Adsorption of Poly (propylene imine) Dendrimers on Glass. An

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Adsorption of Poly(propylene imine) Dendrimers on Glass. An Interplay between Surface and Particle Properties Rene´ C. van Duijvenbode* Gorlaeus Laboratories, Leiden Institute of Chemistry, Leiden University, P.O. Box 9502, 2300 RA Leiden, The Netherlands

Ger J. M. Koper Laboratory of Physical Chemistry, Delft University of Technology, Julianalaan 136, 2628 BL Delft, The Netherlands

Marcel R. Bo¨hmer Philips Research Laboratories, Eindhoven (WA11), Prof. Holstlaan 4, 5656 AA Eindhoven, The Netherlands Received February 16, 2000. In Final Form: April 7, 2000 The adsorption behavior of positively charged poly(propylene imine) dendrimers on glass has been studied by scanning angle reflectometry as a function of generation, pH, and ionic strength. The results indicate that the adsorption is controlled by properties of both the dendrimers (size and charge) and the surface layer (distribution of glass surface sites). At constant pH and ionic strength the collector properties remain the same, and the adsorption scales with dendrimer size. Because of the limited number of glass surface sites there is only 15% surface coverage at maximum. Adsorption increases with decreasing pH and increasing ionic strength, and the effect of pH is much more pronounced for the smaller dendrimers. To explain the adsorption results, not only the size and charge of the dendrimer have to be taken into account, but also the charge density and the roughness of the surface. This implies that the dendrimer size is comparable to the characteristic length scale of the surface roughness.

Introduction A variety of applications are based on polymer adsorption phenomena, such as dispersion stability control, wastewater treatment, paper production, biocompatibilization, etc. For that reason much effort has been devoted to the clarification of the interfacial properties of polymers in contact with a solid wall. A common studied case is polyelectrolyte adsorption on oppositely charged colloidal particles. The polyelectrolytes form a protecting layer, which counteracts the van der Waals forces, that would induce aggregation of the colloidal particles in the absence of electrostatic repulsion. The adsorbed amount of polyelectrolyte is governed by a balance between the electrostatic interactions between the substrate and polyelectrolyte and the mutual interactions between charged segments. pH and ionic strength are important variables to tune this balance, and therefore affect the dispersion stability strongly. Among the small molecules and polymers a new subclass of particles, dendrimers, has been developed over the last two decades.1-3 Dendrimers are synthesized in a stepwise manner, resulting in different generations. Starting from a core, in every synthesis step an extra shell with a new terminal group is added at each end group, the number of functional groups growing exponentially with generation number. The high branching density, the monodispersity, and the well-defined globular geometry, are the features that make the dendrimers special when compared to the ordinary polyelectrolytes. The density of functional groups in both the core and at the surface is much higher than for any other kind of polyelectrolyte. * To whom correspondence should be addressed. (1) Dvornic, P. R.; Tomalia, D. A. Curr. Opin. Colloid Interface Sci. 1996, 1, 221-235. (2) Zimmermann, S. C., Zeng, F. Z. Chem. Rev. 1997, 97, 1681-1712. (3) Bosman, A. W.; Janssen, H. M.; Meijer, E. W. Chem. Rev. 1999, 99, 1665-1688.

Regular polyelectrolytes have flexible chains and can take all kinds of conformational states at the surface depending on the surface charge. However, on the basis of X-ray reflectivity studies on polyamidoamine dendrimers4 and computer simulations on model dendrimers,5 it is concluded that a dendrimer is not able to stretch fully along the surface. The dendrimers can compress slightly, but the additional surface coverage per molecule is hardly comparable to what a flexible chain can do. The number of contacts between the dendrimer and surface will therefore stay low, and it is not expected that the dendrimer adsorption can be explained in terms of trains, tails, and loops along the surface. The number of publications dealing with dendrimer adsorption is growing fast, but a systematic study on the influence of charge on adsorption was not available up to now. In the present paper we study the adsorption of the five commercially available generations of a poly(propylene imine) dendrimer (see Figure 1). The charge on the primary amines in the outermost shell and tertiary amines in the core can be tuned with pH, going from fully uncharged at pH 12 to fully charged at pH 3.11 A completely charged fifth-generation dendrimer holds 126 charges at pH 3 within a radius of 1.5-2 nm.12,13 The interest in the (4) Tsukruk, V. V.; Rinderspacher, F.; Bliznyuk, V. N. Langmuir 1997, 13, 2171-2176. (5) Mansfield, M. L. Polymer 1996, 37, 3835-3841. (6) Saville, P. M.; Reynolds, P. A.; White, W.; Hawker, C. J.; Frećhet, J. M. J.; Wooley, K. L.; Penfold, J.; Webster, J. R. P. J. Phys. Chem. 1995, 99, 8283-8239. (7) Tokshisa, H.; Zhao, M.; Baker, L. A.; Phan, V. T.; Dermody, D. L.; Garcia, M. E.; Peez, R. F.; Crooks, R. M.; Mayer, T. M. J. Am. Chem. Soc. 1998, 120, 4492-4501. (8) Esumi, K.; Goino, M. Langmuir 1998, 14, 4466-4470. (9) Esumi, K.; Fujimoto, N.; Torigoe, K. Langmuir 1999, 15, 46134616. (10) Takada, K.; Storrier, G. D.; Morań, M.; Abrunã, H. D. Langmuir 1999, 15, 7333-7339. (11) van Duijvenbode, R. C.; Koper, G. J. M.; Borkovec, M. Polymer 1998, 39, 2657-2664.

10.1021/la000231j CCC: $19.00 © 2000 American Chemical Society Published on Web 09/02/2000

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Figure 1. A schematic representation of the first and fifth generations of the 1,4-diaminobutane poly(propylene imine) dendrimers. The distance between the amino groups in the core is different from those of all other branches (butylene vs propylene). Each next generation has an additional shell of tertiary amines, indicated with filled circles. The outermost, primary amines are represented with empty circles. In the case of carboxylate-functionalized DAB-dendr-(COO-Li+)64, the empty circles in the outermost shell are COO-Li+ groups.

adsorption properties is closely related to these high charge densities and the very thin film layers that can be formed. The influence of electrostatic interactions on the adsorption of the poly(propylene imine) dendrimers on glass, an oppositely charged surface, is studied using scanning angle reflectometry. To accomplish this, pH and ionic strength are systematically changed. The adsorption is also studied for a fifth-generation carboxylate-functionalized dendrimer, in which the outermost shell of positively charged primary amines is replaced by a layer of negatively charged carboxylate groups. This could give more information about whether only the outermost shell is involved in adsorption. If that is the case, reduced or no adsorption would be expected for the modified dendrimer on the negatively charged glass substrate. To distinguish between the adsorption properties of the dendrimer and the influence of the collector, the adsorption experiments as a function of generation are also performed on a silica substrate. Experimental Section Materials. The 1,4-diaminobutane poly(propylene imine) dendrimers DAB-dendr-(NH2)x (x ) 4, 8, 16, 32, and 64) were used as obtained from DSM (Geleen, The Netherlands). Sample numbers were 4pa-n9652, 8pa-n991051, 16pa-n915651, 32pan961251, and 64am-n97851. The adsorption behavior as a function of pH has also been studied for the highest generation of a carboxylate-functionalized poly(propylene imine) dendrimer, DAB-dendr-(COO-Li+)64.14 A schematic picture of the dendrimers is shown in Figure 1. The refractive index increment (dn/dc) has been determined to be 0.17 mL/g for all dendrimer generations (Wyatt Optilab 903 Refractometer). No significant dependence on pH or ionic strength is observed. The dendrimers were adsorbed onto the optically flat hypotenuse of a rectangular prism made of ordinary glass (Schott BK 7, refractive index 1.5151, Melles Griot). The overall surface charge is negative at pH > 4.15,16 The cleaning procedure of the (12) Scherrenberg, R.; Coussens, B.; van Vliet, P.; Edouard, G.; Brackman, J.; de Brabander, E.; Mortensen, K. Macromolecules 1998, 31, 456-461. (13) Rietveld, I. B.; Smit, J. A. M. Macromolecules 1999, 32, 46084614. (14) van Duijvenbode, R. C.; Rajanayagam, A.; Koper, G. J. M.; Baars, M. W. P. L.; de Waal, B. F. M.; Meijer, E. W.; Borkovec, M. Macromolecules 2000, 33, 46-52. (15) Iler, R. K. The Chemistry of Silica, Solubility, Polymerization, Colloid and Surface Properties, and Biochemistry; Wiley: New York, 1979.

Figure 2. A schematic diagram of the scanning angle reflectometer: (L) light source; (P, A) polarizers, aligned in the plane of incidence; (C) impinging-jet cell; (D) photodetector. A detailed diagram of the impinging-jet cell is shown underneath. It has a distance h ) 1 mm between the collector surface and the inlet tube, and the radius of the inlet tube R ) 0.6 mm. prism involves immersion in piranha liquid, a 6:1 ratio of H2SO4/H2O2, for approximately 10 min at 70 °C. The prism was flushed afterward with 0.1 M NaOH for at least 12 h to reduce the surface layer. This surface layer is created through an ion exchange process in which sodium ions at the glass surface are replaced by protons, especially in the case of concentrated strong acids.15,17 In alkaline conditions silica hairs are removed from the surface and the surface is smoothened.18 The dissolution kinetics on exposure to 0.1 M NaOH were of quality similar to that observed in Figure 9 in ref 19. pH measurements were performed with an HI 8417 pH meter and a HI 1131 combined glass electrode (Hanna Instruments). NaCl (analytical grade) was used to maintain the ionic strength at a constant level throughout an adsorption experiment. NaOH and HCl (Titrisol, Merck) were used to adjust the pH, at concentrations corresponding to the ionic strength in solution. In all cases the contribution of the dendrimer concentration to the ionic strength was negligible. Scanning Angle Reflectometry. The adsorption behavior was studied in an impinging-jet cell by scanning angle reflectometry around the Brewster angle. A schematic representation of the setup is shown in Figure 2. A peristaltic pump (LKB 12000 VarioPerpex, Sweden) was used to flow the solution along the surface. The stagnation point is positioned such that it coincides with the reflection spot of the light source. The light source is a stabilized 1 mW HeNe laser (λ ) 632.8 nm, Melles Griot). The beam passes through two Glan-Thompson polarizers (Melles Griot), which select the polarization state aligned in the plane of incidence. The reflection amplitude of the so-called ppolarization state vanishes at the Brewster angle for a sharp, theoretically flat interface and is thus extremely sensitive to adsorption at the interface. The intensity of the reflected beam is measured by means of a photomultiplier. The angle of incidence is selected by simultaneously rotating the laser and the detector supports, which are fully automated and computer-controlled with an accuracy of 1/10000°. Scanning angle measurements make the technique more sensitive to adsorption, because not (16) Bolt, G. H. J. Phys. Chem. 1967, 61, 1166-1169. (17) Doremus, R. H. Glass Science, 2nd ed.; Wiley: New York, 1994. (18) Vigil, G.; Zhenghe, X.; Steinberg, S.; Israelachvili, J. J. Colloid Interface Sci. 1994, 165, 367-385. (19) Fu, Z.; Santore, M. M. Colloids Surf., A 1998, 135, 63-75.

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Figure 4. The dendrimer adsorption is analyzed in terms of a two-layer model: (i) a glass surface layer and (ii) a layer of dendrimers on top of the theoretical Fresnel glass-water interface. In the matrixes in the Appendix the different media and layers are addressed with m ()0-3).

Figure 3. A typical example of a scanning angle reflectivity curve around the Brewster angle (41.36°) for a glass-water interface is shown for DAB-dendr-(NH2)x (x ) 8, 16, 32, and 64) in 0.1 M NaCl at pH 7. It is compared to the reflectivity for the background electrolyte 0.1 M NaCl. For the first generation no adsorption was observed on this scale. Also depicted is the reflectivity signal for 0.1 M NaOH around the Brewster angle. only is the reflectivity near the Brewster angle measured, but also the shape of the reflectivity curve around the Brewster angle. Furthermore, an angular shift in the Brewster angle due to adsorption can be detected, which would be missed with fixed angle measurements. The measured intensities are related to the reflectivity Rp(θ) by

I(θ) ) I0 + ARp(θ + δ)

(1)

where I0 is the residual intensity at the Brewster angle, A is an instrument-dependent constant, and δ allows for a small systematic indeterminancy of the incidence angle.20 The amplitude reflection coefficient for a simple interface is described by the Fresnel equations in terms of the refractive indices and the angle of incidence θ, and Rp(θ) is the modulus of the amplitude reflection coefficient squared. The three coefficients are obtained from fitting eq 1 to the scanning angle data of a glass-0.1 M NaOH interface. Under these alkaline conditions the glass layer is minimal and the interface is treated as a Fresnel interface. This allows for the determination of the amplification factor A, as well as the residual intensity at the Brewster angle. These are then used to convert the reflection intensity in the adsorption experiments into the associated reflection coefficient. Examples of scanning angle reflectivity curves around the Brewster angle of 41.36° for DAB-dendr-(NH2)x with x ) 8, 16, 32, and 64 in 0.1 M NaCl at pH 7 are shown in Figure 3. These scans were performed after dendrimer adsorption reached a stationary value. For the first generation (x ) 4) the adsorption on glass was below the detection limit. No significant change in Brewster angle was observed upon adsorption. Prior to an adsorption experiment the cell was filled with the background electrolyte at 0.1 M NaCl, and the surface layer was determined. The dendrimers were removed by flushing the cell for 0.5 h with 0.1 M NaOH. The same reflectivity signals as prior to the experiment were obtained, indicating that all dendrimers disappeared from the interface. It was therefore possible to do a series of experiments such as presented in Figure 3 with the same cell and only one set of parameters (I0, A, δ) by flushing with NaOH between the experiments. This minimizes the effect of indeterminancies in these parameters, and excludes a variation (20) Mann, E. K.; van der Zeeuw, E. A.; Koper, G. J. M.; Schaaf, P.; Bedeaux, D. J. Phys. Chem. 1995, 99, 790-797.

in the surface properties while comparing results of different experiments. Dendrimer adsorption was also studied onto another negatively charged substrate, a Si wafer with a 100 nm SiO2 top layer (Philips, Eindhoven). These silica substrates are crystalline, whereas the other glass substrates are amorphous. These experiments were performed with a different optical reflectometer setup. For a detailed description of the equipment, cleaning procedures, and data analysis the reader is referred elsewhere.21,22 The geometry of the impinging-jet cell and the flow conditions are identical to those used for the adsorption experiments on glass. In summary, in a typical experiment one first determines the instrumental parameters in eq 1 using the above-described procedure. Then the cell is abundantly flushed with background electrolyte, after which the dendrimer solution is introduced. The adsorption is followed in time until a stationary (plateau) value is reached, depending on the dendrimer concentration after 10 s to a few hours. Conversion into Surface Coverage. A two-layer model is used to convert the signal into surface coverage and is schematically represented in Figure 4. The hypothesis is that the dendrimers are adsorbed on top of a glass surface layer, which is modeled to be a homogeneous slab with an effective refractive index different from the refractive index in the bulk glass. The existence of such a glass layer and its possible causes are discussed elsewhere19 and is also evident from Figure 3. A comparison between the two reflectivity curves for glass-NaOH(aq) and glass-NaCl(aq) interfaces shows that the intensity at the Brewster angle, I0, is much higher for the latter case. This additional signal is related to an extra layer at the interface, as will be discussed later. A convenient method for the analysis of the adsorption behavior of the dendrimers in terms of a layered system is Abeles’ matrix method,23 which will be described in more detail in the Appendix. The method uses two types of complex 2 × 2 matrixes to describe light propagation through the interface. An interface matrix Im,m+1 deals with the transition from one layer, m, into the next, m + 1, in terms of the Fresnel amplitude transmission and reflection coefficients. A layer matrix Lm describes the propagation of the light through the mth layer. It contains a phase factor involving the optical path, with two parameters, the effective refractive index neff and the thickness dm of the layer. Information about the glass surface layer is determined by fitting the model to the scanning angle reflectivity data for the glass-NaCl(aq) interface. When neff is taken to be equal to 1.50, a value based on the literature19 and close to the refractive index in the bulk, the glass layer thickness (and surface roughness) after treatment with NaOH is found to be on the order of 4 nm. However, because of the indeterminancy in neff, the value for the layer thickness can only be seen as an indication of its size. The adsorbed dendrimer film on the surface is not treated as a uniform layer with an effective refractive index, but as consisting of Rayleigh scatterers. The particles are modeled as being homogeneously distributed in a monolayer. In the above-described matrix formalism this means that the interface matrix Im,m+1 is (21) Dijt, J. C.; Cohen Stuart, M. A.; Fleer, G. J. Adv. Colloid Interface Sci. 1994, 50, 79-101. (22) Bo¨hmer, M. R.; van der Zeeuw, E. A.; Koper, G. J. M. J. Colloid Interface Sci. 1998, 197, 242-250. (23) Azzam, R. M. A.; Bashara, N. M. Ellipsometry and Polarized Light; North-Holland Publishing Co.: Amsterdam, 1989.

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Figure 6. As in Figure 5, but now the layer mass per unit area and the surface charge density are plotted as a function of ionic strength, at pH 7.

Figure 5. The change in collector’s properties as a function of pH is plotted at a constant ionic strength (0.1 M NaCl) for this type of glass. These results are obtained from the change in the residual intensity I0 at the Brewster angle. I01/2 is related to the change in surface layer mass per unit area or surface roughness.19 The layer mass per unit area (filled symbols) is presented in arbitrary units, scaled on the measurements at pH 7. With the open symbols the pH dependence of the surface charge density is plotted as observed in the literature for fused silica,16 which serves to indicate the trend for BK7 glass. The lines are guides for the eye. modified (see the Appendix). The Fresnel reflection coefficients are replaced by layer reflection and transmission coefficients, which in turn involve the polarizability of the particles.22,24 The particles are sufficiently small in comparison to the wavelength of the HeNe laser light, to allow for the Rayleigh approximation.

Results All adsorption experiments presented in this paper yielded reproducible results within 5-10% using the same glass substrate. Experiments performed after rigorously cleaning with piranha liquid gave deviant numbers in adsorbed mass, up to a factor of 4 less of what is reported here. However, trends as a function of generation, ionic strength, and pH were fully reproducible. The advantage of presenting the experiments here of one glass substrate is that throughout the paper the collector’s properties remain constant at constant ionic strength and pH. The instrumental parameter I0, the residual intensity at the Brewster angle determined prior to the adsorption experiments, includes information about the roughness of the substrate. In the ideal case of a Fresnel interface only the background intensity would contribute to I0. A change in I01/2 with pH and ionic strength can thus be related to the surface layer mass per unit area, a measure of the layer roughness (filled symbols in Figures 5 and 6).19 In the same figures the surface charge density as a function of pH and ionic strength is also plotted as open symbols. The values for the surface charge densities are obtained from the literature for fused silica,16 but similar trends are expected for the presently used glass substrate. Dependence on Generation. In Figure 7a the adsorbed mass for the different generations is shown for pH 7 and 0.1 M NaCl on both a glass and a silica substrate. The difference between the two similarly charged substrates is striking. On silica the adsorbed amount for generation 1 is significantly higher than for the other generations, whereas on glass its adsorption is below the detection limit. (24) Koper, G. J. M. Colloids Surf., A 2000, 165, 39-57.

Figure 7. (a) Adsorption behavior for different generations of poly(propylene imine) dendrimers in 0.1 M NaCl at pH 7 in terms of the maximal adsorbed mass. The adsorption behavior at both glass-water (b) and silica-water (O) interfaces is studied. The lines are guides for the eye. Data for the amount of adsorbed particles in (b) for the higher generations are extrapolated to the first generation, which was not detectable experimentally. 4 is a calculation of the adsorbed mass, based on this trend line. (b) The saturation levels in (a) are plotted in the number of particles per unit area. For these calculations the molar masses of the well-defined dendritic structures are used. Lines through the points are eye guides. The extrapolation of the trend line to the first generation at the glass-water interface is indicated with 4. (c) The separation between the adsorbed dendrimers at the interface is plotted for the different generations. Extrapolation of the trend to the first generation at the glass-water interface is indicated with 4; values in (b) and (c) are back-calculated with this separation on glass.

Using the molecular masses of the well-defined dendrimers the data set in Figure 7a is plotted as the total number of particles per unit surface area in Figure 7b. In Figure 7c the mean separation between the adsorbed


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Figure 9. Ionic strength dependence of the adsorbed amount of particles on glass shown for the fifth generation at pH 7, for both cb ) 1 mg/L (O) and cb 30 mg/L (0). The solid line is a guide for the eye. Together with the data the ionic strength dependence calculated with the RSA model in eq 2 is plotted (dashed line). The proportionality constant ξ is calculated with the data at 0.1 M. Figure 8. Adsorption isotherm for DAB-dendr-(NH2)x (x ) 8, 16, 32, and 64) on glass, in 0.1 M NaCl at pH 7. The bulk concentration cb does not show any influence on the saturation level at the glass surface at cb > 0.1 mg/L. Dashed lines are guides for the eye.

particles is plotted as a function of generation. The separations found for the dendrimers adsorbed on silica are closely equal to the radii of gyration obtained from SANS measurements by Scherrenberg et al.12 This indicates that the dendrimer adsorption on silica is close to the jamming limit. This is not the case for the dendrimer adsorption on glass. A linear regression in the particle separations for generations 2-5 extrapolated to the first generation, see Figure 7c, gives particle numbers and adsorbed mass per unit area as plotted in Figure 7a,b. It shows indeed that for the first generation the adsorption on glass is below the detection limit. Also, the linear regression clearly demonstrates that the dependence of the number of adsorbed dendrimers per unit area on generation is almost negligible for the glass substrate. Dependence on Dendrimer Concentration. The adsorption on glass is independent of the concentration of dendrimers for at least four decades, as shown in Figure 8. Rinsing with background electrolyte after reaching a plateau did not result in a change in the amount of adsorbed particles. It is therefore concluded that the desorption rate is negligible compared to the rate of adsorption. However, when a layer of adsorbed dendrimers in a background electrolyte at pH 7 was rinsed with an alkaline solution of pH larger than 12, all adsorbed particles were removed within seconds. No exchange was observed when an already adsorbed dendrimer layer was brought into contact with another generation dendrimer solution. The differences in size were apparently not large enough to accomplish such an effect as regularly observed for (bio)polymers, which is then referred to as the Vroman effect.25,26 Dependence on Ionic Strength. In Figure 9 the influence of ionic strength on the adsorption of the fifthgeneration dendrimer at pH 7 is depicted. Experiments were performed with cb ) 1 and 30 mg/L. As already shown in Figure 8 the concentration of dendrimers in the bulk does not affect the adsorption. A change in ionic strength of the background electrolyte after reaching the adsorption (25) Hoogeveen, N. G.; Cohen Stuart, M. A.; Fleer, G. J. J. Colloid Interface Sci. 1996, 182, 146-157. (26) Ball, V.; Bentaleb, A.; Hemmerle, J.; Voegel, J.-C.; Schaaf, P. Langmuir 1996, 12, 1614-1621 and references therein.

Figure 10. Adsorption in terms of particles per unit area at the glass-water interface in 0.1 M NaCl vs pH, shown for the case of the third (b), fourth (4), and fifth (O) generations of the unmodified poly(propylene imine) dendrimer. Also depicted is the highest generation of a carboxylate-functionalized version (0). The dashed lines through the points are guides for the eye.

saturation level results in a change in the adsorbed amount corresponding to the adsorption level at that particular ionic strength. Assuming that the adsorption of dendrimers on glass can be described by the random sequential adsorption (RSA) model,27 the ionic strength dependence can be taken into account by introducing an increased particle radius H* ) ξ/κa. Here κ-1 is the Debye length, a the particle radius, and ξ a proportionality constant related to the electrostatic repulsion between the particles. The coverage θ is then expressed as

θ ) θ∞/(1 + H*)2

(2)

with the jamming limit θ∞ ) 0.547. The experimentally found change in adsorption with ionic strength is much less pronounced, see Figure 9, than predicted by eq 2. Dependence on pH. In Figure 10 the pH dependence of the adsorption on glass is shown for generations 3-5 as well as for the fifth generation of a carboxylatefunctionalized dendrimer (see Figure 1). The ionic strength (27) Adamczyk, Z.; Warszynski, P. Adv. Colloid Interface Sci. 1996, 63, 41-149.

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is 0.1 M NaCl. Using potentiometric titration results,11,14 the dependence on pH can be translated to a dependence on the fraction of protonated amines. The carboxylate groups are negatively charged for pH > 4. The point of zero charge is around pH 4.5, which means that the overall charge is negative above this pH. A change in pH seems to give a more pronounced effect in adsorption for the lower generations: there is a clear trend going from generation 5 to generation 3. The fifth generations of both the unmodified and the carboxylatefunctionalized dendrimer show the same pH dependence, albeit the adsorption is less and starts only at pH 9 for the latter case. A comparison with potentiometric titration results14 shows that this pH corresponds to complete protonation of the outermost shell of amines. It was already concluded here that it seems as if the outermost shell of amines shields the negative charge of the carboxylate groups, although the amine groups are outnumbered by a factor of 2. For both generations 4 and 5 a minimum in adsorption is measured between pH 9 and pH 10, which is most probably related to the solubility of these molecules. At these pH values there are almost no charges on the dendrimer.11,14 At higher pH the amount of adsorbed dendrimers increases again, which is most likely caused by aggregation. In contrast to the unmodified dendrimers, the carboxylate-functionalized dendrimer does not show an increase in adsorption at high pH again. It has an overall negative charge at high pH, so not only does the double-layer repulsion between the surface and dendrimer prevent the dendrimer from adsorbing, also aggregation is highly unexpected to play a role in that pH domain. A change in pH of the background electrolyte after adsorption led to a change in the adsorbed amount corresponding to the adsorption level at that particular pH in Figure 10. In an adsorption experiment where the pH was continuously varied from 4 to 12 at a rate of 0.1 pH unit/min, the adsorption data for generation 5 of the unmodified dendrimer in Figure 10 were reproduced with the difference that adsorption completely vanished for pH values over 9. This indicates that the rate of aggregation is slower than the time scale of the adsorption experiment. The data points in Figure 10 are based on measurements with solutions equilibrated at the given pH. Discussion and Conclusion The adsorption behavior of positively charged poly(propylene imine) dendrimers on glass were studied as a function of generation and as a function of pH and ionic strength using reflectometry. The outcome of the experiments with a glass substrate cannot be explained in terms of a random sequential adsorption model in which the size of the particle dominates the adsorption behavior. The surface coverage on glass is only on the order of 1015%, the separation between the dendrimers is much larger than the dendrimer size, and the distance between the dendrimers hardly varies with generation, as indicated in Figure 7c. In contrast, the adsorption on silica seems to more closely follow the RSA model predictions. We shall not discuss this latter case any further here. An interpretation of the results in terms of adsorption enthalpy and entropy is also not possible on the basis of the presented experimental results. The fact that rigorously cleaning or changing the prism substrate affects the adsorption is not in line with the idea that it is completely governed by the properties of the dendrimers. Furthermore, the observation that rinsing with background electrolyte did not result in a change in the


adsorbed amount means that the binding between the dendrimer and surface is an irreversible one. pH and ionic strength are parameters that do influence the amount of dendrimers attached to the surface, but the bulk concentration of dendrimers does not induce a change in the adsorbed amount. We shall first briefly summarize the experimental results for the adsorption of the dendrimers onto a glass substrate and then come with a possible explanation for all these results. We find that the number of adsorbed particles decreases with generation, albeit not in terms of RSA model predictions, and the system is not close to the jamming limit (Figure 7). Second, the number of adsorbed dendrimers increases with ionic strength. The RSA model predicts an increase in adsorption, because the electrostatic repulsions between the charged amine groups will be screened more effectively. The Debye screening length and thus the effective dendrimer size decrease, and more particles can attach to the surface. The observed effect is however not so pronounced (Figure 9). We further find that the number of particles that adsorb onto the glass surface increases with the charge on the dendrimer (or a decrease in pH; see Figure 10). The increase seems to be dependent on generation; the change in adsorption is more pronounced for the third than for the fifth generation. The fifth generations of the unmodified as well as the carboxylate-functionalized poly(propylene imine) dendrimer show the same pH dependence. All these above results are changed when substrate is changed, but the trends remain. These results indicate that an explanation must be sought in the interplay between the properties of the dendrimer (size and charge) and the properties of the glass substrate (surface area, number and distribution of glass surface sites). The decrease in the number of adsorbed particles with generation can be ascribed to an increase in dendrimer size. To explain the observation that the adsorption does not reach the jamming limit, it is necessary to take into account the properties of the glass substrate. Lu¨thi and co-workers already demonstrated that the glass surface is highly inhomogeneous, regarding its sorption properties.28 They proved that the adsorption takes place at a limited number of sites and that new sites keep on appearing and disappearing at random positions on time scales of hours. It is this limited availability and the inhomogeneous distribution of glass surface sites that prevent the system from getting fully adsorbed up to the jamming limit. This is in contrast to the silica substrate, which is more crystalline with a more regular distribution of sites in the top layer. The obtained coverages on silica are more according to random adsorption on an ideal substrate. Here we find, just like the RSA model predicts, that it is the size that governs the adsorption on glass, albeit that there are not enough surface sites to realize coverages over 15%. These experiments are all performed with the same substrate. The experimental fact that a change in substrate leads to a change in the number of available sites, and thus a change in the amount of adsorption, but that the size dependence of the adsorption remains, fits perfectly into this picture. A change in pH can radically change the layer roughness as is well documented in the literature.15,18,19,29 Figure 5 demonstrates this for the present type of glass where the layer mass (which correlates with the roughness of the layer) increases by almost 50% while the pH decreases (28) Lu¨thi, Y.; Ricka, J.; Borkovec, M. J. Colloid Interface Sci. 1998, 206, 314-321. (29) Hiemstra, T.; van Riemsdijk, W. H. J. Colloid Interface Sci. 1990, 136, 132-150.


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from 9 to 3. An increased roughness will mean more available binding sites for the dendrimers. The effect is expected to be more pronounced for the smaller dendrimers than for the larger ones, which is experimentally observed. Also, the fifth-generation modified and unmodified dendrimers show the same tendency to bind stronger with decreasing pH, which implies that particle size is indeed the relevant parameter in addition to the collector properties. Not only the surface roughness decreases with increasing pH. Also the dendrimer charge decreases and the collector charge density increases with pH (see Figure 5). The latter two effects oppose each other. The change in surface roughness is however so spectacular that it is expected to dominate the collector properties and the adsorption properties. The ionic strength dependence of the surface roughness and surface charge density (Figure 6) is not so strong compared to their pH dependence. Both effects could very well more or less compensate each other. With increasing ionic strength the electrostatic interactions between the branches of the dendrimer will decrease, and hence the molecule will become more flexible. The more flexible dendrimers will find more binding sites in the rough surface layer, and it may therefore be expected that the adsorbed amount increases with ionic strength. The experimental results show that for these small particles the situation is achieved in which there is an intricate interplay between surface properties and particulate properties. For the larger polyelectrolytes the number of contacts between the surface and molecule are much larger so that predominantly the polymer properties determine the adsorption behavior. Even on glass this will lead to adsorption that can be explained in terms of trains, tails, and loops as frequently reported in the literature.30 On the other hand, in the case of larger and rigid particles, only surface properties will control the adsorption behavior.31 In this sense, dendrimers are between particles and polyelectrolytes. Acknowledgment. We acknowledge Theo van de Ven and Pierre Schaaf for useful discussions. Maurice Baars is thanked for the synthesis and supply of the carboxylatefunctionalized dendrimers, and we are grateful to DSM research for providing us with the poly(propylene imine) dendrimers. Appendix The Abeles matrix method is widely used to describe the reflectivity of stratified layers in terms of an effective refractive index neff and an optical thickness dm. These two parameters appear in the phase factor ∆ ) 2πneffdm cos θ in the layer matrix Lm:

Lm )

(

e-i∆ 0 0 ei∆

)

(3)

The product A of such matrixes is

A ) I0,1L1I1,2L2I2,3

and the amplitude reflection coefficient of interest is calculated by dividing matrix element A21 by A11. However, it is not always clear what the optical thickness means, also because it appears in combination with the effective refractive index. Therefore, the dendrimer layer (see Figure 4) is described with a thin island film theory. This means that (i) the thickness of the layer is small compared to the wavelength of the light and (ii) the system is discontinuous. Bedeaux and Vlieger describe the optical properties in terms of excess polarization and magnetization densities.32,33 Instead of an optical film thickness as mentioned above, the optical properties are described by so-called “optical invariants”. For small spherical particles, these optical invariants can be expressed in terms of particle parameters: size, refractive index, and coverage34 with

γ)

Im,m+1 )

(

1

tm,m+1 rm,m+1

rm,m+1 1

)

γ 2π FR and β ) 4 λ n

(6)

3

where R is the polarizability and F the density of the adsorbed dendrimers. In the Abeles method the interface matrix for the adsorbed layer at hand is not described in terms of the Fresnel equations, but with

F)

(

-r1 1 1 2 t1 r1 t1 - r12

)

(7)

with for p-polarized light

r1(p) )

Y X and 1-X 1-Y t1(p) ) 1 +

X + Y - 2XY (8) (1 - X)(1 - Y)

where

X)

iβn33 sin θ iγ cos θ and Y ) 2 cos θ 2n3

(9)

which replaces I1,2. The resulting matrix A becomes

A ) I0,1L1I1,3F

(10)

The advantage is that with the Lorenz-Lorentz equation35 γ is directly related to the total adsorbed amount of dendrimers Γ, whereas neffdm obviously is not:36

The interface matrix Im,m+1 describes the transition at the interface between medium m and m + 1 for both the transmitted and reflected light in terms of Fresnel coefficients:

1

(5)

Γ)

(neff - n3)dm γλ ) dn/dc 4πn32 dn/dc

(11)

LA000231J

(4)

(30) Takahashi, A.; Kawaguchi, M. Adv. Polym. Sci. 1982, 46 and references therein. (31) Weiss, M.; Lu¨thi, Y.; Ricka, J.; Jo¨rg, T.; Bebie, H. J. Colloid Interface Sci. 1998, 206, 322-331.

(32) Bedeaux, D.; Vlieger, J. Physica 1973, 67, 55-73. (33) Bedeaux, D.; Koper, G. J. M.; van der Zeeuw, E. A.; Vlieger, J.; Wind, M. M. Physica A 1994, 207, 285-292. (34) Haarmans, M. T. Ph.D. Thesis, Leiden University, Leiden, 1995. (35) Born, M.; Wolf, E. Principles of Optics; Pergamon Press: Elmsford, NY, 1959. (36) de Feijter, J. A.; Benjamins, J.; Veer, F. A. Biopolymers 1978, 17, 1759-1772.

Adsorption of Poly (propylene imine) Dendrimers on Glass. An

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