: (a) SDS-SBn system; (b) LiFOS-SBn system.
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Figure 4. Change in the solubility of different compounds with surfactant concentration: (a) SDS-SBn system; (b) LiFOS-SBn system.
tion that normal surfactant micelles start to form) clearly appear and their surface tensions are usually higher than that of the respective γcmc. However, in this study two break points were hardly determined for both systems. Contrary to surfactant-linear polymer systems, the surfactant tensions for the present systems are lower than those of the respective γcmc at low surfactant concentrations. In the case of the LiFOS-SB3 system, the surface tension increased abruptly above 3 mmol dm-3 of LiFOS. At higher surfactant concentrations, it seems that normal surfactant micelles start to form as well as the presence of surfactant-SBn aggregates because the surface tensions for both systems are close to those of the respective γcmc. Such interactions between the surfactants and SBn were also elucidated by the fluorescence probe method.13,14 As for PCA molecule experiences a change in its environment, the maximum fluorescence wavelength (λmax) of the Figure 3. Change in the maximum wavelength of PCA with surfactant concentration for the SDS-SBn system.
pyrrolidone)2,3 that two break points (one is the critical association concentration and the other is the concentra-
(11) Jones, M. N. J. Colloid Interface Sci. 1967, 23, 36. (12) Brewer, M. M.; Robb, I. D. Chem. Ind. 1992, 530. (13) Kalyanasundaram, K.; Thomas, J. K. J. Am. Chem. Soc. 1977, 99, 2039. (14) Milliaris, A. A. Int. Rev. Phys. Chem. 1988, 7, 95.
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Figure 5. Change in the 1H spin-lattice relaxation times for SDS with surfactant concentration for SDS and SDS-SBn systems. The concentration of SBn is 0.5 g dm-3.
monomer emission is affected. It is known15 that λmax of PCA shifts to a shorter wavelength when the PCA is located at lower environmental polarity. In Figure 3, λmax is plotted as a function of SDS concentration in the absence and presence of SBn. In the case of SDS system alone, λmax was almost constant with increasing SDS concentration and then decreased abruptly at some concentration which corresponded to the cmc of SDS. In the presence of SBn, λmax shifted gradually to shorter wavelength with increasing SDS concentration and became close to the λmax of SDS alone. λmax for the SDS-SB5 system was slightly higher than that for the SDS-SB3 system below 10 mmol dm-3 of SDS. Interestingly, the changes in the λmax with the SDS concentration are very similar to the changes in surface tension for the SDS-SBn system. These results suggest that at low SDS concentrations PCA is located in hydrophobic cores of the SDS-SBn aggregates, while at high SDS concentrations PCA is solubilized in the SDS-SBn aggregates and SDS micelles. Accordingly, these fluorescence results are in good agreement with those of the surface tension measurements; at the SDS concentration region where the surface tension for the SDS-SBn system is lower than that for the SDS system alone the micropolarity of the mixed system is lower than that of SDS solution, indicating the presence of some (15) Kalyanasundam, K.; Thomas, J. K. J. Phys. Chem. 1977, 81, 2176.
aggregate in the mixed system. On the other hand, in the case of the LiFOS-SBn system (data not shown), λmax hardly changed with increasing LiFOS concentration because the hydrocarbon PCA is insoluble in the fluorocarbon micelles. Thus, it is expected that the surfactantSBn aggregates show different solubilization behaviors owing to the kind of the surfactant. Figure 4(a) shows the solubilized amount of biphenyl for the SDS-SBn system. In the case of the SDS system alone, biphenyl is solubilized into SDS micelles and the solubilized amount increases with increasing SDS concentration. In the presence of SBn, biphenyl can be solubilized even below the cmc of SDS probably due to the formation of a hydrophobic core caused by SDS-SBn association, although the solubilized amount of biphenyl is low. Above the cmc of SDS, the solubilization curves for the SDS-SBn system are very similar to that of the SDS system alone, where biphenyl is solubilized into both SDSSBn aggregates and SDS micelles. Since fluorocarbon compounds are easily solubilized into fluorinated surfactant micelles compared to the corresponding hydrocarbon compounds,16,17 the solubilization behavior of decafluorobiphenyl was investigated for the LiFOS-SBn system (16) Muto, Y.; Asada, M.; Takasawa, A. Esumi, K.; Meguro, K. J. Colloid Interface Sci. 1988, 124, 632. (17) Muto, Y.; Yoda, K.; Yoshida, N.; Esumi, K.; Meguro, K.; Limbeme, W. B.; Zana, R. J. Colloid Interface Sci. 1989, 130, 165.
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Figure 6. Change in the 19F spin-lattice relaxation times for LiFOS with surfactant concentration for LiFOS and LiFOS-SBn systems. The concentration of SBn is 0.5 g dm-3.
(Figure 4b). It is interesting to note that the solubilization behavior for the LiFOS-SBn system is different from that of the SDS-SBn system at high surfactant concentrations; the solubility of decafluorobiphenyl for the LiFOS-SB3 system is much greater than that for the LiFOS-SB5 system. It seems that the association of LiFOS and SB3 is different from that of LiFOS-SB5. Molecular dynamics of SDS and LiFOS for the SDSSBn and LiFOS-SBn systems were studied by measuring the 1H and 19F NMR spin-lattice relaxation times. Figure 5 shows the 1H spin-lattice relaxation times for the SDS and SDS-SBn systems. In the SDS system alone, the 1H spin-lattice relaxation times for all the peaks decreased with increasing SDS concentration and reached constant value above 20 mmol dm-3. On the other hand, for the SDS-SBn systems the 1H spin-lattice relaxation times for all the peaks were smaller below 10 mmol dm-3 of SDS than those in the absence of SBn. At high SDS concentrations the 1H spin-lattice relaxation times for all the peaks for the SDS-SBn systems were almost the same as those for the SDS system alone, suggesting that the relaxation times are predominantly controlled by SDS micelles rather than the SDS-SBn aggregates at high SDS concentrations. Further, the differences in the relaxation times between SB3 and SB5 were observed at peaks 1 and 4 at
below 10 mmol dm-3 of SDS. Since the decrease in relaxation time can be correlated with the restriction of molecular movement, the change in the relaxation times below 10 mmol dm-3 of SDS is probably due to the association between SDS and SBn; SDS molecules are adsorbed on the SBn surface with an irregular conformation. These adsorbed layers provide a hydrophobic region which can solubilize hydrophobic substances such as PCA. The 19F spin-lattice relaxation times for the LiFOS and LiFOS-SBn systems are shown in Figure 6. Compared to the relaxation times for the LiFOS system alone, the relaxation time for peak 1 for the LiFOS-SB3 system was smaller, while those for peaks 1 and 3 for the LiFOSSB5 system were also smaller below 10 mmol dm-3 of LiFOS. Since the motion of the fluorocarbon chain near to the hydrophilic group of LiFOS is restrained, while the motion of the chain further away from the hydrophilic group is loose, it is suggested that LiFOS molecules adsorb on the SBn surface by orienting their fluorocarbon chains to the aqueous solution at low LiFOS concentration. Thus, the difference in the formation of association between SDS-SBn and LiFOS-SBn systems may be caused by the kind of the hydrophobic chain of the surfactant: as the hydrocarbon chain of SDS is flexible, taking on many conformations, while the fluorocarbon chain of LiFOS is
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Figure 7. Change in the mutual diffusion coefficients for (a) SDS-SBn system and (b) LiFOS-SBn system with surfactant concentration.
Figure 8. Change in the hydrodynamic radii for (a) SDS-SBn system and (b) LiFOS-SBn system with surfactant concentration.
rigid and the aggregation of the fluorocarbon chain is favored. To check the association behavior of the surfactantSBn system, the mutual diffusion coefficients were obtained from dynamic light scattering measurements. Figure 7a and b shows the mutual diffusion coefficients for the SDS-SBn and LiFOS-SBn systems. It can be seen that the mutual diffusion coefficients for both systems decrease with increasing surfactant concentration, where the coefficients for the SDS-SB3 and LiFOS-SB3 systems are smaller than those for the SDS-SB5 and LiFOSSB5 systems except those of SBn alone. It seems that the size of aggregates formed by association between the surfactant and SBn increases with increasing surfactant concentration. The hydrodynamic radius (Rh) of the surfactant-SBn aggregates was calculated using the Stokes-Einstein equation
using CPK model.9 In the SDS-SBn system, the hydrodynamic radius for the SDS-SB3 system increased rapidly to about 2 mmol dm-3 of SDS and then gradually increased with a further increase of SDS concentration, while that for the SDS-SB5 system gradually increased and became constant at above 10 mmol dm-3 of SDS (Figure 8a). The change in the hydrodynamic radius for the LiFOS-SBn system (Figure 8b) was similar to that for the SDS-SBn system except that the hydrodynamic radius for the LiFOS-SB3 system continues to increase with increasing LiFOS concentration. It is worth noting that the hydrodynamic radii of SDS micelles and LiFOS micelles in this concentration region cannot be obtained because they are too small to detect. The difference in the radii by the kind of the surfactant can be interpreted as follows. The adsorption energy change18 from water to air-water interface per -CF2- group is -5.11 kJ mole-1 and that per -CH2group is -2.59 kJ mole-1, so that LiFOS molecules are preferably aggregated in water compared with SDS assuming that the energy change by their hydrophilic groups is equal. These differences in the energy may
Rh ) κT/6πηDt
(3)
where η is the viscosity of the suspending liquid. The hydrodynamic radii of SB3 and SB5 were 2.4 and 4.3 nm, respectively, which are similar to the values calculated
(18) Mukerjee, P. J. Am. Oil Chem. Soc. 1982, 59, 573.
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suggest that the growth of aggregates is much greater for the LiFOS-SBn system than that for the SDS-SBn system. The result that the hydrodynamic radius for the surfactant and SB3 system is greater than that for the surfactant and SB5 system may mainly be caused by two factors; one is the adsorption affinity and the other is the number density of sugar balls. If surfactant adsorbs on the sugar groups of SBn where the intersugar distance of SB5 is shorter than that of SB3, cooperative association between the surfactant and SB5 will occur, while noncooperatively bonded aggregates between the surfactant and SB3 will form. This cooperative association causes aggregation of surfactant on the sugar ball surface. In the noncooperative binding, the distance between the surfactant adsorbed on the sugar ball surface is relatively long so that aggregation of surfactant on the surface hardly occurs. Such cooperative or noncooperative binding has been suggested for adsorption of cationic surfactant molecules to the anionic surface of dendrimers.8 In this study, since the number density of SB3 is about four times that of SB5 at 0.5 g dm-3, the growth of noncooperatively bonded aggregates will be enhanced.
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Conclusions The aggregates between SDS or LiFOS and SBn, at a constant concentration of SBn, are formed through hydrophobic interactions between hydrophobic chains of the surfactants adsorbed on the SBn at low surfactant concentrations. The aggregates for the surfactant-SB3 system show higher surface activity and greater solubilization capacity than those for the surfactant-SB5 system. With increasing surfactant concentration, the size of the aggregates increases, in particular the size for the surfactant-SB3 system is much greater than that for the surfactant-SB5 system. At high surfactant concentrations, the aggregates between the surfactant and SBn as well as normal surfactant micelles are present. The differences in the interactions between the SDS-SBn and LiFOS-SBn systems can be interpreted by the differences in the hydrophobic properties of the surfactants. In addition, the difference in the radii between SB3 and SB5 for the surfactant-SBn systems is probably due to different bindings such as cooperative or noncooperative. LA990347J
