JOURNAL OF COLLOID AND INTERFACE SCIENCE 203, 214–217 (1998)ARTICLE NO. CS985534

Interactions of Poly(amidoamine) Dendrimers with Alumina Particles

Masaya Goino and Kunio Esumi1

Department of Applied Chemistry and Institute of Colloid and Interface Science, Science University of Tokyo,Kagurazaka, Shinjuku-ku, Tokyo 162-8601, Japan

Received December 30, 1997; accepted March 16, 1998

On the other hand, dendrimers, being highly branchedThe interactions between poly(amidoamine) dendrimers with sur- polymers, have become the subject of extensive studies be-

face carboxylic groups for generations G(0.5–5.5) and positively cause their functional groups and specific shape have uniquecharged alumina particles were investigated at pH 5 by measuring properties compared with those of conventional polymersthe adsorbed amount of dendrimers, the z potential, and the disper-

(7) . For example, polyamidoamine dendrimers with surfacesion stability of alumina suspensions. The amount of dendrimerscarboxylic groups in the earlier generations interact weaklyadsorbed increased with increasing concentration of dendrimers andwith cationic surfactant, but the later generations of the den-then attained a saturation, where the saturated amount in weightdrimers can induce cooperative interactions among boundof dendrimers adsorbed became larger from G(0.5) to G(5.5). Insurfactant molecules (8) . Further, dendrimers have abilitiesaddition, alumina particles were flocculated over a wide concentra-

tion region of the earlier generations G(0.5–1.5), but they showed to form organized superstructures at the interfaces (9–12);a dispersion–flocculation–redispersion sequence with dendrimer thus, it is very important to characterize adsorption behaviorsconcentration for the later generations G(2.5–5.5). The z potentials at various interfaces.of alumina particles decreased with increasing concentration of den- In this study, the interactions of poly(amidoamine) den-drimers as well as increasing generations of dendrimers. These re- drimers with alumina particles were investigated by measur-sults indicate that the earlier denderimers operate as electrolytes ing dendrimer adsorption, z potential, and dispersion stabil-and that the later dendrimers behave like an anionic surfactant or

ity of alumina suspensions.polyelectrolyte for alumina dispersion. q 1998 Academic Press

Key Words: dendrimer with surface carboxyl groups; adsorption;alumina, dispersion stability. EXPERIMENTAL

MaterialsINTRODUCTION

Polyamidoamine dendrimers were synthesized by meansof procedure described by Tomalia et al. (13). Here, ethyl-Polyelectrolytes interact strongly with oppositely chargedenediamine was used as a nitrogen core. Methylester-termi-colloidal particles, and the dispersion stability of colloidalnated dendrimers were hydrolyzed with stoichiometricparticles is often influenced by the adsorbed amount and theamounts of NaOH in water to obtain external carboxylateconformation of the polymers adsorbed (1–3). The disper-groups with sodium. A series of n .5-generation dendrimerssion stability also depends on the pH and ionic strength offrom 0.5 to 5.5 was prepared. Their purity was examinedsolutions. The effect of ionic strength of polyelectrolytesby NMR.is clearly demonstrated: as the salt concentration increases,

a-Alumina of 99.995% purity was kindly supplied bypolyelectrolytes with strongly charged groups tend towardShowa Denkou, K. K., and its specific surface area and parti-the behavior of an uncharged polymer, forming loops andcle size were 10.7 m2 g01 and 0.5 mm, respectively. Thetails for polyelectrolytes (1) . It has been reported (4) thatwater used in this study was purified through a Milli-Q sys-the conformation of poly(acrylic acid) adsorbed on aluminatem. The suspension pH was adjusted with HCl. The otherestimated by using a pyrene-labeled polymer changes appre-reagents were of analytical grade.ciably with pH and affects the dispersion stability of alumina

suspensions. In addition, competitive adsorption of differentMethods and Measurementspolymers has been studied from the standpoint of colloidal

dispersion stability (3, 5, 6) . The amount of dendrimers adsorbed on alumina was ob-tained by a depletion method. The pH value adjusted afterthe dendrimers were added to alumina suspensions was 5.All suspensions in the presence of 10 mmol dm03 NaCl in1 To whom correspondence should be addressed.

2140021-9797/98 $25.00Copyright q 1998 by Academic PressAll rights of reproduction in any form reserved.

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215ALUMINA–POLY(AMIDOAMINE) DENDRIMER INTERACTIONS

Figure 1 shows changes in the z potential of aluminasuspensions by adsorption of dendrimers. The positive zpotential of alumina decreased steeply at low dendrimer con-centrations and then became constant with a further increaseof dendrimer concentration, where the G(0.5) dendrimershowed about 20 mV, the G(1.5 and 2.5) dendrimersshowed nearly zero, and the G(3.5–5.5) dendrimers pro-vided negative values. Interestingly, the conversion in the zpotential from positive to negative was observed for G(3.5–5.5) dendrimers. These results suggest that the adsorptionof dendrimers occurs primarily due to electrostatic attractionforces. From the change in absorbance of alumina suspen-sions at 6 h of standing after the adsorption (Fig. 2) , itwas found that the alumina suspensions have low dispersionstability with addition of G(0.5 and 1.5) dendrimers over awide concentration region, while they show a dispersion–flocculation–redispersion sequence with an increase ofG(2.5–5.5) dendrimer concentration. This dispersion se-quence can be correlated with the change of z potential;although alumina suspensions with an appreciable positive zpotential in the absence of dendrimer show a high dispersionstability, they are flocculated by addition of low concentra-FIG. 1. Changes in z potential of alumina suspensions by adsorptiontion dendrimers due to neutralization of surface charge, andof dendrimers.then an increase of further concentration of dendrimers ren-ders the z potential more negatively, resulting in a high stableredispersion due to electrostatic repulsion forces betweenvial glasses were shaken to reach an adsorption equilibriumparticles. The measurements of particle size (Fig. 3) showfor 24 h at 257C. After equilibration, the suspensions werethat the median size of alumina suspension, which is aboutcentrifuged and the concentration of dendrimers in the super-0.7 mm without dendrimers, increases at low concentrationnatant solutions was determined by HPLC.of dendrimers but decreases with a further addition of highThe dispersion stability of alumina suspensions with addi-

tion of dendrimers was evaluated by measuring the ab-sorbance at 600 nm; the higher the absorbance, the higherthe dispersion stability. The absorbance measurements wereperformed at 6 h of standing after the adsorption.

The particle size of alumina suspensions was determinedby using a laser scattering particle size distribution analyzer(Horiba Seisakusyo, LA-910).

The z potential of alumina suspensions with addition ofdendrimers was measured by using an electrophoresis appa-ratus (Pen Kem 500).

All experiments were performed at 257C.

RESULTS AND DISCUSSION

It is known from the pyrene-probe method (8) that thehalf-generation (n .5) poly(aminoamine) dendrimers behaveas ordinary electrolytes for the earlier generations (0.5–3.5)and as a novel type of polyelectrolytes or anionic adsorptionsurface for the later generations (4.5–9.5) . Because the iso-electric point of alumina is about 9.2 and the pKa of acrylicacid is about 4.0–4.5, the interactions between alumina andpoly(amidoamine) dendrimers with surface carboxylicgroups were investigated at pH 5, where the electrostatic FIG. 2. Changes in absorbance of alumina suspensions by adsorption

of dendrimers.interaction would be mainly operational.

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216 GOINO AND ESUMI

FIG. 4. Adsorption isotherms of dendrimers on alumina.FIG. 3. Changes in average particle size of alumina suspensions byadsorption of dendrimers.

acid) (molecular weight Å 50,000) (3) shows that the ad-sorption of the G(5.5) dendrimer is about 3 times that ofconcentration of dendrimers for all the dendrimers studied.poly(acrylic acid) . This difference derives probably fromIn particular, the median size of alumina suspension by ad-the different molecular structures of poly(acrylic acid) andsorption of G(5.5) dendrimer at higher concentrationsdendrimers adsorbed; at pH 5, poly(acrylic acid) adsorbsshowed the smallest size. Furthermore, the adsorption ofpredominantly as train segments and G(5.5) dendrimer isG(5.5) dendrimer provided a very stable redispersion statesuggested to be adsorbed as globular, orienting with only fewafter 1 day of standing, whereas the absorbances of aluminacarboxyl groups to positively charged sites on the alumina.suspensions obtained from the other G(0.5–4.5) dendrimersAssuming a monolayer adsorption of dendrimers on alumina,became almost zero after 1 day of standing. Thus the ob-we calculated the occupied areas of dendrimers from thetained dispersion sequence for G(2.5–5.5) dendrimers hassaturated amount of dendrimers adsorbed and the specificoften been observed for oppositely charged particle–surfac-surface area of alumina (Table 1). It is seen that the occupiedtant (14)/particle–polyelectrolyte (3) systems. Accord-areas of dendrimers increase with increasing generation fromingly, the results of z potentials and dispersion stability of0.5 to 5.5, and they are considerably small compared withalumina suspensions suggest that the earlier generation ofthe corresponding cross-sectional areas for all the dendrimersthe dendrimers operates just as salt for alumina suspension

and the later one plays a role like as anionic surfactant orpolyelectrolyte. TABLE 1

Figure 4 shows the adsorption isotherms of dendrimers Data for Dendrimer Adsorptionon alumina. The amount of dendrimers adsorbed increased

Adsorbed amountsharply at low concentration and then attained a saturationat saturation Occupied area Cross-sectionalwith increasing concentration of dendrimers for all the den-

Generation (mmol/g) (nm2/molecule) areaa (nm2)drimers studied, where the amount in weight of dendrimersadsorbed became larger with increasing generation of den- 0.5 6.0 3.0 6.2drimers. The adsorption of dendrimers is facilitated mainly 1.5 2.5 7.1 10.2

2.5 1.6 11.1 18.1by electrostatic attractive interactions between the negatively3.5 1.2 14.8 34.2charged dendrimer surfaces and positively charged alumina4.5 0.55 32.3 60.8surfaces. Also, the smaller adsorption for the earlier genera-5.5 0.40 44.4 84.9

tions may indicate that the earlier generations interact withthe alumina surface like as ordinary electrolytes. Comparison a Cross-sectional area calculated from diameters determined by size ex-

clusion chromatography in water (8).of the adsorption of the G(5.5) dendrimer and poly(acrylic

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217ALUMINA–POLY(AMIDOAMINE) DENDRIMER INTERACTIONS

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9. Wells, M., and Crooks, R. M., J. Am. Chem. Soc. 118, 3988 (1996).against the alumina surface.10. Saville, P. M., Reynolds, P. A., White, J. M., Hawker, C. J., Frechet,Since poly(aminoamine) dendrimers with surface amino

J. M. J., Wooley, K. L., Penfold, J., and Webster, J. R. P., J. Phys.groups are positively charged below pH 4, the interactions Chem. 99, 8283 (1995).between such dendrimers and negatively charged silica parti- 11. Watanabe, S., and Regen, S. L., J. Am. Chem. Soc. 116, 8855 (1994).

12. Tsukruk, V. V., Rinderspacher, F., and Bliznyuk, V. N., Langmuir 13,cles are also very interesting. Such study is now in progress.2171 (1997).Further, as it is expected that the microenvironmental proper-

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