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Simultaneous Adsorption of Poly(amidoamine) Dendrimers with Surface Carboxyl Groups and Sodium Dodecyl Sulfate at the Alumina/Water Interface Kunio Esumi,* Noriyuki Fujimoto, and Kanjiro Torigoe Department of Applied Chemistry and Institute of Colloid and Interface Science, Science University of Tokyo, Kagurazaka, Shinjuku-ku, Tokyo 162-8601, Japan Received December 1, 1998. In Final Form: March 30, 1999 The simultaneous adsorption of poly(amidoamine) dendrimers with surface carboxyl groups and an anionic surfactant, sodium dodecyl sulfate (SDS), on positively charged alumina particles was investigated at pH 5 by measuring the amount of adsorbed dendrimers and SDS, ζ potential, and sedimentation rate of alumina suspensions. The dendrimers of generations G1.5 and G5.5 were used. Under constant feed concentrations of the dendrimers, the amount of adsorbed dendrimers gradually decreased, while SDS adsorption increased with increasing SDS concentration for both SDS/G1.5 and SDS/G5.5 systems, indicating a competitive adsorption between the dendrimers and SDS. A preferential adsorption of G5.5 over G1.5 in the competitive adsorption with SDS was also observed. The difference in the adsorption of the dendrimers reflected the sedimentation rate of alumina suspensions; the feature of the sedimentation rate with SDS concentration for the SDS/G1.5 system was very similar to that for the SDS system alone, but different from that for the SDS/G5.5 system. In addition, the simultaneous adsorption behavior of dendrimers and SDS on alumina was considerably different from that of a linear polymer, poly(acrylic acid), and SDS that the displacement of poly(acrylic acid) on alumina is easily taken place by SDS.
Introduction The adsorption of a polymer and surfactant on solids depends on their combination as well as surface properties of solids.1 On one hand, if both the polymer and surfactant are favorable for adsorption sites on solids, their competitive adsorption will occur. On the other hand, if the interactions between the polymer and surfactant are strong in aqueous solution, one of them can induce the adsorption of the other which will not adsorb. Such an adsorption often influences the dispersion stability of solids by interactions including the electrostatic force and steric hindrance force induced by the polymer-adsorbed layer. We have been studying the variation of the conformation of the polymer adsorbed on solids by simultaneous adsorption of the polymer and surfactant, using spinlabeled polymers.2-4 It has been found that the conformation of the polymer adsorbed on solids depends on the interactions of the polymer and surfactant; the ratio of train segments in the polymer adsorbed becomes large as the interactions increase. However, until now, all the polymers studied for the simultaneous adsorption of the polymer and surfactant have been limited to linear polymers. Recently, dendrimers, being hyperbranched polymers consisting of a central core and of a number of monomer repeat units bearing a functional group, have become the subject of extensive studies because their functional groups and specific shape have unique properties compared with those of conventional linear polymers.5-9 The adsorption (1) Otsuka, H.; Esumi, K. “Structure-Performance Relationships in Surfactants; Esumi, K. Ueno, M., Eds.; Marcel Dekker: New York, 1997; Chapter 12, and references therein. (2) Otsuka, H.; Esumi, K. Langmuir 1994, 10, 45. (3) Esumi, K.; Oyama, M. Langmuir 1993, 9, 2020. (4) Otsuka, H.; Esumi, K.; Ring, T. A.; Li, J.-T.; Caldwell, K. D. Colloids Surf. 1996, 116, 161. (5) Tomalia, D. A.; Naylor, A. M.; Goddard, W. A. Angew. Chem. 1990, 102, 119. (6) Hawker, C. J.; Frechet, J. M. J. Am. Chem. Soc. 1990, 112, 7638.
of dendrimers on particles is considerably affected by their generation. For example, the earlier generations of poly(amidoamine) dendrimers with surface carboxyl groups interact weakly with positively charged alumina particles, while the later ones can induce a sequence of dispersionflocculation-redispersion of alumina suspension while increasing their concentrations due to a strong electrostatic interaction.10 In addition, the adsorption of poly(amidoamine) dendrimers with surface amino groups on negatively charged silica particles11 also depends on the generations of the dendrimers, similar to those for the alumina-dendrimer system. The results of ζ potential and dispersion stability measurements demonstrate that the earlier generations of the dendrimers behave like electrolytes but the later ones behave like a polyelectrolyte for both alumina and silica suspensions. For a simultaneous adsorption of dendrimer and surfactant, it is also expected that the adsorption of the dendrimer and surfactant on solids depends on their interactions in aqueous solution. It is, in particular, interesting to compare the adsorption behavior of the dendrimer with that of a conventional linear polymer. The aim of this work was to investigate the simultaneous adsorption of poly(amidoamine) dendrimers with surface carboxyl groups and an anionic surfactant, sodium dodecyl sulfate (SDS), on positively charged alumina particles at pH 5 by measuring the amount adsorbed of dendrimers and surfactant, ζ potential, and sedimentation rate of suspensions. Here, the dendrimers of two generations (G1.5 and G5.5) were used. As a comparison, the adsorption of poly(acrylic acid) and SDS on alumina was also examined. (7) Newkome, G. R.; Nayak, A.; Behera, R. K.; Moorefield, C. N.; Baker, G. R. J. Org. Chem. 1992, 57, 358. (8) Jansen, J. F. G.; de Brabander-van den Berg, E. M. M.; Meijer, E. W. Science 1994, 266, 1226. (9) Zeng, F.; Zimmerman, S. C. Chem. Rev. 1997, 97, 1681. (10) Goino, M.; Esumi, K. J. Colloid Interface Sci. 1998, 203, 214. (11) Esumi, K.; Goino, M. Langmuir 1998, 14, 4466.
10.1021/la981664d CCC: $18.00 © 1999 American Chemical Society Published on Web 05/25/1999
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Experimental Section Materials. Poly(amidoamine) dendrimers were synthesized by means of a procedure described by Tomalia et al.,12 where ethylenediamine was used as a nitrogen core. Methylesterterminated dendrimers were hydrolyzed with a stoichiometric amount of NaOH in water to obtain external carboxylate groups with sodium. The dendrimers (Gn.5) used in this study were G1.5 and G5.5. Molecular weights of G1.5 and G5.5 are 2 932 and 52 852, respectively. The number of charged surface groups are 16 for G1.5 and 256 for G5.5. Sodium dodecyl sulfate (SDS) was obtained from Tokyo Kasei Co. and purified by recrystallization from mixtures of 2-propanol and hexane several times. Poly(acrylic acid) (molecular weight ) 50 000) was obtained from Polyscience Inc. and used without purification. R-Alumina of 99.995% purity was kindly supplied by Showa Denkou K.K., and its specific surface area was 30.5 m2 g-1. The water used in this study was purified through a Milli-Q system. The adjustment of suspension pH was carried out using HCl. The other reagents were of analytical grade. Methods and Measurements. The amount of dendrimers and SDS adsorbed on alumina was obtained by a depletion method. The pH value adjusted after the dendrimers and SDS were added to alumina suspensions was 5. All suspensions in the presence of 10 mmol dm-3 NaCl in vial glasses were shaken to reach an adsorption equilibrium in a water bath for 24 h at 25 °C. After equilibration, the suspensions were centrifuged and the concentration of dendrimers in the supernatant solutions was determined using a UV spectrophotometer by measuring the absorbance at 192 nm. The concentration of SDS in the supernatant solutions was also determined using a high-performance liquid chromatograph with an RI-8012 RI detector and a CAPCELL PAK C18 UG column; a mixture of methanol and water (85:15 in volume) containing 0.4 mol dm-3 NaCl was used as the mobile phase. The dispersion stability of suspensions was evaluated by measuring the sedimentation rate using a TURBISCAN MA 2000 (Formulaction). The suspension shaken for 1 day was transferred to a test tube and the sedimentation rate was obtained from an area change in the transmittance of the top portion of the test tube with elapsed time. The ζ potential of suspensions was measured using an electrophoresis apparatus (Pen Kem 500). The surface tension of aqueous solutions of surfactant and dendrimers was measured using a Kruss K12 tensiometer. A similar experiment mentioned above was carried out for the simultaneous adsorption of poly(acrylic acid) and SDS on alumina at pH 5. The concentration of poly(acrylic acid) was determined by means of a UV spectrophotometer. All the measurements were performed at 25 °C.
Figure 1. lp;&3q;1Adsorption isotherms of dendrimers and SDS on alumina.
Figure 2. Variation of the ζ potential of alumina suspensions by the adsorption of dendrimers and SDS.
Figure 1 shows the adsorption isotherms of G1.5, G5.5, and SDS on alumina at pH 5. The amounts of dendrimers and SDS adsorbed increased and reached a plateau with increasing respective concentrations. In the case of dendrimers, the amounts adsorbed at the saturation were 0.9 for G1.5 and 1.45 mg m-2 for G5.5, respectively. These amounts correspond to the occupied areas of 6.3 for G1.5 and 44.4 nm2/molecule for G5.5, respectively. The occupied area by SDS at the saturation level was about 0.24 nm2/ molecule, suggesting a formation of the SDS bilayer. In Figure 2, although the ζ potential of alumina particles alone at pH 5 was positive, the adsorption of G5.5 and G1.5 decreased the ζ potential from positive to negative, indicating that the interaction between the alumina surface and the dendrimers is due to the electrostatic attraction force. The decrement in the ζ potential by the adsorption of G5.5 was greater than that by the adsorption of G1.5. It is reasonable that since the pKa of acrylic acid is about 4.0∼4.5,13 the surface carboxyl groups of den-
drimers are dissociated and the negatively charged COOwould be mainly operational to the positively charged sites of alumina at pH 5. The change in the ζ potential of alumina by the adsorption of SDS alone was similar to that of the alumina-dendrimer system except that the adsorption of SDS alone provides greater negative ζ potentials than that of dendrimers. Figure 3 shows the simultaneous adsorption of dendrimers and SDS on alumina. Since aqueous properties of solutions generally influence adsorption on solids, it is important to note that the interaction between SDS and the dendrimers is very weak in aqueous solution because they have the same negatively charged sites and the surface tension values of SDS with and without the dendrimers are almost unchanged. The simultaneous adsorption was performed by changing the SDS concentration under a constant feed concentration of the dendrimers. Two different feed concentrations of the dendrimers were employed which corresponded to a half and quarter of the respective saturation amounts of the dendrimers adsorbed. The amounts adsorbed of both dendrimers decreased gradually with an increasing
(12) Tomalia, D. A.; Baker, H.; Dewald, J.; Hall, M.; Kallos, G.; Martin, S.; Roeck, Ryder, J.; Smith, P. Macromolecules 1986, 19, 2466.
(13) Cesarano, J.; Aksay, I. A.; Bleier, A. J. Am. Ceram. Soc. 1988, 71, 250.
Results and Discussion
Adsorption of Poly(amidoamine) Dendrimers
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Figure 4. Plots of adsorption selectivity with SDS concentration for the dendrimer/SDS system.
Figure 3. Simultaneous adsorption of dendrimers and SDS on alumina for (a) SDS/G1.5 and (b) SDS/G5.5 systems.
amount of SDS adsorbed; the rate in the decrement of adsorption of G1.5 was much higher than that of G5.5. This result suggests a competitive adsorption between the dendrimers and SDS. In particular, a larger dendrimer prevents more the adsorption of SDS because it occupies several sites for the adsorption on the alumina surface. To compare the effect of SDS concentration on the competitive adsorption, “adsorption selectivity “ can be defined as
A ) (Γd/Cd)/(Γs/Cs) where Γd and Γs are the adsorbed amounts of the dendrimer and SDS, respectively. Cd and Cs are the equilibrium concentrations of the dendrimer and SDS in bulk, respectively. The larger the A values, the more favorable adsorption of the dendrimer than SDS occurs. Figure 4 shows the variation of A as a function of SDS concentration. It is seen that the A values increase with the SDS concentration for all the systems where they are greater for the SDS/G5.5 system than those for the SDS/G1.5 system. This result clearly suggests that G5.5 adsorbs more preferentially on alumina than G1.5 for the competitive adsorption with SDS. This difference between G5.5 and G1.5 is easily understood by a view11 that the earlier generations of dendrimers behave as ordinary electrolytes, while the later ones behave as an anionic surfactant or polyelectrolytes. It is interesting to note that a considerable difference between G1.5 and G5.5 by the simultaneous adsorption with SDS is found in the change of the sedimentation rate (Figure 5). The sedimentation rate of alumina suspensions
Figure 5. Variation of the sedimentation rate of the alumina suspension with (a) dendrimer concentration for G1.5 and G5.5 and with (b) SDS concentration for SDS and dendrimer/SDS systems.
by the adsorption of G1.5 alone increased gradually and remained constant above 2 g dm-3 G1.5 concentration, while that by the adsorption of G5.5 alone increased at a very low concentration of G5.5 and then decreased with increasing G5.5 concentration. In the case of SDS adsorption alone, the sedimentation rate showed a maximum at around 0.2∼0.5 mmol dm-3 SDS and then decreased with increasing SDS concentration. On one hand, this large sedimentation rate is probably due to a flocculation by the hydrophobic interaction between SDS-covered hydrophobic alumina particles. On the other hand, by the
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Figure 6. Variation of the ζ potential of the alumina suspension by the simultaneous adsorption of the dendrimer and SDS.
simultaneous adsorption of the dendrimers and SDS, the sedimentation rate for the SDS/G1.5 system was very sensitive against SDS concentration compared with that for the SDS/G5.5 system; the maximum sedimentation rate for the SDS/G1.5 system was of a magnitude similar to that for the SDS system, while it was small for the SDS/G5.5 system. Thus, the dispersion stability of alumina suspensions is significantly affected by the competitive adsorption of the dendrimers and SDS. Figure 6 shows the change in the ζ potential of alumina suspensions by the adsorption of dendrimers and SDS. The ζ potential of alumina suspensions decreased and became constant with an increasing SDS concentration for the SDS/dendrimer system. As can be expected, the maximum sedimentation rate corresponds to nearly zero ζ potential for all the systems. In addition, the ζ potential of alumina suspensions for the SDS/G1.5 system was much more influenced by SDS adsorption than that for the SDS/ G5.5 system. It is important to compare the adsorption behavior of the dendrimers with that of conventional linear polymers. Although there is a difference that G1.5 and G5.5 have tertiary amines in the core with surface carboxyl groups, while poly(acrylic acid) (PAA) is an acidic polymer, PAA was selected for this purpose. Figure 7 shows the simultaneous adsorption of PAA and SDS, and the adsorption of PAA alone on alumina at pH 5. The amount of PAA adsorbed alone increased sharply at low concentrations and reached a plateau, indicating that the interaction between PAA and the alumina surface is strong because of the electrostatic attraction force between COO- groups of PAA and positively charged sites on alumina. From ESR measurements using spin-labeled PAA,14 it has been found that PAA adsorbs on alumina as mainly train segments at pH 5. Interestingly, the amount of PAA adsorbed at the saturation was quite small compared with that of the dendrimers. This distinct difference in the adsorption is due to a difference of polymer shape and conformation adsorbed between the dendrimers and PAA. In the simultaneous adsorption of PAA and SDS under a constant feed concentration of PAA, the amount of PAA adsorbed decreased significantly and became almost zero
Figure 7. Simultaneous adsorption of poly(acrylic acid) and SDS on alumina. The initial concentration of PAA is 0.1 g dm-3.
above 4 mmol dm-3 SDS, while the amount of SDS adsorbed increased with the SDS concentration, indicating a competitive adsorption. This suggests that compared with the result for the dendrimer/SDS system, SDS adsorbs more preferentially than PAA and displaces completely PAA adsorbed on alumina. Also, from a simultaneous adsorption of sodium poly(styrene sulfonate) (PSS) and SDS on alumina it has been found15 that, under a fixed initial concentration of PSS, the amount of PSS adsorbed decreases and becomes almost zero, while the amount of SDS adsorbed increases with increasing SDS concentration, indicating a preferential adsorption of SDS rather than PSS. Thus, it is seen that the simultaneous adsorption of polymers and surfactants depends on the morphology and shape of polymers. Conclusions Under a constant feed concentration of G1.5 or G5.5, the adsorption of the dendrimer decreases gradually, while SDS adsorption increases with increasing SDS concentration, indicating a competitive adsorption between the dendrimer and SDS. The magnitude in the decrement of the adsorption is much greater for G1.5 than that for G5.5, which is probably due to the action of the dendrimers that G1.5 plays such as an electrolyte and G5.5-like polyelectrolyte. These behaviors appear in the changes of the sedimentation rate of alumina suspensions. From a comparison between the dendrimers and a linear polymerlike PAA it is found that the amount of PAA adsorbed is much smaller than that of the dendrimers and the replacement of PAA by SDS occurs more easily than that of the dendrimers by SDS. Acknowledgment. The authors thank Formulaction for using a Turbiscan MA2000. LA981664D (14) Ishiduki, K.; Esumi, K. J. Colloid Interface Sci. 1997, 185, 274. (15) Esumi, K.; Masuda, A.; Otsuka, H. Langmuir 1993, 9, 284.
