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Simultaneous Adsorption of Sugar-Persubstituted Poly(amidoamine) Dendrimers and Anionic Surfactant at the Alumina/Aqueous Solution Interface Kunio Esumi,* Kentaro Sakagami, Satoshi Kuniyasu, Yukiko Nagata, Kenichi Sakai, 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 June 19, 2000. In Final Form: September 18, 2000 The simultaneous adsorption of sugar-persubstituted poly(amidoamine) dendrimers (sugar ball) and anionic surfactants such as sodium dodecyl sulfate (SDS) and lithium perfluorooctanesulfonate (LiFOS) on positively charged alumina particles was investigated at pH 3.5 by measuring the amount of adsorbed sugar ball and anionic surfactant, ζ potential, and sedimentation rate of alumina suspensions. The sugar balls of generations G3 and G5 were used. Under constant initial concentrations of the sugar balls, the amount of adsorbed sugar balls sharply increases, reaches a maximum, and then decreases, while the anionic surfactant adsorption increases with increasing anionic surfactant. The enhancement in the adsorption of the sugar balls is due to the adsorption of complexes consisting of sugar ball and anionic surfactant. In addition, the dispersion stability of alumina suspensions caused by the adsorption of the sugar ball and anionic surfactant depends on the ζ potential of alumina as well as the adsorption of sugar ball. A significant difference in the adsorption characteristics is not observed for the generation of sugar ball and kind of anionic surfactant.
Introduction Simultaneous adsorption of polymers and surfactants at solid/liquid interface is one of the important topics in the study of dispersion stability of particles.1 The information obtained has been applied to many fields including cosmetics, pharmaceuticals, and paints. Generally, characteristics of simultaneous adsorption of polymers and surfactants significantly depend on the interactions between polymers and surfactants. When the interaction between polymer and surfactant is favorable, enhancement of adsorption of one component would occur by adsorption of another component. If the interaction between polymer and surfactant is very weak, a competitive adsorption might be expected. Until now, simultaneous adsorption for many polymer and surfactant systems has been carried out. In addition, we studied the conformation change of linear polymer adsorbed on solid particles using spin-labeled polymers1 by simultaneous adsorption of surfactant. Dendrimers, being highly branched polymers,2-5 have become the subject of extensive studies, because their functional groups and specific shape have unique properties compared to those of conventional linear polymers. In particular, the structure of dendrimers depends on their generation: earlier generation dendrimers possess an asymmetric shape and open structure, whereas the later generation dendrimers possess a nearly spherical shape and densely packed structure. Furthermore, because dendrimers have various surface functional groups, surface modification of dendrimers can be easily per(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) Tomalia, D. A.; Naylor, A. M.; Goddard, W. A. Angew. Chem., Int. Ed. Engl. 1990, 29, 138. (3) Frecht, J. M. J. Science 1994, 263, 1710. (4) Jansen, J. F. G. A.; Meijer, E. W. J. Am. Chem. Soc. 1995, 117, 4417. (5) Zeng, F.; Zimmerman, S. C. Chem. Rev. 1997, 97, 1681.
formed. Until now we have been studying the simultaneous adsorption of poly(amidoamine) dendrimers with surface carboxyl groups and an anionic surfactant at the alumina/ water interface6 and of poly(amidoamine) dendrimers with surface amino groups and cationic surfactants at the silica/ water interface.7 Recently, Aoi et al.8 synthesized fully sugar-persubstituted globular dendrimers and studied the interaction of the dendrimers and lectin. We also studied9 the interactions of sugar-persubstituted poly(amidoamine) dendrimers (sugar ball) with anionic surfactants in aqueous solution and found that some aggregates between sugar ball and the surfactant are formed, showing a strong surface activity. Accordingly, it is very interesting to determine how these aggregates adsorb at the solid/liquid interface and to compare the results for sugar ball and anionic surfactant with that of conventional linear polymer and anionic surfactant. The objectives of this study were to characterize simultaneous adsorption of sugar ball and anionic surfactants such as sodium dodecyl sulfate (SDS) and lithium perfluorooctanesulfonate (LiFOS) on alumina. Experimental Section Materials. Poly(amidoamine) dendrimers (generations 3 and 5) were prepared by using ethylenediamine as an initiator core according to a previous paper.10 Sugar balls (SBn, n ) generations 3 and 5) were synthesized8 by the reaction of the amineterminated poly(amidoamine) dendrimers with an excess amount of aldonolactone. To obtain lactobionate, lactobionic acid was evaporated several times from methanol in vacuo at 50 °C. Poly(amidoamine) 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 (6) Esumi, K.; Fujimoto, N.; Torigoe, K. Langmuir 1999, 15, 4613. (7) Esumi, K.; Fujimoto, N.; Torigoe, K.; Koide, Y. J. Jpn. Soc. Colour Mater. 2000, 73, 290. (8) Aoi, K.; Itoh, K.; Okada, M. Macromolecules 1995, 28, 5391. (9) Miyazaki, M.; Torigoe, K.; Esumi, K. Langmuir 2000, 16, 1522. (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/la000845o CCC: $19.00 © 2000 American Chemical Society Published on Web 11/21/2000
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Scheme 1
Figure 1. Adsorption isotherms of SDS and sugar balls on alumina. 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 powdery sugar balls were obtained. The purity of these samples was confirmed by 1H- and 13C NMR. In addition, gel permeation chromatography analysis suggested that each sugar ball consists of a single component. The number of sugar residues and the molecular weight were 32, 17765.8 for SB3 and 128, 72252.7 for SB5, respectively. The structure of sugar ball (SB3) is shown in Scheme 1. SDS was obtained from Nakalai Tesque, Inc., and recrystallized several times from ethanol. LiFOS was supplied by Dainippon Ink Chemicals Co., and recrystallized several times from mixtures of tetrachlorocarbon and ethanol. 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. R-Alumina was kindly supplied by Showa Denkou K. K. and its specific surface area and average diameter were 30.2 m2g-1 and 0.3 µm, respectively.
Figure 2. Simultaneous adsorption of SDS and SB3 on alumina: (a) SDS adsorption; (b) SB3 adsorption. The initial concentrations of SB3 are 0.10 and 0.25 g dm-3. The adsorption isotherm of SDS alone is also given. Methods and Measurements. The amount of sugar balls and surfactant adsorbed on alumina was obtained by a depletion method. Mixtures of sugar balls and surfactant were added to alumina. Then, the pH value for alumina suspensions was adjusted to 3.5. 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 sugar balls in the supernatant solutions was determined by the Antron method. The concentration of surfactant in the supernatant solutions was 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 alumina suspensions was evaluated by measuring the sedimentation rate using a Turbiscan MA2000 (Formulaction). The suspension was shaken for 1 day and then
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Figure 3. Surface tensions of SDS alone and SDS-SBn mixed systems. The initial concentration of SBn is 0.25 g dm-3. transferred to a test tube, and the sedimentation rate was obtained from transmittance change of the test tube with elapsed time. The ζ potential of alumina suspensions was measured using an electrophoretic apparatus (Pen Kem 500); ζ potentials were converted from electrophoretic mobilities using Smoluchowski equation. The surface tension of aqueous solutions of surfactant and SBn was measured using a Kruss K12 tensiometer. All of the measurements were carried out at 25 °C.
Results and Discussion Figure 1 shows the individual adsorption isotherm of SDS and sugar balls on alumina. Note that the adsorbed amount of SDS increases with an increase of SDS concentration and then reaches a plateau. From the saturated amount of SDS adsorbed and the specific surface area of alumina, the area occupied by SDS at the saturation was calculated to be about 0.21 nm2, indicating a formation of SDS bilayer. On the other hand, the adsorbed amounts of SB3 and SB5 also increased with an increase of SBn concentration and then reached a plateau. The calculated occupied areas by SB3 and SB5 at the saturation were about 50 nm2. Because the extended areas8 of SB3 and SB5 are expected to range 38-70 nm2, a monolayer adsorption is suggested at the saturation. The driving force for the adsorption of SB3 and SB5 seems to be hydrogen-bonding due to the interaction between hydroxyl groups of alumina surface and hydroxyl groups of SB3 and SB5. It is well known11,12 that when SDS is added to positively charged alumina, the dispersion state of alumina suspension changes from stable dispersion to flocculated state, and then a further increase of SDS concentration allows the suspension to be redispersed. On the other hand, in the presence of SB3 or SB5, the dispersion stability of alumina suspension was high over a wide concentration region of SB. Figure 2 shows a simultaneous adsorption of SDS and SB3 on alumina. Here, the initial concentrations of SB3 were fixed at 0.10 and 0.25 g dm-3. It is very interesting to note that the amount of SB3 adsorbed increases markedly at low SDS concentrations and shows a maxi(11) Meguro, K. Nippon Kagaku Zasshi 1956, 77, 77. (12) Esumi, K.; Ogihara, K.; Meguro, K. Bull. Chem. Soc. Jpn. 1984, 57, 1202.
Figure 4. Simultaneous adsorption of SDS and SB5 on alumina: (a) SDS adsorption; (b) SB5 adsorption. The initial concentrations of SB5 are 0.10 and 0.25 g dm-3. The adsorption isotherm of SDS alone is also given.
mum, and then decreases with an increase of SDS concentration, while the amount of SDS adsorbed in the presence of SB3 is greater than that in the absence of SB3. Such an enhancement in the adsorption of SB3 is probably due to the interaction between SDS and SB3 on the alumina surface. Because aqueous properties of SDS and SB3 may influence adsorption characteristics of SDS and SB3 on the alumina surface, the surface tensions of SDS and SB3 mixtures were measured. Figure 3 illustrates that the surface tension of SB3 alone in aqueous solution is high and is almost the same as that of water, but decreases markedly by addition of SDS, and shows about 30-35 mNm-1 in the SDS range between 0.4 and 8 mmol dm-3. This low surface tension suggests a formation of some complex of SDS and SB3 in which the hydrophilic groups of SDS might be adsorbed on the residue of glycoside of SB3, resulting in the orientation of the hydrocarbon chain of SDS to aqueous phase. When
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Figure 5. Variation of sedimentation rate of alumina suspension for SDS and SDS-SB3 systems. The initial concentrations of SB3 are 0.10 and 0.25 g dm-3.
mixtures of SDS and SB3 consisting of their complexes and SDS monomer contact with alumina particles, SDS monomer adsorbs at first, orienting its hydrocarbon chain to aqueous phase, followed by adsorption of SDS-SB3 complex on the surface. As a result, adsorption of SB3 is enhanced at low SDS concentration. With an increase of SDS concentration, a competitive adsorption between SDS monomer and SDS-SB3 complex against SDS covered alumina surface occurs, and at the same time the state of SDS-SB3 complex changes from monolayer coverage to bilayer coverage of SDS on the SB3 surface. Then, adsorption of SB3 on alumina will decrease. The enhancement in the adsorption of SB3 by SDS was clearly observed when the initial concentration of SB3 was higher. An enhancement in the adsorption of SB5 by SDS at low SDS concentrations on the alumina surface was also observed (Figure 4). For the two systems of SDS-SB3 and SDS-SB5, no clear difference was observed in the adsorption characteristics. A similar adsorption enhancement of a linear polymer, poly(vinylpyrrolidone) by SDS addition on alumina was reported1 in which the surface tension of poly(vinylpyrrolidone)-SDS complex was higher than that of SDS alone. The dispersion stability of alumina suspensions by adsorption of SBn and SDS was evaluated by measuring the sedimentation rate. In Figure 5, the sedimentation rate of alumina suspensions by adsorption of SDS alone increased sharply with the SDS concentration and reached a maximum, and then decreased with a further increase of SDS concentration. This behavior is referred to as the “dispersion-flocculation-redispersion process”.11,12 The maximum sedimentation rate of alumina suspensions may occur due to the formation of SDS monolayer. In the simultaneous adsorption of SB3 and SDS, the sedimentation rate increased and reached a maximum and then decreased, whereas the maximum sedimentation rate decreased with an increase of SB3 feed concentration. From the result of ζ potential measurements of alumina suspensions (Figure 6), it is suggested that because the changes in the ζ potentials in the absence and presence of SB3 are not so different, a relative high dispersion stability in the region below 2 mmol dm-3 SDS in the presence of SB3 is due to a steric hindrance caused by
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Figure 6. Variation of ζ potential of alumina suspension for SDS and SDS-SB3 systems. The initial concentrations of SB3 are 0.10 and 0.25 g dm-3.
Figure 7. Simultaneous adsorption of LiFOS and SB5 on alumina. The initial concentration of SB5 is 0.25 g dm-3. The adsorption isotherm of LiFOS alone is also given.
SB3 adsorbed. A similar dispersion behavior of alumina suspensions was observed for SB5-SDS system (the data not shown). Figure 7 shows the adsorption isotherm of LiFOS alone and simultaneous adsorption of LiFOS and SB5 on alumina. Here, an initial fixed concentration of SB5 was 0.25 g dm-3. In the single adsorption of LiFOS, the adsorbed amount of LiFOS increased and reached a plateau where a bilayer of LiFOS is formed on the alumina surface because the occupied area calculated from the specific surface area and saturated amount of LiFOS is about 0.12 nm2. For simultaneous adsorption of LiFOS and SB5, the adsorbed amount of LiFOS was greater than that of LiFOS alone at lower LiFOS concentration, but was lower at higher LiFOS concentration. On the other hand, the adsorbed amount of SB5 showed an interesting behavior with LiFOS concentration; the adsorbed amount of SB5 increased sharply and reached a maximum, and then decreased with LiFOS concentration. Above 2 mmol
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dm-3 LiFOS, a complete desorption of SB5 from the alumina surface was observed. This simultaneous behavior of LiFOS and SB5 is very similar to that for the systems of SDS and SBn. Because the surface tensions of mixtures of LiFOS and SB5 are much lower than that of LiFOS alone, complexes of LiFOS and SB5 formed in aqueous solution considerably affect the adsorption of SB5 on alumina. From the above results, it is found that no significant difference between SDS-SBn and LiFOS-SBn on the adsorption characteristics on alumina is observed. Conclusions For a simultaneous adsorption of sugar balls and anionic surfactants such as SDS and LiFOS on positively charged
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alumina particles under a constant initial concentration of sugar balls, the adsorption of sugar balls increases, reaches a maximum, and then decreases, whereas the anionic surfactant adsorption increases and reaches a saturation with increasing anionic surfactant concentration. The enhancement in the adsorption of sugar balls is significantly affected by aqueous solution properties of sugar ball and anionic surfactant. In addition, the change in the dispersion stability of alumina suspensions caused by a simultaneous adsorption of sugar ball and anionic surfactant depends on the initial concentration of sugar ball. LA000845O
